Low-angle Normal Fault Zone Research Articles

The Hengshan low-angle normal fault zone: Structural and geochronological constraints on the Late Mesozoic crustal extension in South China

Abstract We document in this paper a significant low-angle normal fault, namely the Hengshan detachment fault, which extends more than 150 km long in NNE orientation and bounds an elongated Early Cretaceous basin in Hunan Province, central South China. Detailed structural and geochronological analyses have been conducted along its southern segment, where a well exposed, ~ 3 km thick, flat ductile shear zone develops along the western margin of the Hengshan granitic massif. This zone is featured by shallow-dipping foliations of varied trend from NE to NW, which bear penetrative stretching lineations varying from NW to SW trend. Shear sense criteria indicate top-to-the-NW and top-to-the-SW motions along its northern and southern parts, respectively. Quartz c-axis orientations of mylonitic rocks from the shear zone exhibit asymmetric single or crossed girdle patterns, and the distributions of fabric point maxima suggest a simultaneous operation of basal and prism slips, indicating a deformation temperature ranging from 400 °C to 550 °C. One zircon U–Pb age of the sheared albitite and three muscovite 40 Ar/ 39 Ar ages of the mylonitic rocks indicate that the ductile shearing initiated at 136 Ma and lasted till 97 Ma. The zircon U–Pb dating results of five granitic samples from the Hengshan granitic pluton yield two phases of crystallization ages of 232–228 Ma and 150–151 Ma respectively, indicating two preceding magmatic events prior to the ductile shearing in the lower plate. Our structural and geochronological data allow defining the Hengshan massif as an extensional dome, rather than a metamorphic core complex as previously predicted, due to the lack of syn-tectonic plutonism and the origin of detachment fault associated with the reactivation of the inherited thrust. This study provides crucial evidence for understanding how the mid–upper crust deformed during the process of extension, which testifies to a coeval occurrence of non-coaxial deformation of the ductile flow in the middle crust and brittle faulting in the upper crust. It also sheds new insights into the timing of the regional crustal extension in South China, and permits to constrain its onset time to be ca. 136 Ma.

Rethinking conditions necessary for pseudotachylyte formation: Observations from the Otago schists, South Island, New Zealand

Pseudotachylytes and two distinct types of cataclasite in the Otago Schist at Tucker Hill, South Island, New Zealand, provide evidence for both seismic slip and aseismic creep on a normal fault zone during regional crustal extension in late Cretaceous time. Regional geologic evidence suggests that the fault had its present low-angle dip (ca. 10°) at the time it was active. ‘Type A’ cataclasites, presumably aseismic, can be recognized by bi-fractal grain size distributions, monomict composition, angular clasts of uniform textural maturity, and a crude fabric defined by oriented grains and transgranular fractures. ‘Type B’ cataclasites, possibly cosesimic, have characteristics consistent with fluidized grain flow, including heterogeneous clast shapes and types, a bimodal grain size distribution, intrusive relationships with other rocks, and the absence of any fabric or transecting fractures. Pseudotachylyte, which occurs as fault veins, injection veins and more complex types of intrusive structures, consistently cuts across and invades Type A cataclasites but is both intrusive into and included as clasts in Type B cataclasites. These relationships are consistent with a fault evolution model in which the development of a damage zone through aseismic cataclasis (Type A) facilitates the formation of pseudotachylyte in a subsequent seismic event by providing a permeable matrix through which fluids can drain in the early stages of slip, thereby maintaining frictional contact between rock surfaces. The formation of pseudotachylyte, in turn, may seal the fault zone and lead to thermal pressurization in a later seismic cycle, forming fluidized (Type B) cataclasites. Seismic slip on the low-angle normal fault zone at Tucker Hill may have occurred by two distinct modes of dynamic weakening — melt lubrication and thermal pressurization — in successive seismic events. Although there is a perception among geologists that pseudotachylyte is most likely to form in intact, crystalline rocks, geophysical models of fault zones clearly demonstrate that pseudotachylyte formation is actually suppressed in low-permeability rock because any fluids present would be unable to escape the fault zone and thermal pressurization would rapidly reduce frictional resistance. The paradigmatic occurrences of pseudotachylyte in otherwise pristine crystalline rocks probably represent somewhat exceptional circumstances (single rupture events at very high effective stress in dry rock). Coseismic frictional melts may actually be more common in hydrated rocks like the schist at Tucker Hill, but harder to recognize and also vulnerable to overprinting as a fault zone matures. In such rocks, pseudotachylyte may represent an intermediate stage in the evolution of a fault zone, the period between the formation of a high-permeability damage zone and the development of a low-permeability fault core.

