bluuebunny/crossref_metadata_2025 huggingface dataset:
Cenozoic collision of the Lesser Antilles Arc and continental South America and the origin of the El Pilar Fault
Authors: Speed R. C.
Published: February 2008

It is proposed that the Cenozoic tectonic record of the southern Lesser Antilles arc and northeastern continental South America can be explained by ongoing right‐oblique collision between the arc and continent. The collision has proceeded by the transport of the leading edge of the arc across the slope and outer shelf of a former north facing passive margin of the South American continent. The overriding began in the study region near the Gulf of Cariaco in eastern Venezuela in late Eocene or Oligocene time and has migrated with a generally SE vector. Suturing has occurred between the arc and continent after the attainment of a critical distance of overlap; today's point of suturing lies in the Paria Peninsula. East of there, overriding continues. Major tectonic elements engaged in or created by the collision are the southern Lesser Antilles magmatic arc, forearc basin, the Araya‐Tobago terrane, a South American foreland thrust and fold belt, and a foreland basin. The Araya‐Tobago terrane is thought to consist of sediments of South American provenance that were accreted to the Lesser Antilles forearc during its transit of an ocean basin and the continental slope and outer shelf. The emplacement of the magmatic arc and the Araya‐Tobago terrane caused tectonic imbrication of shelf strata to propagate ahead of the arc front as a foreland thrust and fold belt. Tectonic loading of the shelf also caused subsidence of a major foreland basin on the continentward side of the thrust belt. It is proposed the El Pilar fault exists between the Gulf of Cariaco and the Paria Peninsula as an active right slip fault but not east of Paria. It is not a throughgoing transform fault between the South American and Caribbean plates. The El Pilar fault exists where the overlapping arc and the continent are sutured and takes up a suture‐parallel component of convergence between arc and continent. The eastern tip of the fault propagates east with the point of suturing. Reconstructions of the Cenozoic collision of the Lesser Antilles arc and the South American continent suggest that the arc lay somewhat north and west of its present position in the Eocene. This conclusion differs from that of plate reconstructions that assume that the arc was the leading edge of a Caribbean plate that has moved east from Pacific longitudes since the Eocene.
Uplift of Tibetan Plateau
Authors: Zhao Wu‐Ling , Morgan W. Jason
Published: February 2008

A history of the elevation and crustal thickness of the Tibetan Plateau since the continental collision at 45 Ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume. In our reconstructions, the Indochina block was attached to the southern margin of Eurasia before being “extruded” to the southeast as evidenced by the opening of the South China Sea. This opening affected the mass input into Tibet so that Tibet did not uplift to an elevation higher than 1 km until early Miocene (when spreading of the South China Sea ceased). The model then gives a steady rise of Tibet from Middle Miocene to its present elevation.
Tectonics of slow spreading mid‐ocean ridges and consequences of a variable depth to the brittle/ductile transition
Authors: Harper Gregory D.
Published: February 2008

Geophysical evidence, especially microearthquakes that extend into the upper mantle beneath the inner floor of the Mid‐Atlantic and Gorda Ridges, indicates that the axial magma chamber episodically freezes beneath slow spreading ridges. Freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences. Most importantly, large near‐axis faults will intersect as the brittle/ductile transition deepens, causing some of the faults to become locked. The topographic effect of fault locking will be a variation in the width of the inner floor from narrow (< 2 km) when a magma chamber is present to wide (>10 km) when a magma chamber is absent. The deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle. The nature of these faults at depth is unknown, but the following hypotheses are presented: (1) the faults may be planar and pass downward into aseismic ductile shear zones, and (2) the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow. Such low‐angle normal faults are common in continental extensional domains. Large‐scale tilting (50°–70°) is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks (similar to toppling dominoes). Calculations of temperatures at the brittle/ductile transition are made assuming olivine and diabase rheologies for the mantle and crust, respectively. The temperature is approximately 800°C when the transition is at 8 km depth, and approximately 600° when the transition is at 2 km depth. The strength of the lithosphere varies dramatically with the rise and fall of the brittle/ductile transition as magma chambers form and subsequently freeze. Strong lateral temperature gradients will be present when a magma chamber is formed, resulting in localization of faulting within thin weak crust above the chamber. When the magma chamber freezes, lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area; this results in active faulting occurring over a width of as much as 60 km. Some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges. The entire crustal section of the Josephine ophiolite was tilted by approximately 50° prior to deposition of overlying pelagic sediments. This tilting probably took place by faulting, suggesting that oceanic faults extended into the upper mantle. Possible oceanic faults have been identified in the Josephine ophiolite, but their oceanic origin cannot be established beyond doubt.
Geologic and gravity evidence for thrust emplacement of the Smartville ophiolite
Authors: Ricci Margaret P. , Moores Eldridge M. , Verosub Kenneth L. , McClain James S.
Published: February 2008

