Synchronous 29–19 Ma Arc Hiatus, Exhumation and Subduction of Forearc in Southwestern Mexico

Synchronous 29–19 Ma arc hiatus, exhumation and subduction of forearc in southwestern Mexico

J. DUNCAN KEPPIE*, DANTE J. MORA´ N-ZENTENO, BARBARA MARTINY & ENRIQUE GONZA´ LEZ-TORRES Instituto de Geologı´a, Universidad Nacional Auto´noma de Me´xico, 04510 Me´xico D.F., Mexico *Corresponding author (e-mail: [email protected])

Abstract: The geology of southwestern Mexico (102–968W) records several synchronous events in the Late Oligocene–Early Miocene (29–19 Ma): (1) a hiatus in arc magmatism; (2) removal of a wide (c. 210 km) Upper Eocene–Lower Oligocene forearc; (3) exhumation of 13–20 km of Upper Eocene–Lower Oligocene arc along the present day coast; and (4) breakup of the Farallon Plate. Events 2 and 3 have traditionally been related to eastward displacement of the Chortı´s Block from a position off southwestern Mexico between 1058W and 978W; however at 30 Ma the Chortı´s Block would have lain east of 958W. We suggest that the magmatic hiatus was caused by subduction of the forearc, which replaced the mantle wedge by relatively cool crust. Assuming that the subducted block separated along the forearc–arc boundary, a likely zone of weakness due to magmatism, the subducted forearc is estimated to be wedge-shaped varying from zero to c. 90 km in thickness; however such a wedge is not apparent in seismic data across central Mexico. Given the 121 km/Ma convergence rate between 20 and 10 Ma and 67 km/Ma since 10 Ma, it is probable that any forearc has been deeply subducted. Potential causes for subduction of the forearc include collision of an oceanic plateau with the trench, and a change in plate kinematics synchronous with breakup of the Farallon Plate and initiation of the Guadalupe–Nazca spreading ridge.

Based upon the truncated character of the present subduction of the Farallon Plate along a WNW- southwestern margin of Mexico and space and trending trench parallel to the present Acapulco time constraints related to Caribbean Plate recon- Trench, but located farther to the south, is respon- structions, interpretation of the Cenozoic history sible for the Upper Eocene–Lower Oligocene arc. of southern Mexico has been dominated by the In this paper we investigate the Upper Eocene– hypothesis that the Chortı´s Block (mainly Honduras Recent geological record of southwestern Mexico and northern Nicaragua) lay adjacent to southwes- in order to relate geological events to potential tern Mexico in the Paleocene moving along the plate tectonic mechanisms. Recent reviews by Motagua fault zone to its present position between Morań-Zenteno et al. (2007), Gome´z-Tuena et al. 45 Ma and the present (e.g. Ross & Scotese 1988; (2007) and Nieto-Samaniego et al. (2006) allow us Pindell et al. 1988; Schaaf et al. 1995; Meschede to limit the paper to the main points. et al. 1997). However, Keppie & Morań-Zenteno (2005) proposed that, in the Paleocene, the Chortı´s Cenozoic geological record of Block lay SW of its present position, rotating clock- wise about an average pole in the southern hemi- southwestern Mexico sphere near Santiago, Chile (Pindell et al. 1988), Arc magmatism and reconstructions and was bounded on its NW side by transform faults bordering the Cayman Trough. This latter Post-80 Ma magmatism in southern Mexico may reconstruction is supported by the undeformed be divided into two spatial and temporal belts: a Upper Cretaceous–Recent sequence in the Gulf of c. 80–29 Ma belt in the Sierra Madre del Sur Tehuantepec that sits astride any westward projec- parallel to the present coast, and the 19 Ma to tion of the Motagua fault zone (Sańchez-Barreda Present Trans-Mexican Volcanic Belt running 1981; Keppie & Morań-Zenteno 2005), the lack from the Pacific coast near Puerto Vallarta to the of significant movement on the Motagua Fault Gulf of Mexico (Figs 1 & 2). An exception to this zone (measured displacements vary from zero to general distribution is the 22–13 Ma arc magmatic ,200 km), and the presence of a continuous Upper rocks located in eastern Oaxaca State, east of what Eocene–Lower Oligocene arc parallel to the we call the Veracruz-Oaxaca Line. Early geochro- present southwestern margin of Mexico (Fig. 1). nological results from the Sierra Madre de Sur One consequence of the latter reconstruction is that suggested that the Late Cretaceous-Oligocene arc

