bluuebunny/crossref_metadata_2025 huggingface dataset:
Deformation along the Yammuneh, The restraining bend of the Dead Sea Transform: Paleomagnetic data and kinematic implications
Authors: Ron Hagai
Published: February 2008

A simple geometrical model shows that the plate margin along the restraining bend segment of the transform plate boundary be deformed in order to accommodate overlap of the crust. This model predicts both the maximum width of the deformed zone and the magnitude of deformation when the general geometry of the plate margin is known as well as the plate slip vector and the amount of cumulative displacement. This idea has been tested along the plate margin of the Yammuneh restraining bend of the Dead Sea transform. The plate margin has been deformed by NNE folding parallel to the restraining bend and by east‐west trending, right‐lateral strike‐slip faults. Paleomagnetic measurements yield counterclockwise rotation of R±ΔR=61.0°±9.6° and F±ΔF=23.4°±17.2°. The paleomagnetic rotational data and fault kinematic suggest that the mechanism which accommodates regional left‐lateral shear is simultaneous right‐lateral strike‐slip faulting on secondary faults and block rotation. The magnitude of deformation is the same as that predicted by the model, which is 100% shortening in a direction parallel to the plate slip vector and negligible deformation in a direction parallel to the transform. The large amount of rotation implies that probably more than one fault set is involved and present day seismic activity is the current manifestation of this crustal deformation process. If this prediction is correct, then the current deformation of the plate margin is accommodated by more favorable newly formed NNW right‐lateral strike‐slip faults.
Constraints on Arabian Plate motion and extensional history of the Red Sea
Authors: Hempton Mark R.
Published: February 2008

Analysis of the geologic and geophysical data from the western and northern margin of the Arabian plate combined with data from the Red Sea/Gulf of Aden leads to an interpretation of Arabian plate motion that supports an episodic extensional history in the Red Sea and Gulf of Aden. Phase 1 extension began in medial to late Eocene and was caused by the NW propagation of the reoriented Central Indian Ridge spreading center. It stretched and thinned continental crust along numerous wrench and normal faults that reactivated preexisting structures. First‐phase extension stopped when the Arabian plate terminally sutured to Eurasia in the medial Miocene and halted northward movement of Arabia relative to Africa. The combined African/Arabian plates moved north more slowly, constricting and thickening continental crust in the Bitlis/Zagros sutures while the Red Sea remained technically quiescent and accumulated salt. By the early Pliocene, continued northward movement was accommodated by lateral extrusion of large continental wedges north of the Bitlis/Zagros sutures. This allowed Arabia to move northward faster than Africa and reopen the Red Sea where phase 2 extension was expressed by seafloor spreading. Extension within the Red Sea/Gulf of Aden rift system has been controlled by the interactions and effects of two factors: (1) convergent processes between the north margin of the Arabian plate and the Eurasian plate to the north and (2) northwest propagation of extension associated with the medial Eocene reorganization of the Central Indian spreading center.
Implications of continental crust extension for plate reconstruction: An example from the Gulf of Mexico
Authors: Dunbar John A. , Sawyer Dale S.
Published: February 2008

The prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup. Continental extension is not expected to be pervasive throughout the plate, but to be confined to a zone of up to several hundred kilometers in width. The direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates. The “best fit” between rifted continental margins is then described by the rotation angle about an Euler pole which best closes both the oceanic crust and the extension within the continental crust, along small circles about the Euler pole. As an example application of these concepts, the pattern of Late Mesozoic crust extension within the Gulf of Mexico basin is used to constrain the location of the Euler pole and angle of rotation of the Yucatan block with respect to North America. Both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point‐by‐point basis from bathymetry and basement depth. The root‐mean‐square (rms) misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening. A contour map of misfit versus pole location reveals a global minimum rms misfit of ±47 km for a 45° counterclockwise opening of the Gulf of Mexico basin about an Euler pole at 25°N, 79°W. An elongate trend of poles, for which the misfit is less than 100 km, extends from the Bahama Islands eastward across the Atlantic and through North Africa. The reconstruction of the Gulf of Mexico for the best fit pole places the palinspastically restored Yucatan block in contact with North America along the Texas and Louisiana shelf. The resulting restored southern margin of North America fits against the the northern margin of South America in its prerift position to within the uncertainty in the North America‐Africa‐South America plate circuit. We conclude that the prerift placement of the Yucatan block between North and South America along the Texas‐Louisiana shelf is consistent with both the pattern of extension within the Gulf of Mexico basin and seafloor spreading history of the central Atlantic basin.
Origin of eclogite‐bearing, domed, layered metamorphic complexes (“core complexes”) in the D'entrecasteaux Islands, Papua New Guinea
Authors: Davies H. L. , Warren R. G.
Published: February 2008