Detailed structural relationships at the margins of ophiolites can provide important information about the mode of emplacement of ophiolitic terranes. In this study we use geologic mapping and gravity data to determine the structural and tectonic relations between a Middle to Late Jurassic volcanic arc complex (the Smartville Ophiolite block) and a pre‐Late Jurassic melange terrane in the northern Sierra Nevada foothills. The volcanic arc complex overlies the melange along a complex fault zone. Although this fault zone is steeply dipping wherever it is observed, map patterns suggest that the Smartville Upper Volcanic unit was originally emplaced along a subhorizontal fault that has been subsequently deformed. Two‐dimensional interactive computer analysis of gravity data from over 200 stations shows that the Smartville Upper Volcanic unit is a relatively shallow feature with a maximum thickness ranging from 0.5 to 3 km and that it was thrust as a sheet over the previously deformed melange. Both units were subsequently folded, giving rise to the steeply dipping faults observed at the surface. Recognition of low‐angle thrusting along the northern margin of the Smartville Ophiolite block has important implications for the mode of emplacement of the block. The most plausible mechanism for the thrusting of this particular oceanic island arc sequence onto the continental margin involves a subduction zone dipping westward beneath the Smartville Ophiolite block, similar to the present‐day situation in the Molucca Sea collision zone.
east‐trending dextral faults in the western Great Basin: An explanation for anomalous trends of pre‐Cenozoic strata and Cenozoic faults
Authors: Stewart John H.
Published: February 2008

Two postulated major east trending strike‐slip fault zones, here named the Coaldale and Excelsior fault zones, are delineated mainly on the basis of faults in Cenozoic rocks in eastern California and western Nevada and coincide with apparent major right‐lateral offsets in the distribution and facies of pre‐Cenozoic rocks. Apparent right‐lateral offset of pre‐Cenozoic rocks on the Coaldale fault zone is 60 to 80 km and on the Excelsior fault zone is 45 to 55 km. This offset accounts for major disruption in trends of pre‐Cenozoic rocks in eastern California and western Nevada, a disruption previously interpreted as the result of either an originally curving continental margin or tectonic distortion of originally linear trends by large‐scale bending (oroflexural folding) or by crustal‐scale folding related to north‐east‐southwest compression. Main offset on the Coaldale and Excelsior fault zones appears to be late Mesozoic in age. A pre‐mid‐Cretaceous age is indicated because mid‐Cretaceous and younger plutonic rocks are not cut by major strike‐slip faults on line with the fault zones. Major movement is probably younger than the Dunlap Formation, which is dated paleontologically as Early Jurassic but which may also contain rocks as young as Cretaceous. Local reactivation of the faults in the Cenozoic, perhaps under a different stress regime, accounts for offset of Cenozoic rocks. The Coaldale and Excelsior fault zones terminate major northwest trending late Cenozoic and possibly older faults. The Owens Valley‐White Mountain fault system in eastern California and the Furnace Creek fault zone in southeastern California and westernmost Nevada terminate northward at the Coaldale fault zone. A system of south‐east trending right‐lateral and high‐angle faults in western Nevada, including the Bettles Well fault, terminate southward at the Excelsior fault zone. Major movement on northwest trending faults such as the Furnace Creek and Bettles Well fault may be mostly late Cenozoic in age, but present information does not preclude the possibility that these fault zones initiated prior to, and are offset by, the Coaldale and Excelsior fault system. The unusual trends of pre‐Tertiary rocks and of the Coaldale and Excelsior fault zones in eastern California and western Nevada appear to be restricted to the Walker Lane belt, a broad northwest trending structural zone characterized by right‐lateral shear in western Nevada and eastern California. Possibly initial shear produced the northwest trending faults, and a later “kink” in this system produced oroflexural folding and the east‐trending right‐lateral Coaldale and Excelsior fault zones. This “kink” may have resulted from a change in the stress regime and structural complexities where the shear zone obliquely crossed the Paleozoic continental margin.
Paleomagnetic results from Honduras and their bearing on Caribbean tectonics
Authors: Gose Wulf A.
Published: February 2008

Paleomagnetic results have been obtained from 37 sites in central and western Honduras and one site in north‐western Nicaragua. The rocks are limestones, sandstones, and granites and range in age from 140 to 60 m.y. Regional fold‐tests demonstrate that the rocks carry a prefolding (pre‐Laramide) magnetization. The data indicate a very large clockwise rotation of the sampling area during the Early Cretaceous followed by a counter‐clockwise rotation in the Late Cretaceous. The changes in latitude are minor during this time period. It is highly unlikely that these large rotations apply to the entire Caribbean plate. Rather, they reflect the motion of a smaller area known as the Chortis Block. This interpretation is supported by paleomagnetic data from neighboring areas. The results also suggest that the Chortis Block sutured onto southern Mexico/Guatemala in Latest Cretaceous time in agreement with geochemical data and several plate tectonic models. These new data together with published paleomagnetic data from around the Caribbean imply that a rigid Caribbean plate did not exist until well into the Tertiary.
Petrologic constraints on imbrication, metamorphism, and uplift in the SW Tauern Window, eastern Alps
Authors: Selverstone Jane
Published: February 2008

