Dante J. Morán-Zenteno

Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence

The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW–SE decreasing granitoid intrusion age trend (∼1.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW–SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (∼7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40–25 Ma, with a velocity of 6.5–4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large-scale right-lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more important in the southeastern segment.

Uplift and subduction erosion in southwestern Mexico since the Oligocene: pluton geobarometry constraints

Details of the late Oligocene to Middle Miocene uplift and tectonic erosion episodes of the southwestern continental margin of Mexico can be inferred using Altot geobarometry of igneous hornblendes, geochronology, and field relations. On the basis of such analyses carried out between Acapulco and Huatulco we find the following: (1) Calc-alkaline batholiths exposed along the coast from Acapulco to Huatulco, mostly in the 35-25 Ma age range, were emplaced at depths between 13 and 20 km. (2) The contact relationships between these plutons and their host rocks, and the exposure of volcanic counterparts, 70 km from the coastline, indicate a landward decrease in the amount of uplift. (3) A comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were, in general, higher between Acapulco and Huatulco than those along the margin between Puerto Vallarta and Manzanillo, 700 km northwest of Acapulco. (4) The uplift of this coastal belt occurred during the late stages of magmatism and after its cessation, triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the Xolapa Complex. These findings, in conjunction with the geometry of the present continental margin, as well as the offshore tectonic and stratigraphic features, support previous interpretations of very active late Oligocene to Middle Miocene subduction erosion after the onset of strike-slip tectonics related to the detachment and subsequent eastward displacement of the Chortis block. Subduction erosion involved both trench sediments and crystalline (continental framework) rocks. Different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism. Subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust, whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework. Comparing the calculated depths of pluton crystallization with the present depth of the continental crust-subducted slab boundary, interpreted using previously published seismic refraction and gravity models, we conclude that onshore basal erosion played a subordinate role during Miocene episodes of subduction erosion. Major removal of lower crustal sections was probably restricted to offshore regions. Plate reconstructions of the Cocos plate and its predecessors with respect to North America indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the Guadalupe plate beneath southwestern Mexico.

Chicxulub impact: The origin of reservoir and seal facies in the southeastern Mexico oil fields

Stratigraphic and mineralogic studies of Cretaceous-Tertiary (K-T) boundary sections demonstrate that the offshore oil-producing breccias and seals from oil fields in the Campeche marine platform are of K-T boundary age and that their mode of formation is probably related to the K-T impact event at Chicxulub. The oil-producing carbonate breccia and the overlying dolomitized ejecta layer (seal) found in several wells on the Campeche marine platform contain typical Chicxulub impact products, such as shocked quartz and plagioclase, and altered glass. These offshore units are correlated with thick (∼50–300 m) onshore breccia and impact ejecta layers found at the K-T boundary in the Guayal (Tabasco) and Bochil (Chiapas) sections. Regionally the characteristic sequence is composed of, from base to top, coarse-grained carbonate breccia covered by an ejecta bed and typical K-T boundary clay. The onshore and offshore breccia sequences are likely to have resulted from major slumping of the carbonate platform margin triggered by the Chicxulub impact. Successive arrival times in this area, ∼350–600 km from the crater, of seismic shaking, ballistic ejecta, and tsunami waves fit the observed stratigraphic sequence. The K-T breccia reservoir and seal ejecta layer of the Cantarell oil field, with a current daily production of 1.3 million barrels of oil, are probably the most important known oil-producing units related to an impact event.

