Crustal Contraction Research Articles

The role of extension in the Miocene denudation of the Nevado‐Filábride Complex, Betic Cordillera (SE Spain)

The Internal Zone of the Betic Cordillera, SE Spain, consists of a nappe stack of three complexes, the deepest of which is the Nevado‐Filábride Complex. The zone is separated from the overlying Alpujárride Complex by a crustal scale shear zone that has variously been interpreted as a thrust or an extensional detachment. A suite of 74 new apatite and zircon fission track results have been obtained from the Nevado‐Filábride Complex and these have been used to define regional cooling patterns for the complex. Rapid cooling (105°C–200°C Ma−1) is spatially related to the tectonic contact with the overlying Alpujárride Complex‐Cooling to near‐surface temperatures occurred first in the east (Sierra de los Filabres) during the mid‐Serravallian (12±1 Ma) and was completed by the early Tortonian (9–8 Ma) in the west (Sierra Nevada). There is no correlation between fission track age and sample elevation. These results are consistent with tectonic unroofing of this complex, a finding that favors extension as the mechanism by which the two complexes were brought into contact. Extension spans the middle and earliest upper Miocene (12–8 Ma) in the study area and therefore lasted much longer than previously documented. A hypothesis is advanced which links oblique convergence between the Iberian plate and the Betic Internal Zones, resulting in crustal contraction at depth, with orogen parallel extension in the middle and upper crust.

Dynamic interactions between crustal shortening, extension, and magmatism in the North American Cordillera

This paper examines the first-order dynamic interactions between crustal shortening, extension, and volcanism in tectonic evolution in the North American Cordillera. The protracted crustal compression in the Mesozoic and early Cenozoic (110–55 Ma) contributed to the subsequent Tertiary extension by thermally weakening the lithosphere and producing an overthickened (>50 km) and gravitationally unstable crust. In addition to post-kinematic burial heating, synkinematic thermal processes including conduction are shown significantly because of the long period of crustal contraction and the slow shortening rates (<4 mm/yr). The effects of shear heating were probably limited for the same reasons. Localized delamination of the lithospheric mantle may have contributed to the abundant plutonism and high crustal temperature in the southeastern Canadian Cordillera at the end of the orogeny. Most early-stage extension in the Cordillera, characterized by formation of metamorphic core complexes, resulted from gravitational collapse of the overthickened crust. Plutonism may have facilitated strain localization, causing widespread crustal extension at relatively low stress levels. Crustal collapse, however, was unlikely the direct cause of the Basin-Range extension, because the gravitational stresses induced by crustal thickening are limited to the crust; only a small fraction of the gravitational stresses may be transmitted to the lithospheric mantle. Nor could core complex formation induce the voluminous mid-Tertiary volcanism, which requires major upwelling of the asthenosphere. While the causes of the asthenospheric upwelling are not clear, such processes could provide the necessary conditions for the Basin-Range extension: the driving force from thermally induced gravitational potential and a thermally weakened lithosphere. The complicated spatial and temporal patterns of volcanism and extension in the Basin and Range province may be partially due to the time-dependent competing effects of thermal weakening and rheological hardening associated with intrusion and underplating of mantle-derived magmas.

The reflection Moho beneath the southern Canadian Cordillera

The transition from the crust to the mantle beneath the Canadian portion of the North American Cordillera varies in depth, geometry, and tectonic age across the orogen. These variations are rarely spatially related to the positions of morphologic or tectonic belts based on surface geology, nor to nearly 25 km of structural relief identified in outcrop and on seismic reflection data. The Moho in this region is thus interpreted to be a long-lived feature, perhaps as old as Proterozoic in the eastern part of the Cordillera, that probably has been active as a structural boundary during periods of crustal contraction and subsequent crustal stretching. Recognition of the Moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural levels beneath the interior of the orogen: they likely project to the base of the crust, where they flatten and cause imbrication of crustal rocks.

