SYNCHRONOUS OLIGO-MIOCENE TECTONIC EXHUMATION OF METAMORPHIC CORE COMPLEXES IN NW MEXICO: PRELIMINARY 40AR/39AR THERMOCHRONOLOGIC CONSTRAINTS

Large regions of the crust in northern Mexico were significantly extended during the Cenozoic. However, little is known about this major extensional province, including basic information on the timing, magnitude or kinematics of extension. This lack of data has hindered the development of even a basic understanding of the processes that drove this extension. Specifically, it is not known if extension occurred mainly during subduction (pre-12 Ma), possibly related to back-arc spreading or gravitational collapse of previously thickened crust. Alternatively, extension may have accommodated transtensional Pacific-North American plate motion after subduction had ceased but prior to the localization of rifting in the Gulf of California (12-6 Ma).

Mexican core complexes include the Magdalena, Aconchi, and Mazatán regions and are key targets for understanding the tectonic development of the region. Preliminary ⁴⁰Ar/³⁹Ar thermochronologic results show that the footwall of the Magdalena core complex cooled >200 °C from 25-22 Ma, which we relate to a period of rapid slip on the detachment fault. Results from the Aconchi core complex are similar but suggest a poly-phase unroofing history with early slip from 26-23 Ma and a later pulse of slip that began at ca. 21 Ma. This poly-phase unroofing history is nearly identical to that of the better-studied Mazatán core complex (Wong and Gans, 2003; 2005).

These preliminary results add to a growing database that suggests that most of the crustal extension in the region predates the change from subduction to transtension at ca. 12 Ma. Therefore, this extension cannot be kinematically linked to relative Pacific-North American plate motions. The degree of synchroneity of core complex development in NW Mexico is surprising and suggests a common trigger for the initiation of “core-complex style” extension regionally. A magmatic trigger is suggested at the Mazatán core complex but this has not been demonstrated at the other core complexes so far. Such a trigger may not be limited to regional forces but may instead be Cordilleran-wide, as core complexes from Utah to NW Mexico show a striking similarity in the timing of extensional unroofing (21-15 Ma). The trigger for nearly synchronous core complex extension on both a regional and orogen-wide scale remains an unresolved question.