From:JAMES, K. H., LORENTE,M.A.&PINDELL, J. L. (eds) The Origin and Evolution of the Caribbean Plate. Geological Society, London, Special Publications, 328, 169–179. DOI: 10.1144/SP328.7 0305-8719/09/$15.00 # The Geological Society of London 2009. 170 J. D. KEPPIE ET AL.

Fig. 1. Geological map of southern Mexico (modified after Morań-Zenteno et al. 2007). LT, Los Tuxtlas; CF, Chacalapa Fault. magmatism migrated from west to east between sections appears to be a series of high-angle c. 80 and 29 Ma. The part between 45 and 29 Ma faults (Sańchez-Barreda 1981; Keppie and Morań- was interpreted as result of the passage of the Zenteno 2005). Between 1008300W and 968W, trench–trench–transform triple junction that U–Pb granitoid ages migrate from c. 35 to 29 Ma, accompanied the southeastward migration of the that is a rate of c.75km/Ma (Fig. 1), much faster Chortı´s Block (Herrmann et al. 1994; Schaaf et al. than the c. 20–30 km/Ma rate deduced in the 1995). However, the more comprehensive geochro- Cayman Trough (Rosencrantz et al. 1988; Ross & nological database now available (Morań-Zenteno Scotese 1988). et al. 2007 and references therein, Western North Using the empirically derived negative corre- America volcanic and intrusive rock database: lation between the angle of subduction and the http://navdat.geongrid.org/) shows a c. 220 km widths of the arc and forearc derived by Tatsumi wide coast-parallel 38–29 Ma (Upper Eocene– & Eggins (1995), the dip of the subduction zone Lower Oligocene) magmatic arc between longi- and the forearc width during the Upper Eocene– tudes 101 and 978. Between 97 and 968 this arc is Lower Oligocene are estimated to have been represented by a narrow band of coastal plutons; 11 + 98 and 280 + 30 km, respectively (Figs 3 & however, undated andesites stratigraphically below 4a). These estimates are complicated by factors, Miocene rocks could be the continuation of the such as: (i) variations in the dip of the Benioff Lower Oligocene magmatic belt to the west (Fig. 1). zone for example a steep dip near the trench could Along-strike to the east, the arc is replaced by an have changed to a shallow dip under the arc, result- undeformed Upper Cretaceous to Recent sedi- ing in a reduction in the width of the forearc; (ii) the mentary basin in the Gulf of Tehuantepec with original southern margin of the arc may have lain no evidence of Upper Eocene–Lower Oligocene farther south and been removed by subduction lavas or plutons: the contact in reflection seismic erosion; and (iii) neogene extension of the Sierra FOREARC IN SOUTHWESTERN MEXICO 171

Fig. 2. Geochronology in two NNE-trending transects across southwestern Mexico, from (a) Acapulco (age data from Herrmann et al. 1994; Ducea et al. 2004; Herna´ndez-Pineda 2006) and (b) Pinotepa Nacional (age data from Ferrusquı´a-Villafranca et al. 1974; Herrmann et al. 1994; Galina-Hidalgo et al. 2003; Martiny et al. 2000; Cerca et al. 2007; Martiny unpublished data).

