Thinly laminated diatomaceous sediments from the Miocene Monterey Formation contain intrastratal microfractured zones (IMZs) with a layer-specific stratigraphic distribution. IMZs are a hybrid fault/vein structure that are stratally bounded by undeformed sediments. IMZs record synsedimentary extension, brittle fracture, and en echelon normal faulting, and are intimately associated with a well-ordered record of sediment and fluid redistribution in sigmoidal, mud-filled vein arrays. Outcrop, microscope, and SEM observations suggest that IMZs are attributable to intrastratal faulting and vein propagation within shallowly buried diatomaceous sediments. We interpret that IMZs formed in a setting of bedding-parallel shear, on a submarine slope proximal to a region of active tectonism. Measuring the spatial orientation of IMZs and correctly interpreting their vergence direction permits accurate estimation of paleoslope direction. The unique physical properties of organic-rich diatomaceous sediments contributed substantially to the formation of IMZs. Associated sediments record contemporaneous fluidization, slumping, disharmonic folding, complex fracturing, and brecciation, suggesting that IMZs are part of a rheological continuum of synsedimentary slope failure. The presence of cobbles and boulders bearing IMZs and related structures within and directly subjacent to soft-sediment conglomerates and breccias further substantiates the synsedimentary origin of IMZs and suggests that stratigraphic horizons bearing IMZs may have acted as detachment surfaces for some slope failure and subsequent mass-movement events. Although the trigger for initiation of failure remains uncertain, our kinematic model and the apparent restriction of IMZs (and comparable structures) to transpressive and convergent tectonic margins suggests that IMZs and some associated rudite deposits may record the signature of seismic events upon shallowly buried hemipelagic sediments.

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