摘要:

Paleomagnetic work on the Hervey Group in southeast Australia reveals a predeformational magnetization as suggested by the retention of a predeformational compaction produced magnetic fabric and the effect of deformation on both the magnetic fabric and the direction of the magnetic remanence in cleaved rocks. The formation mean direction D=29.2°, I=−19.3° with α95=15.5° gives a latest Devonian‐Early Carboniferous palaeomagnetic pole at 54.4°S, 24.1°E with DP=8.4°, DM=16.2°. Comparing this pole with existing poles from Australia and North America, it is suggested that: (1) The Lachlan Fold Belt of southeast Australia has been part of cratonic Australia since at least the late Devonian, and consequently, the Late Palaeozoic palaeomagnetic data from the LFB may be applied to the whole of Gondwanaland. (2) Cratonic Australia and the New England Fold Belt were all in an equatorial position during the Early Carboniferous. (3) There was probably a rapid anticlockwise rotation of Gondwanaland during the Late Devonian around an axis close to Australia. (4) A “V”‐shaped ocean existed between Laurussia and the northern margin of western Gondwanaland during the late Devonian‐Early Carboniferous. The Appalachian‐Hercynian Orogenic Belt is probably the result of the

ISSN:0278-7407    

DOI:10.1029/TC007i003p00351

年代:1988

数据来源: WILEY

摘要:

The early Proterozoic tectonic evolution of the south central Lake Superior region is complex, owing to the presence of four tectonostratigraphic terranes, which were affected by six phases of deformation. The four terranes are the passive margin of the Superior craton, two paraautochthonous passive margin terranes (Crystal Falls and Florence‐Niagara terranes), and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes. Four of the six deformational episodes accompanied subhorizontal shortening, while two were caused by subvertical shortening. The first and the most penetrative phase of deformation is marked by subhorizontal shortening in a NNE‐SSW direction. The second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area. The third, fifth, and sixth deformations mostly caused open folding, and shortening directions were NW, NE, and W, respectively. Because all the terranes in the south central Lake Superior region share parallel deformational histories, it is suggested that the accretion of these terranes occurred during the first deformational episode. After removal of younger deformational effects, including open folding of the suture zone, the tectonostratigraphic assemblages in this region show the following sequence from NNE to SSW: a platformal assemblage overlying sialic basement, a basinal assemblage of tholeiitic volcanic rocks overlain by deep‐water turbidites, an assemblage of basin floor deposits (Crystal Falls terrane) with apparently no demonstratably underlying crystalline basement, a fault‐bounded terrane with highly strained passive margin strata (Florence‐Niagara terrane), and a calc‐alkaline magmatic arc assemblage. Such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through Phanerozoic accretionary continental margins and therefore supports an arc‐continent co

ISSN:0278-7407    

DOI:10.1029/TC007i003p00369

年代:1988

数据来源: WILEY

摘要:

A computer model is used to estimate patterns of lithospheric deformation caused by obduction and removal of loads within orogenic belts. Regions of lithospheric downwarp become sedimentary depocenters and regions of lithospheric upwarp suffer erosion. By following the deposition and erosion of sedimentary packages through a number of model time steps it is possible to calculate a synthetic stratigraphy for a given study area. This model is applied to the post‐Early Ordovician stratigraphy of the Eastern Interior region of North America. The loading that drives lithospheric deformation is associated with the development of the Appalachian and Ouachita orogens and the independent development of the Michigan and Illinois intracratonic basins. Model results are shown to be consistent with presently observed stratigraphy, lithology, and cumulative erosion and are then used to estimate the configuration of given sediment packages at discrete intervals following their deposition. These estimates are presented in the form of isopach maps, stratigraphic cross sections, and time‐stratigraphic diagrams as a function of time over the last 470 Ma. By tracking the burial and erosion history of sedimentary units we are able to establish a cause and effect relationship between particular tectonic events and the extent of preservation of sedimentary units. In particular, we suggest that the Pennsylvanian–Permian Alleghanian orogeny produced major sediment cover over much of the study

ISSN:0278-7407    

DOI:10.1029/TC007i003p00389

年代:1988

数据来源: WILEY

摘要:

