Search

Earthquakes radiate from slip on discrete faults, but also commonly involve distributed deformation within a broader fault zone, especially near the surface. Variations in rock strain during an earthquake are caused by heterogeneity in the elastic stress before the earthquake, by variable material properties and geometry of the fault zones, and by dynamic processes during the rupture (1,2). Stress changes due to the earthquake slip, both dynamic and static, have long been thought to cause dilatancy in the fault zone that recovers after the earthquake (3-5). Decreases in the velocity of seismic waves passing through the fault zone due to coseismic dilatancy have been observed (6) followed by postseismic seismic velocity increases during healing (5,7,8). Dilatancy and its recovery have not previously been observed geodetically. Here we use interferometric analysis of synthetic aperture radar images to measure postseismic surface deformation after the 2003 Bam, Iran, earthquake and show reversal of coseismic dilatancy in the shallow fault zone that causes subsidence of the surface. This compaction of the fault zone is directly above the patch of greatest coseismic slip at depth. The dilatancy and compaction probably reflects distributed shear and damage to the material during the earthquake that heals afterwards. Coseismic and postseismic deformation spread through a fault zone volume may resolve the paradox of shallow slip deficits for some strike-slip fault ruptures (9)., Several seismic studies have found that the ratio of the velocities of S waves to P waves decreases with time after an earthquake, in addition to an overall velocity increase, [...]

This paper elaborates on the concept of multiple magmatic fabrics in plutons. After a general overview of various types of multiple fabrics that may develop in magmatic rocks, two case examples of porphyritic granite and melasyenite plutons in the Bohemian Massif are examined. In the Jizera granite, complex variations in K-feldspar phenocryst shape-fabric revealed by image analysis of a 200 m long section of an underground tunnel are in contrast with homogeneously oriented magnetic (AMS) fabric carried by coaxial contributions of biotite, magnetite and maghemite. In the Knizeci Stolec melasyenite pluton, emplacement-related margin-parallel feldspar foliation was overprinted by flat-lying foliation; the latter is interpreted to record regional tectonic strain. At the two stations examined in detail, the crystallographic-preferred orientation of biotite and amphibole in the inter-phenocryst matrix (measured using electron back-scatter diffraction--EBSD) differed from both feldspar fabric and also from the AMS principal directions. Multiple magmatic fabrics in these two plutons are interpreted in terms of fabric superposition, where late weak strain is superposed onto a high-strength phenocryst framework, but is accommodated preferentially by small mineral grains (biotite, magnetite) in the melt-bearing matrix. This mechanism may explain the discrepancy between mesoscopic feldspar fabric and AMS. We conclude that multiple magmatic fabrics in plutons may thus result from accumulated strain caused by different processes during final crystallization and, as such, may serve as a sensitive indicator of the evolving interactions between magmatic and tectonic processes in the Earth's crust. Keywords: deformation, fabric, granite, magnetic susceptibility anisotropy, pluton.

Recent studies of India-Eurasia convergence suggest that the entire convergence in the Himalayan wedge is almost exclusively accommodated along its basal detachment fault (Main Himalayan thrust, MHT) and its near-surface equivalent (Main Frontal thrust, MFT). Using direct dating of fault-zone gouge and strath terrace deposits, we conclude the following. (1) The present mountain front in the Darjiling sub-Himalaya was emplaced by ca. 40 ka. (2) Out-of-sequence deformation on surface-breaking faults north of the MFT in the Darjiling sub-Himalaya began ca. 20 ka and has probably continued since. (3) The Tista River responded to the ca. 20 ka deformation by migrating 150 m eastward (average rate ~13 mm [yr.sup.-1]) and by incising 48 m vertically (average rate ~4.4 mm [yr.sup.-1]), creating unpaired, disjointed strath terraces between 11.3 [+ or -] 1.3 ka and 1.4 [+ or -] 0.3 ka. Out-of-sequence, surface-breaking faults in the Himalaya indicate partial accommodation of active convergence within the Himalayan wedge. Using the results from the Bhuj earthquake of 2001, we suggest that active deformation along the out-of-sequence faults is a potential seismic hazard in the Himalaya, and Himalayan seismic hazard models must account for this. We also propose a conceptual model for active deformation in the Himalaya. Keywords: active tectonics, Himalaya, luminescence dating, Darjiling, out-of-sequence faulting. doi: 10.1130/G23869A.1

Analysis of aftershock focal mechanisms of the October 1989, M 7.1 Loma Prieta earthquake reveals details of structure and deformation. Aftershocks below 4-6 km define an alignment plane oriented (131[degrees], 65[degrees]). This alignment includes three planar segments defining a restraining bend in a blind strike-slip fault. Dextral strike-slip dominates in the south, and reverse-dextral slip dominates in the north, a distribution similar to slip during the main shock. Above this fault, dominantly reverse faults striking roughly 110[degrees] lie en echelon along the main fault trend. In the 17 sets of spatially clustered aftershocks, a more complex substructure is defined by local planar hypocenter alignments and by preferred orientations of aftershock shear planes. The maximum shortening axes make angles with the normal to these planes that average [theta] [approximately equal to] 46[degreees] and [theta] [approximately equal to] 53[degrees], respectively. These angles are incompatible with a weak fault, for which 0[degrees] [less that or equal to] [theta] [less than or equal to] 20[degrees]. Orientations of preferred aftershock shear planes resemble Riedel shears relative to local hypocenter alignment planes. The similarity of slip distributions inferred from the aftershock inversions and from the main shock implies that the stress drop during the main shock was incomplete and, thus, that the Loma Prieta fault is not unusually weak. Triaxial brittle deformation is partitioned into a pair of plane strains rotated relative to each other about a common principal axis. The shape of the triaxial strain ellipsoid determines the angle of rotation. Evidence supports our micropolar model of brittle deformation, which predicts that the antisymmetric part of the seismic moment tensor records effects of block rotations. Keywords: Loma Prieta earthquake, aftershocks, inversion, strain, partitioning, strength, brittle deformation, fault zones, rotation.

Geochronologic [sup.40]Ar[sup.39]Ar data of fabric-forming metamorphic minerals, in conjunction with structural and metamorphic studies, are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low-grade orogens. Careful data interpretation is needed to extract meaningful age constraints from neo- and recrystallized minerals affected by isotopic disturbances. In the southern Quebec Appalachians, the Sutton Mountains anticlinorium exposes the metamorphic core of the early Paleozoic continental margin of Laurentia. Orogenesis in this part of the Appalachians was the result of tectonic events that have been classically attributed to the combined effects of the Middle to Late Ordovician Taconian and the Middle Devonian Acadian orogenies; however, evidence of separate and distinct Silurian--Early Devonian tectonism has also been recently documented. Laser step-heating [sup.40]Ar[sup.39]Ar data on single-grains of muscovite from polydeformed greenschist facies samples of the Sutton Mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde Taconian event at ca. 456 Ma and an Acadian overprint at ca. 390 Ma. The integration of [sup.40]Ar[sup.39]Ar age spectra analyses with structural relationships provides precise age constraints on the duration, propagation, and evolution of Silurian--Early Devonian hinterland-directed deformation as it migrated across the anticlinorium, including back thrusting from ca. 433 Ma to ca. 420 Ma and extensional faulting from ca. 417 Ma to ca. 405 Ma. Along the Laurentian margin of the northern Appalachians, such Silurian--Early Devonian tectonism is currently attributed either to the collapse of the Taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri-Gondwanan terranes during the Salinic orogeny. Keywords: Quebec Appalachians, [sup.40]Ar[sup.39]Ar geochronology, polyphase deformation, deformation age, greenschist facies metamorphism, dynamic recrystallization.

