Crustal-scale Strike-slip Fault Research Articles

Exhumation of the Tauern window, Eastern Alps

The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.

New geophysical data from the northern Cordillera: preliminary interpretations and implications for the tectonics and deep geology

In this paper we analyze recently acquired geophysical data from the northern Cordillera and their relation to the mapped geology. A prominent gravity high (> −45 mGal (1 Gal = 1 cm/s2)) coincides with a magnetic low and an aseismic region in west-central Yukon where the underlying geology is dominated by quartzo-feldspathic rocks having moderate densities. Extension (~15%), magmatic underplating, and accretion of the anomalous region onto oceanic crust are three possible explanations.Magnetic, gravity, and seismicity data all show significant differences in the physical state of the crust on either side of the Tintina Fault and, together with geological data indicating large offset, suggest it was once a major crustal-scale strike-slip fault. The new gravity data also delineate an arcuate zone of steep gradients (up to 1.4 mGal/km) in the miogeocline, which may correlate with a west-dipping Proterozoic basement ramp mapped on deep seismic sections farther to the north and a transition from thin (east) to thick sediment cover (west). Seismicity data show that current tectonic activity is concentrated along the Pacific – North America plate margin in southwestern Yukon and adjacent Alaska and, although there is a marked decrease in activity inland of this margin, notable concentrations occur along the Denali Fault System and in the eastern miogeocline. There is a distinct absence of earthquakes in parts of the Selwyn Basin and in the northern Yukon–Tanana Terrane. Limited field studies suggest activity is confined to the upper 10–15 km of the crust.

Restoration of Laramide right-lateral strike slip in northern New Mexico by using Proterozoic piercing points: Tectonic implications from the Proterozoic to the Cenozoic

Early Proterozoic rocks in northern New Mexico can be used to document 100-170 km of right-lateral slip along a network of north-striking Laramide (Late Cretaceous-early Tertiary) faults that formed the precursor of the Rio Grande rift. Piercing lines are defined by the intersection of a subborizontal 4 kbar isobaric surface with steeply dipping stratigraphic markers and regional structures. Restoration of slip provides new insight into the nature of Proterozoic orogenic and crustal province boundaries. The Yavapai crustal province (1.75-1.72 Ga juvenile volcanogenic rocks) is imbricated with overlying Hondo Group sedimentary cover in an originally east-trending ductile thrust belt that formed near the southern margin of the Yavapai province. A series of offset magnetic anomalies (Jemez lineament) is interpreted to mark the thrust-buried boundary between older (Yavapai province) and younger (Mazatzal province) crust. Our microplate tectonic model suggests that Laramide strike-slip movement (New Mexico) was kinematically linked with coeval extensional collapse (Arizona) and foreland shortening (Wyoming) during decoupling and northward movement of the rigid Colorado Plateau relative to the undeformed midcontinent. Crustal-scale strike-slip faults in New Mexico were reactivated during later Tertiary extension and continue to localize high heat flow in the Rio Grande rift and northward into Colorado.