Late Mesozoic and Cenozoic thermotectonic history of the Mexican Pacific margin (18 to 25°N): new...

ELSEVIER Tectonophysics 306 (1999) 163–182www.elsevier.com/locate/tecto

Late Mesozoic and Cenozoic thermotectonic history of the MexicanPacific margin (18 to 25ºN): new insight from apatite and zircon

fission-track analysis of coastal and offshore plutonic rocks

Thierry Calmus a,Ł, Gerard Poupeau b, Jacques Bourgois c, Francois Michaud d,Bernard Mercier de Lepinay e, Erika Labrin b, Ali Azdimousa f

a Instituto de Geologıa, Estacion Regional del Noroeste, UNAM, Hermosillo, Sonora, 83000, Mexicob UPRES-A no. 5025 of CNRS, Institut Dolomieu, Universite Joseph Fourier, 15 rue Maurice Gignoux, 38031 Grenoble cedex, France

c CNRS and Universite Pierre et Marie Curie, Laboratoire de Geodynamique, Tectonique et Environnement, 4, place Jussieu,75252 Paris cedex, France

d Laboratoire de Geodynamique Sous-marine, Universite Pierre et Marie Curie, UMR Geosciences Azur,06230 Villefranche-sur-Mer, France

e UMR no. 6526 of CNRS and Institut de Geodynamique, Sophia Antipolis, rue Albert Einstein, 06560 Valbonne, Francef Departement des Sciences de la Terre, Faculte des Sciences, Universite Mohamed-I, 60000 Oujda cedex, Morocco

Received 27 January 1998; accepted 12 February 1999

Abstract

Apatite and zircon fission-track dating was used to constrain cooling histories on granitic samples taken from theoffshore Acapulco trench batholith and onshore Manzanillo and Puerto Vallarta batholiths located along the southwesternactive margin of Mexico, and from the La Paz batholith of southern Baja California. The apatite data indicate that many ofthe samples in the Manzanillo, Acapulco trench batholiths cooled rapidly below 60ºC shortly after emplacement between70 and 55 Ma. The La Paz batholith and the sample NM-20-08 of the Acapulco trench batholith experienced an older andslower cooling across the apatite partial annealing zone. The zircon fission-track ages obtained from two samples of thePuerto Vallarta batholith indicate a rapid cooling from 250 to 110ºC between 54 and 50 Ma. This non-coeval cooling ofbatholiths at two different depths is probably due to uplift and erosion associated with the Laramide event in southwesternMexico. The three samples from the Puerto Vallarta batholith suggest a younger evolution across low temperatures withtwo flat stages located near the base of the apatite partial annealing zone and above it. The first flat stage suggests that thedenudation or rock uplift rate was reduced at a depth corresponding to the base of the apatite partial annealing zone. Theyounger single apatite ages of the Puerto Vallarta batholith, and the last common cooling of the other batholiths indicatethe mild thermal influences of both the Plio-Pleistocene Trans-Mexican Volcanic Belt and the opening of the Gulf ofCalifornia. 1999 Elsevier Science B.V. All rights reserved.

Keywords: fission-track dating; thermochronology; tectonics; subduction; Mexico; Acapulco trench

Ł Corresponding author. E-mail: [email protected]

0040-1951/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 0 4 9 - 9

164 T. Calmus et al. / Tectonophysics 306 (1999) 163–182

1. Introduction

The coastal plutonic belt, which extends alongthe Pacific coast of Mexico from northern BajaCalifornia to the northwest, to Tehuantepec to thesoutheast (Fig. 1), is believed to have formed dueto the subduction of the Farallon plate under theNorth America plate since Mesozoic time (Damonet al., 1981, 1983; McDowell and Clabaugh, 1981;Schaaf, 1990). Isotopic ages from undeformed plu-tons of this belt indicate that rocks get younger fromnorthwest to southeast (Schaaf et al., 1995), frommiddle Cretaceous (100 Ma) to Late Oligocene (25Ma). This migration of magmatism along the conti-

Fig. 1. Geological and tectonic sketch map of south Mexico. Two plutonic belts are identified along the Pacific margin of southwestMexico: The Cretaceous–Paleocene belt extends northwest of the Manzanillo area, the Tertiary belt is from Manzanillo to Tehuantepec.C D Compostella; CA D Campo Acosta; EPR D East Pacific Rise; M D Manzanillo; PV D Puerto Vallarta; RFZ D Rivera Fracture Zone;T D Tomatlan; TM D Tres Marias islands; TFZ D Tamayo Fracture Zone; Zih D Zihuatanejo.

nental margin which may have occurred as a conse-quence of a trench–trench-transform triple junctiondisplacement (see discussion in Schaaf et al., 1995),associated with the southeastward migration of theChortis block (Anderson and Schmidt, 1983; Pindellet al., 1988) along the Polochic–Motagua transformfault system, located south of the boundary betweenMexico and Guatemala.

Schaaf et al. (1995) showed that the migrationof the Chortis block may be responsible for trun-cation of the continental margin in the southeasternpart of the plutonic belt, but not in the northwesternsegment, north of Zihuatanejo. They propose thatsubduction–erosion was probably the main process

T. Calmus et al. / Tectonophysics 306 (1999) 163–182 165

acting in the north, and resulted in the truncationof the forearc, in the region of Puerto Vallartaand Manzanillo. During deep-sea dives of the sub-mersible Nautile (NAUTIMATE project, 1994), twolarge granitic outcrops were discovered offshore theManzanillo batholith. They are located along the in-ner wall of the Acapulco trench and extend seawardvery close to the trench axis. The presence near thetrench axis of such plutonic rocks belonging to themagmatic arc supports the Schaaf et al. (1995) hy-pothesis that subduction–erosion had worked in thisarea.

In an attempt to constrain the Cretaceous andpost-Cretaceous thermotectonic history of the conti-nental margin between the trench and continent inthis region, fission-track analyses were carried out onapatites and zircons from granites collected duringNAUTIMATE dives or sampled onshore from thePuerto Vallarta and Manzanillo batholiths. We alsoincluded two samples from the La Paz batholith ofsouthern Baja California, which has similar isotopicages and equivalent chemical and isotopic compo-sitions than rocks of the Puerto Vallarta batholith(Bohnel et al., 1992), and which is assumed to havebeen closed to present coast of Jalisco and Nayaritbefore the opening of the Gulf of California (Boh-nel et al., 1992), position corroborated by a recentreappraisal of paleomagnetic data of Baja California(Dickinson and Butler, 1998).

