Geomorphology 124 (2010) 164–177

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Geomorphology

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Deep-seated gravitational slope deformations along the active Boconó Fault in thecentral portion of the Mérida Andes, western Venezuela

Franck A. Audemard a,b,⁎, Christian Beck c, Eduardo Carrillo d

a Venezuelan Foundation for Seismological Research—FUNVISIS, Earth Sciences Department, Apartado Postal 76.880, Caracas 1070-A, Venezuelab Universidad Central de Venezuela, Facultad de Ingeniería, Escuela de Geología, Minas y Geofísica, Los Chaguaramos, Caracas, Venezuelac Université de Savoie, Laboratoire de Géodynamique des Chaînes Alpines—LGCA, 73376 Le Bourget du Lac Cedex, Franced Universidad Central de Venezuela, Facultad de Ciencias, Instituto de Ciencias de la Tierra, Los Chaguaramos, Caracas, Venezuela

⁎ Corresponding author.E-mail addresses: faudemard@funvisis.gob.ve, franck

(F.A. Audemard).

0169-555X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.geomorph.2010.04.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 June 2009Received in revised form 16 April 2010Accepted 21 April 2010Available online 5 May 2010

Keywords:Deep-seated slidingDSGSDSackungBoconó FaultMéridaVenezuelan Andes

Two very large deep-seated gravitational slope deformations in the highlands of the Mérida Andes,Venezuela, are herein described: the Mucubají Pass and Cerro La Camacha. These slope movements have slidin post-Last Glacial Maximum times. In addition, both landslides are in very close association with the activeBoconó Fault trace. The Cerro La Camacha (Camacha Range) landslide is fault bounded along its northwestflank, whereas the Mucubají pass mass movement is even cut by the active fault trace. The almost 10 km longMucubají slide mobilizes LGM moraine deposits along the unconformable basement contact. La Camachaslope movement is a sackung-type landslide, involving two huge masses that affect the entire northwesternslope of the La Camacha Range. This sackung is at least 20 km long, paralleling the active Boconó Fault trace.Combination of high relief energy (gravitational forces) and seismic shaking related to an on-site active faultcould be responsible for the destabilization of the slopes or massif in both cases. Although the seismically-induced (re-)activation of the La Camacha landslide is very likely, there is no proof for that yet. Conversely,the Mucubají slide shows geomorphic, geodetic and sedimentary evidence of episodic activity in recenttimes, which could be ascribed to seismic triggering. In the particular case of the La Camacha sackung, thecombination of dextral slip along the Boconó Fault and a SE-dipping fault plane could additionally favor thedestabilization of the NW slope of the La Camacha Range.

.audemard@ing.ucv.ve

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Field surveys and aerial photograph interpretations have allowedthe recognition and mapping of two large deep-seated gravitationalslope deformations (DSGSD), following the definition proposed byDramis and Sorriso-Valvo (1994). We present both geomorphic andgeologic data to describe these slope movements. Also, we discusspossible reasons for the geographical clustering of these two largeinstabilities along the most active tectonic feature in the MéridaAndes, the dextral strike-slip Boconó Fault, and the role of this fault inthe initiation and evolution of the instabilities. We chose these twoexamples because of their size, conspicuous geomorphic features andproximity to the active Boconó Fault. Besides, both study sites arelocated in areas of high relief-energy that have undergone the effectsof the last glaciation (Last Glacial Maximum—LGM; named locally asthe Late Mérida Glaciation by Schubert, 1974) and deglaciationprocesses. The last deglaciation became effective sometime around18 ka. Furthermore, both study sites are in the same valley (Santo

Domingo–Aracay Valley) and specifically on the same NW mountainflank of the Sierra de Santo Domingo and its northward extension inCerro La Camacha, which was affected by more significant glacialloading and unloading during the late Quaternary than the sun-exposed opposite slope (Figs. 1 and 2).

2. Deep-seated gravitational slope deformations

Large deep-seated mass instabilities on slopes have been definedin different ways worldwide (Jahn, 1964; Ter-Stepanian, 1966;Zischinsky, 1966, 1969; Tabor, 1971; Nemcok, 1972; Mahr, 1977;Radbruch-Hall, 1978; Shimizu et al., 1980; Bovis, 1982; Forcella andOrombelli, 1984; Sorriso-Valvo, 1984; Beget, 1985; Holmes and Jarvis,1985; Savage et al., 1985; Savage and Varnes, 1987; Reimer et al.,1988; Varnes et al., 1989; Bovis, 1990; Chigira, 1992; Dramis andSorriso-Valvo, 1994; Crosta, 1996; Pasuto and Soldati, 1996; Agliardiet al., 2001). Most of the reported cases have been attributed tovarious forms of “slope sagging” (Savage and Varnes, 1987;Hutchinson, 1988), a term derived from the original German word“sackung” (Zischinsky, 1966, 1969). This term carries a similarmeaning to the terms “gravitational spreading” (Varnes et al., 1989)and “deep-seated gravitational slope deformation—DSGSD” (Nemcok,

Fig. 1. Location of the study area in western Venezuela: A) Inset figure in the lower right corner depicts a simplified geodynamic setting of northern South America (modified fromAudemard et al., 2000), where location of panel B is shown. B) The study area (Fig. 2) is indicated by a white box on shaded relief base mapmodified from Garrity et al. (2004), wherethe Boconó Fault (BF) locally exhibits a more easterly trend than at regional scale. The Boconó Fault is marked by the alignment of deeply incised axial valleys along the backbone ofthe Mérida Andes.

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1972; Ter-Stepanian, 1977; Sorriso-Valvo, 1984; Pasuto and Soldati,1990; Dramis and Sorriso-Valvo, 1994). However, slight differencesmay persist among those definitions. For instance, a sackung differsfrom a classical large landslide in the long-term slowness of the slopedeformation, together with the absence of well-defined boundariesand toe bulging (McCalpin and Irvine, 1995). “Sackungen” — theplural form of sackung— are often found along ridge crests in massivecompetent rocks, running parallel to slope contours and to the strikeof main rock discontinuities (bedding, foliation, joints and/or faults).

