bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for east‐west crustal extension in the Eastern Alps: Implications for the unroofing history of the Tauern window
Authors: Selverstone Jane
Published: February 2008

The contact between the western border of the Tauern Window and the overlying Austroalpine nappes (Austria and Italy) is marked by a structural, petrologic, and geochronologic discontinuity that developed in the early stages of unroofing of the window. Within the window, pressure‐temperature‐time paths (J. Selverstone, 1985) indicate that significant ductile thinning occurred shortly after the cessation of thrusting (Paleocene ‐ Eocene). Sense‐of‐shear indicators (rotated porphyroblasts, asymmetric augen and pressure shadows, S‐C fabrics) associated with a west plunging stretching lineation indicate that this thinning occurred in response to top‐to‐the‐west low‐angle normal shear during metamorphism. The same sense of normal shear is present in the Mesozoic members of the Austroalpine sequence west of the window. In these rocks, however, west directed shear occurred at low temperatures (<300°C) after the thermal peak of metamorphism. Structural data and the regional geometry imply that the western end of the Tauern Window is a low‐angle normal fault (Brenner Line) that juxtaposed brittlely deformed rocks of the Austroalpine nappes against more ductilely deformed rocks from the window. Biotite K/Ar and Rb/Sr data suggest a Miocene age for final movement on the Brenner Line. These data indicate a prolonged history of east‐west extension in the Eastern Alps that affected all crustal levels (early ductile thinning of lower crust followed by mid‐to‐upper‐crustal low‐angle normal faulting). A model is presented that relates the young east‐west extension to displacement transfer from dextral movement on the Periadriatic Lineament. In this model, the Brenner Line and, by analogy, the Simplon Line can be thought of as detachment faults that tectonically unroofed the Tauern Window and Lepontine thermal highs, respectively.
The origin of summit basins of the Aleutian Ridge: Implications for block rotation of an arc massif
Authors: Geist Eric L. , Childs Jonathan R. , Scholl David W.
Published: February 2008

It is proposed that many summit basins along the Aleutian Arc form from the clockwise rotation of blocks of the arc massif. Summit basins are arc‐parallel grabens or half‐grabens formed within the arc massif and are commonly located near or along the axis of late Cenozoic volcanism. Geomorphically, the Aleutian Arc appears to consist of contiguous rhombic blocks of varying size, tens to hundreds of kilometers in length. The boundaries between adjacent blocks are delineated by fault‐controlled canyons that cut the southern slope of the arc transverse to its regional trend. Evidence that these blocks have rotated clockwise is provided by the triangular‐shaped summit basins bordering the blocks to the north, oblique physiographic trends, offsets in the summit platform, and broad deflections in the southern slope of the arc. We present a model for block rotation that involves translation of blocks parallel to an arc. It is suggested that block rotation, which appears to have accelerated in late Cenozoic time, is linked to (1) a shift in the Euler pole for the Pacific plate, (2) the consequential start‐up of late Cenozoic volcanism, (3) improved interplate coupling instigated by sediment flooding of the Aleutian Trench, and (4) westward subduction of northeast striking segments of the inactive Kula‐Pacific Ridge.
Mud diapirism and subcretion in the Barbados Ridge Accretionary Complex: The role of fluids in accretionary processes
Authors: Brown Kevin , Westbrook G. K.
Published: February 2008

Mud diapirism has recently been recognized in several modern accretionary wedges. It provides an important means of dewatering accretionary wedges and should be regarded as an important process for producing the melanges found in both modern and ancient accretionary terranes. Mud diapirism affects a large area of the Barbados Ridge Accretionary Complex. The distribution of the mud diapirs appears to be primarily controlled by the presence of underconsolidated terrigenous submarine fan deposits that are being accreted to the complex. The frequency of diapir occurrence decreases northward as the fan becomes thinner. Mud diapirs are absent from the very eastern most part of the complex formed from sediments accreted at its toe, with the exception of a few mud volcanoes on the ocean floor in front of the complex. The initiation of diapirism appears to be spatially coincident with the onset of subcretion, or underplating, of sediment to the base of the complex at a ramp between two levels of decollement. It is proposed that the release of mud and pore water from the subcreted sediments is a direct or indirect cause of most of the mud diapirism in the accretionary complex. There is a range of diapiric form dependent on the viscosity of the mud, from mud volcanoes fed by low viscosity mud, to higher viscosity mud ridges. The diapirs in the eastern areas of the complex are generally mud volcanoes with narrow conduits feeding a surface mound. Mud ridges are prominent in the western parts of the complex. This is interpreted as reflecting a general westward decrease in the fluid content of the accretionary complex. Bottom‐simulating seismic reflectors formed by gas hydrate are commonly developed in the areas of mud volcano occurrences. The presence of the hydrate indicates that large volumes of methane are being generated at depth in these regions. The generation of methane may be contributing to zones of overpressuring in the wedge. Methane may also be partly responsible for driving the diapiric material to the surface to form mud volcanoes. Ridges in the subducting oceanic crust beneath the accretionary complex locally enhance diapirism above their crests and southern flanks. Faults formed later in the development of the complex are more commonly associated with diapirism than those resulting from accretion at the toe of the wedge. These later faults play an important role in controlling the sites of individual mud volcanoes, chains of mud volcanoes, and mud ridges.
Further paleomagnetic evidence for oroclinal rotation in the central folded Appalachians from the Bloomsburg and the Mauch Chunk Formations
Authors: Kent Dennis V.
Published: February 2008

