MID-CRETACEOUS SUTURING OF THE ALISITOS ARC TO NORTH AMERICA – STRUCTURAL EVIDENCE FROM THE SIERRA CALAMAJUE, PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO

The Early Cretaceous Alisitos arc is located south of 31°30’N in the western Peninsular Ranges batholith of Baja California. Near latitude 29°30’N in the Sierra Calamajue, a series of high-angle, SW-vergent reverse faults are part of a fold-thrust belt that juxtaposes Alisitos arc units, Cretaceous through Triassic strata, and Paleozoic units. Alisitos arc units are weakly metamorphosed, openly folded, and display a weak NW-SE-trending moderately NE-dipping cleavage. Deformation increases eastward, where fault-bounded blocks contain Cretaceous, metasedimentary and metavolcanic, greenschist facies strata with well-developed cleavage and increasingly tighter outcrop-scale folds. Farther east, lower amphibolite grade metasedimentary assemblages of Triassic-Jurassic and Paleozoic age are intensely deformed. While Mesozoic units record only a single deformation event, Paleozoic units show strongly disrupted layering, mostly obliterated depositional features, and evidence for refolding of earlier formed structures.

Deposition and deformation are constrained by four detrital zircon analyses and two U-Pb pluton crystallization ages. Inferences about maximum depositional ages allow separation of Cretaceous, Triassic, and Ordovician units that can be correlated with similar units north of the study area. Alisitos arc strata are intruded by plutons as old as 144±2 Ma and elsewhere in the study area a 95±2 m.y. age was obtained from an undeformed tonalite pluton. While the former is one of the oldest plutons in the western Peninsular Ranges batholith, plutons similar to the latter are commonly interpreted as LaPosta-type plutons.

Our data support the interpretation that the Alisitos arc accreted to North America in the mid-Cretaceous. While our data do not constrain the origin of the Alisitos arc, detrital zircons in Cretaceous, Triassic, and Ordovician strata east of the arc show strong continental affinities, most likely the North American continent. Furthermore, observed structures can mostly be explained by Cretaceous deformation associated with arc collision. Early Paleozoic units, however, record complex deformation that suggest an additional period of deformation possibly during the late Paleozoic as an active plate margin developed along the margin of SW North America.