Broad Shear Zone Research Articles

Deformational and strain patterns of an intracontinental collision ductile shear zone—an example from the Higher Garhwal Himalaya

The Higher Himalayan metamorphic belt of Garhwal appears to have evolved in a major 15–20 km wide, NE-dipping ductile shear zone of the overthrust-type due to intracontinental crustal shortening during Cenozoic continental collision within the Indian Plate. Out of the four major distinct deformation phases, the most widespread D 2 deformation phase is marked by a prominent S 2 foliation axial-planar to the reclined and recumbent F 2 folds, a coaxial NE-plunging L 2 stretching lineation and syntectonic growth of index metamorphic minerals. These deformational structures are developed during the SW-directed D 2 ductile shear deformation, irrespective of the orientation of the S 2 foliation and thrust zones. Regional strain patterns in quartzite of the Lesser Himalayan Garhwal Group beneath the Main Central Thrust (MCT) and the Central Crystalline Zone using R f/ φ data on quartz clasts and aggregates reveal that the maximum values of finite strain ε s are attained within narrow mylonitic shear zones within the broad ductile shear zone. During its progressive translation towards higher structural levels in late D 2 and D 3 deformation phases, the shortening was accommodated by development of thrust sheets with the MCT and Jutogh Thrust surfaces coinciding with zones of mylonite and maximum strain. At higher levels, ductile shear zones became compressional brittle-ductile zones which resulted in the development of SW-verging F 3a and F 3b folds during the southward migration of the thrust sheets in the Lesser Himalaya.

Victoria Lower Glacier and Ross Sea Glaciation, Dry Valleys Area, South Victoria Land, Antarctica

AbstractVictoria Lower Glacier is a complex structure of ice from two distinct sources (Schultz Glacier to the north and a localnévéof Victoria Lower Glacier) that join at a broad median shear zone. Evidence from the margins suggest that both are currently retreating. Algae in a block of frozen stratified sediment from within the ice of the terminal margin has a radiocarbon age of 20 200 ± 2400 year BP (NZ 6531 A), indicating that the glacier has advanced since that time. Superposition of ice levels of Ross Sea I glaciation on a radio echo–sounding profile of bedrock beneath the glacier indicates that it is unlikely that Ross Sea I ice entered the valley. The radiocarbon date supports this finding.

Victoria Lower Glacier and Ross Sea Glaciation, Dry Valleys Area, South Victoria Land, Antarctica

AbstractVictoria Lower Glacier is a complex structure of ice from two distinct sources (Schultz Glacier to the north and a local névé of Victoria Lower Glacier) that join at a broad median shear zone. Evidence from the margins suggest that both are currently retreating. Algae in a block of frozen stratified sediment from within the ice of the terminal margin has a radiocarbon age of 20 200 ± 2400 year BP (NZ 6531 A), indicating that the glacier has advanced since that time. Superposition of ice levels of Ross Sea I glaciation on a radio echo–sounding profile of bedrock beneath the glacier indicates that it is unlikely that Ross Sea I ice entered the valley. The radiocarbon date supports this finding.

Tectonic boundaries within the Grenville province of the Canadian shield

Recent work in a previously little-known part of the Central Gneiss Belt of the southwestern Grenville Province has outlined a regional structural pattern made by a number of geologically distinctive domains that are separated by broad ductile shear zones. Characteristics rock assemblages, metamorphism, geophysical signatures and structural trends are modified or truncated at domain margins. Bounding high strain zones contain mylonite and other forms of tectonically modified gneiss, formed under at least middle amphibolite facies conditions, within which kinematic indicators imply a northwesterly sense of overriding or lateral sliding between adjacent crustal masses. Similar kinematics also apply at the margin of the Central Metasedimentary Belt to the southeast and along the Grenville Front to the northwest. A scenario involving northwesterly stacking of large crustal blocks and slices at relatively deep level can account for the observed relationships and implies a period of crustal thickening that may represent the culmination of the Grenvillian Orogeny in this region.