Renewed paleomagnetic investigations of red beds of the Upper Silurian Bloomsburg and the Lower Carboniferous Mauch Chunk Formations were undertaken with the objective of obtaining evidence regarding the possibility of oroclinal bending as contributing to the arcuate structural trend of the Pennsylvania salient. These formations crop out on both limbs of the salient and earlier, but less definitive paleomagnetic studies on these units indicate that early acquired magnetizations can be recovered. Oriented samples were obtained from nine sites on the southern limb of the salient and eight sites from the northern limb in the Bloomsburg. The natural remanent magnetizations are multivectorial, dominated by a component (B) with a distributed spectrum of unblocking temperatures ranging up to 670°C, and a component (C) with a higher and very discrete distribution of unblocking temperatures. The B component is uniformly of reverse polarity, shows a statistically significant synfolding character, and represents a Late Paleozoic remagnetization. The C component passes fold tests with normal and reverse polarity site means. The C component directions from the southern limb (345.1°/−31.6°) and the northern limb (359.3°/−29.7°) are significantly different in declination (14.2°±10.4°) but not in inclination (1.9°±9°). Samples were also analyzed from seven additional sites in the Mauch Chunk on the southern limb of the salient. Inclusion of these new data gives a revised estimate of the difference between southern and northern limb mean directions of prefolding magnetizations in the Mauch Chunk of 23.3°±12.5 in declination and 4.8°±11° in inclination. Paleomagnetic data from the Bloomsburg, Mauch Chunk, and revised results recently reported for the Upper Devonian Catskill Formation together indicate 22.8°±11.9° of relative rotation, accounting for approximately half the present change in structural trend around the Pennsylvania salient. The oroclinal rotation can be regarded as a tightening of a less arcuate depositional package that developed across a basement reentrant, to achieve a curvature closer to that of the earlier zigzag continental margin outline.
Deflection, gravity anomalies and tectonics of doubly subducted continental lithosphere: Adriatic and Ionian seas
Authors: Moretti Isabelle , Royden Leigh
Published: February 2008

Three profiles across the southern Adriatic and northern Ionian seas span an intermediate foreland area that is or has recently been subducted both westward under the Apennine‐Calabrian thrust belt and eastward under the Hellenide belt Gravity and deflection data (from the base of the Pliocene) along the three profiles are qualitatively similar to those predicted for a thin elastic sheet subject to end loads at both plate ends. More detailed analysis of the profiles shows very good agreement between observations and predictions if the intermediate foreland lithosphere is treated as a thin elastic sheet with an effective elastic thickness between about 15 and 20 km (equivalent to a flexural rigidity between 2×1022 and 6×1022 Nm). These results, coupled with seismicity data, suggest that active convergence within the Hellenides extends as far north as southwestern Yugoslavia. Analysis of the northernmost profile, across the southern Adriatic Sea, shows a well‐constrained elastic plate thickness of 14 km and an initial water depth, prior to loading, of about 1 km. The distance between the two subduction systems on this profile is estimated at about 275 km. Analysis of the two more southerly profiles yields a minimum effective elastic plate thickness of about 15 km, but no maximum value could be determined. The initial water depths for these two profiles are well constrained at 1.3 and 1.6 km prior to loading, and the distance between the two subduction systems is estimated at about 175 km on both profiles. Along the northwest trend of the two opposing subduction systems the change in distance between the two systems occurs abruptly, and the Hellenic subduction system appears to be segmented into a series of northwest trending, roughly linear subduction segments. The calculated deflection profiles also show pronounced asymmetry on the northernmost profile and an opposite sense of asymmety on the southernmost profile.
Evidence for shear zones in the lower crust offshore Britain
Authors: Reston T. J.
Published: February 2008

