Tectonophysics xxx (2008) xxx-xxx

TECTO-124150; No of Pages 16

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Tectonophysics

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Trench investigation on the main strand of the Boconó fault in its central section, atMesa del Caballo, Mérida Andes, Venezuela

Franck A. Audemard M. a,⁎, Reinaldo Ollarves a, Michel Bechtold a, Gustavo Díaz a, Christian Beck b,Eduardo Carrillo b,c, Daniela Pantosti d, Hans Diederix e

a FUNVISIS, Earth Sciences Department, Apartado Postal 76.880, Caracas 1070-A, Venezuelab Université de Savoie, Laboratoire de Géodynamique des Chaînes Alpines, 73376 Le Bourget du Lac Cedex, Francec Universidad Central de Venezuela, Facultad de Ciencias, Instituto de Ciencias de la Tierra, Caracas, Venezuelad Istituto Nazionale di Geofisica e Vulcanologia, Via de Vigna Murata, 605, 00143 Roma, Italye NMCP, Bezuidenhoutseweg 12, 2594 AV The Hague, The Netherlands

⁎ Corresponding author. Also at Geology DepartmVenezuela. Fax: +58 212 2579977.

E-mail address: faudemard@funvisis.gob.ve (F.A. Aud

0040-1951/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.tecto.2007.08.020

Please cite this article as: Audemard M., F.Adel Caballo, Mérida Andes, Venezuela, Tect

A B S T R A C T

A R T I C L E I N F O

Article history:

The Mesa del Caballo trenc Received 6 March 2006Received in revised form 26 April 2007Accepted 22 November 2007Available online xxxx

Keywords:Exploratory fault trenchingPaleoseismologyPaleoseismic historyActive tectonicsBoconó faultMérida AndesVenezuela

h assessment confirms the Holocene activity of the main strand of the Boconófault at the Apartaderos pull-apart basin. Fifteen earthquakes, of which fourteen have been radiocarbondated, have been recognized, spanning the last 20,500 yr. Recurrence intervals of these ≥7 magnitude eventsare variable. The dominant mode of recurrence is 400–450 yr, and the second one is 900 yr. Eventually someevents are 1400–1800 yr apart. We suspect that our seismic record may be incomplete. This could be easilyjustified by several conditions: most of the earthquake recognitions is based on open-crack filling and theysuperpose spatially (eventually masking or destroying older fills), trenching may miss some events becausethe fault is made of en echelon Riedel shears, and a short return period may lead to faint differences betweenpaleosoils few hundreds years of age apart. This trench also images an older activity of the fault, as evidencedby plentiful earthquake-triggered liquefaction features, as well as slumping and rotational sliding.By comparing paleoseismic results between the Morro de Los Hoyos and Mesa del Caballo trenches, itappears that both fault strands bounding the Apartaderos pull-apart basin move simultaneously. Besides, themain strand also coseismically slips twice in between those common events. In other words, the seismicscenario could be that the northern strand recurs every 1200–1350 yr while the southern does every 400–450 yr. This is also in agreement with a respective slip share of 25 and 75% of the 9–10 mm/yr average slip ofthe Boconó fault in the Mérida Andes central sector.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Venezuela, though being crosscut by the major plate boundarybetween the Caribbean and South America, exhibits a rather moderateinstrumental seismicity both in frequency and magnitude (Audemardand Singer, 1996). Therefore, large earthquakes are rare in the seismiccatalog with very few exceptions (e.g., the Cariaco 1997 earthquake).Neither is the 500-year-long historical record of Venezuelan seismi-city long enough to provide reliable return periods for the mostdestructive earthquakes (Audemard and Singer, 1996, 1997; Aude-mard, 1998, 2005), although it does extend the record into the pastand reveals events of magnitude unrecorded instrumentally. Paleo-seismic assessments by trenching have helped to overcome theselimitations in Venezuela, as to magnitude and frequency of the largestearthquakes (refer to Audemard, 2003a and 2005 for more details).

ent, Universidad Central de

emard).

l rights reserved.

., et al., Trench investigationonophysics (2008), doi:10.10

This paper presents the results of one of such trench assessmentsperformed in early 2004 across the main strand of the Boconó fault, atMesa del Caballo, located south of Apartaderos and west of LakeMucubají. The Boconó fault here shows a releasing bend (Soulas,1985), where the underfed Apartaderos pull-apart basin sits (Aude-mard, 2003b). This study aims at, on one hand, determining the recentseismic history of this fault strand in its central Mérida Andes portion.On the other hand, these trench results should complement thoseobtained by Audemard et al. (1999a) on the northern and secondarystrand, thus allowing a better understanding of the seismogenicbehavior of both strands bounding the abovementioned pull-apartbasin. By comparing results between the two trenches, it should alsoshade light on the (independent and/or simultaneous) seismogenicactivation of both strands and whether the Apartaderos pull-apartbasin functions as a “leaky” or a perfect geometric barrier to earth-quake rupture propagation. In a larger perspective, this present study,as part of a broader project (FONACIT 2001002492: Seismic historyand seismogenic segmentation of the Boconó fault from the geologicanalysis of recently deformed sediments — through trenching and

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

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recovery of continuous cores), also intends to improve the currentsegmentation of the Boconó fault — preliminarily proposed byAudemard et al. (2000) — on the basis of identifying different seismichistories from several trench assessments at a set of pre-selected sitesalong the 500-km long active traces of the fault.

2. The Boconó fault

The Boconó fault is a spectacular NE–SW trending right-lateralstrike–slip (RLSS) fault that extends for about 500 km, between theTachira depression, at the border between Colombia and Venezuela,and the town of Morón located on the Caribbean coast of Venezuela.At the latter locality, the Boconó fault exhibits a 45° clockwise bend,thus prolonging into the east–west striking San Sebastián–El Pilarfault system (Figs. 1 and 2). At the southern tip, the Boconó faultconnects with the Colombian Llanos foothills fault system through theChinacota–Bramón fault system, after undergoing two opposite right-angle bends; structure known as the Pamplona indenter (Boinet,1985). In turn, the Colombian Llanos foothills fault system (CLFFS inFig. 1) appears to extend as far south as the Jambelí graben (Guayaquilgulf, Ecuador), thus splitting the northwestern corner of SouthAmerica from the rest of the continent (Stephan, 1982). Consequently,the Boconó fault seems to play a major role in the escape tectonics tothe NNE of this microplate, as well as of other internal minor blockssuch as the Triangular Maracaibo block (Audemard and Audemard,2002; Fig. 1). In addition, this minor block is directly bounded on thesoutheast by the Boconó fault. This escape is in turn taken up asshortening and overthrusting by the Venezuelan Borderlands terranesonto the Caribbean plate at the South Caribbean Deformed Belt (Fig.1).

The Boconó fault in the northern portion runs at the southern edgeof the Sierra de Aroa, splitting the recent fill of the Yaracuy depressionfrom the Mesozoic rocks of the Sierra de Aroa on the north (Fig. 2).Instead, in the Mérida Andes, the fault essentially runs along thebackbone of the chain. Within the Andes itself, the fault exhibitsdifferent relative locations: In the north, it splits the chain into twoalmost even halves; on the contrary, to the south, the fault is clearlyshifted north (Audemard and Audemard, 2002). Regardless of itsposition in the Mérida Andes, the trace at regional scale, in radar(SLAR) images, is particularly highlighted by the continuous align-ment of axial valleys (Fig. 2) of different rivers (e.g., Turbio,

Fig. 1. Schematic geodynamic map of the Caribbean–South America plate boundary zone, shoBoconó fault (BF), as part of a major fault system that extends farther south into the Colomstrike–slip motion. Relative location of Fig. 2 is also displayed. Other acronyms: EPF: El PilarMarta-Bucaramanga fault, SSF: San Sebastián fault.

