FY 1988

Morphology, structure and resource potential of the Blanco Transform Fault Zone

Embley, R.W., L.D. Kulm, G. Massoth, D. Abbott, and M. Holmes

In Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins—Beaufort Sea to Baja California, D.W. Scholl, A. Grantz, and J.G. Vedder (eds.), Earth Science Series, Vol. 6, Circum-Pacific Council for Energy and Mineral Resources, Amer. Assoc. Petro. Geologists, 549–561 (1987)

A series of rhombohedral basins along the 350-km-long Blanco transport fault zone (BTFZ) are oceanic analogs to pull-apart basins along major divergent wrench fault systems on the continent. In plan view, the BTFZ is similar to the Gulf of California; a series of long strike-slip faults (as much as 100 km long) are separated by short extensional basins (<20 km) that have internal structures oriented at a large angle to the master faults. In the Blanco region, continual reorientation of the transform fault in response to changes in plate motion during the past few million years provides a mechanism for the formation of these basins. A series of back-tilted, inward-facing fault blocks covered with uplifted turbidite beds brackets the largest of the depressions (Cascadia), which may be a sea-floor spreading center. Deformed Holocene and Pleistocene turbidite sequences are present within the depression, and there is substantive morphologic and sedimentologic evidence of rapid Holocene subsidence of the floor of the Cascadia Depression. A large water-column methane signal of unknown origin is present, but to date there is no direct evidence of a sea-floor hydrothermal system. However, the probable presence of intrusive bodies within the sedimentary sequence of the Cascadia Depression could give rise to sediment-hosted sulfide bodies such as those of the Guaymas Basin in the Gulf of California. The depressions at either end of the BTFZ (the Gorda and Blanco Depressions) are largely unexplored, but sulfide-veined greenstone breccias and hydrothermal sediments have been recovered from them. Oceanic transform fault zones such as the Blanco may also have hydrothermal systems associated with deep fracturing and high topographic relief.