A model is presented for deformation, uplift, and exhumation in convergent orogens based on the following premises: (1) foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen; (2) the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton; on the seaward side of this transition, up to 20 km of overthrusts can be accumulated without creating topography; (3) critical shape theory can be applied to orogens as a whole, allowing predictions to be made concerning dynamic balance between mass addition and loss, uplift, exhumation, and growth of topography; and (4) pressure‐temperature‐time (P‐T‐t) data can be used to reconstruct particle paths within the deforming orogen.Three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition, as monitored by the foreland. Rapid exhumation of metamorphic terranes is most likely in the ‘steady state’ where erosion balances accretion; a ‘constructive state,’ where mass addition is greater than mass loss, results in crustal thickening and initially little exhumation; a ‘destructive state,’ where mass loss outweighs mass addition, results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension. P‐T‐t data on peak metamorphism, pluton emplacement, uplift, exhumation, and cooling can be used in conjunction with data from the foreland to distinguish among these various states. The topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen. Application of this model to the central and southern Appalachians suggests that during the constructive phases of the Taconian and Acadian orogenies, overthrust loads accumulated mainly on the seaward side of the ocean‐continent transition as marked by the Bouguer gravity gradient. Between the Acadian and Alleghanian orogenies, the Appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton. The constructive stage of the Alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton; this resulted in widespread uplift and westward transport of older metamorphic rocks. Post‐orogenic unroofing of the southern Appalachians is compatible with the predictions of flexural models; the post‐orogenic unroofing history of the central Appalachians and Ouachitas probably reflects Meso

ISSN:0278-7407    

DOI:10.1029/TC007i003p00417

年代:1988

数据来源: WILEY

摘要:

Two end‐member models of extension involving detachments have been developed. One model incorporates a fault that soles at mid‐crustal level overlying a broad region of pure shear in the lower crust. The second, referred to as the simple shear model, includes a detachment continuing through the entire crust and terminating in a region of concentrated extension in the lower crust. Both models predict basins with no localized thermal effect. With the inclusion of flexural isostasy, both models predict footwall uplift whose amplitude and wavelength are controlled by the detachment geometry and the lithospheric strength. A gravity anomaly over the hanging wall block distinguishes the simple shear model from the intracrustal detachment model. The early Mesozoic basins of the eastern North America, believed to have formed as the result of the normal‐slip reactivation of a Paleozoic thrust system as the Atlantic opened, are associated with distinctive hanging wall gravity highs. These gravity highs, the basin geometry, the lack of a thermal subsidence phase in the rift basins, and the presence of a highly extended and heated region to the east, suggest that the simple shear model may be applied. The simple shear model fits the outer hanging wall anomaly and permits a region of lower crustal extension to be mapped. These basins contain an abundance of basalt flows and diabase sills despite the lack of evidence for regional heating or thermal subsidence, implying that a source, external to the basin, must exist for this magmatic material. The detachment fault may facilitate the movement of the molten mafic material into the rift basins from an offshore region of greater heating and extension. The addition of 2 km of mafic material along the model detachment accounts for the observed inner gravity high and the lack of a negative gravity anomaly across these basins. The match of this modified simple shear model to the observed gravity suggests that the region of greater extension seaward of the hinge zone is the source for the widespread dikes and sills within the basins and the coastal plain and helps explain the geochemical homogeneity of these intrusives and extrusives along 2000 km of the eastern sea

ISSN:0278-7407    

DOI:10.1029/TC007i003p00447

年代:1988

数据来源: WILEY

摘要:

Mesozoic intrusives, 71 sills and 12 dykes, from different structures in the folded eastern Sverdrup Basin contain stable, well‐grouped paleomagnetic directions of both normal and reverse polarity. The mean direction is 274°, +79°, k=173, α95=3.7°, with a corresponding pole position at 157°, 67°N, k=61, A95=6.6°. The paleomagnetic fold test shows that the major deformation, involving tilting of strata around strike, postdated the emplacement of sills and dykes. Minor preintrusion deformation may also have occurred, but, more importantly, an offset of the paleopole from the Cretaceous section of the polar wander path permits a postintrusion anticlockwise rotation of eastern Sverdrup Basin of about 15+/−20° with respect to stable North America. If the intrusives are Cretaceous, the degree of rotation is close to that (11°) estimated for Greenland during the opening of the Labrador Sea. No positive evidence has been found to support models of distributed shear with block rotations to accommodate the contentious northeast translation of Greenland along the

ISSN:0278-7407    

DOI:10.1029/TC007i003p00463

年代:1988

数据来源: WILEY

摘要:

Acadian (Late Caledonian) structures in Wales record the inversion and strong deformation of the Welsh Basin during docking of the Eastern Avalonia microcontinent with Laurasia in Late Silurian to Mid Devonian time. A basement‐involved deformation model is developed in which deep faults and buried granites or horsts controlled the main arcuate trends of Welsh structure. A south southeast directed shortening across the basin is implied by the variation in amount and sense of transection of folds by cleavage around the major arcs. This shortening induced sinistral transpression and clockwise transection across northeast or north northeast trending folds but more orthogonal shortening and zero or vaak anticlockwise transection across east northeast or east trending folds. The south southeast shortening direction parallels mineral lineations in north Wales. It is an estimate of the Avalonia/Laurentia convergence vector but is about 35° anticlockwise of that derived by Soper et al. [1987] from northwest England. Displacement partitioning within the convergence zone may explain this discrepan

ISSN:0278-7407    

DOI:10.1029/TC007i003p00483

年代:1988

数据来源: WILEY

摘要:

