Zone Of Shortening Research Articles

Cenozoic evolution of the Central Andes in Bolivia and northern Chile

Abstract The Central Andes in Bolivia and northern Chile form part of a wide and obliquely convergent plate-boundary zone where the oceanic Nazca plate is being subducted beneath the continental South American plate. In the latest Cretaceous and Palaeocene, this part of the Central Andes formed a volcanic arc along what is today the forearc region of northern Chile, with a wide zone of subsidence, as much as 400 km wide, at or close to sea level behind the arc. In the Eocene, the central part of the behind-arc basin was inverted to form a zone of uplift (proto-cordillera), about 100 km wide and along what is today the western margin of the Eastern Cordillera of Bolivia. The Altiplano basin and an early foreland basin were initiated at this time, receiving sediment from the Eocene proto-cordillera. Subsequently, the proto-cordillera widened, as the rate of deformation increased and deformation spread westwards into the early Altiplano basin, and also eastwards towards the Brazilian Shield. In the Late Miocene, deformation essentially ceased in the Altiplano and Eastern Cordillera. An intense zone of shortening was initiated in what is today the Subandean Zone on the eastern margin of the Central Andes, deforming the Oligo-Miocene foreland basin. Shortening in the Subandean Zone accommodated both underthrusting of the Brazilian Shield and also bending of the entire mountain belt about a vertical axis. It is suggested that much of the distinctive Cenozoic tectonic evolution of this part of the Andes is related to pre-Andean strength inhomogeneities in the South American lithosphere.

Sutures and shear zones in the Arabian-Nubian Shield

Deformational belts in the Arabian-Nubian Shield (ANS) are divided into: (1) those associated with sutures, both arc-arc and arc-continental; and (2) post-accretionary structures which include north trending shortening zones and northwest trending strike-slip faults. The arc-arc sutures manifest collision between arc terranes at -800-700 Ma. They are orientated east to northeast in the northern part of the ANS and north to north-northeast in the south. North or south verging ophiolitic nappes are associated with the east to northeast trending sutures. These nappes were steepened by upright folds associated with the final stages of collision between terranes. East or west verging ophiolitic nappes are associated with the north to north-northeast trending sutures. These were deformed by upright folds and strike-slip faults related to oblique collision between terranes and/or post-accretionary deformations. The arc-continental sutures define the eastern and western boundaries of the ANS and are marked by north trending deformational belts which accompanied collision of the ANS with east and west Gondwana at -750-650 Ma. The post-accretionary structures were developed between -650-550 Ma due to continued shortening of the ANS. This produced north trending shortening zones which offset the east to northeast trending sutures in the northern part of the ANS but were superimposed as co-axial deformation on the north to north-northeast trending sutures in the south. The shortening deformation culminated with the development of northwest trending strike-slip faults and shear zones.

Late Quaternary evolution of the Alpine Fault Zone at Paringa, South Westland, New Zealand

Abstract Recent mapping of the Alpine Fault trace in the Paringa region has revealed the existence of an extensive Haast Schist‐derived thrust nappe resting on Western Province basement rock and moraine. Erosion of the nappe by the Paringa River and its tributaries, however, has resulted in eastward propagation of the active fault zone, forming southeast‐dipping thrust faults linked by swarms of steeply dipping strike‐slip faults. Late Quaternary sediments of the Paringa Formation have been intensely deformed along a newly developed zone of shortening and uplift on the northeast side of Paringa River. Marine, fluviatile, lacustrine, and terrestrial sediments record progressive uplift east of the Alpine Fault. The occurrence of lake deposits rhythmically interbedded with forest horizons may have resulted from damming of the Paringa River behind the zone of rapid uplift. The uplift rate for this region over the last 16 ka has been calculated at 13.7 ± 1 mm/yr. After removing the effects of tilting due to lo...

