Abstract

Structural data and analyses of mineral preferred orientations in the Seiad massif of northern California give evidence for the distinction of two ultramafic assemblages with different oceanic histories. The contact between these two assemblages is a thrust fault. Before the thrusting, the underthrust peridotite was deformed in lithospheric conditions, together with an amphibolite unit not < 1.5 km thick. This episode of plastic deformation, resulting from dextral strike-slip movements, may be related to an oceanic transform zone. In the overthrust ultramafic assemblage, the horizontal foliations that bear east-west to west-north west-east-southeast lineations are thought to have formed during intraoceanic thrusting leading to the emplacement of the ultramafics in the Seiad area. The metamorphic framework of the Seiad area is divided into three distinct deformational units that were produced under amphibolite-facies metamorphic conditions: (1) a mafic unit that was involved in the same deformation event as the underthrust ultramafic assemblage, (2) a composite unit of predominantly sedimentary origin, marking the sole of the thrust fault, and (3) a composite unit of predominantly mafic composition that had structural directions unrelated to the previously described tectonic events. All three units were deformed under amphibolite-facies metamorphic conditions and emplaced in the Seiad area prior to the Late Jurassic Nevadan episode.

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