Abstract
Abstract Large-scale strike-slip faults are fundamental tectonic elements of the continental lithosphere. They constitute plate boundaries (continental transforms), separate terranes with contrasting geological histories within accretionary orogens, or accommodate heterogeneous deformation in intracontinental settings. In ancient orogens, where deeper levels of the crust are exposed, these faults are expressed as shear zones materialized by up to tens of km-wide mylonitic belts. The Borborema shear zone system in northeastern Brazil is one of the largest and best-exposed intracontinental strike-slip shear zone systems in the world, cropping out over 250,000 km2. Here, we review its main geophysical, structural, petrologic, and geochronologic characteristics and discuss the factors controlling its development. This complex continental scale shear zone system is composed of a set of NE- to NNE-trending dextral shear zones from which there are two major E-trending dextral shear zones with horse-tail terminations into the transpressional belt branch, as well as several smaller E-trending dextral and NE-trending dextral and sinistral shear zones. The major shear zones are marked by extensive linear or curvilinear magnetic gradients, implying their continuation at depth. The major shear zones are materialized by migmatite to amphibolite-facies mylonites, but the entire system shows evidence of late deformation at lower temperatures. The system developed during the late stages of the Neoproterozoic Brasiliano (Pan-African) orogeny (mainly from 590 to 560 Ma), postdating by more than 20 Ma the main stage of contractional deformation. Localization of strike-slip shearing in this intraplate setting was controlled by rheological contrasts between blocks with distinct Paleoproterozoic histories, the presence of preorogenic Neoproterozoic rifts, the craton geometry, and zones of enhanced magmatic activity, highlighting the importance of rheological heterogeneity in controlling shear zone nucleation and evolution.
Highlights
Large-scale strike-slip faults and shear zones are prominent features of many orogenic belts
Melt-induced weakening leading to widely distributed deformation may account for a striking feature of the Borborema shear zone system (BSZS), which is the large width of the Patos and, to a lesser extent, West Pernambuco shear zones, >25 and 10 km wide, respectively, relative to the limited displacement that they may have accommodated
The Borborema shear zone system is composed of two main domains: HT dextral NE-trending shear zones predominate in the west, whereas dextral E-trending shear zones and Nto NE-trending transpressional belts predominate in the east
Summary
Large-scale strike-slip faults and shear zones are prominent features of many orogenic belts. Shear zones commonly develop following a period of crustal thickening related to continental collision and their location is strongly influenced or even determined by the presence of large-scale heterogeneities such as craton margins or aborted rifts within the original lithosphere This is, for instance, the case of the Altyn Tagh fault in the Himalayas [8, 9], which separates the Tibetan Plateau from the Tarim Craton, the Além Paraíba shear zone in the Araçuaí belt (southeast Brazil), which records a shift from the orogen-normal to orogentransverse tectonic flow linked to the southern termination of the São Francisco Craton [10, 11], and the Great Slave Lake shear zone in Canada, which accommodated the relative motion of the Slave Craton and the Churchill Province [12]. The work aims at presenting a comprehensive overview of the literature related to the BSZS, reanalyzing aeromagnetic data to deduce the continuation at depth of the shear zones, synthesizing structural, petrological, and isotopic data, and discussing the processes controlling the nucleation and development of intraplate shear zones
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