Tertiary arc-magmatism of the Sierra Madre del Sur, Mexico, and its transition to the volcanic activity of the Trans-Mexican Volcanic Belt

The Tertiary magmatic rocks of the Sierra Madre del Sur (SMS) are broadly distributed south of the Trans-Mexican Volcanic Belt (TMVB) and extend to the southern continental margin of Mexico. They represent magmatic activity that originated at a time characterized by significant changes in the plate interactions in this region as a result of the formation of the Caribbean plate and the southeastward displacement of the Chortis block along the continental margin of southwestern Mexico. The change from SMS magmatism to an E‐W trending TMVB volcanism in Miocene time reflects the tectonic evolution of southwestern Mexico during these episodes of plate tectonic rearrangement. The distribution and petrographic characteristics of the magmatic rocks of the SMS define two belts of NW orientation. The first is represented by the nearly continuous coastal plutonic belt (CPB), which consists of batholiths and stocks of predominantly felsic composition. The second belt is inland of the first and consists of discontinuously distributed volcanic fields with piles of andesitic to rhyolitic flows, as well as epiclastic and pyroclastic materials. These two belts were emplaced along a continental crust segment constituted by a mosaic of basements with recognizable petrologic and isotopic diAerences. These basements originated during diAerent tectono-thermal events developed from the Proterozoic to the Mesozoic. Major and trace element data of the SMS magmatic rocks define a clear sub-alkaline tendency. Variations in the general geochemical behavior and in the Sr and Nd isotopic ratios indicate diAerent degrees of magmatic diAerentiation and/or crustal contamination. These variations, specially in the inland Oligocene volcanic regions of Guerrero and Oaxaca states, seem to have been controlled by the particular tectonic setting at the time of magmatism. In northwestern Oaxaca greater extension related to transtensional tectonics produced less diAerentiated volcanic rocks with an apparently lower degree of crustal contamination than those of northeastern Guerrero. The geochronologic data produced by us up to now, in addition to those previously reported, indicate that the Tertiary magmatic rocks of the SMS range in age from Paleocene to Miocene. The general geochronologic patterns indicate a southeastward decrease in the age of igneous activity, rather than a gradual northeastward migration of the locus of magmatism toward the present-day TMVB. SMS magmatic rocks exposed to the west of the 1008W meridian are dominantly Late Cretaceous to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasing trend. Paleocene and Eocene magmatic rocks of the western region of the SMS seem to keep a general NNW trend similar to that of the Tertiary magmatic rocks of the Sierra Madre Occidental (SMO). In the eastern region of the SMS the Oligocene magmatic rocks show a trend that roughly defines an ESE orientation. The change in the trend of arc magmatism may be the eAect of the landward migration of the trench, for a given longitude, as a result of the displacement of the Chortis block. The

Tectonic Implications of Alternative Cenozoic Reconstructions for Southern Mexico and the Chortis Block

Most current Eocene reconstructions juxtapose the Chortis block of northern Central America against southern Mexico, and invoke ˜1100 km Cenozoic sinistral displacement on the Acapulco-Motagua-Cayman fault zone, the inferred northern margin of the Caribbean plate. Such a hypothesis is incompatible with the presence of undeformed Upper Cretaceous—Recent sediments that cross the projected trace of the Motagua fault zone in the Gulf of Tehuantepec, minimal offset of the Permian Chiapas batholith, and the absence in Honduras of several major features in southern Mexico. These problems may be overcome if the Chortis block is back-rotated anticlockwise about a pole near Santiago, Chile, i.e. ˜1100 km along the Cayman transform faults during the Cenozoic. Such a reconstruction when combined with reconstructions of features in the Pacific Ocean, suggests that Middle Miocene collision of the Tehuantepec aseismic ridge with the Acapulco Trench led to: (1) asymmetric flattening of the subduction zone; (2) an anticlockwise rotation of the Mexican magmatic arc to its present location by the Middle Miocene; (3) the development of a volcanic arc gap in southeastern Mexico, in which the late Middle Miocene Chiapas fold-and-thrust belt developed: as the Tehuantepec Ridge swept westward, arc volcanism was re-established in the gap. Eocene collision of the Chumbia Seamount Ridge (inferred mirror image of the Moonless Mountains—unnamed seamount ridge between the Molokai and Clarion fracture zones) with the Acapulco Trench followed by its ESE migration during the Oligocene led to: (a) flattening of the subducting slab inducing subduction erosion and exhumation of the southern Mexican margin; (b) anticlockwise rotation of the volcanic arc; and (c) sinistral strike-slip faulting in the Sierra Madre del Sur. This contrasts with the region north of the projected Molokai fracture zone where the dip of the subduction zone appears to have steepened, producing extension. Eocene(—Late Cretaceous) subduction along the southern coast of Mexico explains the remnants of a Late Cretaceous arc in the Gulf of Tehuantepec and neighboring Guatemala.