2. Geological setting and sampling

2.1. Submarine geological setting

Geophysical results of the SEAMAT cruise (Bour-gois et al., 1988a,b; Bourgois and Michaud, 1991),and deep-sea observations from the Nautile NAUTI-MATE cruise off Manzanillo along the inner wall ofthe Acapulco trench, and in the Manzanillo graben(Michaud et al., 1995) were used to constrain the off-shore geology. The Manzanillo graben correspondsto the offshore southern boundary of the Jaliscoblock (Luhr et al., 1985) (Fig. 2). North and outwardsof the Manzanillo graben, a geologic cross-sectionof the lower part of the inner wall was reconstructedfrom dives 11, 19 and 20 (Mercier de Lepinay et al.,1997). This cross-section, which extends from the

trench axis (4400 m water depth) to the middle slopeof the inner wall (2500 m water depth), shows twotectonic units separated by a 1–2-km-long flat areabetween the subduction scarp and the middle part ofthe inner wall, explored during dive 11 (Fig. 2).

The lower unit corresponds to the 400–500-m-high subduction scarp bounding to the north the largedeep sea fan at the mouth of the Manzanillo graben.This subduction scarp consists mainly of fracturedand locally foliated and folded mudstone well ob-served along dive 20 (Fig. 2). The deformation ofthis unit suggests a compressional regime related tothe subduction. The lower part of dive 19, north-west of dive 20, exhibits the same structural patterncharacterized by changes in bedding direction.

The upper unit presents only discrete fracturesand normal faults. Between 3703 and 2823 m b.s.l.(dive 11), the middle part of the inner wall providesa continuous outcrop of fractured plutonic rocks, thatform abrupt slopes (Fig. 3). Plutonic rock crops outalso at the depth of 3950 m, at the bottom of thedive 20. These rocks are referred to in this workas the Acapulco trench batholith. At the location ofdive 20, the Acapulco trench batholith is overlainat 3900 m b.s.l. by a conglomerate and sandstonedipping to the SW. This detrital sedimentary unitwas followed until the end of the dive at 3700 mb.s.l. At the location of dive 11, plutonic rocks arecovered unconformably by a thick sedimentary se-quence consisting of a 10 m thick polymictic massiveconglomerate of granitic and volcanic pebbles at thebase, followed upwards by fossiliferous sandstoneand siltstone until the end of the dive at 2474 mb.s.l. The Late Miocene to Early Pliocene fossilif-erous assemblages are characterized by endofaunicspecies and oxygen-minimum zone indicators, whichsuggest a sedimentation between the lower and up-per parts of middle bathyal zone, approximately to adepth of 1000 m (Saint-Marc et al., 1997; Mercier deLepinay et al., 1997). McCloy et al. (1988) describea similar and contemporaneous sequence of marinesediments some 250 km northward from the divearea, on the Marıa Madre island, the largest islandof Tres Marıas islands (Fig. 2). There the sedimentsare unconformably overlying Cretaceous granite andgranodiorite with a K–Ar biotite age of 75–68 Ma(Delgado-Granados et al., 1996) and Tertiary (?)andesite and rhyolite (Chinas, 1963). If we except


Fig. 2. (a) Location of deep-sea dives NM-16, NM-11 and NM-20 along which plutonic rocks were sampled. EPR D East PacificRise; (b) Sketch map of deep-sea dive NM-11; (c) the cross section of NM-11 deep-sea dive shows mainly continuous outcrops ofthe Acapulco trench batholith unconformably overlied by Upper Miocene bathyal detritic sediments (see text for further explanations).Seabeam map after Bourgois et al., 1988a.

the Tertiary (?) volcanic rocks of Marıa Madre is-land, both sections are equivalent, and present thesame unconformity between two similar units: the

lower one corresponds to the Cretaceous magmaticbasement and the upper one to Late Miocene–EarlyPliocene upper bathyal sediments.


Fig. 3. Plutonic rocks vertical escarpment at 2834 m water depth along the inner wall of the Acapulco trench (dive NM-11). Theunconformity of Upper Miocene rocks is 10 m higher. Black line D 1 m. Numbers indicate local time, depth, elevation above sea floorand the head of the Nautile.

This upper unit recognized along the inner wallof the trench off Manzanillo and along the westernescarpment of the Tres Marıas islands exhibits asimilar Neogene evolution until Pliocene time whena change occurred between both areas (Mercier deLepinay et al., 1997): uplift and emersion in the LatePleistocene of the Tres Marıas islands (McCloy etal., 1988), and tectonic subsidence along the conti-nental margin off Manzanillo. We assume that partof the uplift of the Tres Marıas islands may be dueto a thermal response related to the opening of theGulf of California after the subduction ceased in thisarea. In this N–S-striking part of the Acapulco trench(north of 19º50 N), active normal N–S faults alongthe axis of the trench off Tres Marıas islands sug-gest that this segment of the Acapulco trench recordsextension (Michaud et al., 1996).

2.2. Onland geological settings

The La Paz batholith is located in the Cabo SanLucas block of Baja California (Fig. 4a). The plu-tons between La Paz to the north and Cabo SanLucas to the south intrude amphibolitic rocks andlow-grade metamorphosed sediments, which sug-gests that metasedimentary units of Todos Santos(Aranda-Gomez and Perez-Venzor, 1989) extend toEl Triunfo and San Antonio area to the north. TheTodos Santos and El Triunfo and San Antonio meta-morphic units may represent the northern extensionof the metamorphic rocks of the La Cruz de Loretoarea, south of the Puerto Vallarta batholith (Fig. 4b).The ages for the plutons range between late EarlyCretaceous and Late Cretaceous in age (Gastil et al.,1978a; Frizzel and Ort, written com., 1984, cited inAranda-Gomez and Perez-Venzor, 1989). The two


Fig. 4. Simplified geological map of the three onshore areas of sampling. (a) La Paz batholith; SJLPF D San Juan de Los Planes fault;(b) Puerto Vallarta batholith; (c) Manzanillo batholith.

samples dated by fission-tracks on apatite come fromgranodiorites collected from the northwestern part ofthe La Paz batholith. These include sample TC-93-10

from the south of Sierra El Novillo, at 10 km NNWof the town of El Triunfo, and sample TC-93-12 col-lected 1 km south from the same locality (Fig. 4a).


