TECTONIC INVERSION: REGIONAL VERSUS LOCAL, AND ASSOCIATION WITH MIGRATING STRIKE-SLIP STEP-OVERS

Many examples of tectonic inversion appear to fall into one of two categories: (1) regional inversion related to regional changes in tectonic regime, or (2) local inversion associated with fault complexities such as block rotation and strike-slip step-over evolution. The former category has received the most attention, but plenty of examples of the latter exist. Particularly common is inversion that occurs as a step-over or bend along a strike-slip fault migrates with respect to deposits it formerly affected. This involves the continual creation of new transverse structures or linkages, younging in the direction of step-over migration, and in many cases, the propagation of a fault tip on one side of the step-over and the dying out of activity at an end of a fault on the other. In the San Andreas fault system, that is regional transpressive along much of its length, a migrating releasing bend apparently leaves a "wake" of former basinal deposits trailing behind the active basin; deposits in the wake are subject to shortening. Examples may include the Pleistocene Merced and Olema Creek Formations of the San Francisco Bay area. This process appears to be scale independent for inverted sag-pond deposits are observed in many paleoseismic trenches along strike-slip faults. Along a neutral or transtensional system, a migrating restraining step-over following a migrating releasing step-over will cause inversion of the former basinal deposits. Examples may be found along strand of the Eastern California shear zone. The structural details associated with migrating step-overs and tectonic inversion are still poorly known, for it is difficult to even identify transverse structures associated with active step-overs, but it appears that early structures associated with transtension and deposition are commonly reactivated rather than cut by structures accommodating shortening.