Pulverized rocks observed in outcrops near large faults are considered to be products of intense fragmentation occurring during strong earthquakes. However, the exact mechanism and depth of pulverization are controversial. Here we present an analysis of pulverized rocks along an ∼930 m interval in a deep borehole in the vicinity of a buried fault. Cuttings of carbonate lithology from below 4980 m host well-preserved sets of dense fractures with a hierarchic branching geometry that was never affected by near-surface processes. Branches are typically symmetric with consistent orientations and hierarchic splitting. The main fractures are spaced at ∼0.1 mm, with angular fragments ranging from a few microns to submicrons, indicating further intense localized fragmentation. Splitting of fossils by fractures, in the absence of lateral shear, proves the tensile origin of the fractures. The fracture geometry and density are consistent with dynamic tensile fractures that propagate at critical velocities in laboratory experiments. Thus, these dense branching fractures are the first direct observation that pulverization in fault zones extends to a significant depth and originates from dynamic tensile fracturing.

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