A critical review of tectonic processes at continental margin orogens

A “cordilleran-type” orogen has previously been defined as contractional deformation of a continental margin due to subcontinental subduction. The plate tectonic setting of cordilleran-type orogenesis is fundamentally different from that of “collisional” orogenesis. Intracontinental orogens such as the Himalayas and Alps form by collision between a continental margin and a continental margin subduction zone. Orogens at continental margins may form due to subcontinental subduction (cordilleran-type orogeny), or due to collision of a continental margin with an intra-oceanic subduction zone which is typically followed by a reversal of subduction polarity. In the latter case, collisional orogenesis may proceed along with the reversed, subcontinental subduction. Modern arcs are mostly extensional or neutral, so subduction hangingwall contraction is not a necessary consequence of subduction. Crustal contraction in known collisional orogens lasts for tens of millions of years, so collisional deformation at a continental margin may last for tens of millions of years after arc-continent collision and subduction reversal. Collisional orogenesis is a predictable consequence of subduction, and collisional deformation is kinematically linked to the kinematics of subduction. Noncollisional cordilleran-type orogenesis is not predicted by plate theory. A geologic history that would demonstrate noncollisional orogenesis is a magmatic arc built into a passive margin sequence with strong deformation but without an earlier arc or ophiolite accretion event; this history has not been recognized in the geologic record. The classic examples of cordilleran-type orogens, including the Cretaceous-Cenozoic of the Andes and the Jurassic-early Tertiary of the western United States, may be interpreted as arc-continent collisions. However, the youngest increments of deformation in these two orogens occurred 100 million years after the youngest collisions and are thus difficult to ascribe to collisional processes. Notwithstanding the long time scales, these deformations occurred at the end of space-time continua of probable collisional orogenies. Alternatively, these deformations may represent lithosphere-scale fault-bend folding, duplex folding, backthrusting, and/or out-of-sequence forethrusting during shallow subduction.

The Cowichan fold and thrust system, Vancouver Island, southwestern British Columbia

The Cowichan fold and thrust system is a southwest-verging, linked thrust system formed by northeast-southwest, large-scale, crustal contraction involving the Upper Cretaceous Nanaimo Group and its Wrangellian basement. The geometry of the thrust system from plan and profile perspectives illustrates its thick-skinned structural style. The thrust system is interpreted to be a leading imbricate fan. Thrusting was most likely in sequence, creating an estimated minimum 20%-30% shortening at the basement/cover interface. Both fault-propagation and fault-bend folding are evident, the former being more common. Kinematic indicators and the geometry of the thrust system show no evidence of significant transpressional or transtensional displacement fields. Contraction postdates deposition of the Nanaimo Group and is indirectly dated as late Eocene. The sole fault of the thrust system is interpreted to rise from northwest to southeast on a series of deep lateral to oblique ramps and merge with the San Juan fault system. This explains the rapidly westward-thickening wedge of thrusted Wrangellian basement in the study area. In the east, the sole fault is interpreted to have locally permitted the Nanaimo Group to overthrust the San Juan Terrane. Investigation of the Cowichan fold and thrust system reveals how shortening during westward-progressing terrane accretion is accommodated in a forearc region dominated by a large slab of rigid, semicontinental crust.

Large earthquakes and crustal deformation near Taiwan

Source parameters of 15 large (mb ≥ 6) earthquakes which occurred near Taiwan during the last 25 years are determined by synthetic seismogram analysis of teleseismic P and S waveforms. Source depths range from 13 to 88 km. Three earthquakes of intermediate depth are located within the southernmost limit of the Ryukyu arc and show thrust‐type mechanisms with T axes subparallel to a northward dipping Wadati‐Benioff zone. Shallow earthquakes (&lt;30 km) are distributed over a broad region along and off the east coast of Taiwan and show both high‐angle reverse and strike‐slip mechanisms. The P axes of the shallow events located off the east coast of northern Taiwan are oriented near horizontal, trend south to southwest, and may reflect relative motion between the Okinawa platelet and Taiwan or complex deformation at the southwest limit of the Ryukyu Trench. In contrast, P axes of shallow earthquakes both onshore and off the east coast of central and southern Taiwan consistently trend northwest and are interpreted to reflect relative convergence between the Philippine Sea and Eurasian plates. Summation of the seismic moment tensors of eight large shallow earthquakes that occurred along and off the central and southern coast of Taiwan shows that relative plate motion between the Eurasian and Philippine Sea plates, when averaged over the last 25 years, has been accommodated by crustal contraction at a rate of 2.6–5.4 cm yr−1 along an azimuth of about 290°.

