Rolling Hinge Research Articles

Fluid inclusion constraints on the kinematics of footwall uplift beneath the Brenner Line normal fault, eastern Alps

Dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear. Each class of model predicts a different incremental strain history that should be recorded in the footwall. In the Tauern Window (eastern Alps), postmylonitic structures in the footwall of the Brenner Line normal shear zone predominantly consist of closely spaced, steep, west down and east down microfaults. Formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear. In contrast, formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes. The field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed. However, both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion‐lined microfaults with the macroscopic west down and east down faults, obtaining density data for the inclusions, and correlating these data with previously obtained geochronologic data. The results indicate that the west down structures formed at depths of 10–20 km and temperatures >450°C in the mid to late Oligocene and that the east down structures formed at 2‐ to 10‐km depth and temperatures of 300 ± 50°C in the mid‐Miocene. These data support the hypothesis that a "rolling hinge" was present in the footwall of the Brenner Line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear. The depths at which west down and east down faulting occurred, coupled with the angle of dip of the Brenner Line, yield a minimum lateral displacement on the fault of 15–26 km. Approximately coeval ductile shearing and brittle faulting at depths of 15–20 km and temperatures in excess of 400°C may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation.

Is the Sevier Desert reflection of west-central Utah a normal fault?: Comment and Reply

Is the Sevier Desert reflection of west-central Utah a normal fault?: Comment and Reply

Postmylonitic deformation in the Raft River metamorphic core complex, northwestern Utah: Evidence of a rolling hinge

Much of the recent debate over low‐angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model. The model predicts tilting of seismogenic high‐angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation. This shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex. The predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms. Two end‐members have been proposed: subvertical simple shear and flexural failure. Each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction. Subvertical simple shear (SVSS) should generate subvertical faults and kink bands with a shear sense antithetic to the detachment. For an SVSS hinge, the hinge‐related strain magnitude should depend only on initial fault dip; rolling hinge structures should shorten the mylonitic foliation by >13% for an initial fault dip of >30°. In flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses. Resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation. Extensional sets could predominate as a result of superposition of far‐field and flexural stresses. Strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam. Postmylonitic structures were examined in the footwall of the Raft River metamorphic core complex in northwestern Utah to test these predictions. Observed structures strike perpendicular to the regional extension direction and include joints, normal faults, tension‐gash arrays, and both extensional and contractional kink bands. Aside from the subvertical joints, the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets. Range‐wide, the extensional structures accomplish about 4% elongation of the mylonitic foliation. Contractional structures dip steeply, mainly record shear antithetic to the detachment, and accomplish <1% contraction of the foliation. These observations are consistent with the presence of a rolling hinge in the Raft River Mountains, but a rolling hinge that reoriented a high‐angle normal fault by SVSS is excluded. The pattern and magnitudes of strain favor hinge‐related deformation mainly by flexural failure with a subordinate component of SVSS.

Subduction and eduction of continental crust: major mechanisms during continent‐continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides

ABSTRACTDuring continental collision in the middle Silurian, the thickness of the lithosphere under the Caledonides of S. Norway was doubled by subduction of the western margin of Baltica, including the Western Gneiss Region, under Laurentia. Crustal rocks of the Baltic plate reached sub‐Moho depths of near 100 km or more as inferred from the presence of coesite in eclogites. Isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage. The subducted lithosphere experienced vertical constrictional strains as a result of slab‐pull by its heavy and cold root. Eduction of the deeply buried crustal material was initiated by decoupling of the Thermal Boundary Layer in the subducted lithosphere. Isostatic rebound resulted in very rapid uplift (1–2 mm yr‐1), and the deep crust was exhumed, mainly by tectonic extensional stripping over a period of 30–40 Myr. The eduction was probably related to a rolling hinge, footwall uplift mechanism, and the early high‐pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels. The model explains the present‐day normal crustal thickness under the exhumed deep rocks without necessarily invoking large‐scale lateral flow of material in the lower crust or igneous underplating.

Tertiary extension and contraction of lower‐plate rocks in the Central Mojave Metamorphic Core Complex, southern California

Several authors recently have proposed the “rolling‐hinge” model for the formation of extensional detachment faults in metamorphic core complexes. In this model, isostatic uplift of the denuded footwall of a major normal fault causes the near‐surface part of the fault to rotate from a higher to a lower dip. For the rolling‐hinge model to be acceptable, the footwalls of detachment faults must have undergone the strain pattern dictated by the rolling hinge. In this paper, the structural development of the footwall of the central Mojave metamorphic core complex is used as a field test. Tertiary structures in the lower plate of the core complex record NE‐directed ductile extensional shear followed by cataclastic NE‐side up shear with a significant component of contraction along the previous extension direction. This strain history agrees with the rolling‐hinge model, and particularly with a version of the model in which footwall uplift occurs by a combination of elastic flexure and flexurally driven upper‐crustal failure. Similar late‐stage layer‐parallel contraction appears to be present in some other tectonically denuded terranes of the Basin and Range, suggesting that the flexural‐failure rolling‐hinge model for extensional terranes may be widely applicable.