Geology, structure, and tectonic history of the Tualatin Basin, northwestern Oregon

Graduation date: 1997The northwest-trending Neogene Tualatin basin in northwestern Oregon is a pull-apart\ud basin with approximately 750 m of post-middle Miocene structural relief. Gently\ud tilted uplands capped by Columbia River Basalt Group surround the synclinal basin on all\ud sides. Integrated gravity, aeromagnetic, and seismic reflection data indicated that the\ud basin is underlain by 3 to 4 km of sub-horizontal, upper lower Eocene to middle Miocene\ud age sedimentary and volcanic rocks. Up to 300 m of Columbia River Basalt overlie these\ud older strata, in turn overlain by up to 400 m of late Miocene to Pleistocene, dominantly\ud fluvio-lacustrine sediments.\ud Late middle Eocene strong oblique convergence across the continental margin\ud generated regional northeast-trending fissures in the forearc through which the Tillamook\ud Volcanics and basalt of Waverly Heights erupted. Sinistral strike-slip faults, including\ud the proto-Gales Creek-Mt. Angel structural zone, segmented the forearc into discrete\ud blocks which rotated clockwise independently. A right-step in the Gales Creek-Mt.\ud Angel system generated northeast-directed folding and faulting adjacent to the restraining\ud bend, deforming rocks as young as upper Tillamook Volcanics. Regional uplift\ud culminated in a late Eocene unconformity which removed the arkosic Spencer Formation\ud northeast of the Gales Creek-Mt. Angel system, prior to subsidence of the forearc and\ud burial by late Eocene and Oligocene tuffaceous sedimentary strata.\ud The distribution of middle Miocene Columbia River Basalt Group flows shows no\ud evidence of a structural or topographic Tualatin basin at that time. The syncline initiated\ud along the present southwestern margin of the basin in late Miocene time, in response to\ud uplift of the Coast Range and Portland Hills anticlines to the southwest and northeast,\ud respectively. A late Pliocene decrease in convergence and consequent relative increase in\ud the oblique component of convergence reactivated the Gales Creek-Mt. Angel structural\ud zone as a dextral shear zone. The right-step acted as a releasing bend, allowing\ud extension, increased subsidence and normal faulting along the basin margins, and\ud volcanism within the basin. Subsequent development of transverse folds and reverse\ud faults, northwest-trending riedel shears, northeast-trending antithetic faults, and basin-marginal\ud normal faults, is consistent with classification of the Tualatin basin as a pull-apart\ud basin in a dextral wrench system