All DSGSD, in its larger sense, show distinctive morpho-structuralfeatures, such as: double or multiple ridges, scarps and counterscarps—also known or described as scarplets, uphill-facing scarps, back-facingscarps, obsequent scarps, or antislope scarps-, ridge-top depressions,ridge-crossing depressions, trenches, troughs, open-tension cracks ortoe bulges, among others (Jahn, 1964; Ter-Stepanian, 1966; Zischinsky,1966; Beck, 1968; Zischinsky, 1969; Tabor, 1971; Patton and Hendron,1974; Mahr, 1977; Radbruch-Hall et al., 1977; Bovis, 1982; Bovis and

Evans, 1996; Agliardi et al., 2001; Ambrosi and Crosta, 2006; Hippolyteet al., 2006).

These large, deep-seated, slow slope movements (sackungen orDSGSD) commonly occur on high relief-energy hillslopes, often withsizes comparable to the whole slope. Affected slopes are steep andhave high local relief (frequently greater than 500 m) and thedisplaced masses are large (areas N0.25 km2, volumes N0.5 km3,thickness N100 m; Ambrosi and Crosta, 2006) and have ill-definedmargins. Theymay show a lack of a well-defined basal gliding surface.The movements, although slow (long-term gravity-driven creep), cancontinue for long periods, producing significant cumulative displace-ments (Cruden and Hu, 1993; Ballantyne, 2002). These displacementsare relatively small in comparison to the slope itself. Reactivation canalso happen after long periods of quiescence or inactivity. Surfacedisplacement rates typically range from a few millimetres to severalcentimetres per year (typically in the order of 0.4 to 5 mm/year, afterVarnes et al., 1989), and are commonly close to the detection limit of

Fig. 2. Ortophotograph showing the relative location of the two described slope movements along the seismogenic Boconó Fault (BF) trace (dotted line) in the Santo Domingo–Aracay valley. Location of Figs. 3 and 8 are highlighted, as well as profile depicted in Fig. 9.

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monitoring equipment (Bovis, 1990). When triggered or reactivatedby earthquakes, they generally evolve in a step-like way, withalternating rather long periods of quiescence and abrupt events ofreactivation in connection with seismic shocks (Carton et al., 1987;Bisci and Dramis, 1993; Dramis et al., 1995; Gutiérrez et al., 2008).

As mentioned earlier, joints, faults, shear zones, and otherstructural lineaments are important in deep-seated slope movements(Crosta and Zanchi, 2000; Agliardi et al., 2001). Consequently theinstabilities commonly cluster around major tectonic features. Toillustrate this, sackungen are common in the Central Italian Alps(Forcella, 1984a,b; Crosta and Berto, 1996; Crosta and Zanchi, 2000;Agliardi et al., 2001; Colesanti et al., 2004). In particular, they occurwithin the Metamorphic Basement south of the Insubric Line in theOrobic Alps (northern Italy, Southern Alps). Ambrosi and Crosta(2006) indicate that such structural features may play an active and/or passive role in those slope movements. In earthquake prone areas,seismic shaking, or fault slip, may be the main triggering agent ofthese recurrent DSGSD (Beck, 1968; Solonenko, 1977; Dramis et al.,1995). Then, they deserve to be considered as potential seismo-gravitational surface effects, which essentially result from thecombination of both seismic shaking and gravitational stress(Solonenko, 1977; Keefer, 1984; Carton et al., 1987; Crozier, 1992;Bisci and Dramis, 1993; Mc Calpin, 1996). The main triggering agentsof the seismically-induced mass wasting processes are: orientedaccelerations (dynamic loading); earthquake-induced changes in thegroundwater table; cyclic changes of pore pressures and liquefaction;and seismotectonic surface fracturing and faulting (Radbruch-Halland Varnes, 1976; Solonenko, 1977; Dramis and Blumetti, 2005). Thetriggering of landslides, as well as other seismo-gravitational effects,is also favored by lithostructural contacts and geomorphologicalsettings (e.g. narrow ridges and valleys, peaks, etc.), which may

locally amplify seismic shaking (Dramis et al., 1986; Blumetti et al.,1987).

From the existing literature, many researchers point out thatDSGSDmay be clearly (re)activated bymoderate to large earthquakes.In many cases, the cause–effect relation is proposed withoutconfirmation. For instance, Gutiérrez-Santolalla et al. (2005) suggestthat the Vallibierna and Estós sackungen (central Spanish Pyrenees)might have been triggered by seismic shaking in mid-Holocene times,because their activation lagged too long behind the last deglaciationonset. Initially, Beck (1968) interpreted high mountain antislopescarps as coseismic features produced by both gravity and tectonicsalong “gravity faults”. In addition, relying on historical accounts, themultiple deep-seated sliding that involved most of the town ofBisaccia (Italy) appears to have a seismic origin (Dramis and Blumetti,2005). However, the ascription to seismic shaking really seems to bereliable when these DSGSD actually occur during contemporaryearthquakes, such as: sackungen produced by the December 16, 1954,M 7, Dixie Valley, Nevada, USA (Slemmons, 1957), El Chelif slide (ElAsnam, 1980, earthquake; Philip and Meghraoui, 1983), Tsaolinglandslide (Chi-Chi earthquake, 1999, Taiwan, Chigira et al., 2003).Even in some of these cases, a reasonable doubt about the seismicorigin still persists. Large mass movements have also been reportedduring earthquakes in the Mérida Andes (Venezuela), such as in LaPlaya (state of Mérida) during the Mocoties 1610 earthquake and inSan Josecito (state of Táchira) during the 1981 event (Singer et al.,1983).