Compositionally layered metamorphic rocks of the D'Entrecasteaux Islands, Papua New Guinea, are folded into domes and antiforms bounded by faults parallel to metamorphic layering and foliation. The structures are broadly similar to the metamorphic “core complexes” of western North America. Lenses of ultramafic rock lie on the bounding faults, and the same faults have served as loci for Quaternary andesitic volcanic activity. Metamorphic grade in the northern islands (Goodenough and Fergusson) is amphibolite facies, with pockets of eclogite (Fergusson Island only) and granulite, and is greenschist facies in the southern island (Normanby). In all three islands there is a characteristic tectonostratigraphic sequence (FMU sequence) from felsic metamorphic rocks at base, or internally, through mafic metamorphic rocks to ultramafic rocks at top, or externally. The association of metamorphic and ultramafic rocks apparently developed in a north dipping Paleogene subduction system and was exhumed to upper crustal level in the Oligocene‐‐Early Miocene, possibly by reversal of movement on faults in the former subduction system. Vigorous uplift and development of domes and antiforms in the Pliocene was triggered by westward propagation of the Woodlark Basin spreading ridge and was accompanied by rifting, rift‐related magmatism, rapid erosion, and deposition of coarse sediment in the adjacent Trobriand Basin.
Andean‐age structure of Eastern Cordillera (Province of La Paz, Bolivia)
Authors: Roeder Dietrich
Published: February 2008

A Moho root beneath the Bolivian Andes, 40 km deep, is consistent with 230 km of overlap of Neogene age on a single, trenchward dipping transcrustal thrust fault with 10° of finite ramp cutoff (Main Andean Thrust (MAT)). Only 10% of the Andean crustal volume is ascribable to magmatic addition. The MAT is located within South American crust of full thickness. It intersects the basement top 450 km inland from the Neogene crustal margin. It is not a collision suture as shown by persistent pre‐Neogene facies continuity. Thrusting is not accompanied by terrane accretion. The present bilaterally symmetrical thrust belt responds to elastic line loading and to Coulomb rheology. In the hanging wall of the MAT, a deep high‐stress wedge base builds a steep critical slope. In the footwall, the foredeep response is fast subcritical growth by progradation and blind thrusting on a low‐stress decollement. Interaction is maintained by out‐of‐sequence renewal of movement on the MAT.
The continent‐ocean boundary at the rifted margin off eastern Canada: New results from deep seismic reflection studies
Authors: Keen C. E. , de Voogd B.
Published: February 2008

Seismic reflection data were collected and processed to 20 s two‐way travel time along four lines which cross the rifted continent‐ocean boundary off the Grand Banks region of eastern Canada specifically to examine the origin, age, and nature of this fundamental boundary. This represents the first regional study of its kind. The most important result is the presence of landward dipping reflectors near the foot of the continental slope. These occur where oceanic crust appears to terminate against the continent. We suggest that the dipping reflectors mark the continent‐ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary. Sedimentary basins lie just landward of the continent‐ocean boundary. Their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting, and this may be an important stage leading to continental breakup. Rift basins formed further landward on the Grand Banks do not exhibit this thinning. Other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent‐ocean boundary. Also, the oceanic crust exhibits a complexity of reflections, some of which may be due to compositional zonation during magmatic crystallization. Finally, our results may have important consequences for continental predrift reconstruction, as oceanic crust appears to extend farther landward in the Newfoundland Basin than some recent studies have indicated.
Seismic signature and tectonic cross section of the Variscan Crust in western France
Authors: Matte Philippe , Hirn Alfred
Published: February 2008