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

Chabasquén, Saguás, Boconó, Burate, Santo Domingo, Chama, Moco-tíes and Torbes, from NE to SW) along the longest chain axis.

The total offset across the Boconó fault is still an issue underdebate. Stephan (1982) has proposed as much as 70–80 km ofcumulative dextral offsets in Mesozoic rocks. Audemard (1993) vividlyargues against this value because of being measured on a sub-horizontal marker such as a nappe front of the Caribbean tectonicphase. Amore reliable value appears to be about 30 km (Giraldo,1989;Audemard and Giraldo, 1997). Audemard and Audemard (2002) haveproposed an additional argument in favour of the 30-km-cumulativeright-lateral slip along the Boconó fault, based on the relative presentlocation of both Bouguer anomaly minima of the adjacent flexuralbasins to the Mérida Andes.

Since the pioneering work of Rod (1956b), several authors havemade estimates of slip rates along the Boconó fault for different timeintervals (see Schubert, 1982, for a complete review), with a very highvariability. However, for the Late Quaternary, the average slip rate intheMucubají area has been constrained between5 to9mm/a, basedon60 to 100 m of dextral offset of Los Zerpa moraines (Schubert, 1980;Soulas, 1985; Soulas et al., 1986; Audemard et al., 1999a), which areradiocarbon dated at a minimum of about 13 ka old. These rates areessentially consistent with those predicted by plate motion models ofabout 1 cm/yr, assuming that the Boconó fault is part of the mainboundary between the Caribbean and South American plates (e.g.,Molnar and Sykes, 1969; Minster and Jordan, 1978; Soulas, 1986;Freymueller et al., 1993; Trenkamp et al., 2002). Besides, Audemardet al. (1999a) have refined the slip rates of the Boconó fault at theApartaderos basin. Offsets of Late Pleistocene (Late Mérida glaciation)moraines yield a dextral slip rate of 2.3–3.0 mm/yr for the northernstrand, and of 5.0–7.7 mm/yr for the southern strand, roughlyrepresenting 25 and 75% of the overall slip rate for the Boconó faultof the 7 to 10 mm/yr in the past 15±2 kyr. Along strike and at largerscale, the Boconó fault slip rate decreases towards both ends. South ofwhere the fault is the fastest (near Mucubají), average slip ratedecreases to 5.2±0.9 mm/yr betweenMérida and San Cristobal (Fig. 2;Audemard,1997) and as little as a 1mm/yr at the Venezuela–Colombiaborder (Singer and Beltrán, 1996). Complexity of the Boconó faultsystem in the southern Andes (three active strands at least, in somecases), combined with existence of other sub-parallel active faults,such as: Queniquéa, San José, Uribante–Caparo, Seboruco among

wing the main active features (modified from Audemard et al., 2000). Among those, thebian Llanos foothills fault system — CLFFS — accommodates a large fraction of dextralfault, MB: Maracaibo Block, OAF: Oca-Ancón fault, PI: Pamplona indenter, SMBF: Santa

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 2. Relative location of the study area inwestern Venezuela indicated by white open ellipse (Shaded relief base map fromGarrity et al., 2004), where the Boconó fault (BF) locally exhibits amore easterly trend. The Boconó fault ismarked by the alignment of deeply incised axial valleys along the backbone of theMérida Andes (MA), extending from San Cristobal (SC) toMorón (Mo).

ig. 3.Macroseismic epicenterandVIII isoseismal for themajorhistorical earthquakes in the southernand centralMéridaAndes thatmaybeconfidentlyattributed to theBoconó fault (1674om Palme and Altez, 2002; 1812 from Altez, 2005; other events from Audemard, 1997). All macroseismic assessments use the MMI scale, except for the 1674 event, which is evaluated

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Ffr

using the MKS intensity scale. In the inset figure (lower right handside), the current fault segmentation of the Boconó fault based on geometric and/or kinematic criteria is shown (afterAudemard et al., 2000), on shaded relief map by Garrity et al. (2004), as well as relative location of previous trench investigation sites on the Boconó fault (after Audemard, 2005).

Please cite this article as: Audemard M., F.A., et al., Trench investigation on the main strand of the Boconó fault in its central section, at Mesadel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.1016/j.tecto.2007.08.020

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others (refer to Beltrán, 1994, for relative location), may account forsuch rate reduction. This is reflected on a longer returnperiod betweenequivalent earthquakes on the Boconó fault near Cordero (Audemard,1997). Similarly, sub-parallel and branching faulting along the north-ernmost portion of the Boconó fault may explain the slip rate drop(1.5–3mm/yr) reported by Casas (1991), along the Yaracuy depression.

Most of the largest Mérida Andes earthquakes have been ascribedto the Boconó fault zone, as Rod (1956a) has initially stated (Fig. 3).Cluff and Hansen (1969) have also indicated the same conviction forboth historical and instrumental earthquakes (refer to pages 5–13through 16, and 5–45). Among the large historical earthquakes in theMérida Andes (Table 1), Cluff and Hansen (1969) have credited theBoconó fault for the 1610, 1812, 1894, 1932 and 1950 earthquakes.Besides the 1610, 1812 and 1894 events, Aggarwal (1983) — latercited textually by McCann and Pennington (1990) and Suárez andNábelek (1990)— also ascribed the 1644 and 1875 earthquakes to theBoconó fault. Clear seismotectonic associations of those events areactually missing and several of these are still uncertain. For instance,Grases (1990) indicates a probable association of the Tocuyo 1950earthquake with the Boconó fault. So did Cluff and Hansen (1969).But Choy (1998) and Audemard et al. (1999b) propose that otherneighboring faults could also be potential sources because of thestructural complexity in the epicentral region, located west of theBoconó fault. Furthermore, Singer and Beltrán (1996) associate the1644 Pamplona and 1875 Cúcuta earthquakes with the Pamplonaindenter (refer to Fig. 2 for relative location), and particularly the1875 event with the Aguas Calientes fault (an active feature runningsub-parallel and north of the Boconó fault), but with the Boconófault. To date, only the association of the 1610 and 1894 earthquakeswith the Boconó fault, and particularly with its southern segment(Fig. 3), has been confirmed by paleoseismic studies (Audemard,1997). The 1812 earthquake devastated Mérida (Fig. 3) and producedextensive damage to Barquisimeto and along the southern flank ofthe Sierra de Aroa (B and Sa in Fig. 2), and as far as Caracas, which islocated about 500 km to the northeast of the study area. Althoughthis event has been interpreted as a combination of two to threeseparate events based on damage reports, many authors (e.g., Soulaset al., 1987; Grases, 1990; Rodríguez et al., 1997; Altez, 2005) suspectthat one of the events ruptured the Boconó fault from Mérida to thenortheast.

Table 1Major historical earthquakes credited to different segments of the Boconó fault, indicating thet al., 2005)

Event After Palme et al. (2005)

Long. (W) Lat. (N) Mag. (Mw) N° intensity reports

1610 71.65 8.45 7.60±0.63 4

1674 70.80 8.95 7.40±0.63 4

1812 71.05 8.65 6.00±0.63 11

1849 72.25 8.05 6.35±0.63 4

1894 71.70 8.70 7.56±0.33 32

1932 72.00 8.30 6.50±0.31 44

Their corresponding epicenters and VIII isoseismals are depicted in Fig. 3.