The Puerto Vallarta and Manzanillo batholiths(Fig. 4b,c) represent the southeastern continuation ofthe granitic belt of the La Paz batholith, separatedas a consequence of the opening of the Gulf of Cal-ifornia. They are composed of calc-alkaline diorites,granodiorites, granites and minor gabbros. The plu-tonic rocks intrude metamorphic rocks of Jurassicand=or Cretaceous age and Aptian–Albian marinesediments (Lange and Carmichael, 1991; Calmus etal., 1996). Whole-rock and mineral K=Ar and Rb=Srradiometric ages range between 108 and 59 Ma(Zimmermann et al., 1988; Schaaf, 1990; Schaaf etal., 1995). K=Ar radiometric ages of silicic ash flows,especially in the Puerto Vallarta batholith area varybetween 114 Ma and 54 Ma (Gastil et al., 1978b;Wallace and Carmichael, 1989; Righter et al., 1995),which suggests that volcanism and plutonism arecoeval. Table 1 shows radiometric ages for samplescollected from the three batholiths and one age forthe plutonic rock of Marıa Madre island.

3. Fission-track data interpretation

The fission-track analysis of metamorphic base-ment or intrusive rocks is based on the determi-nation of (1) an apparent fission-track age and (2)the distribution of the total etchable track lengths.Fission-track dating is usually done with the exter-nal detector technique, where, whenever possible, atleast 20 grains per sample are dated individually.The fission-track age (FT age) of the sample is thencalculated as the geometric mean, or ‘central age’of these grains (Galbraith and Laslett, 1993). Themeaning of this age depends basically, for a givenmineral phase, of its cooling history.

Tracks are subtle defects of the crystalline latticesubject to reorganization under the influence of tem-perature, and the interpretation of fission-track datauses the concept of partial annealing zone (PAZ).In this concept, one considers that above a certaincritical temperature, fission tracks are very unstableand essentially no tracks are etchable; below another(lower) temperature, they are recorded with nearlyno alteration of their etchable length. Fission tracksproduced in between these two temperatures, in theso-called PAZ, will have variable etchable lengths,depending on their time–temperature .t–T / history

since the beginning of track recording. In apatites thePAZ is considered to be in the 110–60ºC temperaturerange; recent estimates for the zircon PAZ suggest a220–300ºC range.

Total etchable lengths are determined by the mea-surements of confined tracks, i.e. tracks totally in-cluded in the grains. Confined tracks can be etchedonly if they intersect another track or a fracturereaching the surface of a grain and acting as con-ducts for the etching solution. The shape of the totaletchable track length distribution vary as follow: forfast (a few my at most) cooling through the PAZ,confined tracks will have a mean value higher than14 µm and a standard deviation typically lower than1.5 µm. In the case of a slow cooling through thePAZ, the length distribution will be biased towardsshorter values, with a shorter mean value and a largerstandard deviation. In the case of a reheating event,depending on its maximum temperature and dura-tion, the distribution may tend towards a bimodaldistribution, with a mean length down to less than 12µm and a standard deviation exceeding 2 µm.

The distribution of single grain ages is givenin radial plots, where both the age and precisionof each grain in a sample are given (Galbraith,1990). All grains in a hand-specimen metamorphicor intrusive rock have same cooling history. Hence,the ages of all apatite grains might belong to asingle population. However, variations in chemicalcomposition, as in the Cl=F ratio (Green et al.,1989), the Mn ratio (Ravenhurst et al., 1992) or theREE contents (Burtner et al., 1994), induce slightdifferences in track thermal stability. As a result,if apatites with different compositions are presentwithin a rock, several populations or a continuumof ages may appear in a radial plot, dependingof the time versus temperature .t–T / history ofthis rock through the PAZ. An analysis of the agedispersion, using either the �2 test of Green (1981)(see below) or the standard deviation of the centralage (Galbraith, 1990), allows to recognize whenmore than one population of ages are present.

4. Apatite and zircon fission-track dating

Twelve samples collected from offshore outcropsof plutonic rocks during NAUTIMATE dives 11, 16


Tabl

e1

Rad

iom

etri

can

dap

atite

and

zirc

onFT

com

para

tive

ages

for

sam

ples

ofL

aPa

z,Pu

erto

Val

lart

a,an

dM

anza

nillo

and

Aca

pulc

otr

ench

bath

olith

s

La

Paz

bath

olith

Puer

toV

alla

rta

bath

olith

sM

anza

nillo

and

Aca

pulc

otr

ench

bath

olith

s

Sam

ples

Apa

tite

FTK=A

rA

ges

Sam

ples

Apa

tite

FTZ

irco

nFT

Rb=

Srag

esK=A

rag

esSa

mpl

esA

patit

eFT

Rb=

Srag

esag

esag

esag

esSc

haaf

ages

Scha

af(1

990)

(199

0)

Lay

ered

horn

blen

de11

5T

C-9

3-46

(750

mhe

ight

)31:0š

2:2

54:0š

1:9

TC

-93-

53(2

0m

heig

ht)

54:6š

3:3

dior

ite(L

aPa

zfa

ult)

Hau

sbac

kM

S4(b

iotit

e-W

R)

77:6š

2M

S22

59:1š

1:2

(198

4)C

FE12

(who

lero

ck)

70:9š

5:3

TC

-93-

10(3

00m

heig

ht)

71:6š

4:2

CFE

12(b

iotit

e)84:7š

2:7

NM

-11-

0658:0š

2:2

TC

-93-

48(2

0m

heig

ht)

32:5š

1:7

65š

15Z

imm

erm

ann

etal

.(3

705

mw

ater

dept

h)(1

988)

TC

-93-

12(4

70m

heig

ht)

68:4š

3:6

1162

(hor

nble

nde)

90:6š

1:8

NM

-11-

1160:1š

3:7

Scha

afet

al.

