ABSTRACT This chapter expands upon a model, first proposed in 1998 by Busby and others, in which Mesozoic oceanic-arc rocks of Baja California formed along the Mexican continental margin above a single east-dipping subduction zone, and were extensional in nature, due to rollback of an old, cold subducting slab (Panthalassa). It expands on that model by roughly tripling the area of the region representing this fringing extensional oceanic-arc system to include the western third of mainland Mexico. This chapter summarizes the geologic, paleomagnetic, and detrital zircon data that tie all of these oceanic-arc rocks to each other and to the Mexican margin, herein termed the Guerrero-Alisitos-Vizcaino superterrane. These data contradict a model that proposes the oceanic-arc rocks formed in unrelated archipelagos some 2000–4000 km west of Pangean North America. Following the termination of Permian–Triassic (280–240 Ma) subduction under continental Mexico, the paleo-Pacific Mexico margin was a passive margin dominated by a huge siliciclastic wedge (Potosí fan) composed of sediments eroded from Gondwanan basement and Permian continental-arc rocks. I propose that a second fan formed further north, termed herein the Antimonio-Barranca fan, composed of sediment eroded from southwest Laurentian sources. Zircons from these two fans were dispersed onto the ocean floor as turbidites, forming a unifying signature in the Guerrero-Alisitos-Vizcaino superterrane. The oldest rocks in the Guerrero-Alisitos-Vizcaino superterrane record subduction initiation in the oceanic realm, producing the 221 Ma Vizcaino ophiolite, which predated the onset of arc magmatism. This ophiolite contains Potosí fan zircons as xenocrysts in its chromitites, which I suggest were deposited on the seafloor before the trench formed and then were subducted eastward. This is consistent with the geophysical interpretation that the Cocos plate (the longest subducted plate on Earth) began subducting eastward under Mexico at 220 Ma. The Early Jurassic to mid-Cretaceous oceanic arc of western Mexico formed above this east-dipping slab, shifting positions with time, and was largely extensional, forming intra-arc basins and spreading centers, including a backarc basin along the continental margin (Arperos basin). Turbidites with ancient Mexican detrital zircons were deposited in many of these basins and recycled along normal fault scarps. By mid-Cretaceous time, the extensional oceanic arc began to evolve into a contractional continental arc, probably due to an increase in convergence rate that was triggered by a global plate reorganization. Contraction expanded eastward (inboard) throughout the Late Cretaceous, along with inboard migration of arc magmatism, suggesting slab shallowing with time.

The boundary between the western Jurassic-Cretaceous arc terrane and the eastern, pre-Cretaceous cratonally derived metasedimentary rocks extends along the axis of the Peninsular Ranges. In the southern Sierra Calamajue (lat. 29°25′N), where this boundary is exposed, detailed stratigraphic and structural analyses of these lower greenschist grade rocks indicate that the western and eastern terranes are structurally interleaved in a 5- to 7-km-wide zone. Five northwest-trending tectonostratigraphic units are mapped in this area. The structurally lowest (southwesternmost) unit consists of 1,300 m of chert and fine-grained clastic rock, with minor limestone of Late Mississippian age. These rocks are structurally overlain by 2,600 m of Upper Jurassic basaltic to andesitic volcanic and volcaniclastic rock and Lower Cretaceous (Alisitos Group) volcaniclastic and epiclastic rocks. Medium- to thick-bedded quartz sandstones in the upper part of the Jurassic sequence appear to be redeposited in the overlying Alisitos Group, suggesting a depositional contact between these rocks. Two of the three structurally overlying units consist of variably tectonized limestone and fine- to medium-grained clastic rocks of unknown (Cretaceous?) age. These units are in sharp fault contact with, and are separated by, Lower Cretaceous volcaniclastic and pyroclastic rocks. Three phases of deformation have been identified in these rocks. The oldest deformation, D 1 , produced steeply east dipping reverse faults (which separate the rock units), tight to isoclinal folds, and an associated, northwest-trending axial plane cleavage. Progressive flattening strain during D 1 rotated fold hinge lines and the X-axis of deformed lapilli into the direction of tectonic transport, which is defined by a steeply plunging extension lineation. Strain analyses indicate that shortening perpendicular to D 1 foliation probably exceeded 60 to 70% in some areas. The timing of this deformation is constrained by a 103 ± 4-Ma Rb/Sr whole-rock age for deformed metavolcanic rocks (Alisitos Group), and a U-Pb zircon age of circa 100 Ma for an undeformed granite that crosscuts the early fabric. The second recognizable deformation, D 2 , produced a conjugate set of kink bands that indicate shortening parallel to the earlier formed fabric. A map-scale sinistral flexure, which occurs at the join between the Sierra Calamajue and the Sierra la Asamblea, deforms structures produced during D 1 and D 2 .