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

The present-day seismicity along the Boconó fault occurs within abroad zone that generally comprises the entire width of the Andeanrange, suggesting that other faults internal to and bounding the rangemay be seismogenic (Audemard and Audemard, 2002, and pertinentfigures therein). The depth distribution of seismicity associated withthe chain build-up is consistent with an entire (first 20–25 km) brittlecrust being deformed at present (Audemard and Audemard, 2002).

At least five different portions or seismogenic segments have beenidentified along the Boconó fault based on past seismic activity andpotential geometric barriers or complexities to rupture propagation(see Audemard et al., 2000, for more details; Fig. 3 inset).

3. The Boconó fault near Apartaderos

The study area partly lies in the northeastern end of theApartaderos basin that forms at a releasing bend of the Boconó fault(Fig. 4; Soulas, 1985; Audemard et al., 1999a). In fact, this part of thefault shows a slightly more easterly strike with respect to the overallNE–SW trend (Fig. 2). Here, the Boconó fault is well preserved along a3500-m-high drainage divide that separates the streams flowing SEinto the Orinoco basin from those flowing NW into the Maracaibobasin (Figs. 2, and 4–6). This divide is centred in the area of lakeMucubají, belonging to the Sierra Nevada National Park (Fig. 4).Because of sitting in this high-elevation divide, this Apartaderos basinis being deeply dissected at present by both the Chama and SantoDomingo river headwaters (Figs. 4 and 5), even though this depressionseems to have accumulated a rather thick sequence of Pleistoceneglacial deposits as those preserved at Mesas del Caballo and Julián(Audemard, 2003b; Fig. 6). The Boconó fault here comprises twoconspicuous sub-parallel basin-bounding active strands located atabout 1–1.5 km apart (Figs. 4–6), both exhibiting magnificentgeomorphologic expression preserved in latest Pleistocene andHolocene deposits (Fig. 4). Even though the vertical component ofslip appears to be significant at some localities (e.g.: near El Cerrito, inthe village of Apartaderos, Los Zerpa and near Las Tapias), most of thefault geomorphology is typical of a RLSS fault (Fig. 4). Both faultstrands offset latest Pleistocene and Holocene deposits, but faultscarps along the southern strand appear much fresher and morerecently formed than in the northern strand. This seems tomatchwitha faster slip rate on the southern strand. Both strands are easily

eir magnitudes andmacroseismic epicenters after several sources (modified from Palme

After other authors

Epicenter (Long/Lat) Magnitude References

71.8/8.3 7.3 MI Fiedler (1961)71.8/8.3 Cluff and Hansen (1969)

7.2 Ms Ferrer and Laffaille (1998)7.1–7.3 Ms Audemard (1998)

70.64/9.3 Grases (1980)70.9/8.9 6.8 MI Palme and Altez (2002)71.3/8.5 7.1 mC Fiedler (1961)71.05/8.65 aprox. 5.1–5.4 MI Laffaille and Ferrer (2003)72.3/7.3 6.0 MI Fiedler (1961)

6.6 mC Fiedler (1961)72.2/7.9 6.7 MI Cluff and Hansen (1969)72.2/7.9 Ramírez (1975)71.7/8.5 7.1 MI Fiedler (1961)

7.0 mC Fiedler (1961)7.1–7.3 Ms Audemard (1998)

71.69/8.55 7.1–7.4 MI Rengifo and Laffaille (2000)71.75/8.25 6.75 Ms Gutenberg and Richter (1954)71.9/8.2 6.5 MI Fiedler (1961)71.88/8.29 6.75 Ms Dewey (1972)71.7/8.3 6.75 MI Ramírez (1975)72.03/8.15 6.5–6.7 Escobar and Rengifo (2003)

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 4. Geomorphic mapping of the two Boconó fault strands bounding the Apartaderos pull-apart basin, based on aerial photo interpretation and field survey. Trench site locationsare also shown (indicated by black solid rectangles): (1) Morro de Los Hoyos study (Audemard et al., 1999a) and (2) Mesa del Caballo assessment (this study).

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mappable on the basis of earthquake-related morphology. Shutterridges and linear drainages, offset drainages, ridges and fans, sagponds, fault saddles, fault scarps and trenches are present at severallocations (Fig. 4).

4. Trenching across the Boconó fault

Several paleoseismic trenches have already been excavated andstudied across the Boconó fault at different sites of its 350-km-longAndean section, but are still too few to build a reliable seismogenicsegmentation of the fault. The relative location of those trench studiesis provided in Fig. 3 of Audemard (2005) and reproduced here in Fig. 3.Each of them has individually aimed at establishing, in the best case,the seismic history of the portion under evaluation but they haveusually allowed to determine the return period of the larger earth-

Fig. 5. NNW-directed bird-eye view of the Mesas de Julián and Caballo, nearby Lake Mucubsouthern main strand of the Boconó fault and Mesa del Caballo are also shown.

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

quakes. The first two paleoseismic assessments ever performed wereon the southern portion of the fault in late 1986–early 1987, whoseresults were published by Audemard (1997). Here, the two mostrecent earthquakes are about 300 years apart as found in La Gritatrench (refer to Audemard, 1997; T2 in Fig. 3 inset). Further south,where the fault splits into three strands, one of these strands recordedonly three paleoearthquakes during Holocene time (Mis Deliriostrench; T1 in Fig. 3 inset; Audemard, 1997), suggesting an averagerecurrence interval that is one order of magnitude longer than wherethe fault is a single strand, and thus that ruptures may alternateamong the three strands. Later, the Buena Vista trench (San Miguel,SW of Barquisimeto, Lara state; T4 in Fig. 3 inset), financed by theBarquisimeto city municipality, was dug across the northernmostAndean portion of the Boconó fault in March 1987 (Beltrán et al.,1990). This trench could only confirm the Holocene activity of the fault

ají, Mérida state. The Pedregal fan complex is outlined. Its relationships with both the

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

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in its northern Andean portion (Beltrán et al., 1990; Audemard, 2005).More recently, a trench was dug on the northern (secondary) strand ofthe Boconó fault, within the Apartaderos basin and close to LagunaMucubají, on the southern slope of Morro de Los Hoyos, in early 1997(T3 in Fig. 3 inset; also in Fig. 4). Audemard et al. (1999a), from thistrench cut across a linear shutter ridge and associated sag pond,indicate that their estimate of the return period for 3–4 m slipearthquakes for the northern strand should be three times as long asthat of the southern strand, based on the abovementioned slipdistribution between both strands. This also means for these authorsthat themain (southern) strand should record both different andmorefrequent events. However, they also keep open the possibility of bothstrands moving simultaneously since they bound a single releasingbend and may likely represent a single fundamental fault at depth.

