Geology of the southern Sierra Calamajue area : structural and stratigraphic evidence for latest Albian compression along a terrane boundary, Baja California, Mexico

Abstract

The boundary between the western Jurassic to Cretaceous volcanic arc terrane and the eastern, pre-Cretaceous cratonally derived metasedimentary rocks (Peninsular Terrane), extends along the axis of the Peninsular Ranges. In the southern Sierra Calamajue (29° 25' N lat.), where this boundary is exposed, detailed stratigraphic and structural analyses of 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 ~1300 m of Late Mississippian chert and fine-grained clastic rocks, and minor limestone. These rocks are structurally(?) overlain by ~2600 m of Upper Jurassic basaltic to andesitic volcanic and volcaniclastic rocks and Lower Cretaceous (Alisitos Group) volcaniclastic and epiclastic rocks. 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 highest 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, D1, 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 strains during D1 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 D1 foliation probably exceeded 60-70% in some areas. The timing of D1 is constrained by a 103±4 Ma Rb/Sr whole-rock age of deformed metavolcanic rocks (Alisitos Group), and the 92-97 Ma age of undeformed, La Posta facies plutons, which cross-cut the D1 fabric. The second recognizable deformation, D2, produced a conjugate set of kink bands which indicate shortening parallel to the earlier formed D1 fabric. A map-scale sinistral flexure, which occurs at the join between the Sierra Calamajue and the Sierra la Asamblea, deforms structures produced during D1 and D2. A minimum age for the D2 and D3 events has not been determined. The late-Early Cretaceous age of D1 coincides with the change from orthogonal to right-lateral oblique convergence between the Farallon and North American plates and an abrupt increase in the subduction velocity. Thus, it is suggested that the Dl event resulted from collapse of the Jurassic to Cretaceous arc against the craton due to the interaction of the subducting plate with the overriding lithosphere as the subduction angle shallowed.