DEFINING THE MAIN MÁRTIR THRUST IN BAHĹA DE LOS ÁNGELES, BAJA CALIFORNIA, MEXICO

The goal of this study is to locate the ductile Main Martir Thrust (MMT) in the vicinity of Bahia de Los Angeles in Baja California, and determine its structural The Main Mártir Thrust (MMT) fault in northern Baja California is a major intra-batholithic thrust fault that accommodated large scale east-west shortening deformation across the Peninsular Ranges batholith (PRB) in mid-Cretaceous time. The goal of this study is to map this major structure farther south from where it was first defined and determine its character through field mapping, structural analysis, geochronology and petrography.

The MMT in northern Baja is the eastern boundary of the Alisitos Arc (AA), a Jurassic-Cretaceous oceanic arc complex. The arc is proposed to be either an exotic island arc that was sutured to the eastern PRB across the MMT, or alternatively is the submarine extension of the sub-aerial Santiago Peak Volcanics in southern California, which together form an Aleutian-style oceanic-to-continental arc system. The MMT fault has not been mapped south of Sierra Calamajué, and several tentative locations have been proposed for the southward extension of this structure. In the Bahía de Los Ángeles area, mapping and structural analysis, allowed us to characterize the likely extension of the MMT into this region. Lineations and fold hinges are sub-parallel indicating a constrictional strain regime, and flattening and elongation of clasts in a meta-conglomerate suggests a plane strain regime. Lineations, foliations and a variety of kinematic indicators consistently record nearly pure dip-slip in this east-dipping top-to-the-west thrust.

The hanging wall Cantera Gneiss and staurolite-grade schists are being thrust relatively undeformed onto the AA, forming a km-scale footwall syncline with superposed parasitic folding. Rocks in the AA are increasingly deformed and metamorphosed closer to the fault with tighter folds. Metamorphic grade in the footwall increases towards the fault, with rocks that preserve sedimentary structures ~2 km from the fault and cordierite-bearing metamorphic rocks adjacent to it. U-Pb dating of zircons shows that the Cantera Gneiss in the hanging wall and a felsic tuff in the footwall are the same age within error. This suggests that both lithologies could be cogenetric and thus the MMT would be an intra-arc thrust, and the AA a continuation of the Santiago Peak Volcanics.