5. The trench site

Mesa del Caballo is a flattop relief that lies west of the LatePleistocene lake Mucubají moraine complex, northwest of the LatePleistocene (Late Mérida Glaciation — LMG — in Fig. 6, or Last Glacial

Fig. 6.Main geomorphic units in the close region to the Mesa del Caballo trench site, which istop figure, as well as both strands of the Boconó fault bounding the Apartaderos pull-apart bMesa del Caballo region. This pattern is also visible at smaller scale in the bottom figure, pa

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

Maximum — LGM) Caballo moraine and north of the even younger LaCañada ravine (Figs. 5 and 6). TheMesa del Caballo seems to bemade ofolder Pleistocene glacial deposits (Early Mérida Glaciation — EMG —

and/or even older?); older than the Late Mérida Glaciation, as definedby Schubert (1974). Evidence to this may be derived from Dirszowskyet al. (2005), who describe in detail a N40-m thick sequence exposed inthe La Cañada ravine. This sedimentary sequence— named Pedregal-5or by the acronym PED5 — rests unconformably against the southeastflank of the Mesa del Caballo positive relief (Fig. 6). PED5 provides arecord of the Early and Late Mérida Glaciations or stades, as well as ofthe interstadial period; termed by the authors as the Pedregalinterstade, after the village of El Pedregal (Dirszowsky et al., 2005;for location, refer to Fig. 5). The interstade sequence attests to a lakePedregal that lived between about 60 kyr (age of peat VII at the base ofunit VIIb; also limit of the radiocarbon dating technique applied by theIsoTrace facility of Toronto, Canada) and younger glacier re-advancesduring the LateMérida Glaciation (Dirszowskyet al., 2005), which tookplace during the LGM (between 25–13 kyr, after Schubert andClapperton, 1990). Very roughly, the main strand of the Boconó faultin the surrounds of the excavation site flanks the Mesa del Caballo on

highlighted by a star in the bottom figure. The Pedregal fan complex is delineated in theasin. Also notice the R-shear pattern exhibited by the main strand in the lake Mucubaji–rticularly at the trench site.

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 7. South-looking ground view of the trench site. Note its location in relation withthe earthquake-related landforms (pop-up, sagpond and deflected little creek) alongthe Boconó fault main strand, as well as with the Pedregal fan complex and the deeplyincised La Cañada ravine or creek.

Fig. 8. View to the north of the trench interior, from the ravine edge. Black bars indicatethe grid spacing on walls, as well as width of the trench bottom.

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the southeast and runs near — and frequently uphill of — the sedi-mentary boundary between the Pedregal “fan complex” (as named byDirszowsky et al., 2005) and Mesa del Caballo glacial deposits (Fig. 6).Only small relict bodies of the Pedregal fan complex are preservednorth of the La Cañada ravine and on the southeastern slope of Mesadel Caballo. Of note is that the La Cañada ravine, instead of its locationbeing controlled by the main strand of the Boconó fault, deeply incisesalong the inclined basal unconformity of PED5, slightly offscrappingthe Mesa del Caballo older glacial deposits.

Following the criteria developed by Audemard (2005) for trench siteselection, the excavation site was placed, from the structural viewpoint,where the active main strand of the Boconó fault at the Apartaderosbasin appeared rather simple because it showed a single trace (trenchsite 2 in Fig. 4). This trace, in detail, displays en echelon synthetic Riedelshears of several tens of meters in length. The Riedel shears are con-nected at each overlap by modest (few meter long) pop-ups (Fig. 6).

The trench at Mesa del Caballo was dug across one of those Riedelshears and its related pop-up in April 2004 (Figs. 6 and 7). In fact, thispop-up structure also acted as a small shutter ridge, behind which a15–20-m-across sagpond used to bear water. Deactivation of thesagpond occurred when a breach in the western tip of the ridgeopened (on right handside of Fig. 7). In the time spanned betweenshuttering and breaching, the little creek feeding this pond has beendextrally offset in 9 m apparently. This site selection ensured not onlythe accumulation of young but also datable materials in the pond andbehind the pop-up. In addition, other uncertainties regardingtrenching site selection were ruled out. Since the southern flank ofthe pop-up has been affected by gully erosion and retrograding slidingby the La Cañada ravine headwaters, beds of the Pedregal fan complexhave been exposed. In other words, the small pop-up is partly one ofthose relicts of the Pedregal fan complex flanking the Mesa delCaballo, north of the ravine. Conversely to the general tendency of thefault running off—but very close to— the fan boundary, the fault at thetrench site actually splits a relict of the Pedregal fan complex fromdifferent deposits resting on the Mesa del Caballo southern flank.

The trench was hand-dug normal to the Riedel shear orientation—

along the N15°W direction — extending from the sagpond into theravine crown, across a single Riedel shear and the related shutterridge. Meeting the owner's requests, no machinery was employed,regardless of the excellent access to the trench site for any type ofvehicle during the entire study (March–April, corresponding to theend of the dry season). Final dimensions of the Mesa del Caballotrench were: 14 m long, 2.5 m deep and 0.75 m wide at the trenchbottom (Fig. 8). As manual excavation progressed, the trench walls

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

were scrapped from top to bottom. Trench drainage was ensured bythe gentle dipping of its bottom into the ravine located at the trenchsouth end (Figs. 7 and 8), although rain was almost completely absentduring excavation except for very fine occasional drizzles.

6. Trench stratigraphy and structures

As expected from surface geology, a single fault zone is exposed intrench walls (Fig. 9a and b). In addition, this fault juxtaposes twoclearly different sedimentary units. As predicted, the fault appearedbetween the northern foot of the pop-up and the sagpond. Thesouthern end of the trench, corresponding to the pop-up, displays asequence of light-colored, verywell well-stratified beds (Figs. 8 and 9).This unit shows alternations of clays, silts, sandy silts, fine-grainedsands, and pebbly coarse sands. Some clay beds are finely laminated(Fig. 9a and b). The entire sequence is gently folded. There is a goodmatch between the gentle anticline imaged in trench walls and thepop-up displayed on the topography (Fig. 9a and b). Consequently, thismorphology is a transpressional feature related to a restraining overlapbetween contiguous Riedel shears, from the genetic viewpoint. Atop ofthe lower third part of this unit appears a bed comprising a mixture ofdiverse materials of very different grain size. It even contains a 60-cm-across block (a dropstone?; at SSE tip of trench in Fig. 9a). Besides, thisbed is highly deformed and contorted internally. The sequence abovethis “mixed” layer fines upward. Two organic-rich, few-cm thick layersare interspersed into this clay/silt dominated top package. Thispackage as a whole is affected by south-directed (towards the ravine)rotational sliding rooted on top of the “mixed” layer, with overall listricgeometry (Fig. 9a and b). Besides sliding, this unit shows plentifulevidence of other soft-sediment deformations. To illustrate this, let usmention the following. The base of the deepest of all fine-grained sandbeds shows ball-and-pillars structures. In addition, nearing the faultzone, irregularly shaped pockets of liquefied, structureless sandscontain angular clasts of the light-colored sequence. Besides, these

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

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8 F.A. Audemard M. et al. / Tectonophysics xxx (2008) xxx-xxx

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Please cite this article as: Audemard M., F.A., et al., Trench investigation on the main strand of the Boconó fault in its central section, at Mesadel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.1016/j.tecto.2007.08.020

Fig. 10. Earthquake-triggered, irregularly shaped, structureless liquefied-sand body inwest wall, just north of the fault zone, which injects upward and then parallel tobedding (sill shape) in upper part. Compare to trench wall log depicted in Fig. 9b.

