Xolapa Complex Research Articles

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 Al tot 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.

The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico)

Terrane analysis has established differences in lithology, style of deformation, metamorphism, and radiometric ages among several basement complexes in southern Mexico. In this paper, we identify similarities in the geologic history of the adjacent Acatlán, Oaxaca, and Xolapa complexes through a U/Pb zircon study on crustal sources and timing of magmatic, metamorphic, and tectonic activity in the Xolapa complex. Our regional tectonic scenario is based on both new chronologic and previously established structural data. Inherited zircon in the Xolapa complex indicates the presence of a Proterozoic (1.0–1.3 Ga) crustal component with an age range that overlaps Grenville crystallization dates (1.0–1.2 Ga) from the Oaxaca basement, and an inherited Grenville crustal component (1.0–1.1 Ga) identified in zircons from the Acatlán complex. The Proterozoic component in the Xolapa complex indicates either that it received sediments from a continental region of Grenvillean age, for example the Oaxaca basement, which is the closest exposed Grenville crust in southern Mexico, or that the Xolapa complex has a Grenville basement. In the latter case, the Xolapa complex has been modified by widespread high‐grade metamorphism and large‐scale migmatization. Metamorphism and migmatization occurred from 66 to 46 Ma and locally continued into Oligocene time. Magmatism in the Xolapa complex terminated with crustal growth by plutonism, which is characterized by a systematic pattern of eastward‐younging crystallization ages, from 35 Ma in the west (west of Acapulco) to 27 Ma in the east (east of Puerto Angel). Metamorphism and migmatization in the Xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the Farallon and North American plates (e.g., rate and direction of subduction) and the early evolution of the Caribbean. This plate reorganization triggered sinistral transtension distributed across the southern Mexican continental margin and the eastward translation of the Chortis block. The eastward younging of the Xolapa plutons also is consistent with the motion of the Chortis block during eastward displacement of the Caribbean plate. This induced a shift of the magmatic arc from its Cretaceous to early Tertiary position along the Xolapa and Chortis blocks to its present mid‐Mexican position. We conclude that a single tectonic framework accounts for the Mesozoic and Cenozoic geologic history of the southern Mexican terranes.