Other triggering factors for large slope mass movements have alsobeen proposed, which determine their temporal occurrence. Besidesthe short-lived, almost instantaneous, seismic trigger, other factorsinclude: quick mountain chain uplift (eventually, in combination withfast river incision), meteorological events (such as heavy rainfall

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events, and air temperature warming), volcanic eruptions, and humanactivity (excavation, overloading, concentrated water infiltration, etc.).In the longer term, basal erosion, which can be produced by channeldowncutting, lateral displacement of channels and sea-cliff erosion,among others, as well as slope debuttressing induced by river incisionand by glacier retreat (or deglaciation over a longer timespan), are alsocommon causes of DSGSD. Examples of slides initiated by fluvial incisionhave been recognized in the Williams Fork Mountains near Dillon(Colorado, USA; Kellogg, 2001), in the Aspromonte Massif (ValloneColella Torrent, Calabria, Italy; Parise et al., 1997) and in the Pizzoto-Greci slope (Calabria, Italy; Sorriso-Valvo et al., 1999). On theother hand,mass movement of bedrock and surficial sediments developed imme-diately after the retreat of valley glaciers is a widely accepted paraglacialprocess that occurs in response to deglaciation (Shroder, 1998), butexamples of bedrock-involved landslides directly following deglaciationhave rarely been reported, except for few well-studied cases (e.g.,Panizza et al., 1996; Berrisford and Matthews, 1997; Matthews et al.,1997; Soldati, 1999). Other examples of significant sackung-type slidestriggered by deglaciation debuttressing have been studied in themiddlepart of Valfurva, east of Bormio (Rhaetian Alps, Italy; Agliardi, et al.,2001), as well as in Campo Vallemaggia and Cerentino (Ticino valley,Switzerland; Heim, 1898 cited by Seno and Thüring, 2006). Also, theinitial activation of the Columbia Mountain slide, USA, has beenattributed to this cause (Smith, 2001).

3. Study area

The Mérida Andes is a prominent mountain range in westernVenezuela. It extends in a SW–NE direction for some 350 km from theColombian–Venezuelan border to the city of Barquisimeto (north-eastern corner of Fig. 1). Its highest peaks reach almost 5000 m inelevation in the central portion, near Mérida. The present chain build-up results from Pliocene–Quaternary transpression due to obliqueconvergence between two continental blocks: South America andMaracaibo Triangular Block (Audemard and Audemard, 2002). Thispresent geodynamic setting is responsible for ongoing strainpartitioning along the chain, where the foothills and the mountainbelt have been shortened transversely in a NW–SE direction, whereasthe Boconó Fault, roughly located along the axis of the Mérida Andes,accommodates dextral slip (Audemard and Audemard, 2002).Audemard (2003a) recognizes that this rather young mountainchain exhibits four distinct types of active deformations, amongwhich seismically-induced mass wasting (slides, avalanches/flowsand so forth) and deep-seated slope instabilities (i.e., gravitationalspreading) play a major role in high mountain landscape evolutionand mass transfer from slopes into river systems.

3.1. The Boconó Fault and its Quaternary activity

The Boconó Fault is a spectacular NE–SW trending dextral faultthat extends for about 500 km, partly along the backbone of theMérida Andes (i.e., Audemard, 2008a). It runs slightly oblique to thechain axis and bounds the Caribbean Coast Range of northernVenezuela on the west, thus extending between the border betweenColombia and Venezuela and the Caribbean coast of Venezuela (Fig. 1inset). This fault has been identified, mapped and characterized rathereasily since the pioneering work of Rod (1956a) by the large numberof along-strike geomorphic features, including a continuous series ofaligned 1–5 km wide valleys and linear depressions, passes, saddles,trenches, sag ponds, scarps and sharp ridges. Among those, thealignment of valleys associated with the fault is the most conspicuousfeature as it makes the fault easily recognizable andmappable in radar(SLAR) images (Fig. 1).

Most of the largest Mérida Andes earthquakes have been ascribed tothe Boconó Fault zone, as Rod (1956b) has initially stated. Cluff andHansen (1969) have also indicated the same conviction for both

historical and instrumental earthquakes. Among the large historicalearthquakes in theMéridaAndes, Cluff andHansen (1969) have creditedthe Boconó Fault for the 1610, 1812, 1894, 1932 and 1950 earthquakes.Besides the 1610, 1812 and 1894 events, Aggarwal (1983) also ascribedthe 1644 and1875 earthquakes to the Boconó Fault. Only in recent years,clear seismotectonic associations of historical events with this fault orindividual fault segments have been possible through paleoseismicinvestigations (Audemard, 1997, 2003b, 2005; Alvarado et al., 2008;Audemard, 2008b; Audemard et al., 2008; Alvarado et al., 2009;Audemard, 2009, 2010), carried out on the different fault sectionsproposed by Audemard et al. (2000). These paleoseismic researcheshave needed cross-checking with historical seismicity studies, eitherexisting or revised by Audemard (2008b, 2009, 2010). This authorconcludes, from the integration of all paleoseismic trenching resultsgathered from 1986 to 2007, that: 1) the Boconó Fault is actuallysegmented (refer to map by Audemard et al., 2000 for fault segmenta-tion), as initially suggested by the distribution of the few large, known,destructive historical earthquakes; 2) the geometric segment bound-aries (as reported by Audemard et al., 2000) appear to be actualseismogenic barriers to rupture propagation, thus supporting thepreliminarily proposed segmentation, although some may act as“leaky” boundaries (between Boconó-B and -C such as during the 1674AD event) once in a while; 3) all segments appear to have ruptured inhistorical times; and 4) there is a northeastward time and spacemigration of the before-the-last event sequence, starting in 1610 AD onBoc-A (south-westernmost segment) and closing the cycle on Boc-E in1812 AD. Segments Boc-B and Boc-C broke during the 1673–1674earthquakes sequence (similar to the Izmit and Duzce 1999 events;Audemard et al., 2008) and Boc-D would seem to have ruptured duringthe 1736 AD earthquake, eventually in combination with the 1801 ADevent.