Lower crustal reflections seen on British Institutions Reflection Profiling Syndicate (BIRPS) deep seismic reflection data, recorded offshore the United Kingdom, are commonly short, convex‐up, and crosscutting. Consequently, the lower crust should not be described as layered, as much of the reflecting structure is not fully resolved and may be quite complex. Moreover, the lower crustal reflective zone (LCRZ) is not a uniformly reflective layer, but contains reflective bands that wrap around nonreflective (“transparent”) zones. Because transparent zones probably correspond to a lack of a reflecting fabric, understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust. Although transparent zones may have a variety of explanations (e.g., large shear pods or frozen magma bodies), the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low‐strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip‐line field theory and by drawing analogies with structures developed in highly tectonised high‐grade metamorphic terrains. This interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms. In particular, extension may be accommodated by the relative movement of low‐strain lozenges along intervening zones of simple shear. The lower crust may be stretched during extension, but it must also transfer deformation between the upper crust and the mantle. The observation on BIRPS data of mantle reflections, interpreted as mantle shear zones, implies that deformation in the mantle is localised. However, the manue reflections are never colinear with faults in the upper crust, requiring, at least locally, a component of sub‐horizontal bulk strain within the lower crust, which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the LCRZ.
The Ecors Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt
Authors: Choukroune P.
Published: February 2008

The 250‐km‐long deep seismic survey across the Pyrenees from the Aquitaine basin to the Ebro basin was sponsored by French and Spanish organizations, and was carried out in 1985 and 1986. This first seismic survey of an entire orogenic belt crosses the deformed boundary between the Iberian and European plates, the geometry of which has been greatly debated during a recent past. The main results are summarized as follows: (1) The entire crust shows well‐defined reflectors with a general fan shape geometry. (2) The Iberian crust seems to be thicker than the European one. Both are limited by a well‐defined deep layered material above the Moho discontinuity. (3) Only the Iberian crust was significantly thickened by a southward overstacking of slabs. (4) Near the surface in the external domains, reflectors define accurately the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees. Using ECORS data (Etude Continentale et Océanique par Réflexion et Réfraction Sismique), the structure of the studied section of the belt can be modeled considering the nature and continuity of reflectors beneath the surface trace of the North Pyrenean fault considered as the initial vertical boundary between Iberian and European plates.
New tectonic divisions of the Grenville Province, Southeast Canadian Shield
Authors: Rivers T. , Martignole J. , Gower C. F. , Davidson A.
Published: February 2008

On the basis of geological, geophysical, and geochronological data, the Grenville Province has been divided into three first‐order longitudinal belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt (APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by three first‐order tectonic boundaries, the Grenville Front (GF), Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone (MBBZ). The belts are subdivided into terranes based on internal lithological character. The GF separates the Archean to Proterozoic foreland northwest of the orogen from reworked equivalents to the southeast. Continuous at the scale of the orogen, its main characteristic is that of a crustal‐scale contraction fault. The PB, although less clearly identified along the length of the orogen, in most places represents upgraded and tectonically reworked rocks of the adjacent foreland. The boundary between the PB and the APB to the southeast, the ABT, is most clearly delineated in the eastern half of the province. It is the locus of major crustal delamination along which high‐grade, mostly middle Proterozoic, polycyclic terranes were tectonically transported northwest toward and onto the PB. The AMB comprises two separate areas underlain by the Wakeham Supergroup and what is currently known as the Grenville Supergroup, respectively; its basal contact, the MBBZ, is a décollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks. This first‐order zonation implies a tectonic polarity to the Grenville Province, superimposed on which are second‐order features evident from contrasting tectonic styles and radiometric ages. These characteristics are consistent with a diachronous or oblique collisional model for the Grenville orogen.
Geodetic and tectonic analyses along an active plate boundary: The central Gulf of California
Authors: Ortlieb L. , Ruegg J. C. , Angelier J. , Colletta B. , Kasser M. , Lesage P.
Published: February 2008

The Gulf of California is traversed by the shear plate boundary between Pacific and North American plates and, because of several islands in its central part, offers the possibility of direct geodetic measurements of plate motion. A geodetic network of 150 km aperture, and comprising 11 stations, was measured in 1982 and 1986 by laser trilateration methods. The deformations deduced from the comparison of the two epochs indicate right‐lateral shear strain covering the entire gulf rather than localized movements. In the eastern part of the network, between the axial islands and the Sonoran coast, significant right‐lateral shear deformation occurs with a relative displacement of about 23 ± 12 cm over 4 years. In the northwestern region (Canal de Ballenas) a right‐lateral displacement of about 17 ± 4 cm is observed, whereas in the southwestern part of the network (Canal Sal‐si‐Puedes), the deformation remains very weak. This suggests that south of the Canal de Ballenas the plate boundary is locked. A tectonic analysis of Neogene and Quaternary faults in Baja California, Sonora, and the central islands of the gulf, permitted the reconstruction of the stress pattern evolution of this area. These data also indicate the predominance of right‐lateral motion on a NW‐SE trending zone within a regional framework characterized by an approximately N‐S compression and an E‐W extension. The geodetic results are discussed in comparison with the neotectonic analysis and the seismic data available in the area. The data suggest a broad strain accumulation zone covering the totality of the central Gulf of California. A NW‐SE relative velocity of about 8 ± 3 cm/yr is found between the two sides of the gulf during the 1982–1986 interval.
Transition from near‐surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps
Authors: Schmid S. M. , Haas R.
Published: February 2008

