Structural, Temporal and Metamorphic Evolution of the Main Mártir Thrust, Baja California, México

The metamorphic and magmatic response to collision of a magmatic arc with the continental margin is one of the least understood processes in plate tectonics. Several responses can occur, including the extrusion of heated material out of the thickened crust and lower crustal melting leading to pervasive magmatism and metamorphism. This study studies both responses. In the central Peninsular Ranges of Baja California, the Main Mártir thrust (MMt) is a non-terminal suture that marks the Cretaceous collision zone between the Alisitos arc and the continental margin magmatic arc. The amount, and the timing of displacement of the MMt are diachronous along strike and lead to an imperfect understanding of the response of magmatism and metamorphism after collision. My goal is to refine the timing of metamorphism and to understand the displacement of the MMt. I combine the techniques of mapping, structural analysis, petrography, U/Pb monazite geochronology and aluminum-in-hornblende geobarometry to present the following interpretations that relate to the geologic history of the evolving magmatic arc after collision. (1) The age of deposition of the Huico gneiss in the hanging wall of the thrust suggests it is correlative to other flysch-like wallrocks of the batholith and that it was deposited as a Late Jurassic- Early Triassic arc-marginal basin with sediments sourced off North America. (2) Gabbros intruded at 110 ± 0.9 Ma suggest a mantle component to plutonism into the newly thickened arc. (3) Two ages of metamorphism are recognized in the Huico gneiss: ca. 110 ± 0.9 Ma and 99.0 ± 1.0 Ma, the later approximately 10 m.y. after the Alisitos arc collision and most likely a regional metamorphic event. (4) Structural data suggests that the MMt forms a releasing bend, where extrusion of a small sliver of migmatites occurred at ca. 99 Ma. (5) Syn-metamorphic tonalites give intrusion pressures from 3 to 4.5kbar and suggest the displacement on the MMt after ca. 105 Ma is negligible. The data presented here allow the detailed geologic history of the closing stages of arc collision and its crustal response to be understood and could lead to testing of new crustal models