With respect to the general NE–SW trend of the Boconó Faultexhibited along most of the Mérida Andes, the fault in the SantoDomingo–Aracay Valley appears to be pushed off towards thesoutheast, showing an overall very wide open V-shape with vertexto the southeast (Fig. 2). This vertex corresponds to the lowestaltitudinal point of the fault trace in this valley, where the Aracay andSanto Domingo rivers meet, before jointly crossing the structuraltrend of the southeastern Mérida Andes. This passage across thesouthern half of the Mérida Andes takes place along a very recent andoccasionally over a 1000 m deep wine-cup shaped gorge (Audemard,2003a). The entrance of this gorge was chosen as the site for theconstruction of the Santo Domingo hydroelectric dam (Fig. 2). Weinterpret that this smooth kink is not a change in fault strike butreflects the fault dip to the southeast. If this assumption were correct,the Boconó Fault dip could be as low as 45–50° S. If not that low, thefault definitely dips south in this Santo Domingo–Aracay Valley, asdetermined by Audemard (2008a), which has significant implicationsas we shall discuss later in this paper.

The section of the Boconó Fault under study herein extends fromthe Apartaderos pull-apart basin (as defined by Soulas, 1985;Audemard et al., 1999) to the Llano Corredor Pass (Fig. 2). At bothlocalities, the active fault trace is preserved across magnificent LGMmoraine complexes, sitting between 3000 and 3500 m in elevation.Both localities correspond to high-altitude water divides. At Aparta-deros, the Boconó Fault is well preserved along a drainage divide thatseparates the southeasterly flowing streams (Orinoco Basin) from thenorthwesterly flowing streams (Maracaibo Basin). This divide isnestled in the area of Lake Mucubají, belonging to the Sierra NevadaNational Park, one of Venezuela's most spectacular alpine glaciatedlandscapes. The Boconó Fault here comprises two conspicuous sub-parallel active strands 1–1.5 km apart, both exhibiting excellentlypreserved tectonic landforms in latest Pleistocene and Holocenedeposits (Figs. 2 and 3). Even though the vertical component of slipappears to be significant at some localities (strictly related to smalltranstensional basins, such as those cutting across Las Tapias, Los

Fig. 3. Map of the Mucubají slope movement and its relations with the Late Pleistocene moraine deposits in the area, as well as with the active trace of the Boconó Fault. Relativelocation is shown in Fig. 2. Location of cross-section depicted in Fig. 5 is also indicated.

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Zerpa and Mucubají LGM moraine complexes in Fig. 3; Audemard,2008a), most of the fault geomorphology is typical of a strike-slipfault. At present time, this high-altitude basin is being deeplydissected by both the Chama and Santo Domingo river headwaters,being the drainage divide exactly at the Lake Mucubají. This currentbehaviour is in oppositionwith the fact that this depression appears tohave accumulated a rather thick (over 400-m thick) sequence ofPleistocene (pre-LGM) glacial deposits, as those preserved at Mesasdel Caballo and Julián (next to the south of Apartaderos; Audemard,2003a). This sedimentary evolution seems to be strongly influencedby marked hydrologic changes between glacial and interglacialperiods. There is no doubt though that the deactivation of sedimen-tation in this perched basin is due to generalized chainwide uplift thatinduces parallel-to-chain axis deep incision along the heavilyfractured bedrock or brecciated material generated by the presentactivity of the axial Boconó Fault. The Boconó Fault exhibits its highestslip rate at this basin, where the southern and northern strandsrespectively carry about 75% and 25% of the 7 to 10 mm/a net slip ratemeasured in this sector (Audemard et al., 1999; 2008).

In the Llano Corredor Pass, as occurs in the Lake Mucubají area, thefault shows very fresh scarps and other landforms both in bedrock andin LGM and Holocene deposits, proving the recentness of the latestslip event on this fault. From historical accounts, distribution of largemass movements and paleoseismic results, Audemard et al. (2008)correlate the historical 1674 earthquake to the rupture of thissegment of the Boconó Fault (segment C after Audemard et al.,2000). This pass corresponds to the water divide between the BurateRiver flowing to the northeast and the Aracay River flowing inopposite direction.

Between these two high-altitude passes, the Santo Domingo andAracay rivers deeply incise valleys along the current active fault traceof the Boconó Fault (Fig. 2), descending from 3500 m to 1700 m inelevation in less than 10–20 km (6° to 12° average river profilegradient). This implies that both rivers have very high stream power.Particularly for the Aracay River section, it leaves a very prominent

bedrock relief on the southeast river margin, against which someMérida Glaciation moraine complexes rest on its northwestern flanknear and at the Llano Corredor Pass. The rapid incision seems to bevery recent as suggests the high number of hanging staircased alluvialterraces abandoned by the Santo Domingo River between the villagesof Santo Domingo and Las Piedras. A similar situation occurs along thePueblo Llano River, between Pueblo Llano and Las Piedras, beforeconverging with the Santo Domingo and Aracay rivers at the dam site(Fig. 2).

4. Large mass movements in the Santo Domingo–Aracay Valley

Two large deep-seated gravitational slope deformations (DSGSD)have been recognized and mapped in the Santo Domingo–AracayValley, whose lowest point coincides with the Santo Domingo damsite. Each DSGSD sit at the headwaters of the Santo Domingo andAracay rivers, located at the Mucubají and Llano Corredor passes,respectively. Both mass movements are over 9 km in length but arevery different. First, we shall describe the one sitting at the Mucubajíwater divide, located to the SW of the study area, and then the other atthe Llano Corredor water divide, located to the NE of the study area(Fig. 2).