West directed thrusting along the Schlinig thrust amounts to a minimum displacement of 45 km. A discrete plane of brittle thrusting is observed in the frontal part, where the Ötztal unit overrides the sediments of the Engadine Dolomites. Both shortening of the more ductile Lower and Middle Triassic and very substantial stretching by domino‐type normal faults in the brittle Upper Triassic dolomites are contemporaneous with, and caused by, thrusting. Extension is related to extrusion of parts of the sediments in the footwall of the Schlinig thrust. A relatively rapid transition into an intrabasement shear zone, which is as much as 2‐km‐thick, occurs farther to the east in a region where a paleotemperature of 300°C is indicated for Cretaceous metamorphism. Still farther to the east, localized shearing is transformed into a wider area affected by large‐scale folding. This second transition corresponds to estimated temperatures in excess of 550°C. Since the changes in structural style and deformation mechanisms toward the hinterland go hand in hand with increasing metamorphic grade, we regard thrusting as contemporaneous with metamorphism. This view is supported by geochronological data. The age of thrusting and metamorphism (90 Ma) indicates very rapid heating after the deposition of the youngest sediments (Cenomanian) involved in Cretaceous deformation. Our data suggest that localized detachment along a discrete thrust plane and within broader mylonite zones cannot always be traced indefinitely to greater depth. The transformation of localized shearing into large scale folding indicates a fundamentally different deformational style for lower crustal levels.
Correlations between Nazca/Farallon Plate kinematics and forearc basin evolution in Ecuador
Authors: Daly Michael C.
Published: February 2008

The Tertiary evolution of the forearc basins of Ecuador shows a close correlation between the changing convergence rate of the Farallon, and later Nazca, oceanic plates and continental South America. The correlation occurs during the subduction of a relatively young slab and, in the Late Miocene, onset of the subduction of the Carnegie aseismic ridge. The Ecuador forearc basins lie on a basement of oceanic crust known as the Pinon terrane. The accretion of this terrance occurred in the Paleocene as the leading edge of the Farallon plate, the Macuchi island arc, collided with South America. In the Middle Eocene this forearc terrane was the site of major pull‐apart basin formation and turbiditic sedimentation, coincident with a phase of very rapid convergence between chron 21 and chron 13 (48–37 Ma). This deformation was bounded by the trench and a major dextral strike‐slip fault zone and resulted in the northward translation of the forearc with respect to continental South America. During the Oligocene a phase of extension normal to the trend of the active margin occurred, coincident with a phase of relatively slow convergence (chron 13 to chron 6, 37–20 Ma). This extension was followed in the Middle Miocene by inversion of most of the forearc basins, coincident with a return to relatively fast convergence from chron 6 (20 Ma) to the present day. Subduction of the Carnegie aseismic ridge occurred during this period (circa 8 Ma to present) and may have enhanced the compressive event. Further, northward translation of the forearc silver accompanied this later deformation. The relationships outlined for the forearc may be modeled in terms of a dynamic orogenic wedge which responds directly to changes in convergence rate at the subduction zone. The convergence rate appears to be an important control on the coupling between the downgoing slab and overriding continental plate.
Cretaceous geotectonic patterns in the New Zealand Region
Authors: Bradshaw John D.
Published: February 2008

In the mid‐Cretaceous, around 105±5 Ma, the tectonic regime in New Zealand changed significantly. From the Permian to the Early Cretaceous most rocks formed under the influence of convergent margin tectonics and comprise incomplete remnants of magmatic arcs, forearc basins, trench slope basins, and accretionary complexes. This tectonic pattern was ended by the approach and collision of the spreading ridge between the Phoenix and Pacific plates. Crustal extension, leading eventually to fragmentation of the continental crust, commenced immediately after ridge collision. Precollisional and syncollisional events deformed and thickened the accretionary prism, with concurrent folding and thrusting of more arcward elements. Cretaceous granulites in southwest New Zealand are thought to result from the closure of a small backarc basin or marginal sea slightly earlier than the main ridge collision. Postcollisional block faulting led to thick nonmarine sedimentation in new basins that cut across old terrane boundaries, except in the outboard area where a thick wedge of marine rocks was deposited over the subsiding accretionary prism. After oblique ridge trench collision along the New Zealand margin, the Phoenix‐Pacific Ridge propagated to the southwest to link with zones of incipient spreading in the Tasman Sea and south of Australia. Subsequently, the eastern part of the ridge continued to migrate south with increasing offset along the Udinsev Fracture Zone. In the Late Cretaceous a duplicate ridge developed. The southern branch continued to move southward and collided with the trench west of the Antarctic Peninsula in the Cenozoic. The northern branch is the extant East Pacific Ridge.
The Triple Junction of the North America, Cocos, and Caribbean Plates: Seismicity and tectonics
Authors: Guzmán‐Speziale Marco , Pennington Wayne D. , Matumoto Tosimatu
Published: February 2008