Reinterpretation of seismic reflection data from the Moutere Depression, Nelson region, South Island, New Zealand

Abstract Reinterpretation of an open‐file, reconnaissance seismic reflection survey of the Moutere Depression, integrated with information from two exploration wells, geological maps of the region, and a geophysical survey, has revealed three sub‐basins beneath post‐late Miocene gravels; they are subparallel to Tertiary basins of the West Coast region and have preserved thicknesses of up to 2500 m. One sub‐basin is directly related to late Tertiary outcrop geology, but its extent and geometry were formerly unknown; the other sub‐basins are early Tertiary half‐grabens. Their southeastern margins have been overthrust during mid‐Miocene‐Pliocene, WNW‐directed compression. An axial zone of shortening can be traced from the coast to the Kawatiri Fault, separating Rotoroa Complex and Separation Point Batholith basement rocks in the Moutere Depression. Shortening across the depression was previously more widely distributed, but is now concentrated along the Waimea Fault System, where tectonic loading since the P...

Use of longitudinal strain in identifying driving and resisting elements of landslides

Observations of deformation at the surfaces of landslides in Utah and Hawaii indicate that the upslope parts of the land-slides have stretched and the downslope parts have shortened parallel with the direction of movement. The maximum displacement of each landslide occurs in a relatively undeformed zone between the zones of shortening and stretching. The pattern of deformation at the surface of these landslides may be useful in analyzing their mechanics by helping to constrain the longitudinal forces in limit-equilibrium stability analysis. We used earth-pressure calculations to determine the range of possible longitudinal forces (per unit width) for active failure in the zone of stretching and for passive failure in the zone of shortening of one of the Hawaiian landslides. Longitudinal forces computed by stability analysis, assuming homogeneous strength, exceeded the possible forces in much of the upslope half of the landslide. Consequently, we assumed inhomogeneous strength and adjusted shear-strength parameters at each segment of the slip surface until the longitudinal forces computed by stability analysis agreed with those computed by earth-pressure theory, and the factor of safety approached unity. The distribution of longitudinal forces computed for inhomogeneous strength indicated that the boundary between driving and resisting elements of the landslide is near the thickest part of the slide, in agreement with a simple formula for the location of the boundary.

Post-collisional oblique convergence along the Thelon Tectonic Zone, north of the Bathurst Fault, NWT, Canada

The Thelon Tectonic Zone (TTZ) separates the Slave from the Churchill (Rae) structural provinces of the northwest Canadian Shield. Along the TTZ in the Tinney Hills-Overby Lake area, collision was initiated with northwestward overthrusting of granulite-facies gneiss onto the Slave province accompanied by thinskinned shortening of the Proterozoic cover. In the succeeding transpressive stage, shortening structures assumed a SE-, rather than NW-, vergence; a deformation-front migrated westward into the Slave craton-edge, causing thick-skinned folding of the cover, probably including the detachment; and transpression was partitioned, in craton edge and allochthon, into zones of shortening, shortening + strike-slip, and strike-slip. Ductile deformation along the trace of the late NW-striking brittle Bathurst Fault occurred at this time; this zone links dextral NNE-trending transpressive zones, in which the shortening structures have opposite vergence (SE north of the fault, and NW to the south). The last episode of convergence, tectonic escape, may have been accomplished during brittle deformation on the Bathurst Fault. In the Slave Foreland and the reworked Slave craton-edge, the role of heterogeneous basement deformation is very important. The basement had a complex Archean structure, which was overprinted by systems of Proterozoic shear zones. The bulk strains accomplished by these are expressed as upright folds in the stratiform cover, causing characteristic synclines.

