THE UPS AND DOWNS OF STRIKE-SLIP FAULTS - REVISITING LOCALIZED YO-YO TECTONICS IN LONG-LIVED FAULT SYSTEMS

All intracontinental strike-slip and transform faults have zones of transpression and transtension ranging on scales of a few kms to 100 kms or more. The vertical component of these zones can produce multiple cycles of burial and exhumation, sometimes in the same locale, a process that Paul Umhoefer and others referred to as yo-yo tectonics. Recognizing that the vertical motion is coeval with strike-slip can be challenging in the crystalline rock record. Several locales in Argentina and Alaska provide examples of highly localized (few kms scale) rapid burial and exhumation of rocks from mid-lower crustal depths adjacent to major strike-slip fault systems. The process results in narrow zones of medium-high grade rocks immediately adjacent to rocks that underwent minimal burial during the same event. We compare these highly localized zones of extreme vertical tectonics with larger (kms to 10s kms) zones of longer-lived burial or extension occurring coevally along the same strike-slip systems.

A prominent example of yo-yo tectonics on a local scale is preserved along the sinistral Valle Fértil fault system which developed during the waning stages of Ordovician – Silurian collision between the Precordillera terrane and the Famatina arc system on Gondwana. One locale (less than 5 km across) immediately adjacent to the main fault shows evidence for two stages of migmatite development – one at ~ 465 Ma and one at ~ 410 Ma. The latter event is expressed in rocks that did not experience the first event. These rocks were exhumed rapidly by 403-401 Ma, based on a zircon age of 403 in localized melt in the immediate footwall of an oblique-sinistral shear zone and a muscovite age of 401 in the mylonite that forms the hanging wall fabric. This localized transtensional fabric is coeval with crustal shortening occurring on a regional scale, both along strike, adjacent to the Valle Fértil fault system, and across strike, as seen in coeval discrete thrust faults in adjacent mountain ranges. These locales document fabrics formed an apparently contradictory tectonic environments, which reflects the difference between localized step-over processes and regional-scale, longer-lived oblique convergence that occurred for over 20 myrs along the Valle Fértil fault system.