The triple junction of the North America, Cocos, and Caribbean plates is ambiguously defined, mainly because the North America‐Caribbean plate boundary does not clearly continue beyond its known surface trace (the Motagua fault zone) in western Guatemala to intersect the Middle America trench. Well‐located regional shallow earthquakes (h≤70 km) show that there is no intermediate or large‐magnitude seismic activity associated with a presumed extension of the North America‐Caribbean plate boundary to the west, beyond its well‐defined surface trace. There is, however, a clear zone of shallow seismic activity from the western section of the fault system through southern Mexico. Fault plane solutions for these events indicate a left‐lateral strike‐slip displacement, which is in good agreement with surface faulting. We suggest that these strike‐slip faults, together with the Salina Cruz fault in the isthmus of Tehuantepec, mark the boundaries of a broad zone of deformation in southern Mexico and northern central America which takes up the interactions of the three plates. In this sense, no single point constitutes the triple junction. The geologic record suggests that the Motagua fault zone developed because the westernmost portion of the Caribbean plate became locked against North America.
Tilting of continental interiors by the dynamical effects of subduction
Authors: Mitrovica J. X. , Beaumont C. , Jarvis G. T.
Published: February 2008

A mechanism is proposed to explain epeirogenic motions of craton interiors in terms of the response of the lithosphere to subduction. The effects of changes in sea level are distinguished from subsidence of the basement by analyzing the tilt of chronostratigraphic sequences in which the bounding horizons were deposited at approximately the same elevation with respect to sea level. As an example, the Late Cretaceous subsidence and Tertiary uplift of the western interior of North America is examined, and a maximum tilt amplitude of 3 km, with a horizontal deflection scale of approximately 1400 km, is inferred. The link between platform sedimentation and subduction is tested by using numerical models of mantle convection which mimic the subduction and by examining the horizontal scale of the deflections to the overlying lithosphere. It is found that this scale is relatively insensitive to the temperature contrast between the slab and the surrounding mantle, the flexural rigidity of the lithosphere, and even the physical process assumed to govern the subduction. The most important factor affecting the scale is the dip of the subduction zone, and shallower subduction angles (less than 45°) can produce horizontal deflections of the order of 1000 km or more. In contrast, the vertical scale of the deflection is sensitive to all the above parameters. Using these results, two subduction models are introduced which predict both the time and length scales of the North American tilt, and it is conjectured that the process may be responsible for other regions of platform subsidence where subducted lithosphere may have passed beneath the continent at a shallow angle.
Orogen‐parallel extension in the North Cascades Crystalline Core, Washington
Authors: Brown E. H. , Talbot J. L.
Published: February 2008

Metamorphic stretching lineations throughout the Crystalline Core of the North Cascades, Washington, show a strong preferred orientation in a horizontal NW‐SE direction. Foliations strike NW and dip steeply. Preliminary observations of shear sense features indicate noncoaxial right‐lateral motion. We interpret these observations to indicate that the Crystalline Core was a broad, right‐lateral shear zone. The ages of the fabrics, as recorded by metamorphic minerals and syntectonic plutons, range from pre 85‐Ma in the south to post 45‐Ma in the north; thus the shear regime was active during an extended interval of metamorphism and plutonism in the orogen. A similar kinematic pattern is suspected for at least part of the Coast Plutonic Complex, British Columbia, which lies on strike with the Crystalline Core to the northwest.
The Paparoa Metamorphic Core Complex, New Zealand: Cretaceous extension associated with fragmeotation of the Pacific margin of Gondwana
Authors: Tulloch A. J. , Kimbrough D. L.
Published: February 2008