Madre del Sur may have increased the width of the c. 19 Ma (Figs 1 & 4c). East of the Veracruz– arc. As these factors cannot presently be quantified, Oaxaca Line, 22–10 Ma arc magmatism continued the 280 + 30 km width for the forearc is used in this closer to the coast in the eastern Oaxaca and paper. In this scenario, the trench would have been Chiapas states, jumping northwards to Los Tuxtlas located 210 + 30 km south of the trench in the at c. 7 Ma (Figs 1 & 4c). The geochemistry of Late Eocene–Early Oligocene, allowing for 50 km volcanic rocks from the Trans-Mexican Volcanic between the southernmost dated arc rocks in bore- Belt includes not only typical calc-alkaline rocks, hole DSDP Leg 66, Site 493 (Bellon et al. 1982; but also ,11 Ma oceanic island basalts, and Fig. 1) and the present trench, and removal of 12–10 Ma adakites in the eastern Trans-Mexican c. 20 km width between 19 Ma and the Present; Volcanic Belt with a few 3.5 Ma adakites near see below (Fig. 4a). Puerto Vallarta and Mexico City (Go´mez-Tuena Geochronological data from south-central et al. 2007). The Trans-Mexican Volcanic Belt arc Mexico (Morań-Zenteno et al. 2000, 2007; Go´mez- is generally c. 150 km wide whereas the forearc Tuena et al. 2007 and references therein) indicates a increases eastwards (c. 150–410 km), the latter hiatus in arc magmatism between extinction of suggesting a west-to-east variation in the dip of the arc magmatism in the central Sierra Madre del Benioff zone from 50 + 108 to subhorizontal Sur at c. 29 and the initiation of volcanic activity (Figs 1 & 3; cf. Pardo & Suárez 1995). The recon- in the southern Trans-Mexican Volcanic Belt at struction for the lower Miocene to Recent period 172 J. D. KEPPIE ET AL.

Fig. 3. Width of forearcs and arcs versus dip of Benioff zone (modiﬁed from Tatsumi & Eggins 1995) showing data for southwestern Mexico.

(Fig. 4c) has to take into account the c.23kmof (Clift & Vannucchi 2004) places the lower subduction erosion that has taken place since Miocene trench c. 117 + 40 km south of the present 23 Ma (Clift & Vannucchi 2004). Acapulco Trench, that is offset c. 93 km south of the East of the Veracruz–Oaxaca Line, a c. 120 km position of the lower Miocene Acapulco Trench wide belt of Miocene arc magmatism was located west of the Veracruz–Oaxaca Line (Fig. 4c). If along-strike of the Upper Eocene–Lower Oligocene this offset is real, then the Veracruz–Oaxaca Line arc and migrated from west to east between 22 and may represent a transform fault in the subducting 10 Ma (Fig. 1). Assuming no loss of arc to subduc- plate. Across the Veracruz–Oaxaca Line, the tion erosion, the dip of the Benioff zone may have Benioff zone dip changed from low-angle on the been as high as c.408 with an arc-trench gap of western side to 408 on the eastern side. After 190 + 40 km in width (Fig. 3). Adding a distance c. 7 Ma the alkalic-calc-alkaline volcanism of the of c. 50 km between the southernmost outcrops of Trans-Mexican Volcanic Belt extended east of arc rocks at the coast and the present trench to the the Veracruz–Oaxaca Line to Los Tuxtlas and the assumed 23 km of subduction erosion since 23 Ma Chiapanecan volcanic arc (Figs 1 & 4c; Damon & FOREARC IN SOUTHWESTERN MEXICO 173

conjugate strike–slip faulting (sinistral WNW– ESE to east–west faults and dextral north–south faults), under an ENE–WSW horizontal shortening direction (Fig. 4a; Alaniz-A´ lvarez et al. 2002; Nieto-Samaniego et al. 2006). This was followed by sinistral, 27–23 Ma displacements on east– west faults and river offsets near the coast and along the Chacalapa Fault (Fig. 1; Tolson 2005), whereas farther north horizontal NNE–SSW to east–west extension produced sinistral movements on north–south and NE–SW faults, and normal fault movements on NW–SE faults that post-date the Upper Oligocene (Fig. 4b; Nieto-Samaniego et al. 2006): the difference suggests strain partition- ing across the Chacalapa Fault.

Exhumation and subsidence Using the Al–hornblende igneous barometer, Mora´n-Zenteno et al. (1996) have indicated that the uppermost Eocene–lowermost Oligocene coastal plutons were emplaced at depths of 20–13 km, with the amount of exhumation decreasing northwards. (U–Th)/He thermochronology indicates that these plutons were rapidly exhumed before c. 25 Ma north of Acapulco and before c.17Ma north of Puerto Escondido. This was followed by slow exhumation of the coastal zone of the central Sierra Madre del Sur with 85% of the derived sedi- ments being recycled via subduction erosion (Ducea et al. 2004). The c. 4 km subsidence in the outer forearc indicates c.1km/Ma retreat of the trench (Clift & Vannucchi 2004) over the last 23 Ma.

