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 Mora´n-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 & Mora´n-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 (Sa´nchez-Barreda Present Trans-Mexican Volcanic Belt running 1981; Keppie & Mora´n-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 Mora´n-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 (Sa´nchez-Barreda 1981; Keppie and Mora´n- 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 (Mora´n-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 (Mora´n-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 & Su´arez 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.