In Westland‐Nelson provinces of New Zealand, high‐grade metamorphic and granitic basement rocks showing mylonitic ductile deformation are juxtaposed beneath low‐grade metasedimentary rocks and undeformed granites by uplift on low‐angle detachment faults. Several metamorphic core complexes analogous to those described from western North America are recognized. In the Paparoa Range, basement rocks include late Precambrian(?) paragneiss and granitic rocks of both Paleozoic and Cretaceous ages. Cover rocks include Ordovician turbidites, Paleozoic and Cretaceous granites, and mid‐Cretaceous breccia‐conglomerates. Brittle deformation and hydrothermal alteration (silica, chlorite, hematite, carbonate ± fluorite, uranium) characteristic of the detachment zone are also superimposed on uppermost lower‐plate mylonites. Kinematic indicators in the mylonitic rocks including composite S‐C febrics indicate that the detachment faults on the northeast and southwest sides of the Paparoa Core Complex had opposite senses of shear, with cover rocks on both sides moving away from the metamorphic core. Ductile deformation postdates several 114±18 Ma granitic plutons but by 108 Ma had ceased to affect at least some of the rocks currently exposed. Mylonitic rocks were uplifted to the surface and eroded into evolving half‐grabens by 105–100 Ma. Uplifted basement yields K‐Ar dates as young as 88 Ma, and tilting of the graben sediments indicates detachment continued well into the Late Cretaceous, when both cover and basement were intruded by alkali lamprophyre dykes. The Nelson‐Westland core complexes occur within an Early Cretaceous granitic province characterized by relatively radiogenic strontium. The boundary of this province, the NW trend to mid‐Cretaceous half‐grabens, the NNE trend of stretching lineations in mylonitic rocks, and the ESE trend of late Cretaceous lamprophyre dykes indicate that regional extension was maintained in a NNE direction for much of the Cretaceous. This regional extension may be part of an “extension corridor” which traversed the entire Gondwana continental margin from NE Queensland, Australia, to Marie Byrd Land, Antarctica. Extension preceded opening of the Tasman Sea between New Zealand and Australia at approximately 84 Ma and closely followed long‐lived compression on the Pacific convergent margin of Gondwana. The presence of core complexes in western New Zealand contrasts with the Australian margin to the Tasman Sea and lends support to simple shear models of continental rifting.
Forearc uplift and extension, southern Hawke's Bay, New Zealand: Mid‐Pleistocene to present
Authors: Cashman Susan M. , Kelsey Harvey M.
Published: February 2008

The Maraetotara Plateau‐Cape Kidnappers area in southern Hawke's Bay, New Zealand, lies in the forearc region of the obliquely convergent Hikurangi margin, where the Pacific Plate is presently subducting beneath the leading edge of the Australian Plate. Uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid‐Pleistocene time. Principal strain directions and strain ratios, determined from mesoscale fault arrays associated with major faults, document the regional extension. In the Cape Kidnappers area, fault arrays record plane strain with northwest horizontal extension and vertical shortening. In the Maraetotara Plateau area, fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions, either east‐west and north‐south, or northwest and northeast, and vertical shortening. The morphology, seismicity, and nature and distribution of active faults of the southern Hawke's Bay region suggest that the Maraetotara Plateau‐Cape Kidnappers area is the on‐land expression of an outer‐arc high, which is developed between the accretionary wedge on the east and the forearc basin on the west. The cause of uplift and extension in the Maraetotara Plateau‐Cape Kidnappers area may be either upper plate deformation related to structural evolution of the outer‐arc high and underlying backstop, underplating on the Hikurangi subduction zone, or a combination of these processes. The uplift and horizontal extension occurring in the Maraetotara Plateau‐Cape Kidnappers area contrast sharply with active deformation in adjacent parts of the forearc, reflecting a distinct partitioning of strain in this part of the obliquely convergent plate boundary.
Seismotectonics of the easternmost transverse ranges, California: Relevance for seismic potential of the southern San Andreas Fault
Authors: Williams Patrick L. , Sykes Lynn R. , Nicholson Craig , Seeber Leonardo
Published: February 2008

Earthquake locations, depths and focal mechanisms from the Southern California Regional Network (1977–1985) are used to identify the orientation and sense of slip of active subsurface faults in the Easternmost Transverse Ranges (ETR). The ETR are separated from the Salton Trough province by the southernmost strands of the San Andreas fault (SAF). Much of the seismicity in the ETR is concentrated well northeast of the SAF at relatively shallow depths under the Little San Bernardino Mountains. Many of these earthquakes reflect slip on steeply dipping, left‐lateral faults striking northeast to east, at relatively high angles to the adjacent SAF. Focal mechanisms in the ETR show predominantly strike‐slip, normal, or oblique‐normal faulting, and share common near‐horizontal T axes striking WNW. P axes range from near vertical to near horizontal and strike mostly NNE. In contrast, reverse and strike‐slip focal mechanisms that exhibit persistent north trending, near‐horizontal P axes characterize the San Gorgonio Pass area immediately to the west. These different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the SAF. This boundary, which in the Coachella Valley may be the Mission Creek fault, is also distinguished by abrupt changes in (1) rate and depth of seismic activity; (2) topography; (3) Quaternary vertical deformation; (4) strikes and dips of major branches of the SAF; and (5) seismic velocities in the crust and upper mantle. The preponderance of secondary normal faulting in the ETR versus secondary reverse faulting in the San Gorgonio Pass region suggests that fault‐normal stress is much less across the SAF adjacent to the ETR. If a friction law where strength is proportional to normal stress applies to the SAF, then a smaller tectonic shear stress would be required tor slip in large earthquakes along the Salton Trough segment. In this case, the southernmost SAF may have shorter repeat times, smaller average displacements, and lower moment release rates than earthquakes that rupture through the major restraining bend in San Gorgonio Pass.
Tectonic development of Paleozoic foldbelts at the north margin of the Sino‐Korean Craton
Authors: Tang Kedong
Published: February 2008

