Showing papers on "Fault (geology) published in 1999"

Interpretational applications of spectral decomposition in reservoir characterization

Abstract: Spectral decomposition provides a novel means of utilizing seismic data and the discrete Fourier transform (DFT) for imaging and mapping temporal bed thickness and geologic discontinuities over large 3-D seismic surveys. By transforming the seismic data into the frequency domain via the DFT, the amplitude spectra delineate temporal bed thickness variability while the phase spectra indicate lateral geologic discontinuities. This technology has delineated stratigraphic settings (such as channel sands and structural settings involving complex fault systems) in 3-D surveys.

Fault transmissibility multipliers for flow simulation models

Abstract: Fault zone properties are incorporated in production flow simulators using transmissibility multipliers. These are a function of properties of the fault zone and of the grid-blocks to which they are assigned. Consideration of the geological factors influencing the content of fault zones allows construction of high resolution, geologically driven, fault transmissibility models. Median values of fault permeability and thickness are predicted empirically from petrophysical and geometrical details of the reservoir model. A simple analytical up-scaling scheme is used to incorporate the influence of likely small-scale fault zone heterogeneity. Fine-scale numerical modelling indicates that variability in fault zone permeability and thickness should not be considered separately, and that the most diagnostic measure of flow through a heterogeneous fault is the arithmetic average of the permeability to thickness ratio. The flow segregation through heterogeneous faults predicted analytically is closely, but not precisely, matched by numerical results. Identical faults have different equivalent permeabilities which depend, in part, on characteristics of the permeability field in which they are contained.

Effects of plate bending and fault strength at subduction zones on plate dynamics

Abstract: For subduction to occur, plates must bend and slide past overriding plates along fault zones. Because the lithosphere is strong, significant energy is required for this deformation to occur, energy that could otherwise be spent deforming the mantle. We have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology. By increasing the effective viscosity of either the plate or the fault zone, we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy. We have developed a simple physical theory that predicts this slowing by estimating a convecting cell's total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone. The energy required to bend the slab is proportional to the slab's viscosity and to the cube of the ratio of its thickness to its radius of curvature. The distribution of dissipation among the mantle, lithosphere, and fault zone causes the speed of a plate to depend on its horizontal length scale. Using the observation that Earth's plate velocities are not correlated to plate size, we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity, with higher values required if the fault zone can support shear tractions  50 MPa over 300 km. These subduction zone strengths imply that the mantle, fault zone, and lithosphere dissipate about 30%, 10%, and 60% of a descending slab's potential energy release if the slab is 100 km thick. The lithospheric component is highly dependent on slab thickness; it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km. $ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature, so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the Earth cools. Because the "details" of convergent plate boundaries are so important to the dynamics of plate motion, numerical models of mantle flow should treat subduction zones in a realistic way.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Abstract: Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

Defining seismogenic sources from historical earthquake felt reports

Abstract: We present a method that uses macroseismic intensity data to assess the location, physical dimensions, and orientation of the source of large historical earthquakes. Intensity data contain a great deal of information that can be used to constrain the essential characteristics of the seismic source. In particular, both the seismological theory and its practice suggest that the orientation of the source of significant earthquakes is reflected in the elongation of the associated damage pattern. A plausible and easily manageable way of describing a seismic source is by representing it as an oriented “rectangle,” the length and width of which are obtained from moment magnitude through empirical relationships. This rectangle is meant to represent either the actual surface projection of the seismogenic fault or, at least, the projection of the portion of the Earth crust where a given seismic source is likely to be located. The systematic application of this method to all the M > 5.5 earthquakes that occurred in the central and southern Apennines (Italy) in the past four centuries returned encouraging results that compare well with existing instrumental, direct geological, and geodynamic evidence. The method is quite stable for different choices of the algorithm parameters and provides elongation directions that in most cases can be shown to be statistically significant. In particular, the resulting pattern of source orientations is rather homogeneous, showing a consistent Appennines-parallel trend that agrees well with the NE-SW extension style of deformation active in the central and southern portions of the Italian peninsula.