The south Tibetan detachment system in the Rongbuk valley, Everest region. Deformation features and geological implications

Structural and microstructural investigations into the South Tibetan Detachment System in the Rongbuk valley (southern Tibet) pointed to the presence of a complex extensional setting with the development of two major fault zones with movement in a top-to-NE direction. The lower one is an at least 1000 m wide ductile shear zone that brings into contact the overlying North Col Formation, subject to low grade metamorphism, and the high grade micaschist and gneiss of the Higher Himalayan Crystallines, characterized by polymetamorphism, from Barrovian to low-pressure/high-temperature conditions. The upper low-angle fault zone is characterized by a metric thick level of cataclasites and brings into contact the slightly metamorphosed Ordovician limestones and the North Col Formation. The lower ductile shear zone is thought to accommodate at least 5 km of vertical displacement, whereas the upper low-angle normal fault zone is thought to accommodate at least 1600 m of vertical displacement. The lower ductile shear zone is thus to be considered the main extensional feature of the STDS in the Rongbuk valley.

Field relations, petrogenesis and emplacement of the Bhagirathi leucogranite, Garhwal Himalaya

Abstract The Bhagirathi leucogranite forms a series of low-angle en echelon, lensoidal intrusions at the top of the High Himalayan slab in the central Himalaya of Garhwal, northern India. The leucogranite comprises the assemblage: K-feldspar + quartz + plagioclase + tourmaline + muscovite ± biotite ± garnet. Compared to other High Himalayan leucogranites it is particularly rich in tourmaline. The granite is generally compositionally homogeneous although it is magmatically banded in both the upper and lower portions. The Bhagirathi leucogranite is situated structurally above the kyanite and sillimanite gneisses of the Vaikrita Group which, in turn, overlie the north-dipping Main Central Thrust zone of inverted metamorphic isograds. A pegmatite — aplite leucogranite sill and dyke swarm is present around the margins of the leucogranite. Vaikrita Group gneisses below the leucogranite contain a pronounced northeasterly component to generally randomly orientated mineral stretching lineations. This reflects localized reorientation of early, coaxially constrained, mineral growth by later, non-coaxial deformation. Various shear criteria in the gneisses immediately below the granite document the existence of a major zone of ductile NNE-SSW-directed extension across a northeastward dipping, low-angle normal fault zone. The top of the 1500–1800 m thick leucogranite sheet exposed on the peaks of Bhagirathi, Shivling and Thalay Sagar are intrusive into Martoli Formation metasediments of the Tethyan sequence which locally contain andalusite and staurolite. The roof complex shows numerous stoped blocks, xenoliths and elongate rafts of host rock in the top 100 m together with an extensive zone of layer-parallel leucogranite veining. The northward-dipping Bhagirathi leucogranite was intruded syn-tectonically during the ductile to brittle transition and has been deformed into linked, en echelon bodies within the extensional shear zone at the interface between the Vaikrita Group gneisses and the Tethyan sedimentary cover. The long axes of the leucogranite lenses lie parallel to the y-z plane of the finite strain ellipsoid for this extensional duplex. During extension, sub-orthogonal dilatational forces exceeded sub-horizontal shear stresses thus facilitating the repeated emplacement of sheeted granite melt, a process analogous to low-angle tension gash development. The final emplacement level in the crust must have been ultimately controlled by the density contrast between melt and country rocks, the thermal blanketing of the Tethyan sedimentary cover, and the extensional stress field along the top of the High Himalayan slab.

Archean volcanism and sedimentation in the Bousquet gold district, Abitibi greenstone belt, Quebec: Implications for stratigraphy and gold concentration

Research on the nature and distribution of Archean rock types in the Bousquet gold district, Abitibi, Quebec, provides new constraints on deposition, stratigraphy, and gold concentration. Facies changes and textures indicate that within the Blake River Group of the district, basalt to the east is more proximal to an eruptive center which shoaled upward, andesite was deposited in relatively shallow subaqeuous conditions as pyroclastic flow deposits and turbidites around volcanic complexes formed on the basalt eruptive center, and dacite accumulated as lava which erupted into a very shallow subaqueous setting. Turbidites in the Kewagama and Cadillac groups originated from the volcanic complexes and accumulated in the form of subaqueous fans, whereas those in the Pontiac Group are from an unknown source and accumulated on an abyssal plain. In this context, the Cadillac-Larder break formed initially as a low-angle normal fault zone. Field relations and petrography of the rocks in the district indicate that the stratigraphic section consists of generally well preserved and largely informable rock units. Opposing stratigraphic tops in the volcanic rocks of the Blake River and Piche groups suggest that they could be correlatives disposed on opposite limbs of a regional-scale syncline, the axial plane of which passes through the Cadillac Group. Accordingly, the rocks of the Blake River, Piche, and Cadillac groups may be part of a vast, largely subsided and reworked volcanic complex. The shallow subaqueous depositional environment of the andesite and dacite in the Blake River Group could have economic significance because boiling of hydrothermal fluid may have contributed to the concentration of gold in the Bousquet gold district.

Mylonite-dominated footwall geometry in a shear zone, central Pyrenees

AbstractA major ductile shear zone in the gneissic core of the Saint-Barthélemy Massif, central Pyrenees, is characterized by an asymmetric shear strain profile defined as mylonite-dominated footwall geometry. The shear zone is part of a low angle fault system in the massif which caused thinning of a sequence of lithologic units and isograds. The shear zone is interpreted as a low angle normal fault zone of probably Cretaceous age, predating Alpine crustal shortening in the central Pyrenees.