Deep seismic reflection profiling (ECORS program vertical and wide‐angle survey) coupled with field and subsurface geology are used to propose a complete NNE–SSW section of the Variscan Belt in western France from Belgium to southern Brittany. On this 800 km long profile, which probably crosses two Paleozoic sutures, the Variscan Belt appears as a broad fanlike orogen characterized by large (100 km) northward and southward overthrusts with polyphase deformation and metamorphism. The central part of the Belt (Central Armorican zone) suffered a simpler dextral shearing parallel to the strike of the belt with only green schist metamorphism. Deep reflection profiling shows that all the large thrusts visible at the surface root deeply in the lower crust or even crosscut the Moho. In the same way, all the large strike‐slip faults parallel to the belt appear at depth as narrow discontinuities which crosscut the whole crust and, in some cases, the Moho. The seismically well‐layered crust develops essentially at depth in the most deformed and metamorphic internal parts of the belt, in contrast with the relatively transparent Brabant foreland basement, which escaped the Variscan deformation. The Moho appears as a relatively flat boundary at an average depth of 35 km with different characters: It is sharp and strongly reflective both by wide angle and vertical seismics below the internal thrust zones, much less distinct in the central Armorican zone, and only defined by low frequency signals at wide angle below the Brabant foreland. All these characteristics led us to consider the layering of the lower crust and the Moho itself as initially tectonometamorphic features essentially produced by shearing and crust mantle decollement during the Variscan stacking of the crust by intracontinental lithospheric subduction.
Thermal consequences of lithospheric extension: Pure and simple
Authors: Buck W. Roger , Martinez Fernando , Steckler Michael S. , Cochran James R.
Published: February 2008

Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two‐dimensional conductive and advective heat transport through time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listric faults which sole into a subhorizontal detachment. The pure shear results show that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall extension rate, a pure shear model with a narrow zone of extension can result in pressure release melting of the mantle long before low angle simple shear models. These results are compared with topographic and heat flow data from the northern Red Sea rift, a Neogene continental rift which is close to initiating seafloor spreading. The long wavelength topographic asymmetry across the Red Sea, which has been cited as evidence for simple shear extension of the lithosphere, is not matched by any of the models. The observed high heat flow anomalies in the Red Sea require a large component of pure shear lithospheric extension centered under the region of maximum crustal extension. In contrast, at the plate separation rate of the northern Red Sea, simple shear extension of the lithosphere along a shallow (<30°) dip detachment is ineffective in reproducing the observed heat flow anomalies. Only a narrowing region of pure shear extension can satisfy the width of the rift, and the peak heat flow values and generate pressure release melting.
Orogeny and stratigraphy: Numerical models of the Paleozoic in the eastern interior of North America
Authors: Beaumont Christopher , Quinlan Garry , Hamilton Juliet
Published: February 2008

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 area.
Acadian tectonics of Wales during Avalonia/Laurentia Convergence
Authors: Woodcock N. H. , Awan M. A. , Johnson T. E. , Mackie A. H. , Smith R. D. A.
Published: February 2008

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 discrepancy.
Thermal evolution of the Greater Himalaya, Garhwal, India
Authors: Hodges K. V. , Silverberg D. S.
Published: February 2008

The hanging wall of the Main Central Thrust (MCT) in Garhwal, India (roughly 79°N–80°E; 30°N–31°N), exhibits an inverted metamorphic gradient: sillimanite ± potassium feldspar assemblages near the top of the hanging wall, or Greater Himalayan sequence, are underlain by kyanite grade rocks near the fault. Textural relationships in pelitic samples from the Alaknanda and Dhauli river valleys indicate that the “inversion” is the product of two distinct metamorphic events: an early Harrovian event (M1), which affected the entire Greater, Himalayan sequence and a later Buchan event (M2), the effects of which are most obvious in the upper part of the sequence. Rim thermobarometry, garnet inclusion thermobarometry, and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak M1 conditions of >900 K and >960 MPa (roughly 36 km depth) before following an “erosion controlled” uplift path (e.g., England and Richardson, 1977). M2 metamorphic temperatures in the upper part of the sequence also exceeded 900 K, but maximum pressures (317–523 MPa) indicate paleodepths of only 12–19 km. Calculated pressure‐temperature paths indicate that M2 was characterized by temperature increases of >80 K and roughly 5 km of tectonic burial We attribute M1 to tectonic burial of the Greater Himalayan sequence during the early stages of India‐Eurasia collision. We believe that the uplift and cooling path of the sequence was interrupted in late Oligocene(?) ‐ Miocene time by a second burial and heating event (M2) related to thrust imbrications in southern Tibet. This burial was coincident with the generation of leucogranites, which are abundant near the top of the Greater Himalayan sequence but are virtually absent near the MCT. Field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the Greater Himalayan sequence and were injected at their present structural level, or were melted in situ. If the granites were injected, then they may have provided some of the heat necessary for M2 metamorphism. Although our data suggest no direct relationship between the Main Central Thrust (as mapped in Garhwal) and metamorphism in the Greater Himalaya, anatectic melting of an unexposed portion of the Greater Himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped MCT in central Nepal (cf. Le Fort, 1981). If the granites were produced by in situ M2 melting, then we must appeal to a heat source within the upper part of the Greater Himalayan sequence such as locally high concentrations of heat‐producing elements (cf. Pinet and Jaupart, 1987).
Strike‐slip fault geometry in Turkey and its influence on earthquake activity
Authors: Barka A. A. , Kadinsky‐Cade K.
Published: February 2008