Dynamic 3D simulations of earthquakes on En Echelon Faults

Abstract: One of the mysteries of earthquake mechanics is why earthquakes stop. This process determines the difference between small and devastating ruptures. One possibility is that fault geometry controls earthquake size. We test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three-dimensional medium and apply our knowledge to two California fault zones. We find that the size difference between the 1934 and 1966 Parkfield, California, earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 Landers, California, earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event. If there are no linking structures, such as transfer faults, then strike-slip earthquakes are unlikely to propagate through stepover s >5 km wide.

Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico

Abstract: Salt-related faults and fault welds in the northern Gulf of Mexico are classified based on the three-dimensional geometry of the faults or welds, deformed strata, and associated salt. Kinematic or genetic criteria are not used in the classification. Only documented fault styles are considered; those styles produced by experimental or numerical modeling, but not yet observed in the Gulf, are not included. Extensional faults comprising symmetric arrays include peripheral faults, which occur at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults, which occur at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to the dominant dip direction. Faults that dip primarily basinward include roller faults, which are listric growth faults that sole into a subhorizontal salt layer; ramp faults, which extend upward from the landward margin of bulb-shaped salt stocks; and shale-detachment faults, which sole into a shale decollement that merges into a salt layer. Counterregional faults are landward-dipping asymmetric arrays that link cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs with uplifted and rotated roof strata, and rollover faults, which occur at the hinges of monoclinal folds. Two families of contractional faults are described: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale decollement, and break thrusts, which are high-angle reverse faults that cut one or both limbs of detachment folds. Fault arrays that strike parallel to the regional dip direction are termed lateral faults. Six types of fault welds are defined: primary welds are those at the autochthonous level; roho welds are subhorizontal, allochthonous welds into which roller faults detach; counterregional welds comprise both subhorizontal and landward-dipping segments beneath growth monoclines; bowl welds are elliptical and upwardly concave; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep and strike parallel to the regional dip direction. Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to each other and to associated salt bodies and welds. Linked fault systems can contain extensional, contractional, and strike-slip components. Extensional fault families are formed by basinward translation, subsidence into salt, or folding. Those fault families that accommodate basinward translation are balanced by salt extrusion or contractional fault families. Strike-slip fault families commonly provide hard links, although various fault components also can be soft linked. We illustrate five associations of linked fault systems that are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.

The Chi‐Chi, Taiwan earthquake: Large surface displacements on an inland thrust fault

Abstract: In the early morning (01:47 local time) of September 21, 1999, the largest earthquake of the century in Taiwan (Mw=7.6, ML=7.3) struck the central island near the small town of Chi-Chi. The hypocenter was located by the Central Weather Bureau Seismological Center at 23.87°N, 120.75°E, with a depth of about 7 km. There were extensive surface ruptures for about 85 km along the Chelungpu fault with vertical thrust and left lateral strike-slip offsets. The maximum displacement of about 9.8 meters is among the largest fault movements ever measured for modern earthquakes. There was severe destruction in the towns of Chungliao, Nantou,Taichung, FengYuan, and Tungshi, with over 2300 fatalities and 8700 injuries.

Late Cenozoic tectonics of the Kepingtage thrust zone: Interactions of the Tien Shan and Tarim Basin, northwest China