The pressure‐temperature‐time‐deformation (PTtd) history of two tectonic units in the southwest Tauern Window (Austria/Italy) is evaluated from P‐T paths of metamorphism and uplift, geochronologic data, and observations on fabric development. Petrologic data indicate burial of the Lower Schieferhülle (LSH) to depths of ≥ 35 km and the adjacent Upper Schieferhülle (USH) to only 24–28 km; the units are now separated by only 2 km of section. Metamorphism was postkinematic in the LSH and synkinematic in the USH. Correlation of P‐T path reversals calculated from zoned garnets in the two units, however, indicates that juxtaposition of the LSH and USH occurred prior to metamorphism. Temperature‐time and depth‐time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface. Both the petrologic data and the depth‐time profiles for the LSH and USH point to a history of differential uplift in the early stages of decompression. Textural features indicate that the LSH behaved as an essentially rigid body during uplift, whereas the USH experienced extensive ductile shearing. The PTtd data for the LSH and USH are incorporated into a model for continental collision in the Eastern Alps. Initial tectonic uplift of the LSH was accompanied by ductile thinning of the USH and overlying Austroalpine units. Final uplift and erosion affected all of the units “en masse”.
Strain patterns in models of spreading‐gliding Nappes
Authors: Brun Jean‐Pierre , Merle Olivier
Published: February 2008

Three experiments have been carried out in order to study the progressive and finite strain in nappes where gravitational spreading and gliding occur together. These experimental models were made from analogue materials able to collapse under their own weight on an inclined plane. Throughout most of the models the stretch trajectories in a vertical plane parallel to the flow show a low‐amplitude sigmoidal pattern consistent with a previous theoretical model. Other stretch trajectories exist at the front and back ends of each model. The kinematic significance of all these trajectories is discussed in detail. Highest strain intensities are always found next to the base of the model. Strain paths calculated at different stages of flow indicate clearly that nowhere in the model is the motion simple. There is a complex combination in time and space of simple and pure shear, except at the base of the model where the motion approximates a simple shearing. Near the two parallel lateral sides of the model a lateral boundary effect can be observed from the strong curvature of the transverse markers (surface grid and vertical layers). Strain paths have also been estimated in this complex zone where wrench shearing is superimposed upon vertical shortening and thrust shearing. The geological implications of these experimental models are discussed in relation to recent theoretical studies and field work.
Model for Late Mesozoic‐Early Tertiary tectonics of coastal California and western Mexico and speculations on the origin of the San Andreas Fault
Authors: Beck Myrl E.
Published: February 2008

Cretaceous and early Tertiary arc and forearc rocks found along the coast in southern California and Baja California have been shown by paleomagnetic measurements to have originated many hundreds, or even several thousands, of kilometers south of their current locations. Northward transport also is found in Cretaceous batholithic rocks near the edge of the continent in Washington and British Columbia. The consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of North America have been translated along the coast by strike‐slip faulting. Strike‐slip faults are shown to be an expected response to north‐oblique subduction of the Farallon and/or Kula plates. Faulting and subduction probably were concurrent activities; i.e., the arc and forearc grew and were displaced simultaneously. Because displacement was dominantly parallel to subduction‐related lithic belts, arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts. Subduction‐related strike‐slip faulting is shown to be favored by high angles of oblique convergence, shallow dip of the subducting slab, relatively easy slip on potential faults within the arc or forearc, and the existence of a place for the moving sliver to go. It is suggested that these conditions are not difficult to satisfy and that subduction‐related strike‐slip faulting may thus be a common feature of convergent orogens. The location of the San Andreas fault inboard from the continental margin may be attributable to localization of the Pacific‐North American transform by a system of precollision subduction‐related faults.
Overlapping spreading centers: Implications from crack growth simulation by the displacement discontinuity method
Authors: Sempere Jean‐Christophe , Macdonald Ken C.
Published: February 2008

Overlapping spreading centers (OSC’s) are a fundamental aspect of accretionary processes at intermediate and fast‐spreading centers and typically occur at deep points along the axial depth profile. They have a characteristic geometry consisting of two en echelon overlapping, curving ridges separated by an elongated depression. The length to width ratio of this overlap basin is typically 3∶1. We have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity. A boundary element displacement discontinuity method was used to solve this problem. Our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region. For small OSC’s, there is a very good agreement between calculations and observations but, for large ones, the overlap basin tends to be longer than our predicted results indicate. This suggests that the assumptions made in the model (i.e., perfectly elastic, isotropic and homogeneous medium) are perhaps valid for the brittle lid above the magma chamber that underlies OSC’s with small offsets (< 2 km) but oversimplified for OSC’s with large offsets. Our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach. The deflection will be the greatest for small misalignments of the fracture systems, thus even minor misalignments of the spreading centers may result in the development of OSC’s. Where the misalignment is less than the width of the cracking front, the fracture systems may meet head‐on creating a saddle point along the axial depth profile. Our results support the hypothesis suggested by Macdonald et al. [1984] in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned.
Paleomagnetic evidence for tectonic rotations in the Variscan Mountain Belt
Authors: Perroud Hervé
Published: February 2008

