Polymict melt‐bearing breccia dikes in the Morokweng impact structure formed by slip‐induced mechanical mixing of pseudotachylite and cataclasite along large‐displacement impact faults

Abstract

Abstract A core drilled through shocked and faulted Archean granitoid gneisses and dolerites in the eroded peak ring of the 70–80 km diameter Morokweng impact structure intersects multiple centimeter‐ to meter‐wide clastic‐matrix breccias containing a polymict clast population of lithic and mineral clasts and altered, millimeter‐ to centimeter ‐size, melt clasts. These polymict melt‐bearing (PMB) breccias occur both as discrete dikes and in meter‐ to decameter‐wide composite breccia intersections where they are intimately associated with cataclasite and pseudotachylite. Petrographic and bulk‐rock geochemical analysis confirms that the PMB breccias comprise fragmental and melt material derived exclusively from the granitoid and doleritic wallrocks, with local geochemical deviations attributable to metasomatic hydrothermal alteration. Notwithstanding their almost complete replacement by smectite and zeolite assemblages, the melt clasts display textural and compositional characteristics identical to the pseudotachylite dikes. Composite lithic‐melt clasts indicate an intimate association of melting with cataclasis and comminution prior to their incorporation into the PMB breccias. While most melt clasts display sharp, angular shapes, indicating brittle fracturing, local preservation of delicate filaments intruding the adjacent clastic matrix and bulbous to cuspate‐lobate melt clast margins against the matrix indicate incorporation into the breccias while still molten and/or plastic. We propose that the PMB breccias formed by a combination of dynamic injection of friction melt into the cataclasite portions of large fault zones and the development of shear‐induced Kelvin–Helmholz instabilities along the melt‐cataclasite interface during ongoing fault slip. Melt injection into brecciated wallrock and smaller fractures hosting incoherent cataclasite may have been assisted by a pumping‐suction mechanism driven by complex, rapidly changing, block movements during crater wall collapse and peak ring formation. Cooling of the pseudotachylite melts during continued shear or compression of the breccia zones led to their embrittlement and mechanical entrainment as fragments into the incohesive cataclastic fault material, producing the hybrid PMB breccia type. Although the complex strain patterns during peak ring formation could have played a role in extending the duration of shear movements affecting the breccias, we propose that the sequence of cataclasis, frictional melting, melt injection, quenching, brecciation of quenched melt, and melt clast entrainment necessary to produce the PMB breccias can be reconciled with a single, continuous, long‐duration, large‐magnitude, slip event during collapse of the transient crater wall.