4.1. The Mucubají landslide

The Mucubají region exposes a set of moraine complexespreserved at and around the Mucubají water divide. These well-developed moraine complexes rest on the northwestern flank of theSierra de Santo Domingo, on the side less exposed to sunlight. LakeMucubají is located at the Santo Domingo–Chama water divide(Fig. 2). This former glacial lake is nested behind the terminal moraineof this late Pleistocene moraine complex. The Mucuñuque River,running along this moraine bottom, used to discharge into the ChamaCatchment on the southwest before shifting to the Santo Domingobasin on the northeast, as attested by the abandoned spillway of Lake

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Mucubají preserved on the southwestern side of the outermost frontalmoraine (Fig. 3). The abandonment of this spillway has been relatedto a major shift in the Lake Mucubají evolution, recorded in the lakesediments and dated at around 14–14.5 ka (refer to Carrillo et al.,2008 for further details). This moraine system and three others (LaVictoria, Los Zerpa and Las Tapias, from SW to NE; Figs. 2 and 3) areoffset by the main strand of the active Boconó Fault (Fig. 3).

The Mucubají complex shows the particularity of hosting an activelake (Lake Mucubají) behind the outermost moraine ridges (Fig. 3)instead of being in a more rangeward position, as expected in aretreating moraine system; likewise in the La Victoria moraine.Besides, the present Mucuñuque River, which drains the Mucubajícomplex, flows against the NE lateral moraine. This is furthersupported by the presence of abandoned river gaps west of thepresent Mucuñuque River course, preserved in four retreating frontalmoraines, which lie inside theMucubají moraine complex.We believethat these three issues are related to the down-slope motion of the

Fig. 4. Ground views of the headscarp across the Mucubají moraine complex, from the MucuFig. 3 by dark arrows. A) View towards the south of the right lateral moraine. Here is one ofcontact. B) Southwest view towards the left lateral moraine.

entire moraine complex that has also been slightly tilted to the north–northeast, inducing a) the spillway shift from the southwest to thenortheast, b) the present water pounding against the frontal morainefarthest to the northwest and c) the northeastward lateral migrationof the Mucuñuque River, with the associated abandonment of theformer river gaps across the frontal moraines and the drainage of allsmall shallow lakes behind four higher frontal moraines. Thisgravitational motion of the moraine complex is attested by a 5 to6 m high northwest-facing scarp that cuts across (offset) theMucubajímoraine complex near its contact with the Precambrian bedrock(Fig. 4). This feature can be confidently followed from the bottom ofthe Mucubají moraine complex towards the NE and SW, as illustratedin Fig. 3. It stretches for over 9 km, always very close to or at the abovementioned contact (Figs. 3 and 4), resembling a sort of rimaye-likestructure. It runs above the moraine complexes of Las Tapias, LosZerpa, La Victoria, Mucubají and partly of El Caballo, located just SW ofthat of Mucubají (on the right edge of Fig. 3). The northeastern tip of

ñuque valley floor (from inside the moraine). Direction of photo taking is indicated inthe very few locations where the rimaye (scarp) does not match perfectly the lithologic

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the slide scar ends with the outcrops of LGM moraine deposits.Conversely, the other end is not well defined but can still be seen atthe bottom of the El Caballomoraine complex. This feature is typical ofDSGSD. In addition, the northeastern landslide tip reaches anelevation of 2800 m and nears the Santo Domingo River course,whereas the southwestern tip lies at above 3500 m in elevation awayfrom any important river incision (Fig. 3). All these facts suggest thatdestabilization of the Late Quaternary moraine complexes occurstowards the north–northeast, making use of high-relief energyintroduced by the significant incision of the Santo Domingo River.This could be additionally favored by the fault cutting the toe of thesliding mass, besides the debuttressing effect incorporated by the SEdip of the Boconó Fault plane. Very probably, the coseismic disruptionand displacement along the seismogenic Boconó Fault wouldaccelerate, if not trigger, episodically the sliding of this huge massmovement. Due to its length, and the involvement of coalescentmoraine deposits at very different elevations, ranging from 2800 to3500 m high, we consider this slide to be deep-seated. Taking intoaccount that the landslide headscarp runs close to the moraine-bedrock contact, we believe that this DSGSD is rooted at such arheological boundary, as interpreted in Fig. 5, implying that thegliding surface of the slope movement can be over 100 m deep (as inthe Los Zerpa complex; Audemard et al., 2002; Carrillo et al., 2006)and sometimes as deep as 500 m, as in the Mucubají Pass (Audemardet al., 2002).

The age of activation of this slide is unknown, but it appears tohave moved episodically in Late Quaternary times, primarily in thevery Late Pleistocene and Holocene, for several reasons: a) all LGMdeposits in the Mucubají area are affected by the slide. The oldestknown slip of this DSGSD is the one that induced the shift of thespillway of the Mucubají complex from the Chama basin to the SantoDomingo basin, at around 14–14.5 ka; b) Along the Mucuñuque River,running at the bottom of the Mucubají moraine complex, the side ofthe valley floor above the slide headscarp exhibits a set of seven rathersmall Holocene staircased alluvial terraces, implying that sliding hasoccurred stepwise through time, forcing theMucuñuque River to keepcatching up with the river profile of the downthrown side. This step-by-step behaviour suggests a seismic-induced origin; and c) Not onlythe LGM moraine complexes are cut by the slide scar, but also theyounger post-LGMmoraine complexes, such as the Canoa and Canoitamoraines (Fig. 3).

Fig. 5. Interpreted NW–SE cross-section across the Mucubají slide at lake Mucubají (completwo strands. Notice the thickness of moraine deposits that could be involved in the sliding pTill deposits are represented by boulders.