A critical review of Devonian and Carboniferous paleomagnetic results from the Hercynian basement of Western and Central Europe reveals large rotations. Their geographic distribution suggests a tectonic origin for large scale curved structures, such as the Ibero‐Armorican arc or the Variscan V, together with a coherence of the Hercynian domain with time. Geodynamic models which can have produced the observed rotation pattern are discussed. It appears that lateral compression, due to the accommodation of continental collision as in the India‐Asia convergence, is a plausible mechanism although Variscan and Himalayan mountain belts have different characteristics.
Deformation and motion in the Western Alpine Arc
Authors: Choukroune Pierre , Ballèvre Michel , Cobbold Peter , Gautier Yann , Merle Olivier , Vuichard Jean‐Paul
Published: February 2008

We attempt to reconstruct the kinematics (deformation and motion) of the Western Alpine Arc between the Mercantour and Aar Massifs. First, we compile a map showing principal extension (X) direction within three major structural units. In the external zone, X directions are broadly radial to the Alpine Arc; in the internal zone, they are uniform or smoothly varying in some domains; whereas in the intermediate zone they often show much scatter. Next, we use geochronological data and P‐T conditions, estimated from metamorphic mineral reactions, to time the deformation. On each of a series of interval maps, we show X‐directions for strain accumulated during a chosen time interval. We argue that these incremental X‐directions are approximately equivalent to segments of particle paths, showing displacements relative to stable Europe. For 120–100 Ma, the displacement direction was about N150 and coeval with eclogite facies metamorphism; it is interpreted as a direction of overthrusting by the African Plate. Around 40 Ma, the displacement in the south part of the arc became westerly. Finally, between 25 and 15 Ma, displacement directions were generally radial to the arc.
Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere
Authors: Malinverno Alberto , Ryan William B. F.
Published: February 2008

Previously proposed models for the evolution of the Tyrrhenian basin‐Apenninic arc system do not seem to satisfactorily explain the dynamic relationship between extension in the Tyrrhenian and compression in the Apennines. The most important regional plate kinematic constraints that any model has to satisfy in this case are: (1) the timing of extension in the Tyrrhenian and compression in the Apennines, (2) the amount of shortening in the Apennines, (3) the amount of extension in the Tyrrhenian, and (4) Africa‐Europe relative motion. The estimated contemporaneous (post‐middle Miocene) amounts of extension in the Tyrrhenian and of shortening in the Apennines appear to be very similar. The extension in the Tyrrhenian Sea is mostly accomplished in an E‐W direction, and cannot be straightforwardly related to the calculated N‐S Africa‐Europe convergence. A model of outward arc migration fits all these constraints. In a subducting system, the subduction zone is expected to migrate outward due to the sinking of the underthrusting plate into the mantle. The formation of a back‐arc or internal basin, i.e. of a basin internal to the surrounding belt of compression, (in this case the Tyrrhenian Sea) is then expected to take place if the motion of the overriding plate does not compensate for the retreat of the subduction zone. The sediment cover will be stripped from the underthrusting plate by the outward migrating arc of the overriding plate, and will accumulate to form an accretionary wedge. This accretionary body will grow outward in time, and will eventually become an orogenic belt, (in this case the present Apennines) when the migrating arc collides with the stable continental foreland on the subducting plate. An arc migration model satisfactorily accounts for the basic features of the Tyrrhenian‐Apennine system and for its evolution from 17 Ma to the present, and appears to be analogous to the tectonic evolution of other back‐arc settings both inside and outside the Mediterranean region. An interesting implication of the proposed accretionary origin of the Apennines is that the problematic “Argille Scagliose” (scaly clays) melange units might have been emplaced as overpressured mud diapirs, as observed in other accretionary prisms, and not by gravity slides from the internal zones.
The deep crustal structure of the Mojave Desert, California, from Cocorp seismic reflection data
Authors: Cheadle M. J. , Czuchra B. L. , Byrne T. , Ando C. J. , Oliver J. E. , Brown L. D. , Kaufman S. , Malin P. E. , Phinney R. A.
Published: February 2008

