摘要:

Examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style, topographic elevation, denudation, metamorphism, postcollisional convergence, and foredeep basin geometry and facies. The distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary. At subduction boundaries where the rate of overall plate convergence is less than the rate of subduction (termed here retreating subduction boundaries) the transmission of horizontal compressive stress across the plate boundary is small, and regional deformation of the overriding plate is by horizontal extension. The tectonic expression of these retreating subduction boundaries includes topographically low mountains, little erosion or denudation, low‐grade to no metamorphism, little to no involvement of crystalline basement in shortening, little to no postcollisional convergence, anomalously deep foredeep basins, and a protracted history of flysch deposition within the adjacent foredeep basin. Analysis of deflection and gravity data across three retreating subduction boundaries (Apennine, Carpathian and Hellenic systems) shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km (Carpathians), 50 and 150 km (Apennines) and 50 and 250 km (Hellenides). The total mass anomalies represented by the slabs are approximately 3×1012, 6×1012and 12×1012N/m, respectively. The slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland, and partially supported by dynamic (viscous) stresses in the asthenosphere. At subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction (termed here advancing subduction boundaries) the transmission of horizontal compressive stress across the plate boundary is large, and regional deformation of the overriding plate is by horizontal shortening. The tectonic expression of these advancing subduction boundaries includes topographically high mountains, antithetic thrust belts, large amounts of erosion and denudation, exposure of high‐grade metamorphic rocks at the surface, extensive deformation of crystalline basement to midcrustal depths, protracted postcollisional convergence (tens of millions of years), and a protracted history of molasse deposition within the adjacent foredeep basins. Analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere (Western to Eastern and Southern Alps and Himalayas) shows that the thrust sheets have been translated for great distances over the foreland lithosphere (relative to the point at which the subduction forces are applied), thus obscuring any flexural and gravity signals from the subducted slab. However, it appears that far‐field stresses, presumably related to global plate motions, drive most of the convergent motion across these subduction boundaries. The concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved.Appendix B is available with entire article on microfiche.Order from the American Geophysical Union, 2000 FloridaAvenue, N.W., Washington, D.C. 20009. Document T92‐004; $2.50. Payment must accom

ISSN:0278-7407    

DOI:10.1029/92TC02248

年代:1993

数据来源: WILEY

摘要:

Recent quantitative studies of deformation in western North America including Pacific and North American relative plate motion, estimates of Neogene displacement in the southern Basin and Range, and anomalous paleomagnetic declinations of the Pacific Northwest are self‐consistent and, together, determine a simple kinematic model of the deformation that is an extension of the 1970 plate tectonic model of Atwater. A large continental lithospheric block, called here the Sierra Nevada/Klamath block, was displaced about 300 km to the W‐NW relative to the North American craton during the Neogene. The southern end of the overriding plate in the subduction zone along the Pacific Northwest has been shifted the same amount, resulting in the subduction zone and overlying accretionary complex rotating clockwise about 30°. The pivot is located near Seattle and the several tens of kilometers of N‐S shortening required to accommodate the rotation is taken up in the major thrusts of the Juan de Fuca Strait and Olympic Pen

ISSN:0278-7407    

DOI:10.1029/92TC02249

年代:1993

数据来源: WILEY

摘要:

The Lithoprobe seismic reflection transect across the southern Coast Mountains of the Canadian Cordillera images fundamental crustal structures presumably related to collision of the Intermontane and Insular composite terranes, and deep levels in the upper plate of the offshore Cascadia subduction belt. The eastern part of the Coast Mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors; they are truncated by subhorizontal to west dipping middle and upper crustal reflectors. These geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust, and a later phase of east directed shortening caused by wedging of the Intermontane belt into the lower and middle crust of the tectonic stack. Subsequently, the Coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the Intermontane belt and surface in the Omineca and Foreland belts. Extensional faults bounding the east flank of the Coast Mountains and west flank of the central Nicola horst in the Intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage. Within the western Coast Mountains, east dipping reflections spanning the middle crust to upper mantle are traced updip to Vancouver Island and the underlying Cascadia subduction zone. The C reflector on Vancouver Island is believed to separate Wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the Moho. If the Moho is not a young feature, then some accreted material appears to have wedged into the continental framework above the crust‐mantle boundary, possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data. The structurally lower E reflections, interpreted as shear zones, originate at the subduction contact offshore and project landward into sub‐Moho reflections within the upper plate on the Mainland. The region between the E reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and man

ISSN:0278-7407    

DOI:10.1029/92TC00598

年代:1993

数据来源: WILEY

摘要:

