The Sierra Calamajue study area in Baja California, Mexico, exposes a series of units in several fault-bounded blocks. Units include the Jurassic–Cretaceous Alisitos island arc, Cretaceous continental-margin volcanic arc units, Mesozoic North America–derived sedimentary rocks, and North American Paleozoic passive-margin units. Deformation and metamorphism increase eastward. Units in the west are weakly deformed and metamorphosed, while upper-greenschist- to lower-amphibolite-grade assemblages farther east are intensely deformed. Most tectonic structures are probably related to the accretion of the Alisitos arc to North America in the mid-Cretaceous, although Paleozoic units contain an older fabric that supports deformation between the latest Mississippian and Late Permian. Overall, the study area marks the transition from an accreted ocean-island arc to the North American continental margin. Results from the Sierra Calamajue study area located along the eastern side of the Alisitos arc are similar to observations in the arc-continent transition zone farther north, where deformation gradients also exist. However, along-strike variations are recognized. Deformation recorded in the study area occurred at rather shallow crustal levels. In contrast, intense ductile deformation and exhumation from midcrustal levels are described elsewhere. Furthermore, a component of sinistral transpression documented along the northern edge of the arc is not recognized along the east side. We suggest that along-strike variations in the structural style are controlled by (1) tectonic setting (i.e., angle of accretion), (2) the preexisting geometry of the continental margin, and (3) changes in geology and rheologic strength of the units involved in deformation.

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 .