The Central Asian Orogenic System (CAOS) is one of the largest Phanerozoic accretionary orogens in the world and may represent a significant site of continental growth. Its origin has been explained by two competing models: syn-subduction strikeslip duplication of a single (>1000km) long-lived arc (ca. 630-360 Ma) or collision of multiple arcs and micro-continents. Central to the debate are the relative roles of syn-subduction strike-slip faulting versus thrusting. In both models, the Ertix fault figures prominently, either as a roof fault of a large strike-slip duplex system developed during oceanic subduction or as a suture of arc-continent or continent-continent collision. In order to differentiate between the above models, we conducted field mapping, detailed kinematic analysis, and geochronological dating of the Ertix fault zone in the Chinese Altai. Our work indicates that the fault is a crustal-scale thrust that was active in the Permian. Its hanging wall records two pulses of magmatism ca. 450 Ma and ca. 280 Ma and experienced peak pressure and temperature of 6.2-7.7 kbar and 560-670 [degrees]C. Our geologic observations, together with the existing geologic information, favor a tectonic model that involves two episodes of subduction below the Altai arc: first, in the Ordovician, along a south-dipping subduction zone; and second, in the late Carboniferous and early Permian along northdipping subduction of the Junggar ocean. It was during the latter event that a melange complex was underplated below the older Ordovician arc, metamorphosed at lower crustal depths, and then exhumed to the upper crust along the south-directed Ertix thrust zone. Keywords: Ertix, Irtysh, central Asia, magmatism, Xinjiang, deformation

This paper presents a study of Jurassic-Early Cretaceous sedimentary evolution of the Hefei basin in eastern China and explores the relationship between clastic sedimentation and coeval deformation of the Dabieshan to the south and the Taulu fault to the east. The Hefei basin experienced a two-stage evolution. The basin was initiated in the Early Jurassic and expanded in the Middle and Late Jurassic. The synsedimentary Jinzhai normal fault is considered to be a border fault of the basin because it controlled Middle to Upper Jurassic proximal alluvial deposits. Persistent N- to NE-directed paleoflows in the southern Hefei basin indicate that sediments came from the Dabieshan, and the presence of Triassic coesite-bearing detrital zircon in Lower Jurassic sediments documents exhumation of ultrahigh-pressure rocks of the Dabieshan to the surface as early as the Early Jurassic. Occurrences of eclogite clasts in an Upper Jurassic unit indicate continued denudation of the Dabieshan at that time. Thickening of Jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the Jinzhai normal fault on the south and the NE-striking left-lateral transtensional Tanlu fault on the east. Jurassic extensional subsidence of the Hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the Dabieshan, which combines to indicate contemporaneous extension and contraction on the north and south sides of the Dabieshan, respectively. Vigorous volcanism and uplift of the southern Hefei basin characterized the second stage of development of the Hefei basin in the Early Cretaceous, and this led to a synchronous shift of its main depo-center to the north. This younger depocenter is characterized by lacustrine and fluvial-deltaic sedimentation, where alluvial and fan-deltaic coarse-grained deposition mainly occurred along the eastern edge of the basin. Early Cretaceous subsidence is attributed to E-W extension across the middle segment of the Tanlu fault, and the Zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin. A model is accordingly advanced to account for how the Hefei basin developed in response to the tectonic exhumation of the Dabieshan and the deformation of the Tanlu fault in the Mesozoic. It illustrates that the Hefei basin initiated and evolved during Jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultrahigh-pressure rocks of the Dabieshan. Early Cretaceous development of the basin was controlled by magmatism-related uplift of the Dabieshan on the south and orthogonal normal faulting of the middle segment of the Tanlu fault on the east. This study provides an independent constraint upon the exhumation processes of ultrahigh-pressure rocks of the Dabieshan. Keywords: sedimentation, basin tectonics, Mesozoic, eastern China, Dabieshan, Hefei basin.

The Quebrada de los Colorados Formation, at the north end of Caichaqul Valley in Salta Province, northwest Argentina, preserves evidence of syndepositional deformation since the middle Eocene (ca. 40 Ma) that includes (1) an angular unconformity with the underlying Salta Group (Paleogene), and (2) internal unconformities and changes in vertical facies succession and provenance. Its fossil record [mammalian (notoungulates), middle Eocene] is correlatable to the Casa Grande Formation, which also unconformably overlies the Salta Group; both units record middle Eocene deformation along the eastern border of the Puna Plateau and outline previous first-order mechanical heterogeneities related to the Cretaceous rift basin border. Along the western margin of the Puna, Eocene deformation coincides with thermal (magmatic arc) and mechanical (basin inversion) heterogeneities. Thus, the distribution of Eocene deformation followed an irregular pattern as a consequence of the heritage of preexisting heterogeneities. Keywords: Eocene deformation, initial foreland, preexisting heterogeneities, Central Andes.

We have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths. The Rodeo Cove thrust zone, which outcrops north of the Golden Gate Bridge of San Francisco, imbricates two basalt-chert-sandstone sequences belonging to the Marin Headlands terrane (Franciscan Complex). The thrust outcrop is a 200-m-thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop, dominated by basaltic lithologies, where zones of concentration of deformation have been mapped. Disruption is made by variably dense discrete fault systems synthetic to the main thrust (R and P fractures). These faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defined by chlorite and pumpellyite, which also constrain the depth of faulting (8-10 km, T = 200-250 [degrees]C) within the seismogenic zone. The central part of the fault also features the densest system of carbonate-filled veins. Veins occur in the broken formation matrix and fragments, in both cases parallel to the foliation. The veins are either folded, truncated, or pressure-solved along the cleavage. Cementation and hardening of shear surfaces of the fault core may have caused the distribution, as opposed to localization, of subsequent slip events. The fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone. Our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fluids occurring through systems of multiple dilatant fractures grossly parallel to the d6collement zone. The crosscutting relationships between veining and foliation suggest that fluid injection is cyclic and, consequently, that large transient variations in permeability and cohesion may occur. The repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress, or by difference in tensional strength parallel to and perpendicular to the foliation, both of which would require extremely high fluid pressure. We interpret the features of the Rodeo Cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress field in the interseismic phase and an extensional stress field in the immediately postseismic phase. Keywords: Franciscan Complex, accretionary prisms, seismogenic zone, hydroflacture, cataclasis, cyclic processes.

Abstract: The ultra-high-pressure (UHP) Kimi complex (uppermost eastern Rhodope Mountains) is a tectonic mixture of crustal and mantle derived associations. Pressure-temperature (P-T) paths and microtextural and geochronological data reveal that [...]

Isla Santa Maria at the active margin of south-central Chile is the result of earthquake-related uplift and deformation in the forearc since at least late Pleistocene time. Field mapping, dating of key depositional horizons, and analysis of seismic-reflection profiles reveal ongoing deformation in this sector of the Chilean forearc. The 30 [km.sup.2] island is located ~12 km above the interplate seismogenic zone and 75 km landward of the trench. It is situated near the southern termination of the Concepcion earthquake rupture segment, where Charles Darwin measured 3 m of coseismic uplift during a M > 8 megathrust earthquake in 1835. Permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged, landward-tilted abrasion surfaces. Uplift at ~2 m/k.y. and tilting at ~0.025[degrees]/k.y. of the island have been fairly constant throughout the late Quaternary and have resulted in emergence of the island above sea level ~31 k.y. ago. The island is composed of a late Pleistocene upper, tilted surface with two asymmetric tilt domains, and Holocene lowlands characterized by uplifted and tilted strandlines. Industry offshore seismic-reflection profiles covering an area of ~1800 [km.sup.2] and crustal seismicity reveal active reverse-fault cored anticlines surrounding Isla Santa Maria; the principal fault apparently roots in the plate-interface thrust. These reverse faults in the upper plate result from inversion of late Cretaceous to early Pliocene normal faults and rift structure of the Arauco forearc basin. Positive inversion of these inherited structures started between 3.6 and 2.5 Ma and resulted in continuous shortening rates of ~0.8 mm/yr. The seismic-reflection profiles show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the fore-limbs of two converging anticlines, whereas their backlimbs have been removed by cliff retreat. The 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the Arauco-Concepcion region. The close relation among the asymmetric uplift and tilt of the island, modern deformation patterns, and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust influences, localizes, and segments surface deformation during large interplate earthquakes. Furthermore, the link between positive inversion of pre-existing structures, uplift, and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate-boundary deformation. Keywords: Chile margin, forearc deformation, subduction earthquakes, tectonic inversion, reverse faults, crustal seismicity.

