STRUCTURAL TRENDS AND CONTROL ON VOLCANIC FEATURES ON ST. PAUL ISLAND, PRIBILOF ISLANDS, ALASKA

St. Paul Island, a Pleistocene to Holocene volcanic field and one of the youngest eruptive centers in the Bering Sea basalt province, is located on the southern edge of the Bering Sea shelf. The surface of St. Paul is composed mainly of numerous tephra cones surrounded by coalescing, low-viscosity pahoehoe lava flows. A central highland that spans the island from east to west is constructed of relatively young eruptive centers. Localization of volcanic vents appears to be related to E-W and NE-SW trending fault and fissure systems with polygenetic centers, Bogoslof Hill and Cone Hill, located at intersections of major structures. Faulting and fissuring are common on the island; the magnitude of fault separation generally correlates with the stratigraphic age of the faulted unit. Faults with 5-30 m offset are restricted to the older lavas that form the platform of the island. Smallest offsets occur as a series of subparallel fissures cutting stratigraphically high lava flows on the central highland. E-W and NE-SW striking fissures are numerous on Bogoslof Hill and Cone Hill respectively, and these have vertical offsets of less than about 1 m. In places these fissures are open with no surface evidence for associated magmatic activity, whereas in others they contain dikes. The faults and fissures cutting young St. Paul lava flows generally parallel the E-W and NE-SW alignment of volcanic features at both monogenetic and polygenetic volcanoes. Many volcanic vents fall along trends of mapped fissure and fault systems that extrapolate to nearby islands and shoals. In this regard, the E-W alignment of young vents along the central highland is generally consistent with N-S extension inferred for the Pribilof area.