Table 2Radiocarbon ages of samples from the Mesa del Caballo trench, Apartaderos, WesternVenezuela

FUNVISISsample N°

Betaanalytic N°

2 sigmacalibratedage BP (years)

Calibrated calendar years

1 sigma 2 sigma

VEN-01-04a 201215 10100–10090 BC 7950–7755 BC 8150–81409920–9660 BC 7970–7720

VEN-02-04a 201216 7430–7260 BC 5465–5340 BC 5480–5320VEN-03-04a 201217 20560–19610 BC 18530–17740 BC 18610–17660VEN-04-04a 201218 7500–7420 BC 5478–5516 BC 5550–5470VEN-05-04a 201219 4080–3850 BC 2040–1940 BC 2130–1900

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liquefied sands appear injected as little sills within this sequence(Fig. 10), being unequivocal proof of occurrence of liquefaction.Occasionally, very white fine sands are injected cutting across bedding(Fig. 10). These sands seem to be fed from very thin sand layers, whichin turn just vanish or pinch out within clays, implying remobilizationby liquefaction. All these deformations once required that thissequence was still partly unconsolidated and water saturated. Equally,they very likely record an older activity of the Boconó fault than thatimaged by the events we shall describe later in this paper. Fromdifferent lines of evidence, this sequence undoubtedly belongs to thePedregal fan complex. Based on the stratigraphy — sedimentarysequence clearly dominated by well-stratified fine sediments — thisBoconó-fault bounded unit correlates to the interstadial perioddescribed in the La Cañada ravine by Dirszowsky et al. (2005), andmore particularly to their unit VI or VII. This seems supported by thefact that such a sequencewas faulted byan eventon the Boconó fault asold as 20560–19610 yr BP (sample VEN-03-04). At that time thesequence needed to be, at least, consolidated to a certain extent tobehave in a brittle manner. Finally, all forementioned earthquake-induced soft-sediment deformations had to predate sediment con-solidation and took place while lake Pedregal was still functioningduring the Late Pleistocene, roughly between 60 and 25 kyr BP. This

Fig. 11. Detail of the bluish convoluted “varved-type” sagpond sequence, in west wall.Note centimeter-scale asymmetrical folding, indicating transport or remobilization ofthese clays towards the left (to the south) due to slump of overlying mass.

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

sequence belonging to the Pedregal fan complex exposed in trenchwalls is also disturbed byminor faults (Fig. 9a and b). Activation of thisset of faults is post-sedimentationbutwhen sedimentswere still partlyundrained. Some of these fault planes were used as conduits byliquefied sands, since some pockets formed atop of some fault tips(Fig. 9a and b). All these evidence attest to a long-lasting activity of theBoconó fault, spanning the last 60 kyr, but the absence of any datablematerial within this sequence made all of them of no use for paleo-seismic purposes.

North of the fault, the trenchwalls from bottom to top expose fluvialor deltaic deposits, interspersed by lacustrine beds. It also fines upward.It is cappedbya conglomerate containing angular and subangularblocksand boulders floating in an angular pebble conglomerate, probably ofglacial or periglacial origin. The exposed base of the fluvial sequence is asub-rounded cobble unit, in which some sparse large boulders arefloating (Fig. 9a and b). The top of this unit is more fined grained andshows crossbedding structures. Laterally, these topbeds change into siltsthat in turn interfingers with a bluish, finely laminated, anoxic clay ofsome 20–30 cm in thickness. These clays are strongly contorted andconvoluted (Figs. 9a, b and 11). Above this rests a slump, detached andslid over the clays (Fig. 9a and b). Based on the internal deformation, thelaminated clays were very plastic when slump occurred. Furthermore,small asymmetric south-vergent folds in the “varved” clays indicatesense of motion of the slump (Fig. 11). This mass instability very likelyoccurred underwater because the slid mass is a chaotic mixture of theupper part of the fluvial sequence, in which is it also incorporated theoverlying glacial bed. This sequence north of the fault could wellcorrelate with marginal deltaic facies, with some intercalated short-lived glacial episodes, of those described by Mahaney et al. (2004) inunits 2–3 of PED1 or by Dirszowsky et al. (2005) in unit II of PED5. None

VEN-06-04a 201220 4250–4060 BC 2270–2260 BC 2300–21204050–3990 BC 2220–2135 BC 2100–2040

VEN-07-04a 201221 4840–4580 BC 2880–2860 BC 2890–2630BC 2820–2690

VEN-08-04a 201222 5580–5520 BC 3620–3590 BC 3630–35605490–5320 BC 3525–3495 BC 3540–3370

BC 3460–3370VEN-09-04a 201223 4420–4220 BC 2460–2300 BC 2470–2270

4210–4170 BC 2260–2220VEN-10-04 201224 2970–2760 BC 1020–800VEN-11-04a 201225 7500–7420 BC 5516–5478 BC 5550–5470VEN-12-04a 201226 6170–5920 BC 4060–3990 BC 4220–3970VEN-13-04a 201227 7420–7260 BC 5450–5410 BC 5470–5310

BC 5390–5325VEN-15-04a 201228 5740–5600 BC 3765–3665 BC 3790–3650VEN-17-04a 201229 5610–5570 BC 3650–3630 BC 3660–3620

5540–5470 BC 3565–3540 BC 3600–3520VEN-18-04a 201230 5850–5840 BC 3780–3685 BC 3900–3890

5750–5600 BC 3800–3660VEN-19-04a 201231 6540–6320 BC 4530–4450 BC 4590–4360VEN-20-04a 201232 9130–9000 BC 7100–7065 BC 7180–7050VEN-21-04 201233 1530–1300 AD 420–650VEN-25-04 201234 960–730 AD 990–1220VEN-26-04 201235 650–500 AD 1300–1450VEN-32-04 201236 520–290 AD 1430–1660VEN-33-04 201237 910–660 AD 1040–1290

a AMS dating.

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 12. Close-ups of different crevasse fills in east trench wall, within the fault zone ofthe main Boconó strand at the Apartaderos pull-apart basin: A) dark-brown organic-rich open-crack fill revealed during trench digging, nearing top of wall (excavation hadonly progressed 1.2 m deep). Also note the small pocket of white coarse sand; B) detailof tip of same organic-rich fill (just below sample 17 in Fig. 9a), also depicting theearthquake-liquefied sand contained into an older and lighter-colored crevasse; C)youngest crevasse fill of all sampled for radiocarbon dating (sample 21 in Fig. 9a).

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of these sediments north of the fault contain 14C datable materials. Ontop of this, a very organic-rich bed has developed, indicating thereinstallation of a younger sagpond. This sagpond sequence shouldbelong to the deactivated pond that can be still clearly delineated fromthe surface morphology at the trench site (Fig. 7). The base of thissagpond yields a radiocarbon age of cal. 2300–2040 yr BC (sample VEN-06-04). It can be confidently stated that only then the small pop-upsouth of the main strand could completely block that “little creek”draining the southern flank of Mesa del Caballo. The deactivation of thissecond sagpond happened when the pop-up was breached at itswestern end by the feeding little creek (Fig. 7). This can be bracketedsometime between cal. 990–1220 yr AD and cal. 1300–1450 yr AD(between samples VEN-25-04 and VEN-26-04, Table 2).

In trench walls, the fault zone itself is narrow (Fig. 9a and b). Inaddition, it is clearly visible because it stands out for being dark-colored(Fig.12A throughC). Its color results from the collapse of organic-rich soilsinto the open fault plane. Furthermore, several slices with different tonesof brown can be recognized squeezed in it. This implies that the fault atthis particular site has functioned as an open crevasse during eachearthquake (refer to case 1 of Fig. 12 in Audemard, 2005). From thestructural viewpoint, this also points out that the Riedel (R) shears notonly had horizontal slip but also had some across opening component. Inother words, they did not act as pure R shears but a combination of Rshears and T cracks (refer to Fig. 4 in Audemard, 2006). The crevasse fillssometimes also showevidence of associated liquefaction (Fig.12A and B).