The geometry of Turkish strike‐slip faults has been reviewed and described. From this data set it appears that fault geometry (the distribution of discontinuities such as bends and stepovers along the main fault trace) plays an important role in controlling the location of large earthquake rupture segments along the fault zones. Large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30°. More important than the size of each discontinuity, however, is the total “geometric pattern,” i.e., the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size. Certain geometric patterns (restraining single or double bends) are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone. Fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior. For example, large earthquake epicenters often occur near restraining bends or double bends. Furthermore, aftershocks and swarm activity can often be associated with releasing areas (areas along the strike‐slip fault that are undergoing a relatively large amount of extension).
Crustal thinning and nature of extension in the northern North Sea from deep seismic reflection profiling
Authors: Klemperer Simon L.
Published: February 2008

A regional network of deep seismic reflection profiles recorded in the northern North Sea has been used to map Mesozoic and Cenozoic basin thickness and crustal thickness in the Viking Graben and adjacent platform areas. Depth to the reflection Moho varies from about 20 km in parts of the Viking Graben to about 32 km beneath the Shetlands and the Norwegian margin. The shallowing of the reflection Moho beneath the Viking Graben implies crustal stretching factors for Mesozoic extension greater than 2 in the center of the North Sea basin. The axis of crustal thinning is located directly beneath the Viking Graben, the axis of the sedimentary basin. Steeply dipping basin‐bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle, but no continuous reflective feature extending from the sedimentary basin to beneath the Moho is observed on any of the deep profiles. Thus these data do not support the existence of lithosphere‐penetrating low‐angle detachments (zones of simple shear) as the cause of extension in the northern North Sea. Some of the mantle reflectors dip to the west and some to the east, suggesting that simple shear, if it occurs in the upper mantle, is not of uniform sense. Rather, these data suggest a complex, depth‐dependent pattern of brittle extensional deformation in the upper crust; pervasive, ductile extension (bulk pure shear) in the lower crust (which is decoupled from deformation in the mantle); and extension accommodated by discrete, dipping shear zones in the lithospheric mantle.
The development of folds and cleavages in slate belts by underplating in accretionary complexes: A comparison of the Kodiak Formation, Alaska and the Calaveras Complex, California
Authors: Paterson Scott R. , Sample James C.
Published: February 2008

