QUATERNARY STRAIN DISTRIBUTION ON THE ONSHORE PORTION OF THE FOLD AND THRUST BELT AT THE SOUTHERN END OF THE CASCADIA SUBDUCTION ZONE

Plate convergence at the southern end of the Cascadia subduction zone (Csz) is partitioned between the offshore megathrust and a wide upper plate fold and thrust belt that extends onshore in northern California. The fold and thrust belt includes two groups of northeast dipping southwest vergent thrust faults and associated folds, the Mad River fault zone (MRfz) and the Little Salmon fault zone (LSfz). Slip rates have been determined for many of the onshore faults from displacement measurements on three classes of datums: 1) the late Neogene Wildcat Group and Falor Formation sediments; 2) late Pleistocene marine terraces; and 3) Holocene fluvial terraces and sediments. Near the surface the fold and thrust belt faults have shallow dips, commonly less than 30 degrees. Dip slip displacement rates based on surface or near surface measurements for individual faults in the MRfz range form ~1 to ~3 mm/yr and for faults in the LSfz from ~1 to ~9 mm/yr. The total late Quaternary dip slip displacement rate on faults across the MRfz is ~10 mm/yr and across the LSfz is ~11 mm/yr. The dips of the faults at depth and the amount of upper plate shortening represented by folds are not well constrained, producing uncertainties in estimates of the amount of plate convergence accommodated by upper plate contraction. Nevertheless, the contribution to the growth of the fold and thrust belt from plate convergence is large, perhaps as much as half of the ~40 mm/yr relative motion between the plates. This partitioning of strain between the megathrust and structures in the upper plate along the southern part of the Csz suggests the Gorda and North American plates are strongly coupled beneath the fold and thrust belt.