As to present-day activity, it is probable that this slide is still activeand may be also moving slow in a steady manner. In the 1970s and1980s, Henneberg and Schubert (1986) installed benchmark net-works for theodolite triangulation at the Mucubají Pass in order todetermine the slip rate of the Boconó Fault (Fig. 6). Surprisingly, theresults obtained not only showed a rather unclear component ofdextral slip along the Boconó Fault but also a significant andunexpected shortening component in NW–SE direction in the smallnetwork (small pentagon in Fig. 6B) across the fault, which they couldnot explain. In addition, they couldmeasure in the large network up to32 mmof NWmotion of the south block across the fault in respect to afixed north side, between the campaigns of 1975 and 1980–1981(Fig. 15 of Henneberg and Schubert, 1986). This allows estimation ofthe NW–SE shortening rate at about 6 mm/a, almost as much as thecurrent dextral slip rate of the southern strand of the Boconó Faultcalculated by Audemard et al. (1999, 2008). We would attribute bothapparent network shortenings at the Mucubají Pass to gravitationaldeformation induced by the sliding of the Mucubají moraine complexagainst the slope of Morro de Los Hoyos that acts as a backstop(buttress), as depicted in Fig. 5 by the two small opposing arrows).This appears to be obvious for the small network that essentiallymeasures deformation across the Mucubají terminal moraine(Fig. 6c).

Within this huge slide and confined to the LGM moraine deposits,smaller slides also occur, which are not DSGSD. These landslides arealways in close association with small pull-apart basins along theBoconó Fault main strand, forming next to the frontal moraine of LasTapias, Los Zerpa (Audemard et al., 2002) and Mucubají LGMmorainecomplexes. These slides could be resetting gravitational equilibriumeach time these small transtensional basins deepen. Deformationsrecognized in the post-LGM sedimentary fill of Los Zerpa moraine byCarrillo et al. (2006) attest to this.

4.2. La Camacha landslide

The Aracay Valley, which is drained by the Aracay River, shows alongitudinal river profile gradient as high as 12°, descending from3500 m to 1700 m a.s.l. in a distance of about 10 km. The Aracay Riverruns along the foot of the northwestern slope of La Camacha Range(Cerro La Camacha), which is the northeastern prolongation of theSierra de Santo Domingo beyond the gorge of the Santo Domingo

ted from Audemard et al., 2002). Here, the right-lateral strike-slip Boconó Fault showsrocess. No vertical exaggeration. Legend: p : Precambrian bedrock, depicted by crosses;

Fig. 6. Relative displacements across the Mucubají moraine complex between 1975 and 1980–81 from triangulation measurements (modified from Henneberg and Schubert, 1986).A) Relative displacements between geodetic benchmarks of the local network; B) Geometries of the local and regional networks at the Mucubají Pass; C) Relative displacements ofthe local network reported on the topographic map of the Mucubají area. Refer to text for explanation.

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River. The highest peaks of the Cerro La Camacha are just above3800 m a.s.l. and its crestline stays above 3600 m in elevation forsome 4 km. The trace of the Boconó Fault is situated in the lower partof the slopes forming the northeastern flank of La Camacha Range.This fault exhibits a series of huge shutter ridges along the lower reachof the Aracay River, whereas the Boconó Fault directly juxtaposesbedrock against LGM and younger moraine complexes in its upperreach and at the Llano Corredor Pass (Figs. 7 and 8). Bedrock in LaCamacha Range comprises Archean rocks of the Iglesias Complex, asthe Santo Domingo Range, and mid-Paleozoic silica-rich intrusiverocks (Hackley et al., 2005). In this region these rocks show thickweathering profiles except in the highlands of the La Camacha Rangewhere the regolith has been removed due to the Late PleistoceneMérida Glaciation, as observed in aerial photographs (Fig. 8).

Three mass movements can be differentiated in the La CamachaRange; two of them very large and a much smaller one. The largerones are in the eastern and central parts of the Camacha Range and thesmaller one in its western sector, forming the northeastern abutment

and slopes of the Santo Domingo dam and reservoir (Fig. 8). Each ofthe scarps identified in the two larger landslides extend continuouslyfor at least 10 km and overlap in themiddle sector for about 4 to 5 km.These NW-facing scarps run almost parallel to the rather linear traceof the active Boconó Fault. The crown (headscarp) of the third slide, incontrast, shows a semi-circular trace and indicatesmotion to thewest,that is, towards the Santo Domingo reservoir. This rock slide appearsto be rotational and is less than 2 km across. It is affecting thewesternmost tip of the La Camacha range, near the confluence of theAracay, Pueblo Llano and Santo Domingo rivers, where the SantoDomingo dam sits (Fig. 8). The two large landslides jointly extend forover 20 km of the La Camacha Range. The headscarp of theeasternmost of the landslides runs along the La Camacha Rangecrestline for most of its extent, which is in turn parallel to the BoconóFault trace. This crown (master scarp) can be subdivided into threedistinctive portions. The eastern portion runs mostly on thesoutheastern slope of the range, as an uphill-facing scarp. Aftercrossing the range crest, where it exhibits a double ridge top, it

Fig. 7. Panoramic view of the Cerro La Camacha (or Camacha Range). A) General view of the Llano Corredor Pass in the foreground and La Camacha Range in the background. B) TheBoconó Fault active trace is highlighted at the foot of the La Camacha Range.

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becomes an elongated depression. In turn, the middle portion of thecrown is bounded by two antislope scarps at both sides of the range.These failures act as a keystone graben causing the subsidence of the

Fig. 8. Aerial photo-mosaic of the La Camacha Range, assembled from photos 091-100 of mshown in Fig. 2. A) Aerial view of the La Camacha Range, indicating the emplacement of the Sdata were extracted from Schubert (1968, 1969). Note the very sharp expression in aerial vieCamacha sackungen, as well as of the smaller rotational slide affecting the dam. The Bocodextrally in the Llano Corredor Pass, whereas large shutter ridges (SR), and fault saddles (FSfor Fig. 3. OD: offset drainage (dextral offset of the Aracay River).

crestal sector of the range. Compared to the adjoining sectors, the LaCamacha Range has a poorly-defined crestline here. The westernportion of the headscarp of the slope movement appears to be

ission 010455 (Courtesy of Cartografía Nacional; currently IGVSB). Relative location isanto Domingo power supply dam and relative location of Fig. 12. The reported geologicws of the Boconó Fault across different environments. B) Mapping of the translational Lanó Fault trace is also highlighted, which offsets several LGM and post-LGM moraines) nearing the dam underline the fault. Color legend for moraine crestlines is the same as

Fig. 9. Geologic cross-section across the Boconó Fault at the La Camacha Range (modified from Schubert, 1969), on which the location and possible geometry of the sackung aredepicted. This DSGSD appears to be sliding along foliation planes, at least on the upper part of the gliding plane. A possible former basal geometry of the sackung is insinuated by thedotted line, which was probably related to a currently abandoned fault trace of the Boconó Fault along the Aracay River thalweg (mapped by Schubert, 1969; Soulas, 1985). Relativelocation and orientation of this profile is shown in Fig. 3.