The development of folds and cleavages in slate and graywacke belts is commonly attributed to arc‐continent or continent‐continent collisions. However, the Kodiak Formation of southern Alaska and the Calaveras Complex of the western Sierra Nevada, California, are two slate and graywacke belts in which folds and slaty cleavages developed during simple underthrusting and underplating within accretionary wedges. The Maastrichtian Kodiak Formation is composed dominantly of coherent turbidites but includes lesser pebbly mudstone, minor conglomerate, and rare chert. The Kodiak Formation is part of a large accretionary complex that youngs in age seaward, but bedding tops generally show landward younging. A progression of structures has been determined by crosscutting relationships and includes (1) syndeformational depositional features; (2) broken formation; (3) slaty cleavage, folds, and thrust faults; (4) crenulations and crenulation cleavage; (5) late brittle thrust faults; and (6) right‐lateral strike‐slip faults. Broken formation, slaty cleavage, thrust faults, and folds developed during underthrusting and underplating within an accretionary wedge. Crenulations and brittle thrust faults are related to subsequent intrawedge shortening. Based on peak metamorphism in the uppermost zeolite to prehnite‐pumpellyite facies, underplating occurred at a minimum depth of 10 km. The Calaveras Complex is composed of argillite, chert, graywacke, pebbly mudstone, limestone, and volcanic rocks. Its age of deposition has a maximum range from Permian to Early Jurassic. Overall, the unit appears to young westward, but local facing indicators show eastward younging of individual blocks. The sequence of structures developed in the Calaveras Complex is (1) syn‐depositional olistostromes; (2) broken formation; (3) slaty cleavage, folds, and thrust faults; and (4) younger Jura‐Triassic folds and crenulation cleavages. Broken formation and slaty cleavage developed during underthrusting and underplating in an accretionary wedge, in an analogous fashion to the Kodiak Formation. Greenschist facies metemorphism during underplating suggests that the Calaveras Complex was accreted at deeper levels, or in a wedge with a higher geothermal gradient, than the Kodiak Formation. Some other probable examples of slate/graywacke belts that became folded and well‐foliated during underplating include part of the Dunnage Zone (Newfoundland), the Southern Uplands complex (Scotland), the Torlesse terrane (New Zealand), part of the Hamburg klippe (Pennsylvania), and the Taconic allochthons (New York). Peak metamorphism in these belts indicates that they were accreted in accretionary wedges with geothermal gradients higher than usually assumed for such tectonic settings; such wedges may have been fairly common features at convergent margins.
The state of stress in an overriding plate situated above a flat slab: The Andes of central Peru
Authors: Sébrier Michel , Mercier Jacques Louis , Macharé José , Bonnot Didier , Cabrera Justo , Blanc Jean Luc
Published: February 2008

In the Andes of central Peru that are situated above a flat dipping slab, analysis of both structural data collected in the field and available focal mechanisms show that the Quaternary and Recent state of stress is characterized by N‐S trending extension in the Western Cordillera and along the Pacific Coast, whereas E‐W trending compression prevails in the sub‐Andean Lowlands and at the contact between the Nazca and South American plates. The Eastern Cordillera deformations suggest an intermediate state of stress (i.e., strike‐slip faulting) characterized by E‐W trending compression and N‐S trending extension. Thus major fault zones striking NW‐SE, i.e., parallel to the central Peruvian Andes, are normal with a sinistral component in the Western Cordillera (e.g., Cordillera Bianca) and reverse sinistral in the Eastern Cordillera (e.g., Cordillera de Huaytapallana). This state of stress displays similarities with that of southern Peru and may be also interpreted as an effect of compensated high topography. In this model the vertical stress σzz increases with the topography and σHmax is considered as fairly constant, trending E‐W roughly parallel to the convergence. On both edges of the Andes, tectonics being compressional, σzz is σ3 and σHmax is σ1; in the High Andes, σzz becomes σ1, then σHmax is σ2, allowing σHmin striking N‐S to be σ3. Along the Pacific Coast, tectonics is not compressional as expected, and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep Peru‐Chile trench. However, a conspicuous difference occurs in the Andes of central Peru in respect to the southern Peru Andes: compressional strike‐slip tectonics is observed in the Eastern Cordillera. Indeed, the mean elevation of the Eastern Cordillera is 3.7 km, whereas that of the Western Cordillera is 4.2 km, allowing an intermediate state of stress to occur. Therefore in central Peru a lower mean Andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern Peru. Superficial deformations related to the subduction of the aseismic Nazca ridge are restricted to the coastal domain opposite to this ridge. They support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting.
Thermal modeling of extensional tectonics: Application to pressure‐temperature‐time histories of metamorphic rocks
Authors: Ruppel C. , Royden L. , Hodges K. V.
Published: February 2008

