STONEWALL ANTICLINE - AN ACTIVE FOLD ON THE OREGON CONTINENTAL-SHELF

Stonewall Bank is the site of a growing west-verging anticline strikin g north-north-west on the continental shelf at 44.5 degrees N, southwe st of Newport, Oregon. To the east are Pliocene-Pleistocene strata of the Newport syncline, onlapping eastward against gently west-dipping l ate Miocene and older rocks of the Oregon Coast Range. Folding of Ston ewall anticline results in fine-grained middle Miocene strata being ex posed at the sea floor along the anticlinal crest; rocks as old as Eoc ene are encountered in the Unocal P-0093-1 Grebe well, drilled at the anticlinal crest. Rates of folding are based on deformation of an unco nformity between Pliocene and Miocene strata (PM unconformity) and of a stream channel that crossed Stonewall Bank during the last glacial m aximum, The bed length of the unconformity is shortened across the Sto newall anticline and adjacent folds by about 400 m between structures west of Stonewall Bank and the Newport syncline. The PBI unconformity has a vertical separation of about 1000 m between the anticline and th e first syncline to the vr-est. The horizontal shortening and vertical separation imply that Stonewall anticline is underlain by a blind rev erse fault. Retrodeforming the PM unconformity shows that this fault d ips 65 degrees-70 degrees E. A vertical separation of 1000 m on a faul t with this dip yields a slip of 1070-1080 m along the fault, If foldi ng of the PM unconformity is assumed to hare begun 2-3 Ma, this would give a long-term slip rate of 0.4-0.6 mm/yr. Lf most folding began aft er deposition of the entire Pliocene-Pleistocene sequence, the slip ra te would be 1.0-1.1 mm/yr. Stonewall anticline has arched the late Ple istocene lowstand wave abrasion platform since sea level underwent a r apid rise from 14.5 to 8 ka, This arch is crossed by an antecedent str eam channel that is 275-550 m wide and is marked by side drainages and cut banks up to 12 m high. warping of the platform on the eastern lim b of the anticline has back-tilted the stream channel eastward ton ard its present onshore continuation, the Yaquina River. The platform slo pes downward 10-13 m westward from the crest of Stonewall anticline. W e estimate that the platform stopped abrading when sea level reached a bout -40 m at 11-12 ka, Assuming that the west slope of the platform i s controlled by the same blind fault that produced the west dip of the PM unconformity, the Holocene slip rate on this fault would be 0.9-1. 3 mm/yr, comparable to the long-term slip rate. Rupture of the entire 25 km length of the blind fault with 1 m of slip beneath Stonewall Ban k could produce an earthquake with M-w = 6.8 +/- 0.25, which, would re sult in peak ground accelerations close to 0.2g on the central Oregon coast.