COCORP seismic reflection profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low‐angle reflecting horizons within the crust. These complex, though laterally continuous, horizons are interpreted to represent major southwesterly dipping crustal fault zones, and as such they place important constraints on the tectonic evolution of the region. The upper‐most horizon is interpreted to be the Rand thrust, which, where exposed, places Precambrian and Late Cretaceous crystalline rocks over possibly younger Pelona‐Rand‐Orocopia Schist. This reflecting horizon extends for 25 km southwest of the Rand Mountains where it appears to be truncated at a depth of about 7.4 km by another horizon, which may be a later low‐angle normal fault. The other reflecting horizons are not traceable to the surface, and so greater ambiguity remains in their interpretation. The most prominent of these horizons occurs at midcrustal depths (15±6 km), exhibits a “ramp and flat” geometry, and extends over the northern area of the Mojave survey into the Basin and Range Province. A lower horizon, at depths of 20–30 km in the northern part of the survey area, is antiformal and appears to terminate above a flat and relatively continuous Moho‐depth horizon. The crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s (33 km) in the north of the survey and at about 8–9 s (26–29 km) in the south. These reflections are offset in the vicinity of the town of Mojave. The deep intracrustal fault zones inferred from the COCORP survey may represent (1) the deep crustal continuation of the system of Mesozoic thrusts which crops out in southern Nevada and southeastern California, (2) Late Cretaceous to Early Cenozoic, northeast‐vergent thrusts related to the uplift of the Pelona‐Orocopia‐Rand Schist, or (3) low‐angle normal faults related to Early Miocene, northeast‐southwest directed crustal extension. The COCORP survey also traversed the major strike‐slip faults that bound the Mojave block. The San Andreas fault zone appears to truncate reflectors at depths of 6, 8 and 20 km within the Mojave basement, suggesting that it is a major vertical feature which extends to at least 20 km depth. Conversely, the Garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature.
Deep crustal structure and flexure of the Arabian Plate Beneath the Zagros collisional mountain belt as inferred from gravity observations
Authors: Snyder David B. , Barazangi Muawia
Published: February 2008

The Zagros mountain belt region, a Neogene continental collisional zone between Arabia and Iran that is characterized by a suture, a folded belt, and a foreland basin, is unusual among active mountain belts in that its 6–12 km thick sedimentary cover is shortening predominantly by folding while the basement is apparently thickening along numerous high‐angle reverse faults. We have used over 9000 gravity measurements in the region in conjunction with limited seismological observations and geological cross sections of the uppermost 10 km of crust to infer the deeper crustal structure beneath the region. Crustal models constrained by the Bouguer anomalies and other seismological and geological data are characterized by a Moho that dips about 1° to the northeast beneath the folded belt and increases in dip to about 5° near the Main Zagros Thrust (MZT); the Moho depth increases from 40 km beneath the leading edge of the foreland basin (the Mesopotamian foredeep and Persian Gulf) to as much as 65 km beneath the MZT. Alternative models that incorporate a subducted oceanic crust that is attached to the underthrusting Arabian crust deviate little in their Moho configurations from the above simpler models. Negative isostatic residual anomalies are interpreted to indicate local overcompensation beneath the foreland basin and near the folded belt and beneath the MZT. We show that underthrust sedimentary rocks of the converging margins along the suture zone may account for the negative isostatic residual anomalies near the MZT. However, to explain the rest of the isostatic anomalies requires other forces to be acting on this collisional plate boundary. For example, elastic flexure models required a combination of topography and subsurface loads or downward force (the origin of which is not clear) to approximate the Bouguer or isostatic residual anomalies associated with the foreland basin and the folded belt. The topography load of the Zagros mountain belt is insufficient to cause the required deflection of the underthrusting Arabian plate. A combination of isostatic, elastic flexure, and horizontal compression forces acting on the edge of the Arabian craton and the transitional lithosphere of central Iran appears to best model the crust of the Zagros region. We propose a model in which the lower crust of the converging Arabian plate located beneath the Zagros is being shortened and thickened plastically between the seismogenic, rigid upper crust and a rigid uppermost mantle lid. The mantle lid, probably decoupled from the lowermost crust, un der thrusts the Iranian crustal blocks. Thus confined from below, the horizontally compressed, plastic lower crust hydraulically depresses the Moho and raises the faulted and folded, brittle upper crust in isostatic equilibrium.
Eocene underplating along the Kodiak Shelf, Alaska: Implications and regional correlations
Authors: Byrne Tim
Published: February 2008

