Browsing by Subject "Low-angle normal fault"

Now showing 1 - 2 of 2

Continental extension in orogenic belts : modes of extension, origin of core complexes, and two-phase postorogenic extension
(2016-05) Wu, Guangliang, Ph. D.; Lavier, Luc Louis
Continental extension principally occurs in orogenic belts, however, most of numerical simulations use uniform crust that cannot represent an orogenic belt. We simulate lithospheric extension in an orogenic hinterland approximated by a crustal wedge. We first show that the presence of a preexisting weak mid-crustal shear zone dipping at low angle exerts a critical control on whether crustal and mantle deformation are decoupled or coupled. When the lower crust and the mid-crustal shear zone are weak, decoupling occurs and crustal deformation is compensated by lower crustal flow. When the lower crust is strong or a weak shear zone is absent, coupling occurs and crustal deformation is compensated by flow in the mantle. By varying the strength of the lower crust and the weak shear zone in numerical lithospheric extension experiments, we examine structures developed and compare them with structures observed in extended and collapsed orogenic belts. In models with a weak mid-crustal shear zone, we find that decoupling is particularly effective. In these models, we distinguish three modes of extension: 1) localized, asymmetric crustal exhumation in a single metamorphic massif with a weak lower crust, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple metamorphic core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone with a strong lower crust. In models without a mid-crustal shear zone, extension is coupled and structures similar to those observed in continental margins form. We further analyze my model to better explain and understand the core complexes and low-angle normal faults which develop when a preexisting weak mid-crustal shear zone is present. We define three types of detachment systems and present four models which produce core complexes that bear striking resemblance to natural examples: 1) bivergent core complexes, 2) metamorphic core complexes, 3) boudinage structures, and 4) flexural core complexes. We also discuss intracrustal isostasy and the thermal history of material particles sampled in modeled detachment. Finally, based on a geological and geophysical synthesis and using numerical simulations, we propose a two-phase postorogenic extensional scenario that approximates the evolution and the structures observed in the South China Sea margins.
Multi-timescale mechanics of an active low-angle normal fault
(2020-12-09) Biemiller, James Burkhardt; Lavier, Luc Louis; Wallace, Laura, 1973-; Ellis, Susan; Saffer, Demian; Ghattas, Omar
Detachment faults dipping < 30° commonly accrue 10’s of kms of offset and accommodate a large portion of crustal extension in moderately-to-highly extended regions. Slip on these high-offset low-angle normal faults remains perplexing due to their apparent misorientation relative to Andersonian principal stress directions. Classic fault mechanical theory predicts that normal faults should frictionally lock up and become abandoned at dips < 30°, yet geologic, seismological, and geodetic evidence shows that some low-angle normal faults slip actively. Despite evidence for actively slipping low-angle normal faults, few large earthquakes have been recorded on these structures. The scarcity and low long-term slip rates of active low-angle normal faults make it difficult to determine whether these faults rupture in large earthquakes based solely on seismological or geodetic records. In this dissertation, multi-disciplinary studies of the world’s most rapidly slipping low-angle normal fault are integrated to better understand the structural and tectonic evolution of detachment faults as well as to determine whether these faults slip in large earthquakes or predominantly creep aseismically. Bounding the actively exhuming Dayman-Suckling metamorphic core complex, the Mai’iu fault in Papua New Guinea dips 16-24° at the surface and has been estimated to slip at dip-slip rates of 8.6 ± 1.0 mm/yr to 11.7 ± 3.5 mm/yr. Geodynamic models suggest that weak zones and thermomechanical heterogeneities inherited from a previous subduction phase may have facilitated the formation of this long-lived detachment fault system (Chapter 2). Models of seismic-cycle deformation governed by rate-and-state friction show that the spatial distribution of fault rock frictional stability parameters strongly controls whether low-angle normal faults creep aseismically, slip in periodic large earthquakes, or slip in a mix of episodic creep events and earthquakes (Chapter 3). Surveying and U/Th dating of emerged coral reef platforms along the Goodenough Bay coastline show that tectonic uplift is episodic and imply that this segment of the detachment system slips in infrequent (440 – 1520 year recurrence) large (Mw > 7.0) earthquakes (Chapter 4). Velocities from a newly installed network of densely spaced campaign GPS sites reveal horizontal extension rates of 8.3±1.2 mm/yr (~8-11 mm/yr dip-slip) on the Mai’iu fault (Chapter 5). Laboratory friction experiments on exhumed Mai’iu fault rocks showing depth-dependent transitions in frictional stability help constrain inversions of kinematic models of the GPS velocities indicating that the Mai’iu fault is more strongly locked at ~5-16 km depth and creeping interseismically above 5 km depth. This result suggests that large (Mw > 7.0) earthquakes nucleate downdip of the low-angle portion of the Mai’iu fault and can propagate to the surface along the shallowly-dipping segment and/or more steeply-dipping splay faults in its hanging wall. In contrast to previous studies suggesting that active low-angle normal faults predominantly creep aseismically, this work implies that the active Mai’iu low-angle normal fault slips in infrequent large earthquakes accompanied by some shallow interseismic creep.