One‐ and two‐dimensional finite difference models are used to generate theoretical pressure‐temperature‐time (PTt) paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere (“pure shear”) and via movement of crustal blocks along rooted large‐scale low‐angle normal faults (“simple shear”). The temperature‐time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies, although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear, and experience a greater amount of posttectonic cooling. The depth‐temperature paths of rocks uplifted from deep crustal levels (30 km depth) via pure shear extension are often characterized by a period of nearly isothermal (high dP/dT) uplift during the early stages of extension. A modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces PTt paths characterized by a more protracted period of nearly isothermal uplift. In contrast, simple shear uplift along low‐angle normal faults produces almost linear depth‐temperature paths with moderate dP/dT, and isothermal uplift is not observed. For a given initial geotherm, the single factor controlling the PTt paths of rocks unroofed below gently dipping normal faults that dip less than 30° is the rate at which unroofing takes place (unroofing rate is defined as the rate of uplift relative to the surface), and the syntectonic PTt paths are insensitive to fault dip or horizontal displacement rate. Current geothermometric and geobarometric techniques yield typical uncertainties in real pressure‐temperature and temperature‐time data of ±1 kbar and ±50°C. Thus it will be difficult to distinguish between these various extensional mechanisms on the basis of PTt data derived from a single sample or from a single structural level, although use of data from samples collected from a broad range of structural levels may improve resolution. Ultimately, the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to ±0.5 kbar and ±25°C.
An aborted Triassic Ocean in west Siberia
Authors: Aplonov Sergei
Published: February 2008

A small buried oceanic basin named the Obsky paleo‐ocean has been found in West Siberia. It developed from 235 to 218 m.y. during the Triassic by rift propagation and seafloor spreading inside the west Siberian region of the continent. During the postspreading period the Obsky paleo‐ocean did not collapse but instead was covered intact by a layer of sediments with a thickness of up to 15 km. This conclusion is based on detailed interpretation of geophysical and geological data. Striped magnetic anomalies marking the paleo‐ocean spreading have been separated from the total magnetic field. They have been dated and interpreted within the framework of the Vine‐Matthews concept. Seismic and gravitational data have permitted delineation of the Obsky paleo‐ocean depression and contouring of the basement relief within its bounds. Investigation of the Obsky paleo‐ocean basalts by means of deep boreholes has indicated that they are chemically similar to oceanic tholeiites and has also confirmed the presence of strong residual magnetism in them. Spreading of the Obsky paleo‐ocean was a stage in the initial disintegration of Pangea at the Paleozoic‐Mesozoic boundary. The presence of a north trending buried spreading center along the axial zone of West Siberia accounts for many features of the Mesozoic‐Cenozoic history of the region and its present‐day structure.
Can natural faults propagate under Mode II conditions?
Authors: Petit Jean‐Pierre , Barquins Michel
Published: February 2008

Various brittle geological structures form from the tip of preexisting joints. Experiments have been carried out to investigate their mechanical origin and to find out whether a planar defect can propagate in its ownplane under mode II geometrical conditions (displacement parallel to the defect plane and perpendicular to the edge) as supposed by the classical rupture mechanics model, and in particular, whether mode II can be an elementary fracture mechanism, which is not clear in the model and has not been experimentally demonstrated. A biaxial loading device (plane strain) allowed direct observation of fracture propagation from an angled slot in plates of Polymethylmethacrylate (PMMA, Altuglas) and sandstone (low and high porosity). Under uniaxial load, mode I propagation of the classical branch fracture occurred in both materials, but its development was inhibited under biaxial loading (low σ3/σ1 ratio). At higher stress/strain levels, shear zones (clearly distinguishable from the branch fracture) developed from the slot tip. In PMMA they consisted of the prograding development of en echelon shallow microcracks formed in mode I and more or less associated with plastic deformation. In sandstone a narrow shear zone propagated in the prolongation of the defect (slowly in the high‐porosity samples and abruptly in the low‐porosity ones) accompanied by conjugate shear fractures. The deformation mechanism was cataclasis, implying tensile (mode I) microfractures. These experiments suggest that mode II cannot exist as an elementary (primary) fracture mechanism but can only be a macroscopic fracture phenomenon which must necessarily involve tensile (mode I) microcrack formation. The latter is linked to stress concentrations in the prolongation of the existing joint. As such mode II initiated shear zones have not been generated in glass, a high density of small defects which can be mobilized under shear stress would seem essential. These ideas can be used to interpret some types of natural faults.
How much does slip on a reactivated fault plane constrain the stress tensor?
Authors: Célérier Bernard
Published: February 2008

