A PALEOMAGNETIC TEST OF THE ROLE OF CRUSTAL HETEROGENEITY IN CONTROLLING STRAIN TRANSFER IN A WIDE LEFT LATERAL SHEAR MARGIN: SOUTHERN MEXICO

It has been proposed that differences in strength, thickness, and density of the crust controlled lateral and vertical strain propagation during Late Cretaceous (Laramide) shortening and Early Tertiary left-lateral shear that affected southern Mexico. These events resulted in formation of a Late Cretaceous fold and thrust belt and a Tertiary transform boundary that truncated Mexico?s Pacific margin, as rupture of the Chortis Block occurred. Observations in the lower to mid-Cretaceous Guerrero-Morelos platform suggest that Laramide structures were rotated by Early Tertiary (Late Eocene) transpression. A left-lateral transpressive regime was followed by Oligocene transtension and magmatism, as the transform boundary was replaced by subduction. Paleomagnetic data for deformed Lower Cretaceous red beds of the Zicapa Fm. (n=7 sites) and platform carbonate rocks of the Morelos Fm. (n=4 sites), synorogenic siliciclastic rocks of the Mezcala Fm. (n=2 sites), and post-tectonic Mastrichtian-Paleocene intrusive rocks (n=9 sites) indicate large counterclockwise vertical-axis rotations that generally postdate folding. Rotations range from about 55°+/-9° in the red beds, to about 37°+/-12° in the plutons. In the western segment of the platform, no significant differences are observed between synfolding magnetizations observed in pre- Laramide rocks and primary magnetization in post-Laramide rocks, suggesting that most rotation occurred in Tertiary time. Greatest rotation is observed were Laramide structures deviate further from the regional north-south structural trend, near the (present day) southern limit of the platform. Rotations are observed west of the limit of exposures of Paleozoic and Precambrian metamorphic basement terranes, and not on their coverture. This suggests that this limit, the Papalutla fault, is major crustal structure discontinuity in southern Mexico, west of which weaker coupling between upper brittle crust and ductile lower crust has been assumed.