Abstract: The Sacawee block is a narrow belt of Paleo- to Mesoarchean crust that extends for ~70 km across the northern Granite Mountains. It is composed of the ~3.3 Ga [...]

Abstract: The Thor-Odin dome of the Monashee complex, in the southeastern Canadian Cordillera, comprises Paleoproterozoic basement gneiss with infolds of unconformably overlying rocks of a supracrustal cover sequence and is [...]

Neoproterozoic to Paleozoic slope-to-basin facies continental margin strata underlie area ~700 x 200 km across central Yukon Territory, Canada, and collectively define the Selwyn Basin. In a Cordilleran framework, Selwyn Basin strata form a strongly deformed and thrust-faulted package located between the Mackenzie foreland fold-and-thrust belt, and accreted terranes and displaced elements of the ancient North American continental margin. Orogeny commenced in the Jurassic as exotic elements of the composite Ynkon-Tanana terrane overrode the ancient continental margin. Collision-related deformation had ceased by ca. 100 Ma, and was followed by a Late Cretaceous (post-85 Ma) dextral transcurrent regime, which laterally displaced elements of the newly assembled continental margin along the orogen-parallel Tintina fault. In western Selwyn Basin, more than 100 km of structural overlap was accommodated on two main detachments, the Robert Service and underlying Tombstone thrust faults. Internal deformation within the thrust sheets is intense, characterized by shear-related folds and fabrics. Metamorphic grade reaches lower to middle greenschist facies at the deepest structural levels exposed, and is characterized by chlorite-muscovite schists. The onset of deformation is constrained by the Late Jurassic age of the youngest units deformed during orogeny. The end of ductile deformation is constrained by new [sup.40]Ar/[sup.39]Ar ages fur metamorphic muscovite that range from 104 to 100 Ma. Due to the low metamorphic grade, these ages are interpreted to closely follow the waning of deformation. At ca. 93 [+ or -] 3 Ma, isolated granitic intrusions of the Tombstone-Tungsten magmatic belt were emplaced across the western Selwyn Basin in a tensional, postcollisional regime. Restoration of displacement on the Tintina fault places the western Selwyn Basin adjacent to the Yukon-Tanana terrane uplands of east-central Alaska in the Early to mid-Cretaceous. Despite their adjacent positioning in cross-orogen section during orogenesis, the two elements feature some significant differences in Jurassic-Cretaceous deformation. Most notably, the Yukon-Tanana terrane uplands record a significant extensional event at 120-105 Ma, which resulted in NW-SE-oriented extension, exhumation of deep structural levels, and voluminous felsic plutonism. In contrast, western Selwyn Basin did not undergo equivalent uplift and extension, and features temporally and spatially restricted plutonism. Within an orogenic framework, the Yukon-Tanana terrane uplands can therefore be considered to represent an exhumed core characterized by high heat flow, whereas the western Selwyn Basin represents an immediate northeastern salient to the exhumed core. These differences have important implications for the geodynamic setting of mid-Cretaceous plutonism across these two major lithologic-tectonic entities of the northern Cordillera. Keywords: Selwyn Basin, orogenesis, thrust sheets, At-At, Yukon-Tanana terrane.

Deep upper-mantle rocks from the Norwegian Caledonides show evidence for large strain deformation in both olivine and garnet under varying water contents. Using microstructural observations, including lattice-preferred orientation of olivine and subgrain boundaries of majoritic garnet, we infer the following deformation history. At depths exceeding ~150 km, large strain deformation occurred at low stress (~10 MPa) and modest temperature (~1300 K), involving high water content (>1000 H/[10.sup.6]Si in olivine). This was followed by low strain deformation at lower water content (~200-1000 H/[10.sup.6]Si) and modest stress (~40 MPa) in the shallower parts. These observations show that the deep upper mantle in this region had a considerably higher water content than the upper mantle near mid-ocean ridges. Keywords: deformation microstructure, lattice-preferred orientation, water, majoritic garnet, olivine, Norwegian Caledonides.

Basement of the Transantarctic Mountains comprises the Ross orogenic belt, a Neoproterozoic to Ordovician mobile belt located along the palaeo-Pacific margin of Gundwana. Our structural analysis of deformation in the Liv Group, a sequence of Cambrian volcanic, volcaniclastic, clastic and carbonate rocks, and nearby plutonic rocks indicates that the Shackleton Glacier area has a polyphase deformation history that includes development of both contractional and strike-slip structures. We describe evidence for synchronous contraction and strike-slip movement, end suggest that the structural suite in this area developed, at least in part, within a sinistral transpressive kinematic regime. Deformation in the Shackleton Glacier area is inferred to be at least in part younger than 505 Ma, based on the deformation of the well-dated Taylor Formation. New [sup.40]Ar/[sup.39]Ar cooling ages from metamorphosed igneous end sedimentary rocks range from c. 500 to 470 Ma end are interpreted to date orogenic cooling. Based on our structural results, end on consideration of the tectonic environment represented by the Liv Group, we propose that shortening across a subduction-related volcanic arc system caused deformation of the Liv Group end associated plutonic rocks. Oblique convergence and thus. transpression, may have ultimately been related to left-oblique plate subduction. Keywords: Antarctica, Transantarctic Mountains, Ross orogen, [sup.40]Ar/[sup.39]Ar, structure.

Continental extension may occur in two main different modes, narrow and wide rifting, which mainly differ in the width of the deformed region. A third mechanism, the core complex, has been considered either a distinct mode of extension or a local anomaly within wide rifts. In terms of causative processes, continental rifting may be explained by both active or passive mechanisms, which also differ in the volume of magmatic products and in the rheological properties and stratification of the extending lithosphere. Both numerical and analogue models have investigated the main parameters controlling the extension of a rheologically layered lithosphere. In particular, analogue models have highlighted that the style of deformation is mainly controlled by the competition between the total resistance of the lithosphere and the gravitational forces; this competition, in turn, is mainly controlled by boundary conditions, such as the applied strain rate and the rheological characteristics of the extending lithosphere. Magmatic bodies eventually present within the continental lithosphere may significantly affect the process of extension. Both the thermal and mechanical effects related to the presence of magma strongly weaken the lithosphere and localise strain; this effect may have important implications for the mode of continental extension. At a crustal scale, magmatic intrusions may affect significantly the local fault pattern also favouring the development of core complex structures. Results of analogue models, performed taking into account the presence of an initially underplated magma and reproducing various continental extensional settings, suggest a close interaction between deformation and magma emplacement during extension. Particularly, magmatic underplating influences deformation localising strain in correspondence to the low-viscosity body, while on the other hand, rift kinematics and associated deformation has a major control on the pattern of magma emplacement. In particular: (1) During orthogonal rifting, magma is passively squeezed from an axial position towards the footwall of the major boundary faults; emplacement occurs in a lateral position in correspondence to lower crust domes. This process accounts for the close association between magmatism and the development of core complex structures, as well as for the occurrence of off-axis volcanoes in continental rifts. (2) During oblique rifting, deformation causes magma to emplace within the main rift depression, giving rise to intrusions with oblique and en echelon patterns. In nature, these patterns are found in continental rifts and also in some oceanic ridges. (3) Polyphase first orthogonal--second oblique rifting models suggest lateral squeezing and off-axis emplacement in the first phase and oblique en echelon intrusions in the successive oblique rifting phase. This evolution matches the magmatic and tectonic history of the Main Ethiopian Rift. (4) Development of transfer zones between offset rift segments has a great influence on both magma migration and deformation. Particularly, magma accumulates in correspondence to the transfer zone, with a main flow pattern that is perpendicular to the extension direction. This pattern may explain the concentration of magmatism at transfer zones in continental rifts. Overall, analysis of centrifuge models and their comparison with nature suggest that deformation and magma emplacement in the continental crust are intimately related, and their interactions constitute a key factor in deciphering the evolution of both continental and oceanic riffs. Keywords: Continental extension; Analogue modelling; Magma emplacement; Rift kinematics

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland--created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada--shed sediments eastward across the Antler foreland, burying the unconformity. Postorogenic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America. We propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations. Keywords: Paleozoic, tectonics, stratigraphy, Antler, foreland, lithostratigraphy.