Abstract: The Kepingtage (Kalpin) thrust zone, northwest China, is an actively deforming part of the India-Asia collision system. It lies south of the Tien Shan, has an area of ∼16,000 km2, and consists of arcuate, emergent imbricates. Overall vergence is toward the interior of the Tarim Basin to the south. Thrust sheets typically expose Upper Cambrian to Permian platformal strata. Thrusting is largely thin-skinned; thin Upper Cambrian evaporites are likely to be the main decollement horizon. The Kepingtage thrust zone is the only margin of the Tarim Basin to deform in this style in the Cenozoic. It is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the Tarim Basin. Thick (up to 10 km) Mesozoic and Cenozoic clastic successions are present and deformed in these regions. Such successions are not present in the Kepingtage thrust zone or the adjacent Bachu Uplift within the Tarim Basin, because of episodic activity on steep, northwest-southeast trending thrusts which define the margins of the Bachu Uplift. These Mesozoic-Cenozoic strata may have suppressed the ability of regions along strike from the Kepingtage thrust zone to deform by thin-skinned thrusting utilizing the Upper Cambrian decollement. The along strike variation in active thrusting at the southern margin of the Tien Shan is an example of syntectonic sedimentation controlling thrust belt deformation style. A balanced section across the thrust zone indicates ∼28% shortening, equivalent to ∼35 km. This is equivalent to an average slip rate of ∼1.8 mm yr−1 and a strain rate of ∼4.4 × 10−16 s−1, assuming deformation began at circa 20 Ma. Active deformation is focused along the frontal thrust, the Kepingtage Fault. The northern boundary of the Kepingtage thrust zone is formed by the South Tien Shan Fault. This major, north dipping thrust is seismically active, with published earthquake focal depths at midcrustal levels (14 and 18 km). It juxtaposes upper Carboniferous sedimentary rocks of different facies and different Paleozoic deformation histories. Speculatively, it represents a reactivation of a late Paleozoic thrust, which originated at the boundary between the platformal interior of the Tarim Block and a deeper-water foreland basin to its north.

Shear heating in continental strike-slip shear zones:model and field examples

Abstract: SUMMARY A two-layer (crust and upper mantle), finite diVerence steady-state thermomechanical model of a long-lived (several million years) lithospheric strike-slip fault is presented, and its predictions compared with field observations from various major fault zones. In order to estimate the maximum amount of shear heating, all mechanical energy is assumed to be dissipated in heat, in ductile as well as in brittle layers. Deformation follows a friction law in the brittle layer(s), and a power-flow law in the ductile one(s). Variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored. Shear heating is found to be more important in fault zones aVecting an initially cold lithosphere, and increases with slip rate, friction coeYcient and stiVness of materials. In extreme cases (slip rate of 10 cm yr’1, stiV lithosphere), shear heating could lead to temperature increases close to 590 °C at the Moho, and 475 °C at 20 km depth. For more common cases, shear heating leads to smaller temperature increases, but can still explain high-grade metamorphic conditions encountered in strike-slip shear zones. However, modelled temperature conditions often fall short of those observed. This could be due to heat transport by mechanisms more eYcient than conduction. Common syntectonic emplacement of granitic melts in ductile strike-slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle. Besides slip rate, the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust: for hard upper mantle and highly fertile lower crust, partial melting could occur at rates of 1 cm yr’1, while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr’1. As a result of shear heating, partial melting of the upper mantle could occur in the presence of small amounts of fluids. Rise of magmas and/or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone. In nature, shear heating would inevitably cause strain localization in the deeper parts of strike-slip faults, as is often observed in the field for crustal shear zones.

Stress sensitivity of fault seismicity: A comparison between limited‐offset oblique and major strike‐slip faults

Abstract: We present a new three-dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1989 M = 7. 1 Loma Prieta earthquake and to compare fault seismicity rates before and after 1989. The major high-angle right-lateral faults exhibit a different response to the stress change than do minor oblique (right-lateral/thrust) faults. Seismicity on oblique-slip faults in the southern Santa Clara Valley thrust belt increased where the faults were unclamped. The strong dependence of seismicity change on normal stress change implies a high coefficient of static friction. In contrast, we observe that faults with significant offset (>50-100 km) behave differently; microseismicity on the Hayward fault diminished where right-lateral shear stress was reduced and where it was unclamped by the Loma Prieta earthquake. We observe a similar response on the San Andreas fault zone in southern California after the Landers earthquake sequence. Additionally, the offshore San Gregorio fault shows a seismicity rate increase where right-lateral/oblique shear stress was increased by the Loma Prieta earthquake despite also being clamped by it. These responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup; coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes, whereas in more limited offset faults, fluids could rapidly escape. The difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes, since many of these events occur both inside and outside major fault zones.