Recent study shows that Paleozoic orogeny in Inner Mongolia resulted from collisions. An early event was a collision between an island arc and the North China continent in the Late Silurian, and a subsequent event was a collision between the Sino‐Korean and Siberian platforms before the end of the Devonian. Prior to the collisions, an old Asian ocean existed. Its opening in Inner Mongolia is interpreted from the Middle Proterozoic Zartai rift zone. Late Proterozoic‐Early Paleozoic spreading and subduction are represented by a fossil trench‐island arc‐basin system, which constitutes the Xar Moron accreted fold belt along the north margin of the North China platform. Latest Silurian molasse unconformably overlies these belts, constraining the time of collision between the island arc and the North China continent. The old Asian ocean basin persisted but was finally closed by the collision between the Sino‐Korean and Siberian continental platforms. Latest Devonian‐Early Carboniferous molasse and epicontinental carbonate deposits unconformably overlie an imbricated oceanic rock suite bordering the suture zone, indicating that the collision occurred before the end of the Devonian. This collision produced only a weak orogeny with thin, local molasse and moderate deformation. The Silinhot belt marks this collision and represents the suture zone between the two great platforms. During the Carboniferous and Permian, Inner Mongolia and adjacent regions were affected by widespread development of continental rift structures with intense magmatic activity.
Implications of the North and South Panama Thrust Belts for the origin of the Panama Orocline
Authors: Silver Eli A. , Reed Donald L. , Tagudin Jill E. , Heil Darla J.
Published: February 2008

High resolution imaging of the north (NPTB) and south Panama thrust belts, using the SeaMARC II seafloor mapping system and digital seismic reflection profiling, provides an excellent data base with which to determine the kinematics of oroclinal deformation of Panama. Remarkable continuity of structural orientations over 300 km along the NPTB argues for widespread uniformity in direction of thrusting between Panama and the Caribbean plate. Westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence. Two major cross faults cut the wedge in its eastern part, forming two minor blocks with different relative motions with respect to the Caribbean. Frontal thrust orientations change across these faults. Major faults have been proposed to cut the isthmus of Panama, but simple cross‐isthmus faulting alone cannot explain the orientations of the major thrusts of the NPTB. Bending of a flexible beam can explain much of the observed structure, both directions and rates, though the scale of the orocline precludes considering this as an elastic phenomenon. Plastic deformation of Panama may be a useful mechanical explanation, at least for eastern Panama, but consideration of plastic slip lines must include the northern Andes as well. Changing from more rigid, blocklike offsets on faults in western Panama to a continuum of strain in eastern Panama may be the best explanation for observations at present.
The multistage tectonic evolution of the Gulf of Suez and northern Red Sea continental rift from field observations
Authors: Jarrige Jean‐Jacques , Ott d'Estevou Philippe , Burollet Pierre F. , Montenat Christian , Prat Philippe , Richert Jean‐Paul , Thiriet Jean‐Paul
Published: February 2008

Field surveys in the Gulf of Suez‐Northern Red Sea area revealed the structural pattern and evolution of this tertiary continental rift and provided constraints on the continental rifting process. The structure has been controlled, at every scale, by four major trends of faults: N0°–20°E, N100°–120°E, N140°E–160°E and N40°–60°E. Observations of sealed faults and folds and superposed structures and the detailed analysis of the fault surfaces are coherent and indicate that as least two different stages of deformation characterize the evolution of the rift. The initiation of the rift (late Oligocene‐Aquitanian) corresponds to the creation of large rhomboidal basins, weakly subsident and partly bounded by oblique strike‐slip faults. This framework, which resulted from the reactivation of inherited weakness zone in the basement, controlled the subsequent creation of large tilted blocks (upper Aquitanian‐lower Burdigalian). These structures were trangressed by the upper Burdigalian deposits. Then the blocks were sliced by a secondary horst and graben pattern, which was progressively sealed by middle Miocene units. Since the upper Miocene, the deformation has been focused along some major faults located in the central part of the rift. The determination of paleostresses from measurements of striae confirms that the initiation of the rift was controlled by a tectonic regime characterized by an horizontal compressive stress parallel to the rift, and its subsequent evolution was controlled by triaxial and radial extensional regimes with horizontal minimum stress (σ3) predominantly trending N70°–80°E. The rifting process was not induced by regional doming but by a new distribution of intraplate stresses at approximately Oligocene‐Miocene time (passive rift). Uplift of the rift shoulders began in the upper Burdigalian. Cross sections of the rift emphasize its asymmetry: large basement blocks are tilted toward the edge of the rift where a major, recently active fault and a distinct morphological unit, which we call a “banquette,” are located. Several homogeneous tilt direction provinces can be defined. They are separated by transverse hinge zones that correspond to inherited basement trends. Rifting deformation is expressed in the basement by steep faults arranged in a zig‐zag pattern and in Neogene‐Quaternary sediments by steep faults and listric faults associated with decollements. Therefore the style of shallow deformation cannot be extrapolated to deeper levels. Structures formed during the beginning of the rift show different charateristics according to their orientation, but their initial shape was generally complicated by later deformation.
Contemporaneous thrusting and large‐scale rotations in the western Sicilian fold and thrust belt
Authors: Oldow J. S. , Channell J. E. T. , Catalano R. , D’Argenio B.
Published: February 2008