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

7. Earthquake recognition and timing

As mentioned above, several lines of evidence attest to the LatePleistocene and Holocene activity of the main strand of the Boconófault at this trench site. However, the large amount of soft-sedimentdeformations recognized in the trench, such as slumping andliquefaction, if actually induced by any seismic activity, could not beused to timeframe several individual events, which would haveextended the earthquake chronology of the Boconó main strand farback in time, spanning several tens of kyr. Two exceptions to this are: 1)an event can be identified on the evidence of a small colluvial wedge(visible at the southern end of the west wall log in Fig. 9b) thatpinpoints a re-activation of the rotational sliding affecting the upperpart of the sequence forming the southern flank of the pop-up (Fig.13).Time of this event is bracketed by samples VEN-32-04 and VEN-33-04(Table 2). The age inversion between these two dates supports thecolluvial character of the wedge. As erosion scraps off the top of theraised block, older organic soils are progressively exposed,which are inturn later accumulated at the scarp foot, thus inverting age chronologyin the downthrown compartment; 2) another event, younger than theslump, can be recognized. On the west wall (Fig. 9b), a set of sub-vertical fractures cutting the slump mass shows organic-rich infill atthe topmost tip of the crack. These fills should be slightly younger thanthe age of the bottom of the organic-rich sagpond. This event could bebracketed between samples VEN-24-04 and VEN-25-04, but no agedetermination was carried out for sample VEN-24-04. On the otherhand, the set of internal brittle faults affecting the Pedregal fandeposits at the southern end of the trench (Fig. 9),with few-centimeterapparent vertical throw and different from listric sliding surfaces,could not help determine any old event either, because this faultingpostdates the Pedregal sequence as a whole.

The remaining events, except the two latest ones, are all recognizedon the basis of the different color crevasse fills, as well as of variouscrevasse or fissure generations (Fig. 12). Each individual crevasse fillhas been correlated to the filling of an open ground-surface crackformed during an earthquake, as proposed by Audemard (2005) in hisFig. 12. We believe that the opening of these fissures or tension gashes(or tension— T— cracks combinedwith R shears, like those depicted byAudemard, 2006) are coseismic and areproducedbystick–slip. If any ofthese crevasses had to open by creep or aseismically, it should haveaccumulated a single continuous fill, but a set of different superposedor nested fills. As the dating of the crevasse has relied on the age of itsinfill, we had to assume that the obtained age represents the age of thetop soil forming on ground surface at the time when the earthquaketakes place (refer to Fig. 12 in Audemard, 2005). It is then supposedthat, in the years to come after the earthquake, the top soil falls byrunoff into— and fills— the open crack formed in associationwith theearthquake that we intend to date. In regions where rainfall issignificantly high, the time to fill the crevasse is in the order of a fewyears, which is negligible at our time scale (earthquakes recurringevery few hundred years). To assure that the age obtained for eachearthquake occurrence is the closest to the topmost soil age(representing the pre-event horizon), crevasse samples were alwayscollected at the bottom tip of the crack. Due to the nature and availablesize of the datable materials, the radiocarbon dating has always beenperformed by Accelerometer Mass Spectrometry — AMS — on verysmall samples of bulk organic organic-rich sediments (mainly soils andpaleosoils) axially collected into the crack or crevasse bottom tips.

As to individual event recognition, 7 earthquakes are recorded inthe main crevasse on the west wall (Fig. 9b), from the same number ofdifferent crevasse infillings individualized on the basis of their color,crosscutting relationships and/or grain size. From crosscutting and/orsuperposition relationships between crevasse fills on trench walls, itcan be stated that fills on this wall should be chronologically orderedas follows: (1) crevasse fill dated by sample 03 (as labelled in Figs. 9band 14, corresponding to VEN-03-07 in Table 2) should be older than

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 13. Small colluvial wedge at the southern end of the west trench wall (for location, refer to Fig. 9b, between samples 32 and 33) that attests to the re-activation of the rotationalsliding.

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that of 08, which should be older than 07. This one in turn should beolder than 09, which should be older than 10; (2) being on a adjacentand parallel crevasse, it is impossible, without dating, to establish anyrelationship between crevasse fill dated by sample 02 and that of 03,but the former one should be older than 04, which should be olderthan 05 but the latter one being younger than paleosoil sampled byVEN-06-04. Finally, crevasse 5 should be older than that dated bysample 07, which is not the case. This can be attributed to the irregularnon-planar geometry of T-R cracks when opening on an inclinedtopography (refer to Fig. 4 of Audemard, 2006), which must be thecase when a R shear bounds an associated pop-up. On the other hand,samples 02 and 04 are dating the same infill, which is fractioned by aliquefied-sand injection; (3) Two other sub-parallel crevasses are alsorelated to the latter infilling, which are dated by samples VEN-11-04and VEN-13-04, which keep no crosscutting relationship with themain crevasse that underlines the main presently-active fault zone inthe trench; (4) Two other independent crevasse fills have beenrecognized after dating (01 and 12). Besides, crevasse identified bysample 12 appears to be coeval with other filled crevasses to the southof it (sitting on top of the pop-up) that cut down the topmost beds ofthe Pedregal unit.

In the sameway as for thewest wall, crevasse fill dated by 21 in theeast wall (Figs. 9a and 15) should be younger than that of 20 and also

Fig. 14. Blow up of west wall log of the Mesa del Caballo excavation, imaging the fault zone athose indicated otherwise.

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

the youngest of all fills in this wall (dark brown, wetter patch inFig. 12C). As to the other fills, age constraint is required because theyare adjacent to each other with no relationships between them.Nevertheless, two significant aspects can be pinpointed from theassessment of the crevasse fills in this wall: (a) ages of samples 17 and18, collected one above the other in the same crevasse fill, support thefilling evolution of the open cracks abovementioned (runoff scrapstopsoil progressively and fills the crevasse simultaneously), because ofthe age inversion; similar to the crevasse dated in the west wall bysamples 02 and 04; (b) relative position of crevasse fill within themaincrevasse and its verticality does not correlate with its age. This isprobably due to the original inclination of the crevasse walls, which isin turn controlled by the location of the tension gashes on the gentleslopes of the pop-up (refer to Fig. 4 of Audemard, 2006 for a bettervisualization).

The 7 different fills recognized on the trench west wall have beenradiocarbon dated at (Fig. 14 and Table 2): cal. 18610–17660 yr BC(sample VEN-03-04), cal. 5475–5315 yr BC (sample VEN-02-04), cal.3630–3370 yr BC (sample VEN-08-04), cal. 2890–2625 yr BC (sampleVEN-07-04), cal. 2470–2220 yr BC (sample VEN-09-04), cal. 2130–1900 yr BC (sample VEN-05-04) and cal. 1020–800 yr BC (sample VEN-10-04). In other less developed open cracks, also visible in the westwall and all located south of the largest crevasse, the following crevasse

nd 14C sampling sites. Radiocarbon ages are also reported, given in cal. yr BC except for

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 15. Detail of east wall trench log at the Mesa del Caballo site, displaying the faultzone and the 14C ages. Those ages are all reported in cal. yr BC, except for those indicatedotherwise.

12 F.A. Audemard M. et al. / Tectonophysics xxx (2008) xxx-xxx

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fills were also recognized and dated (Figs. 9b and 14 and Table 2): cal.7970–7720 yr BC (sample VEN-01-04) and cal. 4220–3970 yr BC(sample VEN-12-04).

In addition, 3 other fills have been recognized in the east wall(Fig. 15): cal. 7180–7050 yr BC (sample VEN-20-04), cal. 4600–4360 yrBC (sample VEN-19-04) and cal. 420–650 yr AD (sample VEN-21-04).Fill dated by samples VEN-15-04, VEN-17-04 and VEN-18-04 in thiswall coincides in age with that of VEN-08-04 in the west wall.