(324

6m

wat

erde

pth)

(199

5)So

uthe

rnB

aja

Cal

ifor

nia

98to

70M

S2A

(bio

tite-

WR

)77:6š

280:9š

4:9

NM

-11-

1256:4š

2G

astil

etal

.C

FE11

(WR

)90:0š

4:4

(290

1m

wat

erde

pth)

(197

8b)

CFE

11(b

iotit

e)Z

imm

erm

ann

etal

.N

M-2

0-07

59:2š

2:6

(198

8)7

poin

tWR

isoc

hron

99š

4(3

950

mw

ater

dept

h)M

EA

NA

GE

,10

sam

ples

78.7

NM

-20-

0880:3š

3:4

(290

1m

wat

erde

pth)

NM

-16-

01(3

787

wat

erde

pth)

37:3š

1:1

Mar

iaM

adre

id.(

biot

ite)

75to

68D

elga

do-G

rana

dos

etal

.(19

96)

See

text

for

expl

anat

ions

.


and 20 (Fig. 2) and from batholiths of the plutonicbelt were selected for fission-track dating. Six werefrom below sea level, including three from dive 11between 3703 and 2892 m b.s.l. and two others fromdive 20, at depths of respectively 3933 and 3913 mb.s.l. In the northern end of the Acapulco trench, agranitic boulder was collected in a conglomerate ofthe trench axis, during dive 16 at 3783 m b.s.l., tocompare its fission-track age with those of samplesof the Puerto Vallarta batholith. Six samples comefrom the onshore Manzanillo, Puerto Vallarta and LaPaz batholiths.

Apatites and zircons were dated by the externaldetector method (Naeser and Dodge, 1969). Fissiontracks in apatites were etched in a 1 N HNO3 solutionat 20ºC for 30 to 40 s and in zircons in an NaOHC KOH eutectic at 220ºC for 6 to 38 h. All thesamples were irradiated in the Orphee reactor of theCentre d’Etudes Nucleaires of Saclay, France. Thethermal neutron fluence was monitored with NISTfission-track glasses 962 and 963. Confined tracklengths were measured in apatites in a plane parallelto the C crystallographic axis after a 45 to 60 setching time.

The results are presented in Table 2. The agesare calculated as modal ages weighed for the pre-cision of individual crystals ages (‘central ages’ ofGalbraith, 1990). The age dispersion is given by aP.�2/ which is the probability of obtaining a �2

value for n degrees of freedom, where n D number

Fig. 5. Radial plot and distribution of confined tracks lengths for samples from the La Paz batholith. I and II, respectively agemeasurements by T. Calmus and E. Labrin. Histograms: X axis: scale in µm; Y axis: number of tracks; the mean track length andthe standard deviation in µm are indicated on the top of each histogram; N D number of measured confined tracks; radial plots, as inGalbraith (1990).

of crystals dated minus one, and the variance (%) ofindividual ages. When the former is <5% and thelatter >15%, one considers that more than one agepopulation are present in a sample. Apatites couldbe dated in all samples except TC-93-24 in whichuranium distribution was too inhomogeneous. Threeobservers contributed to the dating of apatites. Whenone apatite mount was counted by two or three ob-servers, as for NM-16-01, TC-93-48, NM-20-08 andNM-11-06, the results are in excellent agreement.Zircon fission-track data were obtained for two sam-ples, TC-93-46 (4 crystals) and TC-93-48 (1 crystal)of the Puerto Vallarta batholith.

4.1. La Paz batholith

Both samples from the La Paz batholith presentconcordant central ages around 70 Ma. Sample TC-93-10, from which 45 crystals were dated, did notpass the �2 test and contains a large individual crys-tal-age distribution, from more than 100 to about 50Ma (Fig. 5). In sample TC-93-12, only six crystalswere dated and although it passed the �2 test, thespread in individual crystal ages from 90 to 55 Mawas nearly as large as that of TC-93-10.

4.2. Puerto Vallarta batholith

Two onshore samples, TC-93-46 and TC-93-48were dated, the first of which barely passed the

172T.C

almus

etal./Tectonophysics

306(1999)

163–182Table 2Fission track data

Sample Elevation Latitude Longitude Obs. Min. ²f ð 105 Nf ²i ð 105 Ni N Age dispersion ²d ð 105 Nd Age Mean track SD No. of(m) (N) (W) (cm2) (cm2) (cm2) (Ma) length (µm) measured

š1¦ š1¦ tracksP.�2/ DE(%) (%)

La Paz batholithTC-93-10 C300 23º5105800 110º0904800 1 A 1.21 4512 0.568 2166 45 2.3 12 2.175 14379 71:6š 4:2 12:87š 0:17 1.34 61TC-93-12 C470 23º4703200 110º0605800 2 A 8.11 403 6.54 332 6 20 10 3.384 13245 68:4š 3:6 13:27š 0:16 1.19 57

Puerto Vallarta batholithNM-16-01 �3783 20º0701000 106º1702000 1 A 1.29 2350 1.20 2229 22 12 8 2.175 14379 36:6š 2:2

2 A 8.62 1039 8.43 1016 13 94 <1 2.175 14379 37:6š 1:3

37:3š 1:1TC-93-48 C30 20º3005700 105º1800700 1 A 7.05 992 6.42 924 17 <1 28 2.175 14379 32:6š 2:6

2 A 3.96 402 3.96 402 12 3.8 23 2.175 14379 32:4š 2:3

32:5š 1:7TC-93-48 4 Z 139.53 180 27.9 36 1 0.6484 9605 65š 15TC-93-24 C205 20º4605000 105º2302400 2 A Heterogeneous distribution of uranium 12:91š 0:54 2.70 114TC-93-46 C750 20º2100000 105º1705300 1 A 3.35 471 3.66 527 17 6 11 2.175 14379 31:0š 2:2 13:19š 0:23 1.70 53TC-93-46 4 Z 70.75 1630 24.31 357 3 67 0.6484 9605 55:1š 2:1

