Estimates of Strain Rates and Strain-Related Crustal Thickening from the Alisitos Arc-Continent Collision Zone, Peninsular Ranges, Baja California, Mexico

Collision between North America and the Alisitos arc, located in Baja California, began between 115 and 110 Ma and was largely complete by 105 Ma. A syn-collisional fold-thrust belt (FTB) along the northern and eastern boundaries of the arc exhibits a deformational gradient with tighter folds, and greater fabric and finite strain intensities towards the arc-continent suture.

Rf-Φ analyses of 85 lithic-rich volcanics show that XY-planes of strain ellipsoids parallel regional NW-trending foliation and X-axes parallel moderately to steeply plunging mineral lineations. Finite strain intensity ranges from 0.08 to 2.71 and flattened ellipsoid shapes dominate. Although strain intensity generally increases towards the suture, finite strain is heterogeneous reflecting a FTB characterized by local high-strain zones in ductile shear zones, folds, and pluton aureoles.

Tensor-averaged horizontal shortening and vertical thickening finite strain estimates across the FTB range from -17% to -46% and 12% to 39%, respectively. When removing the strain component of deformation from the FTB using a vertical dimension of 5 km, which represents exposed crustal levels, ductile finite strain accounts for between 0.6 and 10.2 km of shortening and 0.5 to 1.4 km of crustal thickening. Furthermore, timing constraints for the collision suggest that finite strain recorded in the arc accumulated over as little as 5 m.y., yielding shortening strain rates ranging from 1.08 x 10^-15 s^-1 to 2.92 x 10^-15 s^-1.

Our data show that finite ductile strain contributes significantly to bulk deformation in the FTB. However, finite strain and initial fold- and fault-related shortening estimates are insufficient to account for the observed crustal thickness of ~40 km along the east side of the arc. This suggests that additional factors, such as collision-related tectonic underplating, syn- or post-collisional magmatic underplating, and modification of the crust during subsequent tectonism along the North American continental margin, contributed to crustal thickening.