‘Caterpillar tectonics’ in the Harts Range area: a kinship between two sequential Proterozoic extension-collision orogenic belts within the eastern Arunta inlier of central Australia

We propose that in the Harts Range area of the Arunta Inlier of central Australia, a basement and cover relationship previously described by us, represents a thick complexly deformed contact zone between an Early-Middle Proterozoic fold belt, the Strangways Orogenic Belt (SOB) and a younger Middle Proterozoic orogenic belt, the Harts Range Orogenic Belt, both of which contain their own supracrustal sequences. Rocks of the SOB were deposited and possibly metamorphosed to a maximum grade of granulite facies at ∼ 1800 Ma during an early phase of crustal extension. They were subsequently intruded by a significant proportion of acidic magmas at 1767 Ma and compressed to form a distinctively NNE-SSW trending orogenic belt. A WNW-ESE trending shallowly north-dipping zone of intense and repeated deformation, termed the Harts Range Detachment Zone (HRDZ), formed initially during a period of NNE-SSW crustal extension as a major low-angle normal shear/detachment zone within, and crosses at a high angle to the trend of, the basement rocks of the SOB. The HRDZ has been intermittently active as a zone of crustal weakness since its initiation. The first extensional movement on this zone began the formation of the Harts Range Mobile Belt (HRMB) which displays many of the characteristics of modern tectonic zones activated by plate movement. A supracrustal cover sequence, the Harts Range Cover (HRC), formed in the major geosynclinal basin which developed on the subsiding upper plate of the HRDZ. Continued crustal extension juxtaposed the cover sequence, which was then metamorphosed to a maximum grade of amphibolite facies, and granulite facies basement of the HRDZ, before a fundamental change in movement sense led to crustal contraction and a collision-related, thrusting phase. The HRMB which includes the HRDZ and the HRC, was then internally deformed over a relatively very short time span (15–20 Ma), before voluminous granitic magma intruded mainly along the HRDZ, during the Harts Range Orogeny (HRO). The intrusion of this major granitoid at 1747 Ma precisely fixes the time-scale of evolution for the earlier phases of the HRO. The HRO is documented in detail as a peculiar orogenic style of crustal contraction which was largely taken up by reverse-sense, discrete thrusting and shearing on faults and shear zones associated with the HRDZ. Displacement on the HRDZ was a stick-slip process with repeated punctuation of thrust-slip movement by pinning at points located progressively to the south. Each period of cessation of thrust-slip movement was followed by folding of discrete thrusts, of captured basement lobes within the HRDZ and of the overlying supracrustal sequence. The individual discrete movement phases and folding episodes are each named and described, as is a model of this unusual type of Proterozoic tectonic activity.

A comparison of the cratered terrain escarpments on Miranda with the thrust faults on Mercury

Large escarpments on the Uranian satellite Miranda are similar in structure to thrust faults on Mercury result of crustal contraction. The tectonic lineations may be considered evidence of a crustal contraction on Miranda result of differentiation of the satellite.

Late Neogene and Quaternary tectonics associated with northward growth of the San Andreas Transform Fault, northern California