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strongly controlled by foliation (Figs. 8 and 9), whereas theorientation of the scarps in the central sector, where the depressedridge top is located, as well as in the easternmost area, are obliquewith respect to the bedrock foliation. The westernmost portion of thecrown is a well developed and preserved back (uphill)-facing scarpthat runs very close to the crestline. From aerial photographs, thisscarp locally appears to be up to tens of meters high (Figs. 8 and 9).Only this portion of this landslide overlaps the second landslide.

The second slide can be subdivided into two stretches. Theeasternmost of these portions, which overlaps the other largelandslide, runs at mid-height of the northwestern slope of the LaCamacha Range as a downhill-facing scarp. This scarp is parallel to thelocal foliation (Fig. 8). Its general orientation diverges some 10° fromthe general trend of the Boconó Fault. To the west, the scarp remainsat about the same altitude until reaching the crestline, where itconverges with the western tip of the headscarp of the other largeslide. The second segment of this second slide runs on the other side ofthe range (southeastern slope) as an antislope scarp. It also runs nearthe crestline of the La Camacha Range, while the range diminishes inaltitude.

Based on the set of features described, these two large massmovements show a strong resemblance with sackungen describedworldwide. It is clear that the sliding surface of the western part of theeastern slide affecting the La Camacha Range, as well as of that of thewestern slide, is reutilizing the Paleozoic bedrock foliation (Figs. 8and 9). In addition, these large slides also show ill-defined marginsand there is no clear deformation at the toe of the northwestern slopeof the La Camacha Range.Where foliation does not favor sliding due toobliquity with the range flanks, particularly for the central sector ofthe eastern slide, the crown of the slide coincides with an elongatedridge-top depression. Here, the top of the Camacha Range does notexhibit a sharp crestline and is bounded by two opposing uphill-facingscarps (Fig. 8).

The “small” slide affecting the southwestern tip of the La CamachaRange has been instrumented to monitor the behaviour of the SantoDomingo power supply dam. Henneberg and Schubert (1986) werealso responsible for this monitoring (Fig. 10A), that was performedusing theodolite triangulation networks to assess the risk due to theproximity of the dam to the active trace of the Boconó Fault. Theleveling raised even more surprising results than at the Mucubají Pass(Fig. 10B). There was no clear proof of dextral slip along the activeBoconó Fault, but of apparent compression in the NW–SE direction. Asfor the mass movement at the Mucubají Pass, these results could be

interpreted in the light of a benchmark being placed on the slidingmass (e.g., Fc in Fig. 10A), on which the left end of the arched dam isanchored (Fig. 11).

The age of activation of this over 20 km long sackung has not yetbeen constrained. However, if Soulas' et al. (1987) proposal of anabandoned active trace of the Boconó Fault along the Aracay Riverhappens to be confirmed, we could hypothesize that the cumulativeactivity through thousands of years of this large sackung would havefinally disconnected the upper part of the fault plane. Subsequently,the fault had to cut across the sliding mass to rectify the fault dip(simplify the fault plane geometry), as illustrated in Fig. 9. The dottedline in this figure would represent the former sliding surface of thissackung, prior to fault reorganization or simplification. This processwould be partly masked by younger glacial deposits. Despite of thefact that Quaternary deposits can be very locally preserved againstantislope scarps, which appears to be the case at the La Camachasackung, its scarps are mostly preserved in Paleozoic or olderbasement rocks. Nevertheless, we can confidently state that thissackung-type DSGSD has been re-activated after the Mérida Glacia-tion. Some scarps or scarp stretches may look rounded and old, butmost of those reported in Fig. 8 are sharp and young, including someuphill-facing scarps opposed to downhill motion of LGM glaciers.Fig. 12 illustrates one of those scarps, which is present in the middleportion of the eastern sackung. In addition, a small rotational slideaffecting LGM deposits has been mapped in the overlap (stepover)between the two large sackungen (middle of photo mosaic depictedin Fig. 8). No clear relation exists between the two different (rathershallow rotational and deep-seated translational) types of slidesrecognized at the La Camacha Range.

5. Degree of association between DSGSD and the Boconó Fault

The Boconó Fault is the largest seismogenic source in westernVenezuela (Audemard et al., 2000), producing the largest earthquakeswith the shortest return periods (Audemard et al., 1999; Audemard,2008b; Audemard et al., 2008; Audemard, 2009, 2010). The fault runsjust below the toe of these large DSGSD or within the sliding mass.Besides, locally the dextral sense of slip of the Boconó Fault, such as inthe lower Aracay Valley, solely increases the high-relief energy, by justincreasing the La Camacha relief with respect to the valley floor. Inaddition, if the Boconó Fault actually dips to the southeast, asdiscussed earlier, the dextral sense of slip of the Boconó Fault alsoincorporates a void effect in the lower Aracay Valley, which largely

Fig. 10.Geodeticmeasurements at the Santo Domingo power supply dam (fromHenneberg and Schubert, 1986). Arch-shaped dam site is highlighted in grey tone and its relative locationis indicated in Figs. 2 and 8. The dam is a reinforced concrete structure (shown in Fig. 11). A) Geometry of the triangulation networks around the Santo Domingo Dam. B) Displacementvectors measured at this network: dashed arrows correspond to displacement measured between 1973–74 and 1979 and full arrows between 1973–74 and 1982. Refer to text forexplanation.