Neotectonic deformation patterns in the convex-northwards arc of the North Anatolian fault zone

Summary The convex-northwards arc of the North Anatolian fault zone between Çerkeş and Erbaa contains structures and landforms permitting right-lateral displacements for several time intervals to be estimated. Within the Pontus Formation (Late Miocene-Early Pleistocene) in basins along the fault zone, an unconformity, representing a time interval from the latest Tortonian to the earliest Pliocene, is interpreted as marking the transformation of the structure from a broad shear zone to a narrow fault belt. Although the amount of displacement during the shear zone phase is unknown the offset of a sedimentary facies boundary in the Lower Pontus Formation of the Havza-Ladik basin demonstrates that since the latest Tortonian there has been 25 km of right-lateral slip. The offset of valleys and ridges suggests that there has been 8 km of Quaternary displacement, about 2 km of it in the late Quaternary and at least 500 m during the Holocene. Structures in the Pontus Formation indicate that the early history of the western Neogene basins was influenced by regional compression and that the later histories of all basins were dominated by strike-slip displacements. The main active trace is discontinuous, the 2 km-wide belt containing subordinate en échelon faults whose geometry is consistent with development during right-lateral shear.

Neogene non‐marine sedimentation and tectonics in small pull‐apart basins of the San Andreas fault system, Sonoma County, California

ABSTRACTThe San Andreas fault system in northern California forms an 80–90 km wide zone of right‐lateral shear. Extensional tectonism within this broad shear zone is indicated by both Neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north‐west and by numerous Neogene faultbounded structural basins filled with thick non‐marine sequences.The Little Sulphur Creek basins, three well‐exposed 1·5–2 km wide pull apart basins within this shear system, have sedimentation patterns analogous to those of much larger pull‐apart basins. They were formed and subsequently deformed by east‐west extension and by north‐west to south‐east‐orientated right‐slip concurrently with basin filling. Palaeocurrent and maximum‐clast size data indicate both lateral sediment transport from fault‐bounded basin margins and longitudinal transport down the basin axes.The basins are filled primarily with coarse alluvial‐fan and streamflow deposits derived from a surrounding igneous, sedimentary, and metamorphic provenance. Two of the basins contain basin‐plain‐type lacustrine turbidites that grade laterally into distal alluvial fan, fan‐delta, and sublacustrine delta deposits. Talus deposits along the south‐west margin of the basins contain megabreccia indicative of active uplift. Structures indicative of dewatering, liquefaction, and slumping suggest penecontemporaneous tectonism.

Plate margin transition from oceanic arc-trench to continental system: The Kermadec-New Zealand example

The East Coast Fold Belt (ECFB) of the North Island, New Zealand, is the continuation of the Tonga-Kermadec arc-trench system. Structurally its tectonic front to the east defines the Indian-Pacific plate boundary. This, however, is not continuous with the Kermadec Trench. Large-scale fragmentation of the ECFB into segments of greatly varying width, strike and structure may be caused by a strongly segmented subducting plate, individual segments of which strike in different directions and have different dips and rates of subduction. Towards the southwest, regional change of strike with respect to plate motion has resulted in the formation of a broad shear zone marked by a strongly subsiding trough filled with rapidly deposited, largely undeformed sediments in front of the ECFB. This foredeep (Hikurangi Trough), which thus occupies the gap between ECFB (Indian plate) and the continental Chatham Rise (Pacific plate) is gradually being involved in the overall deformation, due to continuing motion of the Pacific plate to the southwest, in a slightly oblique sense along the shear zone. As a result, the Hikurangi Trough is shifting with time to the east-northeast. From a tectonic, structural and morphological point of view, it is unrelated to the Kermadec Trench which terminates in the region of East Cape. The structure of the ECFB is characterized mainly by extension normal to the plate boundary, with regional tilting and down-faulting of the continental margin. Effects of compression are observed only locally, and are often due to diapiric uplifts caused by widespread, undercompacted shale. Such diapirs form elongate structural highs which in many cases have supplied sediments into adjacent basins on their landward side. Overall the continental slope and margin are underlain by land-derived sediments which exhibit in-place deformation. Locally they extend as undeformed sediment aprons beyond the tectonicfront. There is no compelling evidence of a subduction complex of imbricate thrust slices. It is concluded that the tectonic evolution is not controlled by accretion but rather by subsidence and tectonic erosion along the continental margin. The conditions are complicated, however, because of the discrete change from an oceanic arc-trench subduction system to an intercontinental shear zone.