Structural geology and geophysical data from the Kodiak Shelf suggest that the Mesozoic rocks exposed on the shelf are structurally underlain (at about 12 km depth) by several kms of Eocene age strata. Kinematic data from the Late Cretaceous to Paleocene Ghost Rocks Formation indicate that this formation and probably all of the Kodiak Islands, were uplifted vertically to nearly their present elevations. Landward tilting and imbrication are not indicated. The age of uplift is indicated by a regional, angular unconformity of Early Eocene to Early Oligocene age that separates deep‐sea rocks from shallow water to non‐marine rocks. The uplift of the accretionary prism is believed to have been caused by underplating of an Eocene sedimentary sequence because (1) a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and (2) an obductively offscraped sequence of Eocene deep‐sea rocks crops out on the seaward side of the Kodiak Shelf, suggesting that a thick trench‐fill sequence may have been present prior to uplift of the prism. The underplated material is interpreted to be part of either a previously unrecognized turbidite fan of Early Eocene age or a proximal equivalent of the Zodiac fan of Late Eocene to Early Oligocene age. Other possible on‐land remnants of the underplated material may be present in Prince William Sound (the Montague belt), the Gulf of Alaska (lower sections of the Yakutat block) and in the Coast Ranges of Oregon and Washington. The large volume of underplated material beneath the Kodiak shelf suggests that underplating may be the dominant process in the growth of convergent margins.
Subduction erosion versus sediment offscraping at the toe of the Middle America Trench off Guatemala
Authors: Moore Gregory F. , Shipley Thomas H. , Lonsdale Peter F.
Published: February 2008

High‐resolution surface ship and deep‐tow data from the Middle America Trench off Guatemala demonstrate that structures at the base of the landward slope are most simply interpreted as resulting from the offscraping and accretion of the uppermost trench sediments. There is a 250 to 300‐m‐wide ridge elevated 40–60 m above the trench floor at the toe of the trench slope. Uplifted trench sediments are resolved on the ridge in one deep‐tow profiler record. Trench strata beneath the ridge are imaged on migrated seismic reflection profiles, which show evidence for folding of the trench strata. Vergence of the structures is consistent with folding above a landward dipping thrust fault. We therefore interpret the ridge at the base of the slope as the surface expression of folded trench sediments that are presently being offscraped and accreted to the toe of the trench slope. Only the upper hundred or so meters of trench strata are offscraped; the remainder of the trench strata and the underlying plate deposits are subducted beneath the toe of the slope. Trench sediment fill buries much of the horst topography, which is then passively subducted without eroding the base of the slope. Our results indicate that (1) non‐steady state accretion has occurred at this margin, even though Deep Sea Drilling Project drilling suggested otherwise, and (2) the lower‐most slope has been affected by compressional deformation rather than extension or slumping.
Apparent western termination of the North Pyrenean Fault and tectonostratigraphic units of the western north Pyrenees, France and Spain
Authors: Hall C. A. , Johnson J. A.
Published: February 2008

The surface geology of the North Pyrenees, southern France, is markedly different east and west of longitude 0°25′W (a line just west of Lourdes). East of this longitude the North Pyrenean fault (NPF), the major structure in the North Pyrenees, is clearly identifiable, and this fault and other faults in the North Pyrenean zone are subvertical. West of longitude 0°25′W the NPF is not identifiable at the earth's surface, suggesting to some that the NPF is not continuous and is not a regionally significant feature and that low‐angle as well as high‐angle faults are present in the fault zone. While east of Lourdes there is both a relatively continuous belt of Mesozoic metamorphic rock and a geophysically detected 15‐km step in depth to the Moho, neither is clearly recognized west of Lourdes, again bringing into question the continuity of the NPF. However, in order to accommodate opening of the Bay of Biscay, as much as 400 km of translation along a continuous NPF or North Pyrenean fault zone (NPFZ) is required. We have compiled a map covering a 100‐km‐long area in the western Pyrenees. The compilaation is based on our mapping and published maps by others. The compilation and our field work provide the data to delineate four new tectonostratigraphic units. These units are coherent stratigraphic sequences characterized by marked differences in stratigraphic thickness and depositional history. Each unit is bounded by regional faults that delineate four tectonic sheets. The principal stratigraphic differences between the four tectonostratigraphic units were produced during deposition in Cretaceous basins along the northern Pyrenees. The regional bounding faults are reverse and were formed during the main phase of the Pyrenean orogeny, that is, during Late Cretaceous and Tertiary times. Some are reactivated basin‐boundary faults and some of these are reactivated faults of Cretaceous or older age. In the western Pyrenees the NPF, which was an active left‐slip fault during the Late Jurassic and Early Cretaceous, is cut by these faults and is covered by tectonic sheets. These sheets, formed during the Paleogene, account for the apparent termination of the NPF near longitude 0°25′W and obscure the step in the Moho.
Paleomagnetic evidence for Neogene rotational deformations in the Aegean domain
Authors: Kissel C. , Laj C. , Poisson A. , Savaşçin Y. , Simeakis K. , Mercier J. L.
Published: February 2008

New paleomagnetic data are reported from the islands of Skyros and Lesbos in Central Aegea and the Izmir‐Foça region in Western Anatolia. Together with previous results obtained in Northwestern Greece and with results from Northern Aegea reported by others authors, these data suggest that the Lower Miocene arc was almost rectilinear, trending E‐W and that the curvature was acquired by opposed rotational deformations at each termination. Moreover, the observed inclinations, which are shallower than those expected on the basis of a geocentered dipole field, suggest a large northward drift since Middle Miocene.
The Mesozoic‐Cenozoic tectonothermal evolution of the Ruby Mountains, East Humboldt Range, Nevada: A Cordilleran Metamorphic Core Complex
Authors: Dallmeyer R. D. , Snoke A. W. , McKee E. H.
Published: February 2008