For all these 12 earthquakes identified from the crack fill criterium(scenario 1 in Fig. 12 by Audemard, 2005), timing of each event hasbeen fixed slightly younger than the age range of each of the fills(Fig. 16), since the surface soil at the time of the event slightly pre-dates the earthquake during which the cracks instantaneouslyopened. This has been always applied with the single exception ofthe event based on age of sample VEN-08-04. On the east wall, thisevent is recorded by a long sub-vertical crack fill, where three sampleswere collected at different depths (Fig. 15), all being slightly prior tocal. 3500 yr BC (samples VEN-15-04, VEN-17-04 and VEN-18-04). As a

Fig. 16. Proposed chronology of earthquakes for the Boconó fault main strand, derived frommatch with FUNVISIS sample labels provided in Table 2. White larger boxes correspond to

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

remark, the vertical continuity of this crevasse in the east wall isinterrupted by a large liquefied-sand injection (Fig. 15).

Finally, the penultimate event is proposed on the basis of thedeactivation of the second andmost recent sagpond. This deactivationshould have occurredwhen the little creek feeding this pond breachedthe pop-up. Time of occurrence is bracketed between samples VEN-25-04 and VEN-26-04, roughly between cal. 1200 and 1300 yr AD. It isalso interpreted on the basis of the colluvial wedge criterium (scenario3 in Fig. 12 in Audemard, 2005).

In conclusion, 14 events are recognized and dated from differentevidence in the Mesa del Caballo trench along the main strand of theBoconó fault, but mostly (the 12 oldest) using the crack fill criterium(Fig. 12). The two most recent earthquakes are established otherwise.The penultimate event relies on the colluvial wedge criterium, whichalso coincides with a major local environmental change (the deactiva-tion of the youngest sagpond). On the other hand, the latest event isinterpreted on the basis of the re-activation of the rotational slidingaffecting the upper part of the sequence forming the southern flank ofthe pop-up cut by the trench (younger than sample VEN-32-04). Sincethis event is not recognized by an on-fault deformation but by slide re-activation in an unstable retrograding ravine crown, and sliding mighthave been triggered by a distant earthquake, it might have not beennecessarily produced by coseismic slip on this adjacent main strand ofthe Boconó fault bounding the Apartaderos pull-apart basin. This latestevent should be slightly younger than 1660 yr AD (upper bound ofsample VEN-32-04 that pre-dates the colluvial wedge; Fig. 13), fallinginto the timespan of historical earthquakes in the Mérida Andes. As amatter of fact, recent studies by Palme and Altez (2002) have re-evaluated the macroseismic information of the January 1674 events,determining that this sequence of large events produced severedamage to the cities of Mérida and Trujillo (refer to Fig. 2 for relativelocation), which are located almost 150 km apart, SW and NE of theApartaderos pull-apart basin respectively. From this re-assessment,these authors have drawn the 1674MKSVIII isoseismal, shown in Fig. 3,which well corresponds in length and location with segment B of theBoconó fault, as defined by Audemard et al. (2000) and shown in Fig. 3inset (labelled as Boc-b). This would lead to assign this event toBoconó's segment B. Taking into account that segments Boc-b and Boc-c of the Boconó fault respectively correspond to the northernsecondary and southern main strands of the Boconó fault bounding

the Mesa del Caballo trench study. Calendar 14C ages are reported as bars. Bar numbers2-sigma calibration ages and black smaller boxes to 1-sigma calibration ages.

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

Fig. 17. Comparison of earthquake chronologies for the Boconó fault at the Apartaderos pull-apart basin: A) Timing of events on individual fault zones (A, B and C) from the Morro deLos Hoyos trench excavated across the northern secondary strand, as well as tentative correlation of events between fault zones by Audemard et al. (1999a) are shown. Black thin linesnear diagram top are time ranges of dated samples for the three fault zones; thicker lines are time ranges of rupture events on individual fault zones (when only one dated sample isavailable to constrain an event the arrow indicates if it occurred after or before the sample was deposited); various boxes are tentative correlation between ruptures at different faultzones (multiple ruptures that may have participated in an ‘aggregate event’ are joined by a plus sign; in some cases more than one possible correlation is shown); B) Tentativechronology of earthquakes derived from the Mesa del Caballo trench study performed across the main southern strand.

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the Apartaderos pull-apart basin, this earthquake could be then theyoungest earthquake reported byAudemard et al. (1999a) in theMorrode Los Hoyos trench, which has roughly occurred in the last 2000 yr,instead of the 1812 earthquake as previously thought by those authors.However, this would only not explain the heavy damage reported inTrujillo by Palme and Altez (2002), which is imaged by the very largesize of the MKS VII–VIII isoseismal in their Fig. 1, but also themacroseismic epicenter located NNE of the Apartaderos releasingbend. This would require that segment Boc-c in Fig. 3 also ruptured in1674. The eventual activation of both segments, not simultaneously butvery close in time (an event triggering a subsequent one few dayslater), could also be in agreement with the occurrence of several largeearthquakes in a very short time, as expressed by Palme and Altez(2002). Since both strands may have likely ruptured during the 1674earthquake sequence, the off-fault deformation (rotational sliding in

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

the pop-up southern slope) at the Mesa del Caballo trench may havebeen eventually triggered by the activation of the main strand of theBoconó fault at the Apartaderos basin.

Assuming that our record is complete, the event recurrence of themain strand of the Boconó fault at the Mesa del Caballo trench site isvery variable (Fig.16). It ranges between about 400–450 and 1400 yr. Itcan be even as long as 1800 yr (in the period from 8900 to 7200 yr BP).However, the record of events seems more complete after that period.In such a case, except for a long period without a single recorded (orfound) event around cal. 0 yr (about from cal. 800 yr BC to cal. 600 yrAD), the recurrence appears to have two prevailingmodes: 400–450 yrand around900 yr (Fig.16). Themost frequentmode is thefirst one thatrepeats 6 times. Instead, the secondmode only repeats 4 times. On theother hand, onemode curiously doubles the other one. This could leadto several implications: 1) our record, being complete, evidence that

on the main strand of the Boconó fault in its central section, at Mesa16/j.tecto.2007.08.020

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this strand actually has two different prevailingmodes. Explanation tothis variability could be due to interactionwith the secondary northernstrand of the Boconó fault, jumping the activity to the other strandwhen this fault is quiescent (or when the recurrence is longer on thisstrand— 900 yr). This issue shall be dealt with in the following section,by comparing the seismic histories derived from the Morro de LosHoyos (Audemard et al., 1999a) and Mesa del Caballo trenches; and 2)our record is definitely incomplete and the actual returnperiod is 400–450 yr. As amatter of fact, thismaybe the case becausewehave alreadyindicated that the underwater slumpon the northern block underwentlater fracturing (Fig. 9b), which should have occurred sometimebetween the age of samples VEN-24-04 and VEN-25-04. Sample VEN-24-04was not dated for financial reasons, but this event coincidentallyseems to fall within the largest youngest window without knownactivity. If we analyze how the events have been recognized andcharacterized, taking into account the structural style (en echelonRiedel shears) of themain strand of the Boconó fault at the trench side,we can easily argue for missing some events for lack of evidence. Onone hand, the trench location itself may lead to this record incom-pleteness, depending on emplacement of the trench with respect tothe R shears and overlaps. On the other hand, the repetition of opencracking at the approximate same place may induce to errors in thevisual identification of every single crack filling. In addition, if returnperiod between successive events is so short, the color difference oforganic-rich soilsmay be so faint thatmight not be recognizable by thenaked eye.