4 Z 139.53 180 27.9 36 1 0.6484 9605 48š 5

4 Z 4 54:0š 1:9

Manzanillo and Acapulco Trench batholithsNM-11-06 �3703 18º2402700 104º3204500 2 A 17.5 728 17.2 717 9 >99 <1 3.384 13425 58š 2:2 13:43š 0:18 1.32 54

3 A 23.6 983 23.0 956 12 3.384 13425 63š 3 a

NM-11-11 �3246 18º2404700 104º3201100 1 A 2.78 2764 1.57 1598 12 3 10 2.175 14379 60:1š 3:7 13:56š 0:11 1.31 136NM-11-12 �2892 18º2500200 104º3105500 2 A 1.70 984 1.72 997 7 95 <1 3.384 13425 56:4š 2 14:63š 0:10 1.03 108NM-20-07 �3933 18º1904500 104º2700000 2 A 1.21 503 1.15 485 5 98 <1 3.384 13425 59:2š 2:6 14:15š 0:10 0.97 112NM-20-08 �3913 18º1904500 104º1601000 1 A 19.8 1065 12.3 658 9 <1 26 3.384 13425 80:2š 6:2 13:73š 0:33 1.16 12

2 A 1.01 672 0.672 455 8 4.4 16 3.384 13425 80:4š 4:1

80:3š 3:4TC-93-53 C10 19º0700600 104º2302000 1 A 3.84 1592 2.39 1011 50 >99 <1 2.175 14379 54:6š 3:3

Elevation a.s.l.: C; b.s.l.: �. Obs.: number of observers: 1 D T. Calmus, ¾Ap D 320š 18; 2 D E. Labrin, ¾Ap D 339š 7; 3 D N. Lamy, ¾Ap D 365š 10; 4 D A. Azdimousa,¾Z D 374š5. Min.: mineral; A: apatite, Z: zircon. ² and N , fossil . f / and induced .i/ track densities and number of fossil . f / and induced .i/ tracks counted; N D numberof grains dated; dispersion parameters, respectively P.�2/, calculated following Green (1981), and D.E., dispersion of the central age following Galbraith and Laslett(1993). ²d and Nd, track density to the fluence glass monitors 962 of NIST and number of tracks counted; SD; standard deviation of the confined track length distribution.a All ages are calculated as central ages following Galbraith and Laslett (1993), except for NM-11-08 by observer 4, calculated from a pooled statistics (Green, 1981).


�2 test. However, both presented similar individualcrystal age distributions, with extremes at 48 and 17Ma for TC-93-46, and 59 and 16 Ma for TC-93-48,which resulted in the same mean age of 31:0 š 2:2and 32:5 š 1:7 Ma respectively. The confined tracklength distributions in TC-93-46 and the undatablesample TC-93-24, with mean confined track lengthsrespectively 13.19 and 12.91 µm (Fig. 6), and stan-dard deviations respectively 1.70 and 2.70 µm, areindicative of a slow cooling history through the PAZ.The zircons dated came from the two onshore sam-ples, TC-93-46 and TC-93-48. In TC-93-46, whichpassed the �2 test, the zircon age was 54 š 1:9Ma, concordant with the oldest apatite ages in thissample. In TC-93-48, only one zircon crystal wasdated. Its age, of 65š15 Ma, is consistent with thoseobtained for individual crystals in sample TC-93-46.

The dive sample NM-16-01 collected in a Mio-cene (?) conglomerate at 3783 m water depth, closeto the trench axis, passed the �2 test. Two centralages were obtained, 36:6 š 2:2 and 37:6 š 1:3 Ma,with a weighted mean of 37:3 š 1:1 Ma, slightlyolder than that of onshore samples.

4.3. Manzanillo and Acapulco trench batholith

The onshore sample TC-93-53 from the Man-zanillo batholith and samples NM-11-06, NM-11-12and NM-20-07 collected during dives 11 and 20pass the �2 test, giving concordant ages between59:2 š 2:6 and 54:6 š 3:3 Ma. Two other offshoresamples did not pass the �2 test. One of these, NM-11-11, however presented an age of 60:1 š 3:7 Maconcordant with the samples passing the test and adistribution of individual ages which did not differgreatly from the former ones. Data from sample NM-20-08 differ significantly from the others by its olderweighted mean age of 80:3 š 3:4 Ma and a widerrange of individual ages, from 107 to 53 Ma (Fig. 7).

All samples except NM-20-08 have relativelylong mean confined track lengths, varying from13.43 µm (NM-11-06) to 14.63 µm (NM-11-12),which indicates the rocks cooled rapidly throughthe 110º–60º PAZ. The shorter mean confined tracklength of sample NM-20-08 of 13.26 µm, suggests alonger residence time into the PAZ (Fig. 7).

5. Cooling rates

The cooling history of the Cretaceous coastalmagmatic belt is better documented in the PuertoVallarta–Manzanillo region (Gastil et al., 1978a;Pantoja-Alor, 1983; Zimmermann et al., 1988;Schaaf et al., 1995) than in Baja California for whichonly few K–Ar and U–Pb ages ranging between 115and 70 Ma are available (Frizzel et al., 1983, writtencommun. in Hausback, 1984). Biotite Rb–Sr and K–Ar ages, corresponding respectively to cooling below300 š 50ºC (Jager, 1979) or 320 š 40ºC (Harrisonand McDougall, 1980) and 350 š 50ºC (Geyh andSchleicher, 1990) range from about 100 to 70 Ma inthe Puerto Vallarta batholith and down to 59 Ma inthe Cihuatlan–Manzanillo area (Table 1). The K–Arages of plutons which compose the La Paz batholithof Baja California, between La Paz and Cabo SanLucas (Cabo San Lucas block) vary also from lateEarly Cretaceous to Late Cretaceous (Gastil et al.,1978b; Frizzel et al., 1984; Hausback, 1984; Aranda-Gomez and Perez-Venzor, 1989). We give in Table 2some relevant data for the determination of the cool-ing histories of our samples. We will use herein thefollowing closing temperatures for isotopic systems:300 š 50ºC for the Rb–Sr decay system in biotite(Jager, 1979; Harris, 1996) and 320š40ºC (Harrisonand McDougall, 1980) for K–Ar in biotite.