Rhyolitic volcanism in extension zone associated with strike-slip tectonics in the Taxco region, southern Mexico

Abstract The Taxco Volcanic Field (TVF) is part of a broad magmatic province in southern Mexico. It constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of Mesozoic sedimentary and volcano-sedimentary units. A thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the TVF. This volcanic sequence is distributed within a well defined zone, it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults. Geochronologic data indicate that most rhyolitic volcanism in the area is Oligocene in age and synchronous with episodes of strike-slip faulting. We document two successive phases of strike-slip faulting for the late Eocene–early Oligocene interval, the first with NNW extension and the second with NE extension. In both cases pre-existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults. The stratigraphic sequence gives evidence that the TVF is located in an extensional basin associated to strike-slip faults. The evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism. The result of this change was the development of a volcanic pile with elevations higher than the surrounding Mesozoic rocks. According to the fault kinematics, stratigraphy and the volume of volcanic rocks, the rhyolitic volcanism was emplaced in the area of maximum extension, showing that magma flowed into low pressure zones. The small number of faults within the Oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike-slip faulting.

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

Abstract The geology of southwestern Mexico (102–96°W) 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 Chortís Block from a position off southwestern Mexico between 105°W and 97°W; however at 30 Ma the Chortís Block would have lain east of 95°W. 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.

Paleomagnetism of the Acatlan terrane, southern Mexico: evidence for terrane rotation

232 drill samples were collected from 33 sites from the Acatlan Terrane, southern Mexico. The medium-grade metamorphosed basement rocks (schists, granitoid, greenstones) showed magnetic directions that are randomly distributed and no meaningful results could be obtained. However, late Paleozoic red beds, the Ordovician (?) Totoltepec Granite and the early Paleozoic (?) Tecomate Limestone (metamorphosed in Acadian times) give coherent paleomagnetic directions. The samples responded well to thermal demagnetization but not to alternating field demagnetization. The ages of the magnetizations are reasonably bracketed between Carboniferous and Jurassic. Compared with data from the North American craton and from the Oaxaca terrane, conclusive evidence for major north-south displacements of the Acatlan terrane is not present, but significant clockwise rotations of the terrane with respect to the craton and the adjacent Oaxaca terrane are quite evident. These rotations occurred during the Jurassic or Early Cretaceous.

Origin of the Río Verde Batholith, Southern Mexico, as Inferred from its Geochemical Characteristics

The Rio Verde batholith, in southern Mexico, represents a pulse of voluminous magmatism occurring over a relatively short time span, followed by uplift and erosion. The batholith consists predominantly of calc-alkaline rocks ranging from tonalites to granites with biotite and hornblende as the main mafic minerals. Intrusion ages for the three recognized plutons range from 29.9 ± 1.1 to 23.5 ± 0.6 Ma, with a younging trend toward the east. Isotopically, these granitoids are characterized by eNd values between −3.0 and +0.9 and 87Sr/86Sr initial ratios ranging from 0.70419 to 0.70532. The small differences between hornblende and biotite ages suggest rapid cooling rates. Geobarometric determinations performed on hornblendes yield a pressure of 5.3 ± 0.6 kbar, which corresponds to a depth of emplacement of 19.7 ± 2.2 km. Using trace-element abundances as well as the Sr- and Nd-isotopic data, we modeled possible magmatic processes of partial fusion, fractional crystallization, and assimilation assuming differe...

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province, southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.