The Ribblesdale fold belt, NW England—a Dinantian-early Namurian dextral shear zone
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Summary Folds and faults in various en échelon patterns affect the Tournaisian-early Namurian sedimentary rocks of the Craven basin, a rift basin which includes Dinantian and possibly late Devonian strata some 5 km thick. The structures are interpreted as a response to regional dextral shear, which accompanied sedimentation from mid-Dinantian to early Namurian times at least. The principal Ribblesdale folds are viewed as products, in the Carboniferous cover, of primary wrenching on inherited fractures in the Lower Palaeozoic basement. Some en échelon sets of minor folds are regarded as products of secondary wrenching in the axial zones of ‘primary’ folds. The block-basin transition zone south of the North Craven fault appears to have been affected by dextral transtension during the late Brigantian (late Dinantian).The NNE-trending Isfjorden-Ymerbukta Fault Zone is an oblique structural element within the NNW-trending Tertiary transpressional fold-thrust belt of Spitsbergen, Arctic Norway. It can be traced for nearly 50 km, and separates two different structural domains in the fold-thrust welt of Oscar II Land, Central Spitsbergen. The fault zone is more than 500 m wide and contains several segments of highly folded/rotated, faulted and cleaved Triassic through Paleocene rocks. Displacement across the fault zone can be decomposed into (i) a reverse, top-to-the-ENE component with a minimum 650 m vertical throw, and (ii) a horizontal dextral component of approximately 5-10 km. Displacement across the fault decreases northward along strike, where the fault zone merges into parallelism with a ramp-system of the fold-thrust belt. Inherited, underlying Devonian(?)-Carboniferous structures may have controlled the location of the fault zone. Detailed studies of map and mesoscale faults and folds reveal complex geometries and varied kinematic signatures across the fault zone width. Along the southwest portion of the fault zone (Ramfjellet-Erdmannflya) three major fault segments record oblique-reverse (Morenekilen fault), combined oblique-reverse and oblique-normal (Straumhallet fault), and dextral strike-slip (Flydammane fault) movements, respectively. Further northeast (Mehøgda-Bohemanflya), a traverse from west to east of three structural domains shows; (1) thrusts and associated folds that record oblique-reverse kinematics, (2) steep faults with dextral strike-slip and conjugate strike-slip (extrusion) movements, and (3) thrusts and oblique-normal faults. The overall kinematics is consistent with mainly oblique-reverse and dextral strike-slip faulting, and subordinate local fault zone-oblique/parallel extension. Various geometries within the fault zone, as well as the variation in the direction of movement on the segments, can be explained from either (i) synchronous shortening and out-of-the-plane movement partitioning, or (ii) an effect of polyphase changes in the orientation of the overall shortening axes during the fold-thrust belt evolution. Either of these interpretations is consistent with the Isfjorden-Ymerbukta Fault Zone as an oblique-thrust ramp or transfer fault.
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The area around Taemas Bridge in the Gilgandra‐Cowra‐Yass Zone, southwest of Yass, contains Devonian limestone, silicic volcanics and terrestrial sedimentary rocks that are folded by four deformations of inferred Carboniferous age. Interference between folds is well developed, mainly as Type 1 and Type 2 interference patterns. The two most prominent fold trends can be found more widely throughout the Lachlan Fold Belt as northwest/north‐northwest and north/north‐northeast regional trends. Early folds may be more localised. The fold axes associated with consecutive generations display an anticlockwise directional sequence, suggesting that incremental strain axes rotated anticlockwise during the Carboniferous deformation. North‐northwest‐trending faults in the area are inferred to have moved by both sinistral strike‐slip and reverse mode at different times. Small‐scale structures, such as veins, vein arrays and tectonic stylolites, are well developed in the Lower Devonian limestones. These structures can be correlated with, and indicate the 3-D incremental strain directions and kinematics of, the fold events. Early folding shows a predominantly wrench style of deformation and appears to be related to wrench motion on the bounding faults. North‐northeast‐trending F2 folds are 30° clockwise to faults and, together with associated small‐scale wrench indicators, suggest sinistral shear on these faults. Northwest‐trending F3 folds are associated mainly with reverse faulting, indicating a change in kinematic style. These are in turn overprinted by wrench motion associated with ?minor north/north‐northwest compression. The results of this study suggest a multiphase contraction and wrench history that is more complex than previously proposed for this part of the Eastern Lachlan Fold Belt. Keywords: deformation (structural geology)KanimblanLachlan Fold BeltNew South Walesreverse faultsstrike‐slip faultsstylolitesTabberabberanTaemas Bridgeveins
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Summary We interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.
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The north-northeast trending Sellarsville and Rafting Ground faults are southeasterly directed Acadian (Devonian) thrusts in the Québec Appalachians. They are located at the western end of the Grand Pabos fault system, a dextral strike-slip fault system that transects Upper Ordovician to Lower Devonian sedimentary rocks in the southern Gaspé Peninsula. The structural analysis of mesoscopic brittle and brittle–ductile shear zones by graphical methods was used to determine the stress field related to these two faults. The attitude of slip lines was calculated when the slickenside striations were not observed on the movement plane. Conjugate faults, Arthaud's method, and Angelier and Mechler's method were used to determine the paleostress. The maximum principal compressive stress σ 1 , always subhorizontal and striking west-northwest – east-southeast, is perpendicular to the Sellarsville and Rafting Ground faults and was probably the cause of the thrusting motion along the faults. North-northeast-trending regional folds and cleavage could also be related to this same stress. Geological mapping and structural cross sections confirm the southeasterly directed thrust motions, which are well integrated in the Grand Pabos fault system. Sellarsville and Rafting Ground faults with the Restigouche fault may represent a leading contractional imbricate fan in a dextral strike-slip system. [Journal Translation]
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Thrust fault
Devonian
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Hornblende
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Abstract P‐wave azimuthal anisotropic tomography reveals that the July 6, 2019 M w 7.1 Ridgecrest earthquake occurred in a region with clockwise crustal rotation. The rotation together with the sinistral slip on the Garlock Fault is a response to the northwest‐trending, dextral shear within the Eastern California Shear Zone due to the relative motion between the Pacific and North America Plates. The hypocentral area of the Ridgecrest mainshock is characterized by a sharp lateral velocity contrast which has a reversal in contrast polarity at about 5 km depth. We find high Vp/Vs ratio structures covering the rupture zones of the M w 6.4 foreshock and the M w 7.1 mainshock, which may indicate the existence of fluids in the fault zones. We speculate that fluids and crustal rotation may have played important mechanical roles in causing the 2019 Ridgecrest earthquake sequence.
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Shearing (physics)
Echelon formation
Orogeny
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