Low‐ and high‐angle, normal‐sense faults present along the northern margin of the East Humboldt‐Wood Hills metamorphic complex record a protracted history of episodic extensional unroofing. The earliest extension associated with the exhumation of the metamorphic complex occurred in the mid‐Eocene (about 50 Ma) during slip along the west‐rooted Wells Peak fault. Subsequent extension developed during movement along the east‐rooted Black Mountain fault system in the late Eocene (about 35 Ma). Continued exhumation of the high‐grade metamorphic rocks, occurred during the late Oligocene to early Miocene (about 29–23 Ma) along a west‐rooted, normal‐sense, plastic‐to‐brittle shear zone exposed along the length of the East Humboldt Range and much of the Ruby Mountains. The mylonitic shear zone, Wells Peak fault, and Black Mountain fault system were all overprinted by the west‐rooted, low‐ to high‐angle Mary's River fault system which was active from mid‐Miocene to Holocene. Broadly coeval extension in the Windermere Hills was also accommodated by the development of high‐angle, north and east dipping normal faults between

ISSN:0278-7407    

DOI:10.1029/92TC01967

年代:1993

数据来源: WILEY

摘要:

Geologic mapping, structural analysis, petrologic study, and U‐Pb geochronology at Iron Mountain, 20 km southwest of Barstow, California, place important constraints on the paleogeographic affinities of metasedimentary rocks in the area and provide new data to test Mesozoic and Cenozoic tectonic models for the central Mojave Desert. Rocks present at Iron Mountain include: Precambrian(?) and/or lower Paleozoic(?) miogeoclinal rocks, Middle Jurassic tonalite, Middle Jurassic Hodge volcanic series, Late Jurassic hornblende diorite, Cretaceous(?) peraluminous granite, and Cretaceous(?) granodiorite prophyry. Two phases of ductile deformation are present at Iron Mountain. The first phase, which penetratively deforms the miogeoclinal rocks, tonalite, and Hodge volcanic series, developed under amphibolite and greenschist facies metamorphic conditions. Fabric development in the Hodge volcanic series preceded emplacement of 151 ± 11 Ma granite. The second fabric deforms peraluminous granite and Late Jurassic hornblende diorite as well, and consists of spaced mylonitic shear zones. The shear zones predate emplacement of Late Cretaceous dikes (83 ± 1 Ma). The presence of probable miogeoclinal strata indicates that the boundary between allochthonous eugeoclinal rocks and parautochthonous miogeoclinal/cratonal rocks must lie north of Iron Mountain. The older amphibolite facies metamorphism and contractile deformation at Iron Mountain are interpreted to be part of a belt of Middle to Late Jurassic age deformation that runs northeastward through the Mojave Desert and forms the southern continuation of the east Sierran contractile belt. Newly recognized subvertical mylonitic shear zones of Cretaceous age at Iron Mountain have not been documented elsewhere in the central Mojave Desert. No significant Tertiary ductile deformation fabrics are present at Iron Mount

ISSN:0278-7407    

DOI:10.1029/92TC02423

年代:1993

数据来源: WILEY

摘要:

Investigation of key areas of the central Klamath Mountains demonstrates that genetic relationships connect several regionally significant lithologic units that were previously considered to be separate tectonostratigraphic terranes. The rocks embody an intraoceanic tholeiitic to alkaline magmatic arc and contiguous accretionary wedge formed in Permian to Middle Jurassic time. The Eastern Hayfork and St Claire Creek units comprise Permian‐Jurassic oceanic sedimentary rocks deposited nearby and coeval with eruption of the North Fork‐Salmon River volcanoplutonic arc. The more inboard St Claire Creek unit consists of stratigraphically coherent chert and minor argillite probably deposited in a back‐arc setting, whereas the more outboard Eastern Hayfork sedimentary rocks were disrupted during formation of an accretionary wedge. The volcanoplutonic North Fork‐Salmon River arc extends for 200 km in the central Klamath Mountains and was active episodically or continuously from Permian through Middle Jurassic time. Alkalic members of the mafic igneous suite may have erupted following subduction of a spreading center. A varied heating history has produced an amalgam of submarine and regional low‐pressure, moderate‐temperature metamorphism, overprinted locally by contact metamorphism. Geochronology of weakly metamorphosed mafic dikes/sills (10 new40Ar/39Ar analyses) suggests that significant heat was retained regionally into Early Cret

ISSN:0278-7407    

DOI:10.1029/92TC02250

年代:1993

数据来源: WILEY

摘要:

Basalt, gabbro, and peridotite in the western Brooks Range structurally overlie Mississippian through Late Jurassic platform sediments that were deformed during Late Jurassic ‐ Cretaceous time.40Ar/39Ar step‐heating experiments on hornblende, biotite, plagioclase, and potassium feldspar from gabbro, diorite, plagiogranite, and granite yielded complex release spectra. On isotope correlation diagrams, the incremental‐heating data indicate that gabbro, diorite, and plagiogranite cooled below the closure‐temperature for argon in amphibole between 187 and 184 Ma. Because the closure temperature of argon retention in amphibole is relatively high, these ages are interpreted to be the time since crystallization. Biotite and hornblende from metamorphic rock exposed along faults separating peridotite and gabbro from underlying basalt yielded plateau and isochron dates of 169 – 163 Ma; these dates are interpreted to record postmetamorphic cooling following intraoceanic detachment and transport of “hot” peridotite and gabbro over “cold” basalt and sediment. Detachment and faulting of the gabbros and peridotites is interpreted to have occurred approximately 20 Ma after crystallization, and is longer than the 5–10 Ma interval commonly observed in studies of other ophiolites. Fossils in flysch underlying the ophiolites indicate that the ophiolites were emplaced onto the continental margin during the Tithonian, approximately 15–20 Ma after initial detachment. The ages of feldspars (149 – 137 Ma) in the ophiolites and associated volcanic rocks are similar to the fossil ages of the underlying flysch and might record uplift during emplacement of the ophiolites onto the continental margin. A fission‐track age of apatite implies that the ophiolitic rocks were overlain by>4 km of rock until ∼59 Ma ago, the time of regional uplift in the Brooks Range.Supplement Appendix 1 is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. document T92‐005; $2.50.