The spectacular Mam Tor landslip, near Castleton, Derbyshire, formed over 3000 years ago on an oversteepened slope left after the last ice age. A section of the Namurian Edale mudstones and overlying Mam Tor sandstones has collapsed, leaving an 80 m high scar on the eastern side of Mam Tor. The slip is about 750 m long and nearly 300 m wide and has the old main road from Manchester to Sheffield built across it. The slipped mass is in a state of year-on-year creep motion, which over the 190 years since the construction of the highway has led to extensive damage to the road, culminating in its closure in 1979. Since 1996 we have carried out annual monitoring by electronic distance measurement of the movement of a network of some 30 stations on the slipped mass. The average movement rate of the whole mass is about 10 cm [a.sup.-1], with the central region moving significantly faster, at almost 50 cm [a.sup.-1]. Thus substantial readjustments of mass are taking place within the landslip. Local vertical displacements are systematically related to horizontal offsets, and the ratio of the two allows inference of the local attitude of the basal slip surface. The development of surface morphological features (pressure ridges and irregular topography, shear offsets on the highway, bulging of the road surface) reflects the lateral variations in displacement rate. Comparison of our survey of the present-day road position with that recorded in the topographical survey of 1880 shows a total 40 m downhill displacement of the highway over 122 years. This is consistent with extrapolation of our measured displacement rates back to 1880. Displacement rates within this period are, however, clearly higher than during the past 3000 years. There is a fairly clear correlation between vertical and horizontal displacement rates and annual variations in rainfall, with accelerated displacements following winter rainfall above a critical threshold level. Using survey points to define nodal points of a network of triangles, we have analysed the distribution of strain within the slipped mass. This revealed a pattern of continuous strain variations comparable with that found in flowing glaciers. In the lower part of the slip, horizontal strains are contractional and triangle areas are decreasing, causing some uplift of the ground relative to the general downhill flow on the basal slip surface. In the uphill part, strains are extensional and triangle areas are increasing. The new data allow an estimate of the time-dependent theological behaviour of the sheared mudstone in the basal shear zone. The shear stress vs. shear strain rate relationship is very non-linear, with significant shear strain rates occurring only at shear stress levels within 10% of that required for rapid (catastrophic) shearing. Keywords: Mam Tor landslip, creep, mudstones, rheology.

[1] Geologic mapping and structural analysis of Baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high-pressure rocks, where strain partitioning has preserved evidence for interpreting the evolution of late structural features. The earliest of these were extensional detachments juxtaposing eclogite-facies rocks against overlying amphibolite-facies rocks that show no evidence for eclogite-facies metamorphism. These early detachments are strongly overprinted and complexly folded, and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels. Younger more localized mylonite zones formed synchronously with tubular, sheath, isoclinal, tight and open folding that shows a progression from WNW to ENE trends. The earliest mylonite zones, interpreted as originally subhorizontal, range in strike through a 20[degrees] angle from 110[degrees] to 90[degrees]. Later steeply dipping mylonite zones, formed under lower amphibolite-facies conditions, strike 75[degrees] and locally truncate earlier structures. The youngest mylonite zones, formed under lowest amphibolite conditions, strike 50[degrees] and truncate all earlier structures. Folds developed during this progression show the range in orientation from WNW to ENE reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing. These changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation. L > S fabrics, absence of axial planar foliation, and chaotic orientations of axial surfaces of granulite to amphibolite-facies folds indicate formation in a constrictional strain field. This is also supported by estimates of the finite strain accumulated at ~780[degrees]C and 45 km and similar observations by previous workers. Assuming a simple monoclinic deformation for transtension, strain estimates and structural measurements indicate apparent transtensional angles of 9-11[degrees] that increased to greater than 20[degrees] and then decreased to less than 20[degrees]. These changes in transtensional angle agree with changes in the orientation of the X-Y plane of strain during exhumation from 45 km to less than ~20 km depth and appear to reflect changes in the boundary conditions of deformation. This upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of HP and UHP metamorphic rocks. INDEX TERMS: 5475 Planetology: Solid Surface Planets: Tectonics (8149); 8109 Tectonophysics: Continental tectonics--extensional (0905); 8102 Tectonophysics: Continental contractional orogenic belts; 9335 Information Related to Geographic Region: Europe; KEYWORDS: transtension, exhumation, UHP-metamorphism, Western Gneiss Region, Norway, amphibolite, Caledonides. Citation: Terry, M. P., and P. Robinson, Evolution of amphibolite-facies structural features and boundary conditions for deformation during exhumation of high- and ultrahigh-pressure rocks, Nordoyane, Western Gneiss Region, Norway, Tectonics, 22(4), 1036, doi:10.1029/2001TC001349, 2003.

[1] Geologic investigations of how the Tibetan plateau is currently deforming have focused primarily on its boundary faults. Consequently, how the interior of the plateau deforms remains poorly understood. To fill this gap in knowledge, we conducted field mapping, analysis of remote sensing and digital topographic data, and reinterpretation of existing geologic maps in central Tibet. This study reveals a 200-300 km wide and 1500-1800 km long east trending zone of conjugate strike-slip faults across central Tibet. The central Tibet conjugate fault zone is comprised of northeast striking left-slip faults north of the Bangong-Nujiang suture and northwest striking fight-slip faults south of the suture zone. These strike-slip faults are kinematically linked with north trending Tibetan rifts located north and south of the conjugate fault systems. Without exception, all conjugate faults intersect or merge toward one another along the Bangong-Nujiang suture zone. Motion on these faults accommodates coeval east-west extension and north-south contraction. To determine the fault kinematics and the magnitude of fault slip, we investigated three conjugate fault sets in the central Tibet fault zone. These include from east to west, the Dong Co, Bue Co, and Aishi Co conjugate fault systems, which are adjacent to the Bangong-Nujiang suture zone and separated by a distance of 400 and 70 km, respectively. The average magnitude of fault motion on individual strike-slip faults is ~12 km as determined by offsets of Tertiary thrusts and Paleozoic-Mesozoic lithologic units. The conjugate fault configuration requires ~12 km of north-south contraction across the 200-300 km fault zone since its initiation. Because the conjugate strike-slip faults are kinematically linked with the north trending Tibetan rifts which initiated between 14 and 8 Ma, our estimated magnitude of north-south contraction implies a contraction rate of ~1-2 mm/yr across central Tibet. The relatively closely spaced ( INDEX TERMS: 8107 Tectonophysics: Continental neotectonics; 8109 Tectonophysics: Continental tectonics--extensional (0905); 8005 Structural Geology: Folds and folding; 9320 Information Related to Geographic Region: Asia; 8010 Structural Geology: Fractures and faults; KEYWORDS: conjugate strike-slip faults, Tibet, neotectonics, extrusion tectonics. Citation: Taylor, M., A. Yin, F. J. Ryerson, P. Kapp, and L. Ding, Conjugate strike-slip faulting along the Bangong-Nujiang suture zone accommodates coeval east-west extension and north-south shortening in the interior of the Tibetan Plateau, Tectonics, 22(4), 1044, doi:10.1029/2002TC001361, 2003.