Assuming that the recurrence interval is 400–450 yr for thisBoconó fault strand, which is the shortest and most frequent derivedperiodicity, the region should be shaken by a forthcoming event in a100–150 yr time because the latest event occurred around cal. 1700 yrAD.

Record of earthquakes in this trenchmakes complex any magnitudedetermination based on quantification of deformed geologic markers.Open cracking atop a fault plane is not a feature that permits easyquantification of horizontal slip on a Riedel shear. However, fromempirical knowledge it is known that strike–slip faults only rupture theground surface during earthquake magnitude about 7 or larger, afterruling out any creep on this fault plane, based on the forementionedarguments. So, thismagnitude can be taken as a lower bound for the on-fault events recognized at the Mesa del Caballo trench. This order ofmagnitude seems compatiblewith the longest axis of the VIII isoseismalof the 1674 earthquake determined by Palme and Altez (2002), which isabout 130 km long (Fig. 3), if the 1674 event could definitely beattributed to the Boconó fault (at least to segment Boc-b) and VIIIisoseismal longest axis bears equivalence to the rupture lengthof a givenearthquake. More precisely, such a rupture would more likelycharacterize events of Ms 7.2–7.4.

8. Comparison with the Morro de Los Hoyos record

Audemard et al. (1999a), from the trench excavated across thesecondary northern strand of the fault, proposed 7 events in the last10 kyr (Fig. 17A). Except for one event, all other 6 from this latterassessment can be correlated to one or any of several events of thoseidentified in this trench study, due to the large uncertainty in the timeof earthquake occurrence along the northern fault strand (Fig. 17A).Audemard et al. (1999a)'s event III hardly matches any event of thepresent study and essentially falls into one of the 900-yr long returnperiods previously described (Fig. 17A). This could support the like-liness of failing recognition of an earthquake.

9. Simultaneity between the two strands at the Apartaderos basin

The comparison of the two seismic histories derived from the twotrenches excavated across each of the two bounding strands of theBoconó fault is insufficient to determine whether the two strands at

Please cite this article as: Audemard M., F.A., et al., Trench investigationdel Caballo, Mérida Andes, Venezuela, Tectonophysics (2008), doi:10.10

the Apartaderos pull-apart basin move either simultaneously, inde-pendently or both ways indistinctly (compare Fig. 17A and B). On onehand, it would seem that the record at the Mesa del Caballo trench isstill incomplete. On the other hand, the timing of individual events atthe northern strand is not precise enough due to the small number oforganic-rich horizons found during that assessment. Nevertheless, itappears, as proposed by Audemard et al. (1999a), that the northernsecondary strand moves jointly with the main strand, but the mainstrand, as it recurs faster, happens to move also independently.Assuming that the average recurrence intervals for both strands are1100–1500 yr (Audemard et al., 1999a) and 400–450 yr (this study), aplausible seismic scenario could be that both strands move jointlyevery 1200–1350 yr and the southern main strands moves indepen-dently twice in between those events (every 450 yr). This scenario alsofits well with a slip rate distribution of 1/4 and 3/4 between thenorthern and southern strands, respectively.

10. Conclusions

This trench study displays a complete set of features never seen inVenezuela before. Besides imaging the main active fault plane of theBoconó fault main strand at the Apartaderos pull-apart basin, earth-quake-induced liquefaction features, sliding and slumping of LatePleistocene–Holocene age have also been recognized and described.Sand dykes, sequence pieces or clay plugs floating in well sortedstructureless sands, and convoluted bedding are unequivocal evidenceof earthquake-triggered liquefaction. These structures were recog-nized in finely laminated pond deposits as well as in the fluvialsequence of which the pop-up is built. Sedimentation also attests to acomplex evolution in a Holocene tectonically active environment sincetwo different fault-bounded sagpond sequences were identified, andseveral organic-rich paleosoils. Besides, a coarse alluvial sequenceresting on the older sagpond deposits (or interfingered between thetwo sagponds) was affected by water-rich slumping, implying that thenearby fault may have also triggered syn-sedimentary slope instabil-ities. In addition, the free face of the pop-up also shows sliding towardsthe La Cañada ravine, which would not seem to have occurred in dryconditions (as it happens nowadays), because sliding surfaces root in asame softly deformed layer, implying that they are not currentlymoving by regressional erosion but had by shaking during olderearthquakes. This seems supported by the fact that paleosoils atopsliding masses are also “down-faulted”, and they occasionally exhibitsort of colluvial wedges inside the organic-rich paleosoils. Definitely,all these deformations confirm that this is the other Holocene activestrand of the Boconó fault bounding the Apartaderos pull-apart basin,as proposed by Audemard et al. (1999a).

In regard of the main fault plane in trench walls, it matches nicelywith the Riedel shear on surface inferred between the pop-up and itsrelated deactivated sagpond. This dextral shear, which also presents acertain amount of across opening, has functioned as a crevassewhere aminimumof 10 different vertical slices of paleosoils has fallen in (7 and3 recognized in thewest and eastwall, respectively). This is evidence ofa record of as many paleoearthquakes in this largest main crevasse.Two other different events have also been dated in secondary cracksidentified on the southern portion of the west wall. In turn, the twoyoungest events, to total 14 event recognitions at this trench assess-ment, are interpreted on the basis of one event being responsible forthe deactivation of the youngest sagpond, and the other one for theformation of a colluvial wedge. This evidence may be attributed to the1674 earthquake that damaged both Mérida and Trujillo, and canpreliminarily assigned to segments B and C of the Boconó fault. Returnperiod of these events seems variable. However, the first recurrencemode appears to be in the order of 400–450 yr for earthquakes ofmagnitude 7 or larger. The second most frequent return period is of900 yr (exactly the double of the first mode). It is very likely that thislarge variation in return periodmay be attributed to incompleteness in

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the determined seismic record at this locality, being several the causes:structural style of the main strand at the trench site (en echelon Rshears); repetition of coseismic breakage location, overprinting ormasking of former crack fills; and shortness of the return period,making difficult the differentiation among organic-rich soil genera-tions. Regarding the seismogenic behavior of the Boconó fault at theApartaderos pull-apart basin, it appears that both strands movesimultaneously every 1200–1350 yr but the main strand also movesindependently twice in between with a recurrence interval of 400–450 yr. This scenario also matches with a slip rate on the main strandthat is three times of that of the secondary strand (slip distribution of75% and 25% between the main and secondary strand, respectively).

Acknowledgements

The authors wish to thank the Manfredi family, owners of“Agropecuarias Las Briseras”, and their employees for all their helpduring this study. Funds were essentially provided from ProjectFONACIT-2001002492 and FUNVISIS, but FUNVISIS also providedlogistics. These results are also contributions to projects GEODINOS(FONACIT G-2002000478) and FONACIT-ECOS Nord (PI-2003000090).Wewill rememberwithmuch appreciation the “Andino”who stole ourrope grids from the trench, twice in a row. We feel we helped the localagriculture development, particularly the potato collect. Dating is soleresponsibility of Beta Analytic Inc. (Miami, USA), for whichwe are verythankful. Thanks to Marina Peña for her drawings. Our thanks also goto Hotel Santo Domingo, its owners (Elio and Aramila Piva) and staff(Ibis, Nereida, Alfredo, Carlos y Justo), who have hosted us andmade usreally feel at home. We keep good memories of “las parrillas y lascervezas”. Also our acknowledgements are for the actual trenchdiggers: Jorge Quintero, Omar Angeles y Victor Manuel Hernández.Last but not least, wewish to express our most sincere gratitude to thetwo reviewers— Drs. Eulalia Masana and Alessandro Maria Michetti—whogreatly helped us to improve a former version of this contribution.

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