Based on FT data, different regions experienceddifferent histories. The Manzanillo batholith, whereRb–Sr ages are available, experienced obviouslyrapid cooling during Paleocene. However, some ar-eas of the Acapulco trench batholith cooled below110ºC a long time before, as evidenced by FT agesof 80 Ma. These oldest ages data suggest similarcooling history in La Paz batholith (see below). FTages between 59 and 54 Ma were obtained on sev-eral samples passing the �2 test. This is the caseof the zircons of onshore sample TC-93-46 (PuertoVallarta batholith) and the apatites of samples col-lected during the Nautile dives and in the Manzanilloarea (NM-20-07, NM-11-06, NM-11-12, and TC-93-53). Therefore, the fission-track data show thatrapid cooling of these batholiths occurred during thePaleocene. As our samples were not dated by otherisotopic methods, we cannot determine whether theirPaleocene FT ages are related to an episode of cool-ing taking place immediately after emplacement or


Fig. 6. Radial plot and distribution of confined tracks lengths for samples from Puerto Vallarta batholith. Same legend as Fig. 5.


Fig. 7. Radial plot and distribution of confined tracks lengths for samples from Manzanillo and Acapulco trench batholiths. Same legendas Fig. 5.


if they cooled below 110ºC long time after emplace-ment. The exception is sample TC-93-53, which wastaken from a gabbro–dioritic intrusion located westof Santiago (Fig. 4c). This location was previouslydated by Schaaf (1990) who determined a biotite andwhole rock Rb–Sr age of 59:1š1:2 Ma for this intru-sion. Taking into account a mean closure temperatureof 300 š 50ºC for the Rb–Sr decay system in biotite(Jager, 1979; Harris, 1996) and the 54:6 š 3:3 Maapatite FT age for sample TC-93-53, it is possibleto derive a mean cooling rate of 37ºC=m.y. between59.1 Ma and 54.6 Ma (200ºC over 4.5 m.y.).

Data from sample NM-20-08 of the Acapulcotrench batholith do not pass the �2 test. This sample,which contains some apatites having FT ages com-patible, within the analytical precision, with a 100Ma emplacement, would belong to the oldest Creta-ceous intrusion generation. The conservation of suchold ages implies rapid cooling shortly after emplace-ment. This is in good agreement with the isotopicdata that led Schaaf et al. (1995) to admit that theRb–Sr and K–Ar cooling ages biotite or hornblendeof the smaller intrusive stocks are ‘insignificantlyyounger’ than the main intrusion ages, which meansthey cooled rapidly to low temperatures. These au-thors assumed that further reheating, if it occurred,was never important as petrographic observations re-veal no evidence of metamorphism. In the sampleNM-20-08, there is no evidence of reheating, and thepresence of grains with ages as young as 60–50 Masuggests that sample sat at high temperature afterintrusion, then rapidly cooled during the Paleocene.The unimodal track length distribution of samplesNM-11-06, NM-11-11 and NM-11-12 suggests thatthey did not suffer any reheating.

The apatites of La Paz batholith seem to haverecorded a thermal history similar to that of sam-ple NM-20-08 from the Acapulco trench batholith:samples TC-93-10 and TC-93-12 also exhibit apatitesgrains with individual ages distributed in the samerange, between about 100 and 60–50 Ma. For the samereasons as the Manzanillo batholith, these data implya two-stage cooling history: an initial episode afteremplacement around 130–100 Ma (maximum age) tobring rocks to a temperature at which some tracksare retained (hence old ages), followed by major fi-nal cooling below the PAZ minimum temperature. Inthe special case of TC-93-10 which failed the �2 test,

the ages are mixed ages and do not directly record acooling event at 70 MA, and the older grain ages sug-gest an earlier stage of cooling in the PAZ to preservethem. This conclusion is available for the Cretaceous–Paleogene thermal evolution of the La Paz batholith,but a recent unpublished study (J. Fletcher, oral com-munication, 1997) along the San Juan de Los Planesfault (Fig. 4a), southeast of La Paz, shows that someFT ages of rocks sampled close to the fault recordedLate Miocene cooling due to uplift associated withfault tilting, whereas ages of other samples far fromthe fault corroborate the Late Cretaceous–PaleoceneFT ages. Late Miocene FT ages correspond to localuplift of deeper parts of plutons that were below thePAZ before. Also, Mio-Pliocene apatite FT ages fromsamples of Proterozoic and Cretaceous plutonic rocksin Sonora (NW Mexico) record recent tectonic activityassociated with the opening of the Gulf of California(Defaux et al., 1997; Calmus et al., 1998).

A different behavior appears in the apatites ofsamples TC-93-46, TC-93-48 and NM-16-01 of thePuerto Vallarta batholith. In these three samples, theoldest individual FT ages are around 60–50 Ma andthe youngest range from 26 Ma for NM-16-01 to18-17 Ma for the two onland samples. In the PuertoVallarta batholith, the oldest FT ages have to be re-lated to the Late Cretaceous–early Paleocene cool-ing event observed in the Manzanillo and La Pazbatholiths, as they are concordant with zircon FT agesof 54:0 š 1:9 Ma (TC-93-46) and 65 š 15 Ma (TC-93-48). From this concordance, we can conclude thatthese samples cooled quite rapidly during the Pale-ocene from 230š 40ºC which is the critical tempera-ture range for the recording of fission tracks in zircons(Tagami et al., 1996) down to the apatite PAZ. DuringPaleocene cooling, part of the batholith cooled abovethe base of the PAZ and started to retain some tracksin apatite as NM-16-01, while other rocks remained attemperatures higher than the base of the PAZ, as TC-93-46 (Fig. 8). Subsequently, during the Oligocene–Miocene a second event of rapid cooling brought allsamples to temperatures lower than 60ºC.