Transition from Convergence to Escape: Field Evidence from the West Carpathians

A large data base of gravimetric, magnetic, seismic, paleomagnetic, lithostratigraphic, sedimentologic, fission-track, bore-hole, and structural information has been used to analyze the structural development of the West Carpathians. These data support a structural model for the evolution of this orogen from convergence to tectonic escape. The West Carpathians resulted from Cretaceous-Miocene convergence of the European and Apulian plates. Paleocene convergence was northeast directed. With progressive deformation, the central mountain front encountered a shallow wedge-shaped portion of the subducted European plate, which caused a sinistral deflection of convergence trajectories in the western portion of the chain. Beginning with Egerian/Eggenburgian collision and continuing to the present time, the orogen has consisted of two zones: a frontal shortening zone and an internal extensional/strike-slip zone. A cycle of deformation patterns is recognized along the frontal part of the orogen in which {sigma}{sub 1} remained normal to the West Carpathian front: orogen-vergent thrusting, intermediate back-thrusting, and strike-slip faulting. Tectonic escape along strike-slip fault sets oriented sub-parallel to the suture zone begun in early Badenian time in the westernmost West Carpathians. The final phase of thrusting becomes younger to the east. The principal compressive stress orientations in areas affected by escape were sub-horizontal and pointed tomore » the location of the last thrust movements along the orogen front. With increased distance from the active collision-suture zone, the principal compressive stress orientations plunged more steeply, indicating a continuous change from a transtensional to an extensional stress regime.« less

Late Cenozoic strike‐slip faulting in the Mojave Desert, California

Recent tectonic models for southern California treat the entire Mojave Desert Block as the site of distributed simple shear during late Cenozoic time. These models consider that much of the region is composed of a series of narrow blocks, bounded by active NW striking, right‐slip faults that have facilitated the distortion and rotation of the region about vertical axes during translations. As much as 100 km of cumulative right slip is predicted for these faults by some of these models. These kinematic models require that the faults of the Mojave Desert Block merge with the Garlock fault, which is viewed as the intact northern boundary that served to accommodate the distortion of the Mojave Desert Block by simple shear. Map‐scale structural relations are used to test explicit and implicit features of kinematic models proposed for the region. These relationships indicate that late Cenozoic NW striking, right‐slip faults of the Mojave Desert Block possess the following characteristics: (1) the faults are discontinuous, with only the Calico‐Blackwater fault spanning the entire Mojave Desert; (2) the faults terminate before reaching the Garlock fault; (3) faults south of an irregular line extending from near Barstow eastward to Ludlow and to Soda Lake are continuous and well developed and have a cumulative net slip of >40 km, whereas faults to the north are discontinuous and display <12 km of right slip; and (4) there is a northwestward decrease in net slip along most of the faults. A new kinematic model is proposed to reconcile these new observations with existing data. We assert that integrated strain within the province since middle Miocene time is not regionally homogeneous as predicted by simple shear models but is instead partitioned into six major domains. The domains probably have deformed and rotated about vertical axes independently of each other and are separated by zones of shortening or extension or by strike‐slip faults. Strike‐slip faults and folding have likely accommodated internal deformation and rotation of some of the domains. The model predicts that the Mojave Desert has been the site of ∼65 km of right shear since middle Miocene time. The broad network of faults of the Mojave Desert Block along with similar strike‐slip faults of the Death Valley region constitute a regional zone of right shear, named here, the Eastern California shear zone. Because of its probable physical connection to the San Andreas fault system, the Eastern California shear zone may have accommodated a significant portion of Pacific‐North American transform motion. The Eastern California shear zone accounts for 9–14% of the total shear, predicted from plate tectonic reconstructions, along the Pacific‐North American transform boundary since ∼10.6 Ma. The kinematic connection of the normal faults of the Death Valley region, with the San Andreas fault system via the faults of the Mojave Desert accords with the deduction of Atwater (1970) that late Cenozoic extension in portions of the Basin and Range province is related to Pacific‐North American transform shear. Finally, the present arcuate trace of the Garlock fault is ascribed to oroclinal folding within the broad zone of distributed shear of the Eastern California shear zone.

