Humboldt Slide - a large shear-dominated retrogressive slope failure

Humboldt Slide is a large, complex slide zone located on the northern Calif ornia continental margin. Its three-dimensional architecture has been image d by a combination of multibeam bathymetry, Huntec Deep-Tow seismic profili ng, and sidescan sonar. The slide is interpreted to be Late Pleistocene to early Holocene in age and was caused by a combination of factors. The area of the slide is a local depocenter with high accumulation rates of organic- rich sediment; there has been local steepening of slopes by tectonic uplift s; and the entire area is one of high seismicity. Overall, the failure occu rred by retrogressive, shear-dominated, minimum movement apparently as a se quence of events. Failure initially occurred by subsidence extension at the middle of the feature, followed by upslope retrogressive failure and downs lope compression, and finally by translational sliding at the top of the sl ide. Degassing, as evidenced by abundant pockmarks, may have inhibited down slope translation. The slide may still be active, as suggested by offsets i n Holocene hemipelagic sediment draped over some of the shear surfaces. Cro wn cracks occur above the present head of the failure and may represent the next generation of failure. (C) 1999 Elsevier Science B.V. All rights rese rved.