UNRAVELING DISPLACEMENT FIELDS IN ARCS: AN EXAMPLE FROM THE PENINSULAR RANGES BATHOLITH

Based on geologic, geochemical, and isotopic studies, the Jura-Cretaceous Peninsular Ranges batholith is commonly divided into western and eastern zones that are distinguished by island arc and continental-margin arc characteristics, respectively. South of the Agua Blanca fault, a central zone composed of dominantly Cretaceous deposits marks the boundary between the accreted Alisitos arc and North America.

In the Sierra Calamajue, western zone Cretaceous volcanic arc strata, central zone Cretaceous inter-arc basin deposits, and eastern zone Paleozoic deep-water strata of North American affinity are juxtaposed across SW-directed reverse and thrust faults. Western zone strata are generally flat lying or openly folded and show depositional structures and weak cleavage. After correcting for primary fabrics, quantitative finite strain estimates range from 25 to 30% extension and 20 to 30% shortening. In the central zone, deformation is more intense. NW-SE striking, moderately to steeply NE-dipping cleavage transposes tight folds of bedding surfaces. Reverse faults and shear zones show consistent top-to-SW kinematic indicators. Strain ellipsoids plot in both, the flattening and constrictional fields and intensities range from 60 to 90% shortening and 100 to 300% extension. In the eastern zone, strains are moderate to high. Unfortunately, strain markers have not been found to allow for quantification. However, deformation is generally more complex than to the west as layering is strongly disrupted, depositional features are completely obliterated, and abundant isoclinal folds of compositional layering are transposed and locally tightly refolded.

Our strain data from this arc segment fit well with other preliminary data from south of the Agua Blanca fault in the Sierra San Pedro Martir and San Vicente area. Significant increase in finite strain and deformation intensity toward the central zone further support the accretion of the Alisitos arc to North America during the mid-Cretaceous. However, the complexity of deformation in the eastern zone may indicate a prolonged deformation history that is not simply explained by a single deformation episode related to arc accretion. Instead, Permo-Triassic deformation along the southern margin of North America may account for previous deformation.