Given a fault plane and its slip vector, the stress tensor which caused the displacement is sought. Two constraints are considered: first, a geometrical constraint that the shear stress applied to the fault plane is parallel to the slip (Wallace, 1951; Bott, 1959); second, a frictional constraint that the shear to normal stress ratio equals tanϕ0 (Coulomb, 1776). In a first step, the stress tensors that satisfy the geometrical constraint are sought. For tensors belonging to the vectorial space of solutions, shear and normal stress magnitudes become a function of the orientation of the principal stresses,
,
,
, and are mapped, extending a study by McKenzie (1969). In a second step, it is investigated which of these tensors also satisfy the frictional constraint. Within this more restricted vectorial space, there is a relationship between the magnitudes, δ = (σ1 − σ2)/(σ1 − σ3) and s = (σ1 − σ3)/σ1, and the orientations,
,
,
, of the principal stresses. Both the range of s and the spatial distribution of
,
,
are more restricted than when the geometrical constraint alone is considered. As when the geometrical constraint is solely considered (McKenzie, 1969), the orientations of the principal stresses,
,
,
, may lie significantly away from and up to right angle to the P, B, T axes. However, this can happen only in two cases: (1) either the effective stress difference s has reached a high value, which is unlikely to happen if enough preexisting fractures are available to release the stress, or (2) σ2 becomes close to either σ1 or σ3 and therefore barely distinguishable from it; in that case the delocalization of the orientations of the principal stresses is best described by a tendency for
to exchange role with either
or
. When the stress difference remains small and σ2 reasonably away from σ1 and σ3,
,
,
approach positions that we define as the Pf, B, Tf axes and that are obtained from the P, B, T axes by a rotation of angle ϕ0/2 around B and toward the slip vector. This explains why the P, B, T axes gives reasonable estimates of the orientations of the principal stresses (Scheidegger, 1964) despite objections (McKenzie, 1969). However, whenever the fault plane can be distinguished from the auxiliary plane, Pf, B, Tf should give a better estimate (Raleigh et al., 1972). In an area where many fault planes are available and a uniform tensor is assumed, the scatter in the plane orientations contains information about both the relative position of σ2, represented by δ, and the relative stress difference s: the higher s or the closer δ to either 0 or 1, the more scatter. This information could then be extracted by inverse methods. Because a friction law would constrain these inverse method more tightly, it may show the necessity of nonuniform tensor to explain scattered fault plane.
Extension of the Najd Shear System from Saudi Arabia to the central eastern desert of Egypt based on integrated field and LANDSAT observations
Authors: Sultan Mohamed , Arvidson Raymond E. , Duncan Ian J. , Stern Robert J. , El Kaliouby Baher
Published: February 2008

The Najd Shear System in Saudi Arabia extends over 1200 km in a NW‐SE direction and has a width of approximately 300 km. A digital color mosaic, compiled from seven Landsat thematic mapper scenes, was used to delineate characteristic structural features of the Najd System in the Midyan region of Saudi Arabia and to search for similar features in the Egyptian Eastern Desert. The digital mosaic was generated using ratios of Landsat thematic mapper bands (bands 5/4 × 3/4, 5/1, 5/7) that are sensitive to the rock content of Fe‐bearing aluminosilicates, spectrally opaque phases, and hydroxyl‐bearing or carbonate minerals, respectively. The mosaic covers approximately 130,000 km² of late Proterozoic exposures of the Arabian‐Nubian Shield and has the Eastern Desert and the Midyan region placed in their approximate pre‐Red Sea locations. The Ajjaj Shear Zone (AJZ) marks the termination of the Najd System against the eastern margin of the Red Sea in the Midyan region. The AJZ aligns with the central Eastern Desert, based on analysis of pre‐Red Sea locations. Analyses of Landsat data and field observations show that the Ajjaj Shear Zone and the central Eastern Desert exhibit the following features in common: (1) outcrops that are generally elongate in a NW‐SE direction as a result of folding, with fine‐scale lithologic heterogeneity at the outcrop scale related to deformation associated with faulting; (2) NW trending left‐lateral faults and ductile shear zones; (3) subhorizontal, NW trending mineral lineations, and variably dipping NW trending foliations, with local changes in attitude around large competent (e.g., granitic) bodies; and (4) lithologic contacts that are generally tectonic in nature and related to faulting. These features are less common to the north and south of both the Ajjaj Shear Zone and the central Eastern Desert. Results are consistent with the Najd Shear System extending into the Eastern Desert and dominating the structural patterns within the central part of the Eastern Desert.
The Alleghanian Southern Appalachian Piedmont: A transpressional model
Authors: Gates Alexander E. , Speer J. Alexander , Pratt Thomas L.
Published: February 2008