Active continental deformation and regional metamorphism

The vertical and horizontal distribution of present-day continental deformation is examined to see how tectonic movements may be related to large wavelength perturbations to the temperature and pressure experienced by rocks in the crust. Earthquakes are generally restricted to the upper part of the continental crust. The lower crust is usually aseismic and assumed to be weaker. The uppermost mantle beneath continental regions has minor seismic activity that does not account for much deformation, but probably indicates an important strength contrast between the lower continental crust and the upper mantle. The maximum focal depth of earthquakes in any region appears to be limited by temperature, with most restricted to material colder than 350± 100 °C in the crust and colder than 700± 100 °C in the mantle. At length scales long compared with the thickness of the brittle upper crust, the deformation in regions of continental extension or shortening appears to be continuous, even though, in reality, discontinuous movement on faults occurs. This probably indicates that the deformation is dominated by distributed flow in the ductile portion of the lithosphere and not by the behaviour of the thin brittle upper crust. The distribution of seismicity, elevation contrasts and vertical movements at the surface suggests that there is little spatial separation between the brittle deformation in the upper crust and the ductile deformation below on length scales larger than the lithosphere thickness. For this reason, and because of the short thermal time constant of the crust, long-wavelength perturbations to the thermal regime are more influenced by the behaviour of the lithosphere as a whole than by the precise geometry of deformation in the crust. Large-scale regional metamorphism in zones of shortening may result from the re-establishment of the initial geotherm in thickened crust when the lower part of the lithosphere detaches and falls into the asthenosphere. In regions of extension, an increased geothermal gradient is unlikely to result in regional metamorphism unless magmatic augmentation to the heat supply is important. However, if the stretched region is covered by thick sediments, the basement may experience a small increase in temperature and remain significantly hotter than it would be if there were no sediment cover. While unlikely to account for significant metamorphism, this effect may strongly influence the rheological behaviour of the lithosphere in extending regions. The rapid vertical movements associated with syn- or post-orogenic normal faulting in regions of large-scale crustal thickening are probably at least as important in exhuming mid-crustal metamorphosed rocks, and in disrupting patterns of isograds, as those associated with erosion.

Basement balancing of Rocky Mountain foreland uplifts

Restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks. Cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts; local zones of shortening and extension occur in hinge zones above transitions in fault curvature. High-curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover. 20 references, 4 figures.

Microearthquake seismicity and active tectonics of northwestern Greece

We carried out a microearthquake survey lasting for six weeks in northwest Greece using 18 portable seismograph stations to examine a region in which normal and thrust faulting have been reported in close proximity to one another. With this array we located 148 events and determined fault plane solutions for eight events using only rays radiated upwards. The seismicity of the region is diffuse with events extending to depths of nearly 30 km, and there is a minimum in activity near a depth of 15 km. The fault plane solutions exhibit a wide spectrum of fault types and orientations and are not consistent with simple zones of shortening or extension. Neither tractions applied to the edges or bottom of the region nor deviatoric stresses that compensate for lateral variations in crustal thickness can account for the variety of fault plane solutions. We think that the complicated behavior is a manifestation of inhomogeneous deformation due, at least in part, to a pre-existing complicated juxtaposition of structures and formations.

Stress and shear fracture (fault) patterns resulting from a suite of complicated boundary conditions with applications to the Wind River Mountains

The Airy stress function is used, via the Principle of Superposition and the series summation concept, to obtain stress states in a static, self-gravitating elastic beam subjected to boundary stresses. The boundary conditions investigated are more complicated than those previously published and include cases with sawtooth-, step-, and sinusoidally-shaped lower-boundary loads, with and without additional tectonic end leads. Potential shear fracture (fault) patterns derived from the calculated stress fields indicate co-existing (simultaneous) regions of lateral shortening and extension. Application of one of the cases to the study of the structural geometry of the Wind River Mountains of Wyoming yields a good ‘fit’ and forms a possible explanation for the observed rotations and zones of shortening and extension.