Geophysical data The dip of the Benioff zone south of the Trans- Mexican Volcanic Belt has been shown to vary from 308 in the west to nearly zero in the east (Pardo & Sua´rez 1995). Preliminary analysis of recent seismic data from Acapulco through Mexico City to Tampico, although generally conﬁrming this geometry, indicates a 158N dip northwards from the trench to c. 100 km, becoming almost subhorizontal Fig. 4. Cenozoic reconstructions showing the locations between 100 and 275 km at a depth of c. 45 km, and of the trench, forearc, and arc and structures at various dipping at 208N beyond 275 km to Tampico (Kim times. *Cocos Plate pole of rotation from 12.5 Ma to et al. 2006; Husker & Davis 2006). East of Present. TMVB, Trans-Mexican Volcanic Belt. c. 95.58W, the Cocos Plate bends downwards to the east and its dip changes from gentle to moderate Montesinos 1978; Go´mez-Tuena et al. 2007), (408NNE) (Bravo et al. 2004): the change has been suggesting a relative eastward migration of the related to the projection of the Tehuantepec Trans- transform. form beneath the North American Plate (Keppie & Mora´n-Zenteno 2005). P-wave tomography across Structure western Mexico shows a similar geometry with a moderately dipping slab down to c. 400 km; In the Sierra Madre del Sur, the Laramide Orogeny between 400 and 600 km the dip is gentler, and was followed by Upper Eocene–Lower Oligocene below c. 600 km there is a considerable gap 174 J. D. KEPPIE ET AL.

present coastal part of the Upper Eocene–Lower Oligocene arc; (iii) the strain regime north of the Xolapa Complex associated with conjugate strike–slip faults changed from shortening to extension along a WSW–ENE direction; (iv) the dip of the Benioff changed from a uniform c.118 dip in the Upper Eocene–Lower Oligocene west of the Veracruz–Oaxaca Line to an asymmetric dip in the Miocene from 30 to 508 in the western Trans-Mexican Volcanic Belt to ,108 in the eastern Trans-Mexican Volcanic Belt, and 408 Fig. 5. Tomographic section across southern Mexico east of the Veracruz–Oaxaca Line; (v) the Farallon showing subducted plate (see Fig. 1 for location). Plate broke into several smaller plates at c.29– Modified after Gorbatov & Fukao (2005). 25 Ma (Mammerickx & Klitgord 1982); and (vi) c. 210 km width of the Upper Eocene–Lower Oligocene forearc was removed. before reaching a deeper part of the subducted slab (Gorbatov & Fukao 2005). However, across eastern A 29–19 Ma arc hiatus and subduction Mexico, P-wave tomography suggests a vertical step down to the north in the Benioff zone that erosion increases eastwards from zero at c.998W through A gradual change in the dip of the Benioff zone c. 200 km in height at 978W along the Veracruz sec- through time beneath southwestern Mexico could tion (Figs 1 & 5) to c. 350 km in height at 928Win explain most of the arc rotation. However, the the section across Chiapas. This step connects a 10 Ma hiatus in arc magmatism requires another shallow southerly segment from the Acapulco explanation, such as: (i) subduction ceased; (ii) the Trench to a deep northerly segment (Figs 1 & 5). dip of the Benioff zone changed rapidly; and/or Along the Veracruz cross-section, the age of the (iii) the forearc was subducted. Cessation of sub- subducted slab at the top of the step is estimated duction is unlikely because subduction of the to be c. 19 Ma, increasing gradually towards the Farallon and Guadalupe plates appears to have east (Fig. 1). been continuous (Mammerickx & Klitgord 1982), Magnetotelluric data along the Acapulco– as is confirmed by the tomographic continuity Mexico City–Tampico section show bright of the subducted plates beneath eastern Mexico. A anomalies near the trench that have been related to change in the dip of the Benioff zone should be serpentinization (Jo¨dicke et al. 2006). The lack of accompanied by migration of the arc, and this is anomalies in the flat slab segment suggest that not observed (Fig. 2). On the other hand, subduction fluids are absent, whereas those beneath and to the of the forearc may have caused the 10 Ma hiatus by Trans-Mexican Volcanic Belt may be related to replacing the mantle wedge beneath the arc by rela- dehydration of the subducted slab (Jo¨dicke et al. tively cold material. Assuming that the subducted 2006). Along the Veracruz–Oaxaca–Puerto Escon- block separated along the boundary between the dido line, the magnetotelluric data indicate several forearc and the arc, a likely zone of weakness due discrete low resistivity anomalies within 150 km to magmatism, the subducted forearc is estimated of the Pacific coast that have been related to to be wedge-shaped, varying in thickness from release of water at depths of c. 20 and c.40km zero at the trench to c. 90 km at the arc, the depth (Jo¨dicke et al. 2006). required to initiate arc magmatism. Such an under- thrust wedge-shaped slice of continental margin Interpretations and conclusions material varying in thickness from zero to .50 km has been recorded in southern British Columbia The geological record of southwestern Mexico (Monger & Price 2002): that it has not been far (102–968W) reveals several major changes during removed by subduction may be due to the nearly the Upper Oligocene–Lower Miocene (Fig. 6): (i) margin-parallel relative motion between the Juan there was an hiatus in arc magmatism, which sep- de Fuca and North American plates (Engebretson arates a WNW-trending, 220 km wide, Upper et al. 1985). A similar, but smaller, slice occurs in Eocene–Lower Oligocene arc parallel to the present northern Cascadia where a 20 100 km thrust- coast from a more northerly, west-trending bounded slice of crustal rocks is being transported Miocene–Present Trans-Mexican Volcanic Belt arc: downwards by aseismic slow slip (Calvert 2003). 22–10 Ma arc magmatism east of the Veracruz– The apparent absence of such slices beneath Oaxaca Line lies along-strike of the earlier arc; (ii) southern Mexico suggests that they have been 13–20 km of exhumation occurred along the deeply subducted. If the Veracruz–Oaxaca Line FOREARC IN SOUTHWESTERN MEXICO 175