[1] Besides the ~W-E trending Kap Cannon Thrust Zone and Harder Fjord Fault Zone, the 300 km long and NW-SE striking Wandel Hav Mobile Belt represents the third major fault zone in north Greenland. Structural analyses in several areas of this belt suggest that compressive deformation is characterized by transpressive dextral strike-slip kinematics. This is demonstrated by a combination of folding around ~W-E axes, approximately north and south directed reverse faulting, and dextral strike-slip displacements along NW-SE trending fault lines. The initial formation of the long, linear faults is interpreted to be related to deformational events in Late Paleozoic, Triassic-Jurassic, and Late Cretaceous times. Due to the lack of structural evidence, we assume that deformations led to the generation of extension faults which probably were reactivated during dextral strike-slip tectonism. It is suggested that dextral transpressive deformation was coeval with ~N-S compression at the Harder Fjord Fault Zone and Cap Cannon Thrust Zone and took place during Eocene (Eurekan) times. Dextral strike-slip tectonism in the Wandel Hav Mobile Belt was the result of ~N-S compression due to a general northward movement of the Greenland plate. As a zone of crustal weakness, the belt can be interpreted as the onshore equivalent of the main transcurrent fault zone (De Geer Fault) which caused the intracontinental dextral slip of Svalbard (Barents Shelf) relative to north Greenland during Early Tertiary (Eurekan) times and prior to their separation. In this configuration, it represents one part of the belt of Eurekan deformation which extends from the Canadian Arctic islands over north Greenland to the Barents Shelf where compressive deformation is recorded in the West Spitsbergen Foldand-Thrust Belt. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8010 Structural Geology: Fractures and faults; 8110 Tectonophysics: Continental tectonics--general (0905); 9315 Information Related to Geographic Region: Arctic region; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Arctic, North Greenland, Tertiary, structure, Eurekan deformation, transpression. Citation: von Gosen, W., and K. Piepjohn, Eurekan transpressive deformation in the Wandel Hav Mobile Belt (northeast Greenland), Tectonics, 22(4), 1039, doi:10.1029/2001TC901040, 2003.

[1] The Andes of Argentina and Chile between latitudes 33[degrees] and 34[degrees]S are composed from west to east of an Oligocene to Miocene volcanic arcs and the Neogene east-vergent Aconcagua fold and thrust belt of the Cordillera Principal, and the basement-block faulted Cordillera Frontal. A regional cross section suggests that shortening across the Andes was achieved by thrusting along detachments at several levels in the crust. While thin-skinned deformation along newly formed thrusts occurred in Mesozoic sequences of the eastern Cordillera Principal, reactivation of preexisting Jurassic and Oligocene normal faults has resulted in additional hybrid thick- and thin-skinned structures in the western Cordillera Principal. Five major thrusting events are recognized in this part of the Andes: (1) Early to Middle Miocene tectonic inversion of the extensional faults in the western Cordillera Principal, (2) Middle to Late Miocene development of the Aconcagua fold and thrust belt, (3) Late Miocene uplift of Cordillera Frontal, (4) Late Miocene--Early Pliocene out-of-sequence thrusting in the Cordillera Principal, and (5) Pliocene to present deformation of the foreland. INDEX TERMS: 9604 Information Related to Geologic Time: Cenozoic; 9360 Information Related to Geographic Region: South America; 8015 Structural Geology: Local crustal structure; 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: central Andes, Cenozoic deformation, Aconcagua fold, thrust belt. Citation: Giambiagi, L. B., V. A. Ramos, E. Godoy, P. P. Alvarez, and S. Orts, Cenozoic deformation and tectonic style of the Andes, between 33[degrees] and 34[degrees] south latitude, Tectonics, 22(4), 1041, doi: 10.1029/2001TC001354, 2003.

The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that [M.sub.1]-[M.sub.2] and the correlated periods of intense deformation ([D.sub.1][D.sub.2]) are part of a single tectonothermal event between 1,717 [+ or -] 2 and 1,732 [+ or -] 7 Ma. It is considered unlikely that the two metamorphic phases ([M.sub.1], [M.sub.2]) suggested by earlier work represent separate events occurring within 10-25 Ma of each other. Previous higher estimates for the age of [M.sub.1] granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U-Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multigrain U-Pb monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield [sup.207]Pb/[sup.235]U ages between 1,7281 [+ or -] 11 and 1,712 [+ or -] 2 Ma. The age range of the monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of monazite.

Stratigraphic and isotopic data identify the lower 45 m of the Araras Group, on the southwest margin of the Amazon craton, as a Neoproterozoic platform cap carbonate deposited below wave base upon Varanger glacial diamictites of the Puga Formation. The basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tubelike structures and fenestral nonmicrobial planar laminites with tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans (pseudomorphs after aragonite). The basal contact of the Puga cap exhibits soft-sediment deformational structures (principally load casts) that are here attributed to rebound-induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite. These features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball Earth model of Neoproterozoic glaciation. Keywords: Neoproterozoic, Amazon craton, carbonate rocks, glaciation, isotopes, deformation.

The geotectonic setting of Pennsylvanian uplifts and associated basins of the Ancestral Rocky Mountains province has long been unclear because analogy of the deformed intracontinental domain with either arc or collisional orogens is not apt. Diachronous subsidence of Ancestral Rocky Mountains basins was coincident with sequential closure, from east to west, of the Ouachita suture to the southeast. This geotectonic relationship suggests that Ancestral Rocky Mountains deformation was induced by intracontinental stresses associated with continued subduction of westerly parts of Laurentia after more easterly parts had locked against Gondwana. Keywords: Ancestral Rocky Mountains, Pennsylvanian, Ouachita, suture belt, tectonism.

Integrated field structural studies and SHRIMP U-Pb zircon and monazite dating have been undertaken in Renland, west of Scoresby Sund district in the southern part of the East Greenland Caledonides. Southwest Renland is dominated by metasedimentary rocks correlated with the Krummedal supracrustal succession of East Greenland and which on Renland were intruded by augen granites. Krummedal psammite from Renland yielded a spectrum of Mesoproterozoic to Palaeoproterozoic detrital U-Pb zircon dates, the youngest of which indicate deposition of the psammite occurred c. 1000 Ma ago, thus post-dating Grenvillian continent-continent collision in North American Laurentia. These Krummedal metasediments were deformed into regional nappe-scale folds prior to metamorphism, crustal anatexis and genesis of augen granites; an example of the latter has been dated at 915 [+ or -] 18 Ma (U-Pb zircon). This demonstrates early Neoproterozoic high-temperature tectono-metamorphism affecting rocks within the southern East Greenland Caledonides, broadly contemporaneous with similar rocks farther north in East Greenland and with Sveconorwegian events on Baltica. Still in southwestern Renland, a later thermal event led to development of uppermost amphibolite to granulite facies metamorphic assemblages, veins and patches of in situ garnetiferous melt-bearing neosome in both metasediments (432 [+ or -] 6 Ma, U-Pb zircon) and in the augen granites, and contemporaneous biotite-bearing granite sheets in top-down-to-the-E extensional shear zones (434 [+ or -] 5 Ma, U-Pb zircon). Monazites from southwestern Renland record Caledonian thermal events as late as 410-400 Ma. In contrast, southeastern Renland is dominated by quartzofeldspathic migmatites with a strongly Caledonian signature but enclosing relicts of augen granite and retrogressed granulite facies psammitic and pelitic metasediment. There is also a sequence of Caledonian granitoid intrusions. Two samples from a hypersthene monzonite intrusion yielded U-Pb zircon dates of 424 [+ or -] 8 Ma and 424 [+ or -] 6 Ma. This pluton shows the marginal effects of the regional migmatization and was intruded early in the sequence of granitoid emplacement. An amphibolite facies migmatite, textural evidence from which suggests that it had never hosted granulite facies assemblages, records zircon growth at 423 [+ or -] 6 Ma, and closure of monazite by 402 [+ or -] 10 Ma. High grade metamorphism, and the protracted sequence of granitoid emplacement and still younger thermal events which together span the period between 430 and 400 Ma may, in part, reflect complicated lithospheric dynamics associated with subduction outboard of the Laurentian margin. Crustal segments carrying the relict evidence of Neoproterozoic and early Caledonian events must then quickly have been thrust northwestwards in foreland-propagating, northwesterly directed thrusts over Cambro-Ordovician platformal sequences on the Laurentian margin. This records the final closure of Iapetus, encroachment of Baltica and continent-continent collision from late Llandovery times (425-430 Ma). Keywords: Neoproterozoic, Caledonian, orogenesis, East Greenland, zircon.

