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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.

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