From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, February 9, 2011
Assembly Of Western Mexico In Mesozoic Time
Link to streaming video TBA: available 02.09.2011 at 1:55pm
Dr. Cathy BusbyDr. Cathy Busby, UC, Santa Barbara
2010-2011 Oualline Centennial Lectureship, JSG, Elena Centeno-Garcia, George Gehrels
University of California at Santa Barbara
2010-2011 Oualline Centennial Lectureship, Jackson School of Geosciences
Based in part on a paper in press at GSA Bulletin
The western margin of Mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins: accretion of exotic island arcs by the consumption of entire ocean basins vs. accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone. We present geologic and detrital zircon evidence that western Mexico was largely assembled by the latter mechanism, through extensional and compressional processes operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin throughout most of Mesozoic time. This process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension, followed by accretion and thickening of the crust during contraction.
We divide Mesozoic rocks of western mainland Mexico and Baja California into four distinctive tectonostratigraphic assemblages:
I) Triassic-Early Jurassic accretionary complex (Arteaga Complex): this subduction complex forms the basement to arc rocks in western mainland Mexico and includes Grenville, Pan-African and Permian detrital zircon suites that match the Potosi fan of the Mexican mainland. In Baja California, the arc basement consists of arc-ophiolite assemblages of this age, amalgamated with very minimal deformation along Late Jurassic sinistral strike slip faults.
II) Late Jurassic to earliest Cretaceous extensional volcanic arc assemblage, with a Callovian-Tithonian (about 163-145 Ma) peak in magmatism. Extensional unroofing began in that time frame and continued into through the next; arc rifting resulted in separation of parts of the Arteaga accretionary complex from the continental margin, and produced a series of deep marine basins, with primitive arc and MORB volcanic rocks.
III) Early Cretaceous extensional arc assemblage, with two magmatic peaks: one in the Barremian-Aptian (ca. 129-123 Ma), and the other in the Albian (ca. 109 Ma). In some localities, rapid subsidence produced thick, shallow marine to deep marine volcano-sedimentary sections, while at other localities, extensional unroofing of all older assemblages resulted in recycling of zircon from all older units (I, II, III).
IV). Late Cretaceous compressional arc assemblage: arc shortening began by 100 Ma in the west (in Baja California), accompanied by emplacement of a huge ~98-94 Ma batholith, while western Mexico accumulated sediments derived from this contractional arc, in a retro-arc foreland basin. The contractional deformation front then swept eastward into the western edge of mainland Mexico, between 93 and 84 Ma, and continued to migrate inboard, across the western half of mainland Mexico, through the end of Cretaceous time. Intramontane nonmarine compressional arc basins formed in the wake of this inboard-migrating deformation front.
To summarize, western Mexico was largely created by earlier extensional and later contractional processes operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin. The extensional tectonic regime recorded by assemblages II and III (as well as assemblage I in Baja California) was likely controlled by slab age, because the paleo-Pacific at the time of breakup of Pangea was probably composed of large, relatively old, cold plates (Busby et al., 1990, 1998, 2004). Continental margin rifting and addition of new igneous and volcaniclastic material during extension created new crust, which was then accreted to the Mexican margin during a contractional phase (likely the combined result of younger slab age and increased convergence rate). This led to substantial growth of the Mexican margin, and supports models for significant growth of continents in this manner.