The study of soft-rock deformations has received increasing attention during the past two decades, and much progress has been made in the understanding of their genesis. It is also recognized now that soft-rock deformations--which show a wide variety in size and shape--occur frequently in sediments deposited in almost all types of environments. In spite of this, deformations occurring in lithified rocks are still relatively rarely attributed to sedimentary or early--diagenetic processes. Particularly faults in hard rocks are still commonly ascribed to tectonics, commonly without a discussion about a possible nontectonic origin at a stage that the sediments were still unlithified. Misinterpretations of both the sedimentary and the structural history of hard-rock successions may result from the negligence of a possible soft-sediment origin of specific deformations. It is therefore suggested that a re-evaluation of these histories, keeping the present-day knowledge about soft-sediment deformations in mind, may give new insights into the geological history of numerous sedimentary successions in which the deformations have not been studied from both a sedimentological and a structural point of view. Keywords: Soft-sediment deformations; Tectonics; Pseudo-tectonics; Structural history

Pyritized graptolites from the Welsh Basin (United Kingdom) slate belt acted as rigid bodies during cleavage formation, and epizonal white micas formed within the resulting strain shadows, orthogonal to the principal stress orientation. Although the quantities of mica are small, they are a pure synkinematic mineral and have been dated by [sup.40]Ar-[sup.39]Ar infrared laser microprobe as a means to dating cleavage. Four samples of strain-fringe mica from different hand samples yielded ages ranging from 394.4 [+ or -] 3.1 to 397.8 [+ or -] 1.8 Ma (2[sigma]), with a mean age of 396.1 [+ or -] 1.4 Ma (2[sigma]). By focusing on minerals that are unequivocally synkinematic, this technique provides a novel solution to the problems of isotopically dating slaty cleavage. Previous studies have predominantly relied on dating whole-rock slate samples or separated illite grains by [sup.40]Ar-[sup.39]Ar techniques; problems encountered included (1) separating the effects of isotopic contamination by detrital phases, (2)[sup.39]Ar loss during the irradiation of illite mineral separates, and (3) thermally induced [sup.40]Ar loss in nature from fine-grained minerals. By circumventing these problems, this new method provides the first unequivocal and high-precision age data for Acadian deformation in the well-characterized Welsh Basin slate belt. With such precision, the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent-continent collision. Keywords: [sup.40]Ar-[sup.39]Ar laserprobe, low temperature, slate, deformation, geochronology, Acadian.

Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during [D.sub.2] have been dated using SHRIMP U-Pb geochronology. Zircon ages of 424 [+ or -] 8 Ma (Vagastie Bridge granite), 420 [+ or -] 6 Ma (Klibreck granite) and 429 [+ or -] 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional [D.sub.2] deformatiom during mid- to late Silurian time. Titanite ages of 413 [+ or -] 3 Ma (Vagastie Bridge granite) and 416 [+ or -] 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550-500[degrees]C. A mid- to late Silurian age for [D.sub.2] deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of foreland-propagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arc-continent orogeny. The mid- to late Silurian [D.sub.2] nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia. Keywords: Scotland, Moine Supergroup, Caledonian orogeny, zircons, titanites, metagranites.

New mapping and U-Pb zircon geochronology help establish the timing of contractional deformation and magmatism in the White Mountains of California. In the Redding Canyon area of the west-central White Mountains, Mesozoic deformation characterized by east-directed movement along reverse faults as well as recumbent folding was followed by development of upright folds with axes that plunge moderately to the north. This later folding event produced penetrative, vertical, north-striking axial-planar cleavage that is present along much of the western flank of the range. Deformed units include folded and/or boudinaged diorite dikes (ca. 165 Ma; U-Pb zircon) that contain the later penetrative cleavage. The cleavage is clearly cut by the Redding Canyon pluton (ca. 164 Ma; U-Pb zircon), demonstrating that at least some of the intense deformation preserved in the area is Middle Jurassic and correlative with the East Sierran thrust system identified elsewhere in California. Dates for the Beer Creek pluton (ca. 179 Ma; U-Pb zircon) and the Sage Hen Flat pluton (ca. 175 Ma; U-Pb zircon), which cut deformation in their wall rocks, suggest that East Sierran thrust deformation did not propagate as far eastward as these plutons at the present level of exposure. The new dates also cast doubt on the presence of any Late Jurassic Early Cretaceous plutonism in the White Mountains. Throughout the east-central Sierra Nevada and White Mountains, high-precision U-Pb zircon geochronology is resolving significant plutonism into two short-lived events that occurred at ca. 180-165 Ma and 102-86 Ma. Keywords: geochronology, deformation, Mesozoic, White Mountains, California.

The Cretaceous gabbros in the Fujian coastal region, southeastern China, experienced different magmatic and tectonic processes associated with deformation of the Changle-Nanao shear zone. Group 1 gabbros (115 Ma) show marked LILE (e.g., Sr, Ba and K) and LREE enrichment, and HFSE (e.g., Ta, Nb, Zr and Hf) depletion, strongly suggesting an island-arc affinity. The parental magma of group 1 gabbros, arguably derived from an arc-related source mantle modified by previous subduction, rose into and interacted with the coexisting syntectonic granitic magma generated during the main stage of shear zone deformation, and produced the gabbro-diorite-granite complexes. Group 2 and 3 gabbros (106-95 Ma) possibly formed by coupling of the subduction-modified source mantle with part of undepleted mantle (e.g., Nb enrichment) in an extensional environment at the end of deformation. Their parental magmas did not undergo significant magma mixing but produced a cumulate nature. All the three groups of gabbros were contaminated by crustal material during the processes of intrusion and emplacement.

[1] Recent field investigations in the central East Greenland Caledonides (72[degrees]-74[degrees]N) resulted in the identification of an orogen-scale extensional fault system called the Fjord Region Detachment (FRD). Previous geochronologic constraints on this deformation indicated that the FRD was active circa 430-425 Ma, a time when the Baltica-Laurentia collision was thought to be occurring, and continued to be active for up to 80 million years. We present new [sup.40]Ar/[sup.39]Ar thermochronologic data from an E-W transect that cuts across two splays of the FRD. Our data demonstrate that at least two distinct episodes of faulting were responsible for extension in the East Greenland Caledonides: an earlier phase (circa 425-423 Ma) that was synorogenic and penetrated to middle-crustal levels, followed by a post-Caledonian phase of reactivation (~414 to 380 Ma) that affected even deeper structural levels. Furthermore, we present in situ UV laser [sup.40]Ar/[sup.39]Ar data for pseudotachylite collected along the deepest splay of the FRD that indicate this fault was active again as recently as ~357 Ma (coeval with Devonian basin formation). Altogether, our data suggest that rather than being active continuously for 80 million years, the FRD consisted of multiple splays that were active for shorter intervals over discrete time periods separated by as much as 60 million years. Finally, our data provide evidence that young extensional deformation associated with postorogenic collapse in East Greenland was not restricted to the formation of sedimentary basins in the far eastern part of the orogen, but also resulted in deformation of the Archean-Paleozoic crystalline basement. INDEX TERMS: 8109 Tectonophysics: Continental tectonics--extensional (0905); 1035 Geochemistry: Geochronology; 8010 Structural Geology: Fractures and faults; 9315 Information Related to Geographic Region: Arctic region; KEYWORDS: extensional tectonics, East Greenland Caledonides, [sup.40]Ar/[sup.39]Ar, UV laser, thermochronology, synorogenic extension. Citation: White, A. P., and K. V. Hodges, Multistage extensional evolution of the central East Greenland Caledonides, Tectonics, 21(5), 1048, doi: 10.1029/2001TC001308 2002. Received 19 June 2001; revised 11 December 2001; accepted 25 March 2002; published 24 October 2002.

