Analysis of brittle shear faults of IODP-ICDP expedition 364 drill core: testing numerical models of late-stage impact cratering

Drilled rocks from the Chicxulub impact crater are pervasively affected by decimeter-scale brittle shear faults. Measurements of the orientations and slip senses of 602 shear faults in shocked granitoid target rocks resulted in a unique fault-slip data set of unprecedented detail. The shear faults most likely represent a late stage of cratering, during which the crater grew horizontally. Based on numerical models, deformation during this stage is characterized by concentric and vertical extension and uniform radial shortening. Extracting principal strain axis directions from the inversion of brittle shear faults allowed us to test to what extent the observed brittle deformation corresponds to the deformation regime predicted by numerical models. As the fault-slip data were measured on drill core segments in horizontal position, the data had to be rotated back with regard to the in-situ positions of the segments prior to inversion. Results of the inversion show that the principal directions of observed brittle strain adhere to numerically predicted directions in the vicinity of a most prominent deformation zone at a depth between 1220 and 1316 m below the sea floor. By contrast, elsewhere above this deformation zone, principal strain orientations seem rather non-uniform. It remains to be determined to what extent the variation of strain axis orientations with depth may be systematic or abrupt across possible structural or lithological discontinuities. At any rate, the local orientation of the principal strains is more variable than predicted for the apparently simple strain regime during late-stage crater growth predicted by numerical models.