In the western Sicilian fold and thrust belt, large clockwise rotations of allochthons occurred during late Cenozoic contraction of part of the southern Tethyan margin. The magnitude of rotation decreases stepwise from over 120° in the upper sheets, lying on the north coast of Sicily, to no appreciable rotation in the frontal portion of the belt flanking the southern coast of the island. The allochthons are composed of imbricate thrust sheets derived primarily from individual basin and platform assemblages of the old Tethyan margin. Paleomagnetic and structural data indicate that the rotation of the allochthons was accommodated by coherent torsional displacements on relatively low‐angle detachment surfaces. Timing relations for the imbrication history of the Sicilian fold and thrust belt are derived from stratigraphic overlap and local involvement of sediments deposited in a series of foreland and piggyback basins. The locus of deposition within successive foreland basins first migrated easterly then southerly during progressive deformation in the orogen. Imbrication began in the early Miocene (Burdigalian‐Langhian) and continued at least through the early Pleistocene and appears to be continuing today. Rotation is related to thrusting and accompanies a 70° change in the tectonic transport direction from easterly to southerly. Easterly striking, right‐oblique transpressional faults and associated northeasterly trending folds postdate thrust sheet rotation in the interior of the thrust belt and were active contemporaneously with south‐directed thrusting in the foreland region. Pleistocene and possibly older (late Pliocene?) extension strongly modified the older thrust morphology along the Tyrrhenian coast of northwestern Sicily, with the development of down‐to‐the‐north listric normal faults. The extensional structures apparently are related to the opening and subsequent deformation of the Tyrrhenian Sea to the north.
Variation in deformation of the South Panama Accretionary Prism: Response to oblique subduction and trench sediment variation
Authors: MacKay Mary E. , Moore Gregory F.
Published: February 2008

Migrated single‐channel seismic lines and Sea‐MARC II side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south Panama, a region previously believed to be a transform margin. The eastern flank of the Coiba Ridge is being subducted in this region, creating the requisite geometry for along‐strike variation in trench sediment thickness and type. The regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east. In the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments. A wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area. The eastward increase in sediment thickness correlates with the following changes in the accretionary prism: (1) decrease in initial surface slope; (2) broadening of the inner trench slope; (3) increase in thrust spacing; (4) steepening of frontal thrusts; and (5) fold development. Each of the responses reflects, in varying degrees, the increase in volume and changing physical properties of the accreted material, as well as the changing shear strength of the décollement. The increase in overburden pressure and compaction in the deeper sediments, together with the change in lithology from oceanic to trenchfill sediments, should produce an eastward increase in sediment shear strength both within the wedge and along its base. Because the décollement will tend to form where the pore fluid pressure‐overburden ratio is at a maximum, the shear strength within the wedge should increase relative to basal shear strength. Decreased surface slopes will result from the increasing contrast in shear strength between the wedge and its base.
Foreland shortening and crustal balancing in the Andes at 30°S latitude
Authors: Allmendinger R. W. , Figueroa D. , Snyder D. , Beer J. , Mpodozis C. , Isacks B. L.
Published: February 2008

Excellent surface exposures, known Benioff zone geometry, a dynamic morphology, and the availability of industry seismic reflection data all make the Andes at 30°S an excellent transect for investigating crustal‐scale balanced sections. 150–170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland. The thin‐skinned, east‐verging Precordillera of western Argentina accounts for 60–75% of the total shortening and formed mostly since major volcanism ceased at ∼10 Ma. Industry seismic reflection data show that the décollement of the Precordillera belt is located anomalously deep at ∼15 km. The belt is dominated by fault propagation folds and contains several prominent out‐of‐sequence thrust faults. Seismic stratigraphie analysis shows that Miocene strata in the Iglesia Valley, located between the Precordillera and the crest of the Andes, accumulated in a piggy‐back basin. Onlap relations on the western side indicate that the High Cordillera was uplifted as a major fault bend fold over a buried ramp. Thrusting in the two western belts, both in the High Cordillera of Chile, formed during the waning stages of arc volcanism, 11–16 Ma. and account for 25–40% of the shortening. The observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses, indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted Nazca Plate. Our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography, requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the Andes. This non‐unique model provides a simple explanation of the first order morphology of the Andes at this latitude.