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favors the destabilization of the northwestern flank of the La CamachaRange.

In terms of direct evidence, a large gravitational reorganization ofthe Mucubají moraine complex took place at around 14–14.5 ka,driving its general tilt to the north–northeast, as recorded by severallines of evidence (Carrillo et al., 2008 and herein). This morainecomplex also records the relative step-by-step displacement of thesliding mass, through a set of seven small staircased terraces of theMucuñuque River preserved above the headscarp of the landslide.This evidence strongly suggests that the kinematics of such a large

Fig. 11. Ground view to the east of the small slide at the southwestern tip of the La Camachcrown and the sliding mass, which are the closest to the dam, are visible. Shape of the slid

slope movement are controlled by strong earthquakes. In addition,triangulation-network results indicate ongoing shortening at the slidetoe. This geodetic information in combination with the inferredstepwise displacement on the longer term, allow proposing that thishuge landslide may undergo continuous creep deformation punctu-ated by coseismic slip. In the particular case of the La Camachasackung, no direct evidence of sliding has been yet correlatedwith anypre-historic earthquake, but favorable trenching sites for paleoseismicinvestigations have been already identified, both across the BoconóFault and scarps of the La Camacha sackung.

a Range, corresponding to the left abutment of the Santo Domingo Dam. Only the smalle is provided in Fig. 8.

Fig. 12. Enlargement of aerial photo No. 093 of mission 010455 (Courtesy of Cartografía Nacional; currently IGVSB), imaging the “freshness” of an anti-slope or uphill-facing scarp inthe middle portion of the eastern sackung (marked by arrows on photo edges), where LGM glaciers better developed, because being in the shaded flank of the range.

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6. Conclusions

The highlands of theMérida Andes in Venezuela, particularly at theSanto Domingo Range and its northeastward extension the Cerro LaCamacha, northeast of Mérida state, show clear evidence of very largedeep-seated gravitational slope deformations (DSGSD). Two cases areherein presented and discussed: the Mucubají Pass and Cerro LaCamacha. Both slope movements are in close spatial association withthe seismogenic Boconó Fault trace. The Cerro La Camacha (CamachaRange) is bounded by this fault along its northwest flank, which is inturn affected by a 20 km long sackung that is mobilizing the entireNW slope. This slope movement involves the competent Precambrianbedrock, primarily along foliation planes. It could be considered as atranslational slide. The peaks of this range may show a local reliefdifference in elevation of almost 2000 m above the adjoining AracayValley floor, but it usually stands over 500 m high.

The slopemovement at theMucubají Pass is cross-cut by the activeBoconó Fault. Although it can also be considered as a DSGSD, thislandslide involves only the moraine deposits of 5 coeval Last GlacialMaximum (LGM) moraine complexes (from NE to SW, Las Tapias, LosZerpa, La Victoria, Mucubají and El Caballo). The landslide has alsodeformed younger moraine complexes (Canoa and Canoita) formedduring post-LGM deglaciation. Since the headscarp of the slideresembles a rimaye-like feature at the contact between the bedrockand overlying moraine deposits, we believe that the sliding surface isset at this lithologic boundary. It also seems to be a translational slide.

Combination of high relief energy and seismic shaking of an on-site active fault are most likely the main factors responsible for thedestabilization of the fault-bounded mountain ranges in each case.The slope movement of the Mucubají Pass shows geomorphicevidence of long-term episodic displacement plus contemporaryand continuous creep as revealed by repeated measurements ofgeodetic networks, originally installed to determine the slip rate ofthe Boconó Fault. This slide might be combining coseismic slip andinterseismic creep (continuous creep deformation punctuated byepisodes of coseismic displacement). For the La Camacha Range, wecan surely state that its southwestern tip is sliding, most likelyrotationally, as attested by repeated triangulation campaigns. As tothe large sackung, we have no direct evidence of episodic movementin association with either historic or pre-historic earthquakes, but wecan state confidently that this massif has been active in post-LGMtimes. Its size, type of slide and geometry, as well as the direct

association of its toe with the Boconó Fault, suggest a seismicallyinduced origin. In that sense, dextral slip on the fault, in associationwith a fault plane dipping SE (against the northwest slope of therange), would favour the reactivation of the La Camacha landslide ateach coseismic slip on the Boconó Fault. Void effect under an“apparent” thrust fault plane would induce instantaneous slopecollapse or destabilization.

Acknowledgments

The authors thank the Venezuelan Foundation for SeismologicalResearch (FUNVISIS), the Venezuelan National Fund for Science andTechnology (FONACIT), and the French National Council for ScientificResearch (UMR 5025), for funding and logistic support. The FrenchMinistry of Foreign Affairs, through EGIDE and the French Embassy inCaracas (Technological and Cultural Cooperation), supported our jointVenezuelan–French scientific program; we specifically thank YvesYard and Estrella Marciano. This work was largely financed by theFONACIT 2001002492 and ECOS Nord-FONACIT PI-2003000090grants that allowed carrying out research and scientific exchangesbetween both countries, respectively. We are also especially gratefulto the manager, his wife, the family and the staff, of the Hotel SantoDomingo for allowing us the use of the hotel as our home, office andlaboratory, during recurrent field work missions between 2000 and2009. Special thanks to Prof. Jaime Laffaille from Universidad de LosAndes (School of Geophysics), for providing the aerial photographdepicted in Fig. 2, and to Leonardo Figueroa andMaybelis Pérez for thedigital assemblage of the photo-mosaic shown in Fig. 8. Marina Peña isgratefully acknowledged for her high-quality drafting. This is also acontribution to project GEODINOS (FONACIT 2002000478). Tworeviewers, Dr. María Ortuño, and an anonymous one, are also largelythanked for their thorough reviews and the abundant comments andsuggestions given to the authors. Our gratitude also goes to the guesteditor Dr. Francisco Gutiérrez and the Chief Editor Dr. Adrian Harvey,who gave a final revision to this contribution.

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