Strain patterns within the forearc at a convergent margin adjacent to a passing fault‐fault‐trench triple junction show a systematic evolution. We describe late Neogene and Quaternary deformation in northern coastal California in the last 3 m.y. associated with the migration of the Mendocino triple junction. Deformation in the North American forearc near Humbolt Bay consists of a 30‐km‐wide zone of on‐land contraction with a narrower zone of translation to the east. The zone of subaerial contraction is part of the relatively wide, southernmost extent of forearc contraction within the Cascadia subduction zone. Net Quaternary northeast‐southwest contraction across forearc thrust faults is at least 7.9 km, and minimum fault slip rates on the six major thrust faults are 0.8–2.3 mm/yr. Net right slip within the zone of translation is a minimum of 3 km. South of the forearc, translation is predominant in the North American plate and is accommodated along two major right‐lateral fault zones of the San Andreas transform boundary, the Lake Mountain, and Garberville fault zones. Discontinuous exposures of late Neogene sediments along these faults near Covelo and Garberville, respectively, indicate that crustal contraction, expressed by thrust faults and associated folding, predated translational deformation at each site. Inception of contraction near Covelo was related to inception of internal deformation of the Gorda plate at 3 Ma. This contraction, due to coupling of the convergent plates, migrated northward in the forearc in advance of and at the rate of migration of the triple junction. Present‐day deformation near Humboldt Bay thus reflects contraction associated with subduction of the Gorda plate and translation farther east associated with oblique convergence at the plate boundary. As the triple junction migrates, the strike‐slip faults of the San Andreas transform boundary will extend to the north‐northwest into the present forearc, and forearc strike‐slip faults will become faults of the San Andreas system. Contractional structures near Humboldt Bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south.

Earth structural patterns and rhythmic tectonism

Earth structural patterns and rhythmic tectonism

Reinterpretation of Lime Peak thrust as a low-angle normal fault: Implications for the tectonics of southeastern Arizona

Normal separation of offset stratigraphic units across the Lime Peak fault, mapped by Cooper and Silver as a thrust in the Dragoon quadrangle of southern Arizona, indicates that the fault is actually a low-angle normal fault, reflecting crustal distension rather than crustal contraction. The fault has been identified by Drewes as one of the few exposed segments of the supposed Hidalgo thrust of inferred subregional extent. Recognition that the structure is a low-angle normal fault of probable mid-Tertiary age thus casts doubt on the concept that coherent subhorizontal thrust sheets of Laramide age actually exist in southern Arizona.

Circum-Arctic Petroleum Potential: ABSTRACT

The Arctic is one of the most climatically hostile and demanding areas in the world. However, to the petroleum explorationist it is one of the most exciting and promising hunting grounds that remains to be explored. The low-lying areas and the shallow broad shelves of the circum-Arctic are underlain by many large sedimentary basin complexes. From the Late Devonian through the Tertiary, many types of sedimentary basins were formed and filled. Episodes of continental rifting have created large interior basins, passive continental margins, and new ocean basins. Major shear or transform faults have displaced continental segments hundreds of kilometers and formed sedimentary basins in the process. Convergent plate motion has resulted in thrust faulting, magmatism, subduction, and the accretion of exotic terranes to the continents. The processes of crustal contraction have formed sedimentary basins, and in some cases, inverted them. Paleolatitudes have ranged from near the equator to the present polar portion and climates have fluctuated from tropical to arid to boreal. End_of_Article - Last_Page 1203------------

Crustal movement in the Tokai district, Japan

Triangulation and trilateration surveys over the past 90 years have shown considerable crustal contraction (about 1 meter in E-W direction) in and around the Suruga Bay. From the precise leveling data, the crustal subsidence of the west coast of Suruga Bay amounts to about 40 cm in the past 70 years. Horizontal movement in and around the Suruga Bay is relatively small. The west coast of Suruga Bay is still subsiding. A small variation with the amplitude of ± 5 mm is superposed on the uniform subsidence rate of 8 mm/yr.

Shear zone geometry: A review

Shear zones may be classified into brittle, brittle-ductile, and ductile shear zones. The geometry and displacement boundary conditions of these zones are established. The geometric characteristics of ductile shear zones relevant to geological studies are described: orientations and values of principal finite strains, rotation, and deformation features of pre-existing planar and linear structures. Ductile shear zones show a fabric (schistosity and lineation) related to the finite strain state. The methods for determining strains and displacements from field studies are described. Shear zones commonly occur in conjugate sets, but the two differently oriented sets do not seem to be able to operate synchronously. The angular relationships of conjugate ductile shear zones are different from those of brittle shear zones. The termination of all types of shear zones poses complex compatibility problems, some solutions are suggested. A synthesis of shear zone geometry in regions of crustal contraction and crustal extensions is made, and ideas on how deep level ductile shear zones relate to high level brittle shears and gliding nappes are put forward.

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3�25?S

A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25′S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/106 years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts. Comparison of the EPR at 3°25′S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.