Fig. 6. Time and space diagram showing magmatic and tectonic events during the Cenozoic in southwestern Mexico (41 Ma to Present).

represents a transform fault, across which the dip of dehydration continued behind the subducted the Benioff zone changed, it could explain the age forearc. The steeply dipping boundary between of reinitiation of arc magmatism (at c. 22 Ma east cooler, low viscosity serpentinites and the hotter, of the Veracruz–Oaxaca Line compared with normal mantle wedge may have coincided with c. 19 Ma along the Trans-Mexican Volcanic Belt) the crustal viscosity contrast at the boundary in terms of the different time of arrival of the trailing between the arc and forearc produced by upward edge of the subducted forearc to c. 90 km depth magmatic flow. However, continued subduction of where arc magmatism would generally begin. the forearc could have increased the depth of dehy- Geodynamic modeling of subduction incorpor- dration into the mantle wedge causing steepening of ating dehydration of the subducting slab produces the Benioff zone, which perhaps explains the verti- two scenarios (Manea & Gurnis 2006): (i) a cal step in the Benioff zone observed in the tom- release of fluids at shallow depths produces serpen- ography beneath eastern Mexico (Fig. 5). tinization and a low viscosity forearc mantle wedge, Assuming that the 10 my magmatic hiatus which causes a decrease in the dip of the Benioff records the time of passage of the c. 210 km forearc zone; and (ii) dehydration at depths up to 400 km beneath southern Mexico, the rate of relative motion causes steepening of the Benioff zone. Thus, sub- is c.21km/Ma (forearc width/hiatus length). This duction of the cold forearc would have initially is much slower than both the pre-30 Ma rate of replaced the corner of the mantle wedge causing a 99 km/Ma and the 20–10 Ma rate of 121 km/Ma magmatic hiatus. Post-19 Ma gently dipping sub- (Engebretson et al. 1985; Pindell et al. 1988; duction beneath the eastern part of the Trans- Schaaf et al. 1995). By analogy with Cascadia, Mexican Volcanic Belt suggests that shallow this suggests that the forearc was underthrusting 176 J. D. KEPPIE ET AL. by aseismic slip at a slower rate than the sub- Keppie & Morań-Zenteno (2005). Firstly, passage ducting slab. Passage of the trailing edge of the sub- of the Chortı´s Block through the Gulf of Tehuantepec ducting forearc would lead to reinitiation of arc would have removed and deformed the Upper Cre- magmatism in the Trans-Mexican Volcanic Belt. If taceous and Cenozoic basin south of the westward the forearc continued subducting at the 21 km/Ma projection of the Motagua Fault zone: this contra- rate after 19 Ma its trailing edge would presently dicts seismic data that clearly shows a continuous lie 400 km from forearc-arc boundary. In this case, basin astride such a projection (Sańchez-Barreda it might have underplated the overriding plate and 1981). Secondly, using the estimated 15 + 5mm/ be revealed in geophysical data as a relatively cool annum rate of opening of the Cayman Trough since layer. However, such underplating is obscured by 30 Ma (Rosencrantz & Sclater 1988) and the recent magmatic activity that produced fluids and c. 130 km, post-45 Ma stretching of the Nicaragua melt recorded in both tomographic and magneto- Rise (Ross & Scotese 1988), the northwestern tip of telluric data. On the other hand, once the forearc the Chortı´s Block would have lain at 958W off the passed beneath the continental Moho, it may have Gulf of Tehuantepec at 30 Ma, not further west traveled with the subducting slab at a convergence between 1058W and 978W off southwestern Mexico rate of c. 120 km/Ma between 19 and 10 Ma (Fig. 7), that