Comparisons and Contrasts between Boundary Transform Fault Basins and Trench-Linked Transform Fault Basins

B

Busby, Cathy J.¹ (1) University of California, Santa Barbara, CA

Transform faults on continents are dominated by two types: boundary transforms (e.g. San Andreas fault) and trench-linked transforms (which accommodate the horizontal component of oblique subduction). Although linkages between tectonics and sedimentation have been studied in detail along the former, the latter have been neglected, despite the fact that they occur in two-thirds of all modern continental arcs.

Basins are structurally similar in both settings. The first type, releasing-bend basins, are small, deep, highly asymmetrical basins that experience alternating and/or coeval normal and reverse faulting, and attendant “porpoising”. This results in rapid subsidence and sedimentation, alternating with uplift and creation of very deep erosional unconformities. The second type, stepover basins, are symmetrical grabens that subside very rapidly.

Basin fill contrasts between boundary transform and trench-linked transform settings are marked. Releasing-bend boundary transform basins are commonly underfilled in the deep end, where lakes or marine embayments form, and are filled with siliciclastic sedimentary systems that prograde into the basin from the shallow end. Releasing-bend trench-linked transform basins, in contrast, are overfilled in the deep end of the basin, because numerous small volcanic centers are sited along the master fault and its splays, which frequently plumb small batch of magma to the surface. The shallow end of the basin is dominated by fluvial sediment reworked from explosive eruptions at the deep end of the basin, as well as ignimbrites erupted from extrabasinal calderas that form along other segments of the trench-linked transform. Calderas form in stepover basins, where extension is greatest.