Djebel Debadib is a northeast trending anticline in the Tunisian Atlas, near El Kef, Tunisia. The core of the anticline consists of a deformed Triassic, gypsum‐matrix breccia containing clasts of supratidal dolomite, terrigeneous clastic rocks, and ophites (metabasites). Locally, low‐grade metasedimentary rocks, occurring as clasts in the evaporite, may represent fragments of basement rock plucked from salt subcrop during deep‐seated flowage. The multiply deformed Triassic rocks are juxtaposed against Cretaceous or younger rocks, and Jurassic and Neocomian strata are not present in outcrop. These characteristics have been cited in many studies to suggest a history of diapirism to explain both the internal structure of the Triassic rocks and the contact relations with the younger rocks. A previously unreported unconformity, developed on the Triassic rocks and overlain by Aptian‐Albian strata, indicates that an important period of piercement occurred during the Early Cretaceous for the Djebel Debadib diapir. From the mid‐Cretaceous to the late Eocene the continental margin of northern Tunisia was a regional basin in which a thick sequence of carbonate rocks was deposited. Beginning in late Oligocene time, sandy clastic sediments accumulated on the older carbonate rocks above a regional unconformity, and during the middle Miocene Atlas orogeny, numerous northeast‐southwest folds deformed the rocks of the Tunisian trough including the diapiric Triassic masses. These folds, commonly boxlike in geometry, were extensively faulted during the late Miocene and Pliocene. A four phase tectonic model is recognized for the Tunisian Atlas: (1) early Mesozoic rifting of the North African continental margin, (2) diapiric emplacement of the Late Triassic‐Liassic(?) evaporites into overlying strata beginning in the early Cretaceous, (3) folding of the cover strata in response to regional compression culminating in the middle Miocene Atlas orogeny, (4) the development of transvers grabens during late orogenic normal faulting (latest Miocene to Recent). A major regional décollement is not considered necessary to account for the shortening of the sedimentary cover during the middle Miocene deformation. Rather, the principal driving mechanism of regional crustal shortening was the reactivation of early Mesozoic normal faults as reverse faults that displaced basement rocks. This model implies that the small amount of shortening recorded in the Tunisian Atlas involved active shortening and structural thickening of basement and the concurrent deformation of the sed

ISSN:0278-7407    

DOI:10.1029/TC007i003p00497

年代:1988

数据来源: WILEY

摘要:

The Cantabrian Zone constitutes the external zone of the Variscan orogenic belt in the northwest of the Iberian Peninsula. Within it a large number of thrusts and folds can be observed which define the form of the Asturian Arc. Tectonostratigraphically, two units can be distinguished: a pretectonic one and a syntectonic one. The time of the transition between the two units lies close to the Devonian‐Carboniferous boundary. The major allochthonous units of the Cantabrian Zone display varied geometries (duplexes, imbricated thrusts, out‐of‐sequence thrusts, etc.). The allochthonous units were emplaced in a foreland propagating sequence with movement age ranging from Westphalian B to Stephanian. Movement directions changed from one major unit. As a whole, the movement directions converge toward the core of the Asturian Arc, and are best interpreted in terms of a progressive series of rotational displacements, leading to a final disposition of major units similar to that of the leaves of a photographic iris. The folds were initiated at the same time as the motion of each nappe unit, and each type of structure represents the hanging wall adaptation to the development of a lateral and longitudinal staircase thrust surface topography. These folds suffered tightening after the emplacement of the thrust unit in which they occur, as a result of the different direction of movement of the next thrust unit propagating from be

ISSN:0278-7407    

DOI:10.1029/TC007i003p00517

年代:1988

数据来源: WILEY

摘要:

Out‐of‐sequence thrusts (OOST) are those thrusts which do not obey the foreland propagating or in‐sequence deformation style. They include both isolated thrusts which develop hindward of the thrust front and sequences of break‐back thrusts which propagate from the foreland to the hinterland. Two end‐members of a series of OOST types are recognized: (1) older in‐sequence thrusts which are reactivated along their entire length and (2) completely new thrusts which propagate through already deformed thrust sheets. Between the two end‐members are thrusts composed partially of reactivated in‐sequence thrust sequences and partially of new, entirely out‐of‐sequence segments. OOSTs can be initiated for a variety of reasons including: (1) keeping the orogenic wedge at critical taper, (2) break‐back sequences from the suture zone in the overriding plate, (3) ramping to overcome a sticking point which inhibits in‐sequence thrust propagation, and (4) during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust. Many different types of thrust behavior including gravity sliding, plucking, and derivation of isolated horses from ramps may mimic some of the characteristics of OOSTs. Consequently, it may be difficult to conclusively prove an OOST origin for a

ISSN:0278-7407    

DOI:10.1029/TC007i003p00539

年代:1988

数据来源: WILEY