Ductile shear zones: some aspects of constant slip velocity and constant shear stress models

Summary. The thermomechanical differential equations governing deformation in viscous shear zones have been solved for both constant velocity and constant stress boundary conditions. The solutions show that the inertial term in these equations can be neglected everywhere. The starting condition of the constant velocity model has been shown to be a constant velocity gradient and not a Heaviside function. The temperature anomaly produced by shear heating at the centre of the shear zone is shown to increase gradually and continuously with time, not reaching an asymptotic value. Conclusions for the constant velocity boundary condition are otherwise generally similar to those presented by Yuen et d. and agree with Fleitout & Froidevaux. The temperatures reached by constant velocity shears are sufficient for partial melting. Constant stress boundary condition shear zone models show an initially broad shear zone with uniform shear velocity gradient. Depending on the level of applied shear stress and ambient temperature, localized intense shear heating may develop followed by thermal runaway. At lower ambient temperatures relatively high stresses are required to produce thermal runaway. The broadening of the constant velocity shear zone proceeds more rapidly with increased ambient temperature. This can be used to show that shear zones broaden with depth. The merging of parallel shear zone pairs has been investigated and shear zones separated by distances of less than lOkm coalesce to form a single shear zone within 3 Myr. Only shear zones separated by 50 km or more remain distinct over periods of tens of millions of years.

Paleomagnetic Study of Neogene Tectonic History of Baja California, Mexico: ABSTRACT

Paleomagnetic study of 20 Miocene (19 to 5 m.y.B.P.) volcanic flows and one dike in central and southern Baja California has resulted in determination of inclinations too shallow for the present latitude at all sites, suggesting a greater amount of northward movement than can be accounted for by previous End_Page 1697------------------------------ studies. Natural remnant directions from 21 units at five sites (24 to 29° north lat.) suggest an average northward translation of roughly 10° since late Miocene time, and a probable 45° clockwise rotation (post 6 m.y.B.P.) of the San Ignacio flows. The Paleomagnetically indicated rate of absolute motion of the peninsula is 11 cm per year since 5 m.y.B.P. and 3.5 cm per year prior to 5 m.y.B.P., assuming an offset axial dipole. Absolute northward motion, assuming a geocentric axial dipole, is 18 cm per year from 0 to 5 m.y.B.P., and 3.5 cm per year from 5 to 19 m.y.B.P. The rates of northward motion described by Atwater and Molnar, and Dickinson for the same time spans are 3.5 cm per year and 1.5 cm per year, respectively. Possible solutions to this discrepancy are: (1) Baja California is part of a broad shear zone of the plate margin, and has had more movement along faults within the proto-gulf and the present margin of western Mexico than previously deduced, (2) the North American plate has no northward motion, or (3) the North American plate has had northward motion since the Miocene, with the amount of motion of the plate margin being equal to that described by T. Atwater and P. Molnar; thus, the paleomagnetic data show both motions. Studies by M. J. Kamerling and B. P. Luyendyk, and continued paleomagnetic studies at San Diego State University, show a comparable amount of northward motion for southern California and northwestern Mexico during the Miocene. Paleomagnetic results would imply that the present palinspastic reconstructions have not completely resolved the tectonic framework of the Pacific-North American plate boundary. End_of_Article - Last_Page 1698------------

Gravity and structural evidence for the deep structure of the limpopo belt, southern africa

Gravity data have been used together with surface structural geology to examine the configuration of major shear zones and the position of high-grade gneisses beneath the Early Proterozoic Limpopo mobile belt of southern Africa. The structural and geophysical data demonstrate the asymmetry of the Limpopo belt with a suggested model of overthrusting of the Archaean Kaapvaal craton in South Africa over the Rhodesian craton, along a broad ductile shear zone which outcrops along the northern edge of the belt. The granulites at the northern edge of the belt have been uplifted by thrusting along this shear zone. Granulites at the southern margin of the belt may have been uplifted by shearing and faulting along the line of the Soutpansberg fault or they may have been upwarped due to the change in dip of the underlying shear zone. Amphibolites in the central part of the belt rest on a gently dipping shear zone which moved the southern part of the Limpopo belt to the west relative to the northern part. This movement is possibly much later than that which uplifted the granulites.

Deformation and exhumation of the Higher and Lesser Himalayan Crystalline sequences in the Kumaon region, NW-Himalaya based on structural and fission-track analysis

DOI = 10.3126/hjs.v5i7.1294 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.101-102