ATTACHMENT ZONES IN OBLIQUE CONVERGENCE AND DIVERGENCE: CONCEPTS, MODELS, EXAMPLES

The existence of rheologically distinct lithospheric layers necessitates different displacement fields to accommodate oblique convergence and divergence. The subhorizontal boundaries between these layers are critical locations where the layers are decoupled (detached), coupled (attached), or partially coupled. In order to test the degree of coupling between rheologic layers, we model strain in attachment zones developed beneath rigid upper crustal blocks. We investigate two cases: 1) translation of elongate blocks (slivers) parallel to strike-slip faults, and 2) rigid-body rotation of circular blocks of upper crust. We calculate the orientation and shape of the finite strain ellipsoid within attachment zones, and compare them to strain and fabrics observed in natural transpression to transtension settings. Attachment zones beneath translating blocks display steep foliation below the central part of rigid blocks and gently dipping foliation toward the margins. Strain intensity is maximum below the strike-slip faults where there is an abrupt reversal of sense of shear. The models predict constrictional fabrics in transpression attachments and flattening fabrics in transtension attachments. These predictions are robust because results are independent of the angle of convergence/divergence and of the amount of displacement. The linear belt of greenschist-grade metamorphic rocks in Trinidad's Northern Ranges and eastern Venezuela's Paria Peninsula is a candidate for an exhumed attachment zone developed beneath translating upper crustal blocks during Neogene highly oblique convergence between the Caribbean and South America plates. Block rotation allows us to test the top-driven versus bottom-driven models of lithospheric deformation, by comparing paleomagnetic rotations and plate motion vectors (surface deformation) to shear-wave splitting data (upper mantle deformation) from the same region. Applying this approach to Southern California and New Zealand, similar deformation is recorded by the upper crust and lithospheric mantle. Only bottom-driven flow, in which mantle deformation drives upper-crustal block rotation, is consistent with these observations.