A combined K‐Ar and40Ar/39Ar geochronological study of the Ruby Mountains‐East Humboldt Range, Nevada, a Cordilleran metamorphic core complex, has revealed a complex tectonothermal history. The three structural subdivisions of the core complex (migmatitic core, mylonitic zone, and cover) share magmatic and deformational histories in part, but also display important contrasts in structural style, metamorphic paragenesis, and thermal history. During the Late Jurassic (ca 160 Ma), miogeoclinal rocks were polyphase‐deformed, metamorphosed under amphibolite facies conditions, and pervasively intruded by peraluminous granitic magmas, thereby forming an igneous and metamorphic complex. Late Jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks. Late Cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved Cretaceous incremental‐release40Ar/39Ar hornblende spectra and by U‐Pb monazite ages from pelitic schist exposed in the northern Ruby Mountains. Between ca 45 Ma and 20 Ma, the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low‐angle, normal‐sense, simple shear zones. As a result, rocks metamorphosed at depth in the Mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals. Near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation. However, it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 Ma metamorphism or if distinct, locally superposed late Mesozoic and/or middle Tertiary thermal overprints had affected the cooling history. A major extensional simple shear zone is marked by an approximately 1‐kilometer‐thick mylonite zone that experienced late kinematic, superposed brittle deformation. A geometric consequence of the development of low‐angle extensional fault zones is the eventual translation of colder rocks over hotter rocks. Such a history is supported by the40Ar/39Ar data. For example, mylonitic mafic orthogneisses from a major allochthon record40Ar/39Ar hornblende plateau ages between 44 and 48 Ma and biotite plateau ages between 32 and 33 Ma, and are structurally positioned above mylonitic rocks of the northern Ruby Mountains which record biotite cooling ages of ca 22–24 Ma. Such relations suggest either multiple episodes of Tertiary mylonitization or a protracted, multiphase history of mylonitization. However, after ca 20 Ma, all of the presently exposed mylonitic rocks were maintained below 100°C and have only experienced upper crustal brittle deformation. This article contains supplementary material.
Structure and U/Pb geochronology of Central Hoggar (Algeria): A reappraisal of its Pan‐African evolution
Authors: Bertrand Jean‐Michel , Michard Annie , Boullier Anne‐Marie , Dautel Danielle
Published: February 2008

Discovery of large‐scale deep‐seated thrusts in Central Hoggar, with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation, has led us to reconsider the Pan‐African tectonic and metamorphic history in that region. Two areas are described, and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust. The age and kinematics of this structural reworking have been approached using U/Pb zircon dating in the Tamanrasset region. Despite the difficulty of estimating the age of the initiation of the assumed intracontinental A‐type subduction, the results provide a time span of 30–40 m.y. between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes. Some key ages were determined: (1) 2075 ± 30 Ma is the age given by granulite facies remnants which escaped from the refoliation, the corresponding lower intercept at 530 ± 70 Ma confirms the Pan‐African imprint; (2) 615 ± 5 Ma reflecting the age of syntectonic to late‐tectonic granitoids emplaced in reworked gneisses and in preserved granulites; (3) 580 Ma, the concordant age of sphenes and monazites from the same granitoids, which is interpreted as corresponding to the end of medium‐grade conditions. No evidence has been found of a ∼1000 Ma age: a Kibaran event does not appear to exist in Central Hoggar. The age similarity between the observed deep intracontinental evolution of Central Hoggar and the collision‐related tectonics of Western Hoggar and Iforas suggests a common origin for both phenomena.
Cocorp deep seismic reflection traverse of the interior of the North American Cordillera, Washington and Idaho: Implications for orogenic evolution
Authors: Potter Christopher J. , Sanford William E. , Yoos Theodore R. , Prussen Eleanor I. , Keach R. William , Oliver Jack E. , Kaufman Sidney , Brown Larry D.
Published: February 2008

Deep seismic reflection data collected by the Consortium for Continental Reflection Profiling along a transect of the interior of the northwestern U. S. Cordillera indicate the presence of crustpenetrating faults; these faults developed during an orogenic cycle consisting of Jurassic to Paleocene accretion‐related crustal thickening followed by Eocene crustal thinning. The reflection data also suggest that the Moho is a young, dynamic feature which developed a relatively smooth, flat geometry beneath a complexly deformed crust, probably during an Eocene episode of crustal extension. The profiles reveal prominent west dipping reflection sequences (and associated diffractions) interpreted as a Mesozoic thrust system of crustal dimensions that linked accretion‐related shortening in the western Cordillera with the thrust belt in the eastern Cordillera, east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated Eocene crustal extension, and essentially flat Moho reflections at 33–35 km beneath complexly deformed crust. These features are consistent with a sequence of (1) late Mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the North American passive continental margin along a Jurassic thrust (2) propagation of crust‐penetrating thrusts to shallower levels in the east, where they probably formed the sole of the Rocky Mountain Thrust Belt (3) Eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and (4) development of a flat Moho geometry during Cenozoic extension and magmatism. These data provide new information about the structure of the entire crust in the Cordilleran interior and offer a working model, both for evolution of Cordilleran core complexes and possibly for other collisional orogens.
Palaeozoic terrane accretion in the Western Irish Caledonides
Authors: Hutton D. H. W. , Dewey J. F.
Published: February 2008