6. Discussion

The youngest single-crystal fission-track ages de-termined in this work, around 17–19 Ma, were found


Fig. 8. Thermal histories of samples from La Paz, Puerto Vallarta and Manzanillo and Acapulco trench batholith, from fission-trackand geological data. AC: Acapulco trench batholith; LP: La Paz batholith; M: Manzanillo batholith; PV: Puerto Vallarta batholith; PAZ:Apatite partial annealing zone. See discussion in text.

from apatites of the Puerto Vallarta batholith (Fig. 5).At that time, the Cabo San Lucas block of southernBaja California (Fig. 4a) was still located in the NWvicinity of the Puerto Vallarta batholith. The La Pazbatholith may therefore be considered as belongingto the same Mexican Cretaceous Pacific magmaticbelt as the Manzanillo and Puerto Vallarta ones.These intrusions were emplaced during the Creta-ceous, the oldest ones about 140 to 100 Ma beingmostly found in the northwest part of the magmaticbelt, and the youngest in the southeast (Schaaf et al.,1995).

The apatites exhibiting the oldest ages in the LaPaz and Manzanillo batholiths show that fast meancooling rates, of the order of 35ºC Ma�1 between650ºC and 300ºC calculated from data of Schaafet al. (1995) in the Manzanillo batholith may havepersisted down to less than 60ºC, at least for the

Manzanillo batholith. Assuming a relatively highgeothermal gradient of 40ºC, and a mean annualsurface temperature of 20ºC, this means that some ofthe samples were brought to less than 1 km from thesurface by the late Early Cretaceous. If we considera minimum depth of emplacement of 5 to 3 km forthese plutons, we calculate that minimum denudationor erosion of between 2 and 4 km took place withina few million years after pluton emplacement. Inthe case of the La Paz batholith, although a foliatedhornblende diorite yielded a K–Ar age of 115 Ma(Hausback, 1984) close to samples TC-93-10 andTC-93-12, we cannot directly compare these ages, inso far as the latter samples belong to an undeformed,and therefore younger intrusion.

The denudation and=or erosion at that time haveto be related with the Laramide orogeny, which iswidespread in Mexico and western United States.


It corresponds to a change in the evolution of themagmatic arc probably related with the collision andaccretion to the continent of exotic terranes as theGuerrero terrane, the Alisitos magmatic arc, or theArperos oceanic basin (Tardy et al., 1994). To thenorth, different interpretations are given to explainthe Laramide orogeny among which the collisionand northward migration of the Baja BC terranes(Maxson and Tikoff, 1996) or a decrease of thesubduction dip (Dickinson and Snyder, 1978).

The fossil tracks of TC-93-46 sample have a13.19 µm mean length and a standard deviationof 2.70 µm. This relatively long mean length sug-gests that, if the cooling rate of Puerto Vallarta andManzanillo batholiths were similar from high tem-peratures, as the results of Schaaf (1990) indicate,the samples of the Puerto Vallarta batholith wereexposed to temperatures ranging from 250ºC (zirconeffective retention temperature) to 110ºC, from 54Ma (FT zircon age of TC-93-46) until around 50 Ma,i.e. around 4 m.y, and experienced a slower coolingbetween 50 Ma and 35 Ma, at temperatures corre-sponding to the base of the PAZ. Assuming a samegeothermal gradient of 40ºC km�1 along the mag-matic arc during Paleogene time and a mean annualsurface temperature of 20ºC, the analyzed sampleswere between 50 and 35 Ma at a depth of around2.25 km. This depth may correspond partly to thethickness of the Cretaceous co-magmatic volcanicsequence (Lange and Carmichael, 1991) and=or theLower Cretaceous sedimentary unit of the San Se-bastian del Oeste, formation which probably belongsto the Alisitos–Teleoloapan magmatic arc (Calmus etal., 1996). The Late Eocene–Early Oligocene cool-ing age of the Puerto Vallarta batholith is somewhatcorroborated by the FT central age of granodioriteNM-16-01 (37:3š 1:1 Ma).

In the Punta de Mita intrusive complex (Fig. 4b),north of Banderas bay, Schaaf (1990) obtained ahigher cooling rate of 115ºC Ma�1, between 500ºCand 320ºC. This rapid cooling is interpreted as beingrelated to the location of the samples near the bor-der of the intrusion. The mean fossil track length ofsample TC-93-24 located very close to the samplescollected by Schaaf, is 12:91 š 0:54 µm long. Thisvalue suggests that the rapid cooling rate reduced forthe PAZ temperatures in this part of the batholith.Nevertheless, the lack of FT age allows no quanti-

tative conclusion regarding the cooling rate regimebetween 320ºC and the 60ºC upper limit of the PAZ.

The Puerto Vallarta batholith and host rocks madeup mainly of Cretaceous sedimentary and Cretaceousvolcanic units, e.g. in the San Sebastian del Oesteand El Cuale areas (Fig. 4b), are part of the Jaliscoblock. These rocks are overlain by Plio-Quaternaryvolcanic rocks belonging to the Trans-Mexican Vol-canic Belt, which suggests that Oligo-Miocene vol-canic rocks belonging to the Sierra Madre Occidentaldo not exist in the Jalisco block. The existence ofvolcanic units of the Sierra Madre Ocidental is dis-cussed by previous authors (Ferrari et al., 1994;Righter et al., 1995). According to Ferrari et al.(1994), volcanic rocks of the Sierra Madre Occiden-tal are present south of the Tepic–Zacoalco graben(Fig. 2), which represents the northern boundary ofthe Jalisco block (Fig. 2). But the presence of golddeposits and hydrothermal alteration of ignimbritesand tuffs in this region suggests that these rocksare probably of Cretaceous age and were intrudedby the Cretaceous batholith. The northern limit ofthe Jalisco block is inherited from the sharp transi-tion between Cretaceous–Paleogene volcanic rocksof the Jalisco block and younger ash flows of theSierra Madre Occidental to the north. Two hypoth-esis are proposed: (1) the southern boundary of theSierra Madre Occidental corresponds to an impor-tant geodynamic or structural limit during Oligo-Miocene time, (e.g. Sedlock et al., 1993), or (2) theJalisco block is a site of a post-Miocene uplift andcorrelative erosion which stripped off the volcanicrocks of the Sierra Madre Occidental. Sedlock et al.(1993) proposed a boundary between the Tahue andthe Nahuatl terranes masked by the Trans-MexicanVolcanic Belt. Following this first hypothesis, thevolcanic rocks of the Sierra Madre Occidental neverexisted to the south, especially on the Jalisco block;Alternatively Righter et al. (1995) propose an upliftrate on the order of 0.5 mm yr�1 from 4.2 Ma topresent, to remove by erosion the 2000 m of Tertiaryvolcanic rocks (McDowell and Clabaugh, 1979), ifthey were originally present on the Jalisco block.Nevertheless, paleontologic and geologic data ob-tained on coastal plains of the Jalisco block (Durhamet al., 1981) between La Cruz de Loreto and Cihu-atlan (Fig. 4b,c) do not suggest such uplift duringthe past 5 Ma along the present coastline. They de-