is using this model, the Chortı´s Block and c. 60–70 km/Ma after 10 Ma, which would would have been removed before removal of the place its trailing edge at c. 1700 km north of the forearc. Thirdly, the western Chortı´s Block has a tri- forearc-arc boundary. If the subducted forearc was angular shape varying in width from zero to 600 km floored by oceanic lithosphere it would have been measured from the present coast of Mexico: thus converted to eclogite. any Eocene–Oligocene arc would likely have a NNW-strike passing from southern Mexico into the Possible causes of 29–19 Ma subduction Chortı´s Block rather than its WNW-trend within erosion southern Mexico (Fig. 1). Collision of a larger feature, such as an oceanic What initiated subduction of the forearc is unclear. plateau, is a possibility for producing subduc- It does not appear to be related to a change in the tion erosion. Such an oceanic plateau could have rate of convergence, which was 99 mm/annum lain on the inferred Chumbia seamount chain; prior to 28 Ma and increased to 121 mm/annum however, no mirror-image plateau appears along after 20 Ma (Fig. 1). Nor does it correlate with a dis- the Moonless Mountain seamount chain, which continuity in the age of the subducting oceanic Keppie & Morań-Zenteno (2005) deduced to be lithosphere (Mammerickx & Klitgord 1982). the mirror-image of the Chumbia seamount At c. 29 Ma, the East Pacific Rise lay c. 2000 km chain. On the other hand, an oceanic plateau could from the coast of southwestern Mexico and the have been created just on the east side of the subducting Farallon Plate would be c. 25 Ma older mid-ocean ridge by either plume magmatism or (¼ Anomaly 19). Keppie & Morań-Zenteno (2005) between two temporarily overlapping ridges as proposed that subduction of seamounts (Chumbia has been documented by Mammerickx & Klitgord seamounts) could have been responsible for the (1982). The synchroneity of the 29–19 Ma sub- subduction erosion; however, elsewhere in the duction erosion with the 29–25 Ma breakup of world seamounts generally only cause an ephemeral the subducting Farallon Plate into the Guadalupe rise in the rate of subduction erosion (generally Plate and birth of the Cocos–Nazca spreading ,10 km/Ma: Clift & Vannucchi 2004). Average centre at 25 Ma suggests a cause and effect rela- global rates are ,8km/Ma rising to c.10km/Ma tionship. However, it is unclear how such reorgani- where mid-oceanic ridges have entered the sub- zation of the Pacific plates affected the North duction zone (Clift & Vannucchi 2004). Using esti- America Plate. mates calculated earlier in this paper based on the empirical relationship between the dip of the Exhumation and subduction erosion Benioff zone and the width of the arc and forearc suggest that c. 210 km forearc width was re- Lallemand et al. (1994) have shown that whereas moved between 29 and 19 Ma, that is an average overriding of a seamount causes uplift in the outer of c.21km/Ma. The exceptionally high rate of sub- forearc above the seamount followed by subsidence duction erosion at 29–19 Ma in southern Mexico after its passage, the inner forearc is less affected. may be explained in several ways. By analogy, subduction of a forearc wedge could Traditionally, removal of the forearc would be cause exhumation of the present coastal plutons explained by moving the Chortı´s Block eastwards in southwestern Mexico followed by subsidence along the southern coast of Mexico (e.g. Ross & of the offshore section. It might also explain the Scotese 1988; Schaaf et al. 1995); however, several switch from NE–SW contraction to extension further problems arise besides those discussed in farther inland. FOREARC IN SOUTHWESTERN MEXICO 177