The southern boundary of the early Caledonides in the British Isles (The Dalradian Supergroup) is represented by the major Highland Boundary‐Fair Head‐Clew Bay line. In the west of Ireland, accretion along this (Laurasian) margin occurred by sinistral strike slip during two phases. The earliest was responsible for the docking of the Connemara terrane, a displaced part of the Dalradian, outboard of the South Mayo Trough in the Llanvirnian‐Llandeilian. This event is recorded independently by radiometrically dated sinistral strike slip deformation in the Ox Mountains of western Ireland. Following deposition of a Silurian overlap sequence, sinistral movements were renewed between the upper Llandoverian and the Siegennian. This event is part of a major phase of sinistral strike slip throughout the British Isles, involving accretion of the Cadomian terrane and the Devonian accretion of the Avalon terrane in the Northern Appalachians.
Stress fields in the European platform at the time of Africa‐Eurasia collision
Authors: Bergerat Françoise
Published: February 2008

An analysis of Cenozoic brittle deformation in the European platform has been carried out between the Bohemian Massif to the east and the Western Mediterranean Sea to the west. A succession of four main paleostress fields has been defined: (1) N‐;S compression of late Eocene age. This event induced the formation of the “German‐Czech triangle,” bounded on the east by the NW‐SE dextral strike‐slip faults of Pfahl and Franconia and on the west by the NNE‐SSW sinistral strike‐slip faults along the axis of the future west European Rift, (2) E‐W Oligocene extension that opened the west European Rift. (3) NE‐SW compression, early Miocene in age, that reactivated the Pfahl fault line (as a reverse fault) and the main faults of the Rhinegraben (as dextral strike‐slip faults). (4) Finally, since the end of the Miocene, a fan‐shaped distribution of directions of compression has developed at the periphery of the Alpine arc. However, farther from the Alpine chain, a more consistent direction of compression has dominated (first NW‐SE, then NNW‐SSE). A comparison with plate tectonic data demonstrates that this succession of tectonic events is compatible with the reconstruction of relative movements between Africa and Eurasia during the Cenozoic collision. However, some local stress patterns, close to the Alps, are clearly related to the local evolution of the Alpine arc.
Passive margin formation by low‐angle faulting within the upper crust: The Northern Bay of Biscay Margin
Authors: Le Pichon X. , Barbier F.
Published: February 2008

We use a detailed 6500 km commercial seismic reflection survey over the lower northern margin of the Bay of Biscay to discuss the structure of the thinned lower margin crust. Our study demonstrates that the “S” reflecting horizon, at the base of the tilted block layer, progressively cuts into higher levels of the upper continental crust oceanward. This S horizon is best interpreted as a low‐angle normal fault within the upper crust with an original dip toward the continent. We use previously acquired seismic refraction work to demonstrate that the S horizon is the top of a highly thinned high‐velocity lower continental crust layer. The crustal layers above and below S both thicken toward the continent. We consequently interpret the lower crust as a ductile zone of decoupling between the upper brittle crust and the upper presumably brittle lithospheric mantle. We present a model which incorporates those features.
Crustal evolution of northern Alaska inferred from sedimentology and structural relations of the Kandik area
Authors: Howell David G. , Wiley Thomas J.
Published: February 2008

One of the most complete stratigraphic sections in North America crops out in east central Alaska and western Yukon Territory north of the Tintina fault. These uppermost Precambrian to uppermost Mesozoic strata record, in ascending order: a late Precambrian and Paleozoic continental rifting and margin subsidence history, a Late Devonian orogenic pulse possibly coinciding with a eustatic low stand, continental margin uplift of at least 3 km in the Carboniferous, renewed subsidence in Permian and Triassic time, uplift of the ancestral Brooks Range in the Early Cretaceous, opening of the Canada Basin in mid‐Cretaceous to Late Cretaceous time resulting in counterclockwise rotation of all of northern Alaska and parts of western Yukon Territory including the Kandik area, and a crustal sag that evolved into the Yukon Flats. The latter resulted from transtensional slip along the Tintina fault reflecting plate motions dictated by kinematics of the Pacific realm. The rotation of northern Alaska was about a pole located near Mackenzie Delta and transpired between Barremian and Campanian time (circa 120 to 80 Ma).