scribed non-marine Pleistocene sediments croppingout in a creek of the coastal plain the mean eleva-tion of which does not exceed 100 m a.s.l. A recentuplift, if it exists, was therefore less than 100 mduring the Quaternary, that corresponds to an upliftrate of about 0.06 mm yr�1, ten times smaller thanthe rate calculated by Righter et al. (1995). 30 to 40km northwest of Cihuatlan, Upper Cenozoic marinesediments are reported (Durham et al., 1981). Thecollected molluscs possibly indicate a lower thannormal salinity that we can associate with a lagoonalenvironment of very shallow water depth. The con-clusion of a slow uplift of these coastal plains ofthe Jalisco block may be generalized to many otheroutcrops of Upper Cenozoic sandstone, conglom-erate or siltstone present along the coast, betweenManzanillo and Puerto Vallarta.

Furthermore, the Upper Cenozoic marine andnon-marine sediments, that unconformably overliemagmatic rocks of the Puerto-Vallarta batholith, in-dicate that the erosion of the whole cover of extrusiverocks would pre-dated the late Cenozoic time. At 5km west of the town of San Sebastian del Oeste(Fig. 4b), a conglomerate older than 1.5 Ma (Langeand Carmichael, 1991) overlies directly the Creta-ceous granitic pluton and silicic ash flows, withoutOligocene or Miocene volcanic rocks, coeval withthe Sierra Madre Occidental volcanic units.

These remarks do not mean that there was notectonic activity in the onshore part of the Jaliscoblock during Late Pliocene–Quaternary, but manystudies made during the last decade indicate thatthis activity is concentrated along its limits (e.g.Luhr et al., 1985; Barrier et al., 1990; Allan et al.,1991; Michaud et al., 1991, 1993; Garduno-Monroyand Tibaldi, 1991; Quintero-Legorreta et al., 1992;Nieto-Obregon et al., 1992; Ferrari et al., 1994;Rosas-Elguera et al., 1996). Details of results andgeodynamic and cinematic interpretations vary fromone author to the other, but they agree with a mainlyextensional regime along the Tepic–Zacoalco andthe Colima grabens.

7. Conclusions

FT data of the La Paz, Puerto Vallarta, Man-zanillo, and Acapulco trench batholiths provide a

new insight about the evolution of the area beforethe individualization of the Jalisco block and theopening of the Gulf of California.

The deep sea dives of the Nautile off Manzanillohave shown that parts of the lower slope of thissegment of the continental margin of Mexico exhibitlarge outcrops of granodiorite at water depth rangingfrom 2500 to 3500 m: the Acapulco trench batholith.FT data available from surface samples suggest thatthe Acapulco trench batholith is the seaward prolon-gation of the Manzanillo batholith, since they exhibitthe same cooling history and same mean FT age.

FT analysis of Acapulco trench and Manzanillobatholiths indicate that cooling occurred between 83and 54 Ma, which corresponds to the widespreadLaramide orogeny in Mexico. The older FT age ofsample NM-20-08 may correspond to an older in-trusion, which suggests a migration of intrusionsfrom west to east during the Cretaceous. SinceUpper Miocene sediments unconformably overliethe Acapulco trench batholith, we assume that thevolcano-sedimentary cover of the batholith knownonland was removed by aerial erosional processesoccurring between 78–38 and 10 Ma. Subsidence ofthe Acapulco trench batholith occurred subsequently.In the case of the Puerto Vallarta batholith the FTages and track lengths indicate a first uplift or de-nudation which raised the studied samples to depthshallower than 2.5 km by the beginning of Eocenetime, and a second from the beginning of Oligocene,inducing the erosion of Cretaceous host rocks.

The information available offshore, along the in-ner slope of the Acapulco trench, constrains theNeogene tectonic evolution of this segment of theMexican Pacific margin. The unconformity of LateMiocene conglomerate, well documented in the up-per part of dives 11 and 20, as well as the unconfor-mity of Boca de Tomatlan and coastal late Cenozoicsediments, indicate that, if volcanic units belong-ing to the Sierra Madre Occidental accumulated onthe batholith, they were subsequently eroded beforeLate Miocene. Moreover, the unconformity which isdeeper than 3000 m at present time, probably indi-cates aerial or shallower marine conditions necessaryfor the erosion of Cretaceous units. From data of dive11, the calculated rate of tectonic subsidence is 0.35mm=yr (Mercier de Lepinay et al., 1997). The samesituation was documented in the Tres Marıas Is-


lands where Upper Miocene through Pleistocene (?)marine sediments unconformably overlie Cretaceousgranite and granodiorite, and Tertiary (?) volcanicrocks, which means that an erosional event occurredbefore Late Miocene time.

Acknowledgements

The NAUTIMATE cruise was supported by theInstitut Francais de Recherche pour l’Exploitationde la Mer, the Institut National des Sciences del’Univers and Centre National de la Recherche Sci-entifique. We thank the captain and the crew ofR=V Nadir for their work aboard. We thank NathalieLamy and Gloria Toro for their helpful contributionduring this work. Mineral separations and irradia-tions costs were supported by the INSU-GeosciencesMarines 94GEO-1=1.10 and SEP-FOMES grants.This work was also supported by the Mexican-French ECOS-ANUIES program through the projectno. M96U01. We thank an anonymous reviewer andPaul O’Sullivan for thoughtful comments on the firstversion of the manuscript.

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Late Mesozoic and Cenozoic thermotectonic history of the Mexican Pacific margin (18 to 25°N): new...

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