Fig. 7. Reconstructions over 30 Ma showing the position of the Chortı´s Block according to Keppie & Mora´n-Zenteno (2005), and Ross & Scotese (1988) including subtraction of the c. 130 km stretching in the Nicaragua Rise.

Coupling or decoupling between plates from the subducting Cocos Plate that have lubri- cated the Benioff zone (cf. Bostock et al. 2002). It has been suggested that coupling between the On the other hand, Sobolev & Babeyko (2005) plates produces parallelism between the direction have calculated that a 2–3 cm/annum absolute of convergence and the stress directions in the westward motion of the South American Plate com- overriding plate (Tatsumi & Eggins 1995 and refer- bined with the high coefficient of friction (0.05) and ences therein). In apparent confirmation of this crust .40 km are the main factors in producing a hypothesis, Meschede et al. (1997) related struc- fold-and-thrust belt in the Central Andes. Crustal tures in the central Sierra Madre del Sur to stress thicknesses and rates of the absolute westward transmission between subducting and overriding motion of the North and South American plates plates, and concluded that stress and strain direc- are similar in the central Andes and Mexico, tions are parallel. However, Nieto-Samaniego suggesting that a low friction coefficient may be a et al. (2006) show extension axes associated with significant factor in inhibiting coupling. Further- strike–slip tectonics and convergence directions more, the Acapulco Trench strikes WNW, highly are slightly oblique (Fig. 4b), which is consistent oblique to the westwards absolute motion of North with the general case where the axis of convergence America, which may also contribute to the lack of is not parallel to either strain or stress axes in a fold-and-thrust belt. oblique convergent plate scenarios (Jiang et al. 2001). The obliquity could also be due to a We are grateful to CONACyT grant (0255P-T9506) for divergence between the convective flow pattern funding the project, and thank Drs Luca Ferrari and in the mantle wedge and the convergence direction Albert Bally for their constructive comments on an between the plates, although an intervening hydrous earlier version of the paper. peridotite/serpentinite layer may severely limit viscous coupling. Given the high rate of subduction erosion at References 29–19 Ma, the absence of a fold-and-thrust belt ´ in southern Mexico appears to be anomalous. ALANIZ-ALVAREZ, S. A., NIETO-SAMANIEGO, A. F., ´ Compressional deformation could have been MORAN-ZENTENO,D.J.&ALBA-ALDAVE,L. 2002. Rhyolitic volcanism in extension zone associ- restricted to the removed forearc. However, there ated with strike-slip tectonics in the Taxco region, is no evidence of such deformation in the present southern Mexico. Journal of Volcanology and forearc where shallow flat slab of the Cocos Plate Geothermal Research, 118, 1–14. is occurring (Kim et al. 2006), possibly due to BELLON, H., MAURY,R.C.&STEPHAN, J. F. 1982. either sediment subduction and/or fluids released Dioritic basement, Site 493: petrology, geochemistry, 178 J. D. KEPPIE ET AL.

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