A transpressive crustal-scale dextral shear zone is documented in the Variscan Basement of northeastern Sardinia. It indicates the presence of a shear deformation parallel to the belt overprinting previous D1 structures related to nappe stacking and top-to-the-S and -SW thrusting. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2. Phase D1 developed during initial frontal collision, whereas the D2 deformation was characterized by dextral shearing during the increasing curvature of the Ibero-Armorican arc. Transpressional deformation developed in a regime of decreasing pressure. It caused telescoping of the Barrovian isograds and the exhumation of the low- to medium-grade metamorphic rocks. In this sector of the Variscan belt, exhumation is due to continuing compression with an increasing component of horizontal displacement. The overall change of the shortening direction in a large sector of an orogenic belt, with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse. Keywords: structural geology, basement tectonics, Hercynian Orogeny, transpression, shear zones.

Pennsylvanian--Permian synorogenic deposits (Minturn and Sangre de Cristo Formations) of the Central Colorado trough record an interplay of deformation and sedimentation in an Ancestral Rocky Mountains basin. The Central Colorado trough was a north-trending basin bordered by basement-involved highlands of the Uncompahgre uplift on the west and the Ancestral Front Range and Apishapa uplift on the east. Stratigraphic data show that the Central Colorado trough was an asymmetric basin in which coarse-grained sediments were deposited adjacent to the Sand Creek-Crestone thrust fault system of the Uncompahgre uplift. These deposits pinch out eastward against the Apishapa uplift along the eastern margin of the basin. Lithofacies analysis shows that the Central Colorado trough was filled by fan-delta, fluvial-delta, and turbidite deposits of the Middle Pennsylvanian Minturn Formation and by alluvial-fan, braided-stream, and meandering-stream deposits of the Upper Pennsylvanian--Permian Sangre de Cristo Formation. Geologic mapping has identified three syndepositional structures in the strata of the Central Colorado trough that indicate Pennsylvanian-Permian shortening: (1) the Gibson Peak growth syncline in the footwall of the Crestone thrust fault, which formed by syndepositional rotation of the Crestone Conglomerate Member of the Sangre de Cristo Formation during thrust displacement; (2) the Sand Creek thrust fault, which cuts the lower part of the Crestone Conglomerate Member but is covered by younger deposits of the Crestone Conglomerate Member; and (3) an intraformational angular unconformity in the Sangre de Cristo Formation that separates folded strata from overlying less deformed strata. All three structures indicate general east-west shortening during deposition. We interpret the Central Colorado trough as a flexural basin on the basis of syndepositional thrust-related structures, basin asymmetry, and lithofacies distribution. Displacement on the east-verging Sand Creek-Crestone thrust fault system appears to have controlled uplift of the central part of the Uncompahgre uplift and also subsidence in the adjacent basin. The Apishapa uplift, located along the eastern margin of the basin, is interpreted as a possible flexural forebulge related to crustal loading of thrust sheets along the western margin of the Central Colorado trough. Geologic mapping, subsurface data, and identification of syndepositional structures has also led to a better understanding of post-Paleozoic deformation of Pennsylvanian--Permian strata of the Central Colorado trough and the structural evolution of the present Sangre de Cristo Mountains. Keywords: fan deltas, Minturn Formation, Rocky Mountains, Sangre de Cristo Mountains, thrust faults.

The blind reverse Bajo Segura Fault is located at the eastern extreme of the Trans-Alboran shear zone (Betic Cordillera, southeast Iberian Peninsula). The surface expression of recent activity of this blind ENE-WSW fault is represented by coseismic surface anticlines and growth synclines on both sides of the anticlines. In the synclines, the deformation of the most recent Quaternary materials is obscured by a sedimentary unit more than 30 m thick which was deposited during the later part of the Late Pleistocene and the Holocene. The present study reports three high-resolution seismic profiles made in the northern growth syncline, which was the one developed most by the Bajo Segura Fault. In these seismic profiles we recognize the boundary between pre-growth strata and growth strata. This marker, Early Pliocene in age, dates the start of the activity of this blind reverse fault. The geometry observed in the seismic profiles of the syntectonic strata, dating from the Late Pliocene and Quaternary, indicates a limb rotation folding mechanism. On seismic profile 2, the complex geometry of the Benejuzar anticline forelimb can be attributed to several splay faults close to the surface of Bajo Segura Fault. Keywords: active tectonics, blind faults, Betic Cordillera, seismic profiles.

ABSTRACT Detailed mapping in eastern and east-central Maine reveals a complex history of multiple brittle reactivations along the Norumbega fault system (NFS) following initial late Acadian (380 Ma) ductile shearing. [...]

Byline: I. E. Shipovskii (1) Abstract: The dynamic loading of a rock mass during explosion of a borehole explosive is studied using a continuum mechanics approach in two-dimensional plane and axially symmetric formulations with the aid of a modified finite element method [1, 2]. This numerical technique makes it possible to study wave processes in a rock mass owing to explosions of single charges as well as those of systems of borehole explosives under different conditions. These include varying the site at which the charge is initiated and accounting for the propagation velocity of detonations in the explosive, so it is possible to calculate the shape of the stress field created by a charge with a given design. Numerical simulation of the explosion process for multiple borehole explosive charges with delays relative to one another can be used to obtain the optimum delay time for initiation and the distances between the charges. These results can also extend our concepts of the processes taking place in a rock mass during explosive fracture. Author Affiliation: (1) Scientific Research Institute for Geodynamics Problems, Taurian National University, Ukraine Article History: Registration Date: 06/10/2004

The authors suggest that California's Sierra Nevada region and Idaho's Salmon River suture, formerly considered separate entities, may be linked by a structural discontinuity referred to as the western Nevada shear zone. A strike-slip fault is thought to have been reactivated and then obscured during the Cretaceous era, forming the connection.

During the late Miocene east-northeast-directed extension in the Gulf of California extensional province, the western rift margin in northeastern Baja California, Mexico, was segmented at the northwest-striking Matomi accommodation zone. The accommodation zone passed through the northern Puertecitos Volcanic Province and separated pre-6 Ma extension to the north from the unextended region to the south, pre-6 Ma northeast-sidedown displacement is documented across the accommodation zone, which may have undergone dextral oblique-slip motion. The rift margin migrated westward during the latest Miocene or Pliocene, bypassing accommodation-zone structures and incorporating the Puertecitos Volcanic Province into the region of Gulf extensional province deformation. East-northeast- to east-directed extensional deformation is at least post-6 Ma in the northern Puertecitos Volcanic Province and deformation currently continues. Pliocene changes in the Gulf of California spreading center system may have triggered incorporation of the Puertecitos Volcanic Province into the Gulf extensional province as recently as 2-3 Ma. Paleomagnetic analyses of 6.3-6.6 Ma pyroclastic flow deposits show no consistent evidence for rotational deformation, although minor (approximately 10 degrees -15 degrees) clockwise rotation is possible given anomalous declination directions recorded at some sites. Comparisons with paleosecular variation models and the late Miocene paleopole for stable North America imply no statistically significant rotation relative to geomagnetic north, although minor (approximately 10 degrees) clockwise rotation is permissible given uncertainties. Geologic relationships show that pre-6 Ma accommodation-zone structures identified in this study did not mark the southern boundary of later rotational deformation documented to the north. The boundary of Pliocene to Holocene rotations may be a broad, diffuse zone of extensional shear encompassing the northeastern Puertecitos Volcanic Province, which accommodated small rotations. Keywords: accommodation zones, Baja California, extension tectonics, paleomagnetism, plate divergence, volcanics.