Alleghanian dextral transpression produced multiple strike‐slip shear zones forming a contractional mega‐duplex in the central and southern Appalachian Piedmont. Structural upwelling along the zones is identifiable as crustal arching on seismic reflection profiles that roughly corresponds to the Piedmont gravity high. The structural upwelling coincides both temporally and spatially with areas of peak Alleghanian plutonism and metamorphism. The compositionally bimodal plutons generally occur in or around the fault zones and are isotopically primitive. Geobarometry of cognate xenocryst inclusions of coexisting plagioclase and pyroxene from a granitoid indicates 8–11 kbar or 31‐ to 42.5‐km depth to proposed source region. This depth roughly corresponds to the pre‐erosional location of multiple reflections beneath the arches on seismic profiles that are interpreted as the Moho. The granitoids are proposed to have formed by partial melting of the lowermost crust through heat from mafic intrusions and decompression resulting from crustal arching beneath the strike‐slip faults. The granitoids ascended along the strike‐slip faults and were emplaced in or near them. The crustal arching and melting processes appear to be important mechanisms in the development of strike‐slip orogens.
The North American Midcontinent Rift beneath Lake Superior from Glimpce seismic reflection profiling
Authors: Cannon W. F. , Green Alan G. , Hutchinson D. R. , Lee Myung , Milkereit Bernd , Behrendt John C. , Halls Henry C. , Green John C. , Dickas Albert B. , Morey G. B. , Sutcliffe Richard , Spencer C.
Published: February 2008

The Midcontinent rift system is a 1.1‐b.y.‐old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on‐shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low‐angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben‐bounding normal faults were reactivated as high‐angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.
Structural evolution of the Cape Smith Thrust Belt and the role of out‐of‐sequence faulting in the thickening of mountain belts
Authors: Lucas Stephen B.
Published: February 2008

Out‐of‐sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early Proterozoic Cape Smith Belt. The south verging thrust belt is marked by a complex history of accretion, internal deformation, thickening‐induced metamorphism, uplift, and erosion. An overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism. Prethermal peak deformation includes (1) initial piggyback stacking of basalt‐dominated thrust sheets above a basal décollement localized at the basement‐cover contact; (2) greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt; and (3) out‐of‐sequence thrusting, precipitating the metamorphic thermal peak both above and below these faults. Synthermal peak out‐of‐sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal décollement. Postthermal peak deformation is characterized by the continued development of out‐of‐sequence thrusts which appear to young toward the hinterland (north). Internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out‐of‐sequence faults. This suggests that (1) an eroding and accreting thrust wedge may be able to maintain its critical shape by out‐of‐sequence thrusting; and (2) this style of internal deformation is favored in belts dominated by relatively strong material (e.g., basalt, as in the Cape Smith Belt).
Isotopic studies bearing on the tectonics of the West Junggar Region, Xinjiang, China
Authors: Kwon S.‐T. , Tilton G. R. , Coleman R. G. , Feng Y.
Published: February 2008

Isotopic investigations from the Hercynian‐age fold belt between the Kazakhstan and Siberian cratons in the West Junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region. Sphene from the leucogabbro phase of the Tangbale ophiolite melange, the oldest member of the ophiolite sequences in the fold belt, yields an isotopic Pb‐Pb age of 523.2±7.2 Ma. Zircon from a postcollision alkali granite yields slightly discordant isotopic U‐Pb ages that indicate magma crystallization at 321.4±6.7 Ma, dating it in the Lower Carboniferous period. Radiometric dating thus documents a time span of circa 200 Ma for igneous activity in the area. Petrogenetic studies were made to test whether Precambrian crustal rocks might underlie the Junggar sedimentary basin. Initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid‐ocean ridge basalts in Pb isotope correlation diagrams. Sm‐Nd data from sphene and apatite from one of the granites yield an initial εNd(T)=+6.1. The granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks.