Constraints on the strength of faults from the geometry of rider blocks in continental and oceanic core complexes

Abstract

Large offset normal faults, central to the formation of core complexes, require a minimum amount of fault weakening to form according to analytic and numerical models. New work suggests that these faults cannot be too weak and still result in the kind of fault-bounded rider blocks overlying the lower plate of some large offset normal faults. Rider block wedges of upper plate rocks, including syn-tectonic sedimentary rocks, are often seen on continental metamorphic core complexes. Blocks of volcanic rocks are sometimes seen to bury the detachment of oceanic core complexes. We consider extensional faulting in Mohr-Coulomb layers to estimate the conditions that can lead to rider block formation and estimate the size of rider blocks formed. Offset of a single normal fault causes rotation of the fault, with the shallow part of the fault rotating more than the deeper parts. In models, rider blocks form when the shallow section of a normal fault becomes rotated so far from an optimal dip that it is replaced by a new, steeper-dipping splay fault that links with the deeper part of the old fault. Analytic theory predicts a narrow range of fault weakening that leads to large offset normal faults with rider blocks. Infill of sediments or volcanics into the basin formed by offset of a single normal fault also promotes rider block development. For a 10 km-thick brittle layer complete cohesion losses greater than 15 MPa are too large to result in rider-block formation. Significant reduction in fault friction can prevent rider block formation. With sufficient infill the rider blocks up to nearly 10 km 2 in cross-sectional area, similar to those observed, can result. Numerical models confirm the general predictions of the analytic model, but also show that precise relations between block size and amount of fault weakening must await higher resolution models.