Although the Altyn Tagh fault system has played an important role in the Indo-Asian collision, its geometry and tectonic evolution remain poorly known. Between 86 degrees and 92 degrees E, this system is at least 100 km wide and is bounded to the north and south by the North Altyn and Altyn Tagh faults, respectively. Mapping along the Jianglisai reach of the North Altyn fault indicates that Miocene(?) to Pliocene(?) motion was predominantly left to left-reverse slip, with transport vectors trending N45degrees -60 degrees E. Map relationships suggest that total offset on the fault is >120 km. These results are inconsistent with previous models of the Altyn Tagh fault system in which oblique convergence along the northern margin of the Tibetan Plateau is partitioned into thrusting on the North Altyn fault and left slip on the Altyn Tagh fault. An alternative hypothesis is that the North Altyn fault is the northern boundary of a transpressional strike-slip duplex within which the structurally elevated Altyn Mountains were created. Our model suggests that transpressional deformation may be restricted to this strike-slip duplex and need not characterize the entire margin.

From at least 2-4 Ma to present, crust in the southeastern part of the Tibetan Plateau west of the convex-east Xianshuihe-Xiaojiang fault system has deformed internally and rotated clockwise around the eastern Himalayan syntaxis. The northwest-striking Ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78-100 km, of which approximately 60 km is transferred to the Xianshuihe fault zone across a diffuse transfer zone, and approximately 22-40 km is absorbed by bending of older structures and crustal shortening. Crustal shortening is expressed along and east of the eastern end of the Ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the Tibetan Plateau. A similar transfer of left slip into shortening occurs farther south across the Xianshuihe fault in the high mountains around and east of Gongga Shan (7556 m). The northwest-striking, convex-east, left-lateral Litang fault zone lies southwest of the Ganzi-Xianshuihe-Xiaojiang fault zone and appears to be less well developed but otherwise similar to the Ganzi fault zone. The Batang, Chenzhi, and other northeast-striking right-lateral faults of small displacement occur within the rotating crustal fragment. Together with the left-slip faults, they accommodate east-west shortening northeast of the eastern Himalayan syntaxis. South of this region of shortening, the crust is extending to form grabens within the Dali and southern Xiaojiang fault systems and in the Tengchong volcanic province. The progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern Himalayan syntaxis. The assemblage of structures in southwestern Sichuan geometrically resembles structures of Eocene to Miocene age in southern Yunnan that were positioned northeast of the eastern Himalayan syntaxis, similar to present-day southwestern Sichuan, at the time of their development. The similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past approximately 50 m.y. Structures in Sichuan are less fully developed than older structures in southwestern Yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis. Deformation in these areas indicates that plateau formation has been complex, inhomogeneous, and diachronous at scales from 1000 km to less than 100 km. Keywords: Tibet, Sichuan, tectonics, late Cenozoic, syntaxis.

Series of scaled sandbox models are used to simulate the development of thin-skinned simultaneous shortening above adjacent ductile and frictional substrates. These models simulate the evolution of the Potwar Plateau and Salt Range in Pakistan, where Paleozoic to Holocene sediments are shortened partly above a ductile substrate of the Salt Range Formation. In this study, variations in the initial thickness of the ductile substrate and the influence of prekinematic and synkinematic overburden wedge have been systematically investigated. Model results confirm that forward-vergent imbricates forming relatively steep wedges develop above a frictional substrate, whereas low-taper wedges with both foreland and rearward-vergent imbricates develop above ductile substrates. Furthermore, deformation propagates farther and more rapidly above a ductile substrate than above a frictional substrate. The differential rate of propagation of a detachment between adjacent areas with ductile and frictional substrates generates an inflection subparallel to the shortening direction. Transpression develops across this lateral inflection boundary due to differential rates of propagation. Folding of the overburden carries the ductile substrate along the inflection boundary, creating a conduit along which salt walls and stocks develop, which are potentially diapiric. Model results show structures similar to those observed in the Salt Range-Potwar Plateau. Pop-up and/or pop-down structures are characteristic features of the Potwar Plateau, and are observed in all model series. On the basis of model results, it is suggested that the anomalous thickness of salt beneath the hanging wall of the Salt Range thrust is a consequence of buttressing by a basement fault and the southward evacuation of salt resulting from differential sedimentary and tectonic loading, a feature clearly displayed by the models. Keywords: diapirs, imbricate tectonics, Potwar Plateau, detachment, ductile deformation, brittle deformation.

Grain-scale deformation processes during sliding on simulated faults, and in the presence of reactive pore fluids, are examined with in situ, see-through experiments using polycrystalline biphenyl aggregates at 60 degrees C as a rock analogue. Reactive pore fluids were water-alcohol-vapor mixtures and bulk shear strain rates were 5.8 x 10(super -6) s(super -1). Under these conditions, slip at displacement rates around 0.01 (mu)m/s on shear fractures involves a mixture of cataclasis and frictional sliding, together with dissolution and precipitation processes. Simultaneously, dislocation flow processes accommodate distributed shearing away from slip surfaces. The evolution of fault porosity is controlled by a dynamic competition between crack growth and opening and crystal growth into cracks and pore spaces. During progressive sliding, growing crystals and other asperities on opposing fracture walls interact by fracturing, rotation, grain translation, and dissolution and precipitation. This multimechanism behavior in rock analogue materials provides insights about the mechanical behavior of faults in the presence of hydrothermal fluids at seismogenic depths. Models of the strength and slip behavior of natural faults may thus need to consider multimechanism behavior during frictional sliding.

Combined shear-sense criteria, finite-strain data and vorticity analyses were used to study the deformation path in a curved crustal-scale shear zone (Phyllite-Quartzite Series) of the southwestern Hellenides. The results are combined with data on the structural evolution of a cover nappe (Pindos thrust belt) to provide new insights into the orogenic evolution of this region. Ductile deformation within the Phyllite-Quartzite Series was associated with a top-to-the-west-southwest shearing and was partitioned into two structural domains: a root zone and a frontal domain. The root zone is characterized by vertical coaxial stretching, high strain and upward movement of the material, while the frontal domain comprises simple-shear deformation at the base and pure shear at the top. This pattern suggests superposition of pure shear on simple-shear deformation, and implies tectonic extrusion of the material from the root zone. The initiation of brittle deformation in the Pindos thrust belt was associated with westward translation above the sub-horizontal Pindos Thrust. Later, as the mountain range elevated, normal faulting at high altitudes and migration of thrusting to the west occurred, while east-directed folding and thrusting in the belt started to the east. According to the proposed model, crustal thickening was taking place throughout the Oligocene and early Miocene, including the subduction of the Apulian beneath the Pelagonian microcontinent and the intracontinental subduction of the Phyllite-Quartzite Series. During the lower Miocene, vertical buoyancy forces led to the successive steepening of the shear zone and the simultaneous duplexing of its basement, facilitating tectonic extrusion of the material from its root zone. Finally, an indentation process caused vertical expulsion of the orogenic wedge and gravity collapse in the brittle crust.