ISSN:0278-7407    

DOI:10.1029/92TC02640

年代:1993

数据来源: WILEY

摘要:

Fission‐track thermochronologic studies on mid‐Cretaceous dioritic and tonalitic rocks from Tobago, West Indies, provide important temporal constraints on post magmatic Mesozoic cooling as well as the final phase of accretion of these oceanic‐arc plutonic rocks onto the continental margin of northern South America. Zircon fission track data indicate that the plutonic rocks of Tobago cooled through the zircon closure isotherm (250° – 200°C) at about 103 Ma. Apatite ages and track length analysis suggest rapid cooling through the apatite annealing zone (120° – 60°C) at about 45 Ma. Furthermore, the track lengths in apatites from these plutonic rocks are all greater than 14 microns, which implies that these rocks have not been above 50°C since 45 Ma. If a geothermal gradient of 25°C/km is inferred, the analyzed samples have been at depths of less than 2 km since 45 Ma. Tobago has occupied an upper crustal structural position since the mid‐Cretaceous, and significant final uplift of these Mesozoic oceanic‐arc rocks occurred in the mid‐Eocene. The tectonothermal history of Tobago since the mid‐Eocene has not involved deep burial or a high‐temperature thermal flux. This history apparently has chiefly involved translation of the Tobago terrane along the Caribbean ‐ South America Plate Boundary Z

ISSN:0278-7407    

DOI:10.1029/92TC02317

年代:1993

数据来源: WILEY

摘要:

A review of the prebreakup geology of west central Pangea, comprising northern South America, the Gulf of Mexico, and West Africa, combined with a study of the Mesozoic rift trends of the region confirms a relation between the rift systems and the underlying older grain of deformation. The prebreakup analysis focuses attention on the Precambrian, early Paleozoic, and late Paleozoic tectonic events affecting the region and assumes a Pindell fit. Two late Precambrian orogenic belts are observed in west central Pangea. Along the northern South American margin and Yucatan a paleo northeast trending Pan‐African aged fold belt is documented. A second system is observed along West Africa extending from the High Atlas to the Mauritanides and Rockelides. Similar aged orogenies in the Appalachians are compared. During the late Paleozoic, renewed orogenic activity, associated with the Gondwana‐Laurentia suture, affected large segments of west central Pangea. The general trend of the system is northeast‐southwest and essentially parallels the Guayana craton and West African and eastern North American cratons. Mesozoic rifling closely followed either the Precambrian trends or the late Paleozoic orogenic belt. The Triassic component focused along the western portions of the Gulf of Mexico continuing into eastern Mexico and western South America. The Jurassic rift trend followed along the separation between Yucatan and northern South America. At Lake Maracaibo the Jurassic rift system eventually overlaps the Triassic rifts. The Jurassic rift resulted in the “Hispanic Corridor” that permitted Tethyan and Pacific marine faunas to mix at a time when the Gulf of Mexico underwent continental sedi

ISSN:0278-7407    

DOI:10.1029/92TC01002

年代:1993

数据来源: WILEY

摘要:

The Betic Cordillera of southern Spain forms the westernmost of the Mediterranean Alpine orogenic belts and is divided, traditionally, into Internal and External Zones. In the External Zone fold and thrust belt, unmetamorphosed cover rocks of the Iberian margin were deformed from the early Miocene onward. In contrast, Internal Zone rocks are dominantly metamorphic and are generally assumed to have been deformed and metamorphosed in an Alpine event resulting from Late Cretaceous/Palaeogene convergence between Africa and Europe. In the Sierra Espuña, at the northern edge of the Internal Zone in the Eastern Betics, the uniquely preserved Permo‐Triassic to Miocene stratigraphy of the unmetamorphosed Malaguide Complex, offers a rare opportunity to date the Internal Zone deformation. Kinematic data, thrust geometries and structural facing of folds suggest that the imbricate thrust stack of Malaguide rocks formed as a result of NNW‐directed motion, within present geographical coordinates. A fossilized front to this thrust stack, preserved beneath a thick sequence of Oligocene conglomerates, indicates that the timing of the thrust deformation is late Eocene in age. Deformation propagated into the foreland basin in the late Oligocene with renewed thrusting. Subsequently, the whole stack has been folded into a N‐NW vergent regional recumbent fold, the Espuña fold, during the early

ISSN:0278-7407    

DOI:10.1029/92TC02507

年代:1993

数据来源: WILEY