Foliated Breccias in the Active Portuguese Bend Landslide Complex, California: Bearing on Melange Genesis

The active portion of the Portuguese Bend landslide complex is approximately 3 km2 in area and 30-50 m thick. Measured displacement rates range from less than 1 to greater than 30 mm/day on different parts of the landslide, with total displacements over the last 30 yrs ranging from about 10 to greater than 150 m. Six types of breccia, each locally with a foliated matrix, were recognized in the active landslide complex and are absent outside the landslide complex. Slide-body breccias are of two types, the first formed by extensional fracturing during bulk "pure shear" at the top of the landslide (slide-top breccia) and the second by flow of tuffaceous shales and fracture of embedded siliceous shales during simple shear deep in the landslide to the basal decollement (slide-bottom breccias). Slide-margin breccias, also in simple shear, are produced on the lateral margins of individual slide blocks accompanying wrench-fault motion. Other breccias (fault-ramp breccias) are formed during motion over ramps. Colluvial deposits within tension gashes (crack-fill breccias) and at the toe of the slide (slide-toe breccias) represent a fifth breccia type. Diapirs originating from over-pressured zones at the slide base also contain breccia. Recognition of different breccia types in ancient rocks would be difficult, because fabrics in the different types are similar. Foliations are defined by: scaly cleavage, compositional banding and color banding (in shear zones), stretched mud clasts, and aligned hard grains. Foliated breccias are synonymous with melanges. We regard the six breccia types described herein as representing the principal types of melange that occur in ancient accretionary settings. Furthermore, we argue that previous classification of melanges into "olistostromal" and "tectonic" types is artificial, because the Portuguese Bend landslide is clearly an olistostrome type that is currently producing structures indistinguishable from those produced tectonically.