The 17 August 1999 Izmit Earthquake

Abstract: On 17 August 1999, an earthquake of magnitude 7.4 shook northwestern Turkey. Details on the earthquake9s characteristics and effects across northwestern Turkey are provided and are put in the historical context of previous earthquakes in the region. The earthquake was the seventh in a series of earthquakes migrating westwards along the North Anatolian fault since 1939. Information on historical earthquakes, modeling and estimates of the slip rate along the fault indicate that the location and severity of the earthquake should not have come as a surprise.

An active, deep marine strike-slip basin along the north anatolian fault in turkey

Abstract: The Tekirdag depression within the Marmara Sea in the Mediterranean region is an active, rhomb-shaped strike-slip basin along the North Anatolian fault with a basin floor at a water depth of −1150 m. New multichannel seismic reflection data and on-land geological studies indicate that the basin is forming along a releasing bend of the strike-slip fault and is filled with syntransform sediments of Pliocene-Quaternary age. The basin is bounded on one side by the North Anatolian fault and on the other side by a subparallel normal fault, which forms the steep submarine slope. In cross section the basin is strongly asymmetric with the thickness of the syntransform strata increasing from a few tens of meters on the submarine slope to over 2.5 km adjacent to the North Anatolian fault. Seismic sections also show that the slope-forming normal fault connects at depth to the North Anatolian fault, implying that the basin is completely detached from its substratum. The whole structure can be envisaged as a huge, rather flat, negative flower structure. The releasing bend of the North Anatolian fault, responsible for the formation of the basin, is flanked by a constraining bend. Along the constraining bend, the syntransform strata are being underthrust, implying a recent change in the direction of the regional displacement vector. This thrusting is responsible for the uplift of the submarine slope to a height of 924 m, possibly by a mechanism of elastic rebound. Regional geology suggests that most of the syntransform strata are lacustrine with only the topmost few hundred meters consisting of deep marine clays. The anomalous present depth of the Tekirdag depression is due to reduced Quaternary sedimentation coupled with high rates of displacement along the North Anatolian fault, which amounts to 20 mm/yr in the Marmara Sea region.

Plate convergence measured by GPS across the Sundaland/Philippine Sea Plate deformed boundary: the Philippines and eastern Indonesia

Abstract: SUMMARY The western boundary of the Philippine Sea (PH) Plate in the Philippines and eastern Indonesia corresponds to a wide deformation zone that includes the stretched continental margin of Sundaland, the Philippine Mobile Belt (PMB), extending from Luzon to the Molucca Sea, and a mosaic of continental blocks around the PH/Australia/Sunda triple junction. The GPS GEODYSSEA data are used to decipher the present kinematics of this complex area. In the Philippines, the overall scheme is quite simple: two opposing rotations on either side of the left-lateral Philippine Fault, clockwise to the southwest and counterclockwise to the northeast, transfer 55 per cent of the PH/Sundaland convergence from the Manila Trench to the northwest to the Philippine Trench to the southeast. Further south, 80 per cent of the PH/Sunda convergence is absorbed in the double subduction system of the Molucca Sea and less than 20 per cent along both continental margins of northern Borneo. Finally, within the triple junction area between the Sundaland, PH and Australia plates, from Sulawesi to Irian Jaya, preferential subduction of the Celebes Sea induces clockwise rotation of the Sulu block, which is escaping toward the diminishing Celebes Sea oceanic space from the eastwardadvancing PH Plate. To the south, we identify an undeformed Banda block that rotates counterclockwise with respect to Australia and clockwise with respect to Sundaland. The kinematics of this block can be defined and enable us to compute the rates of southward subduction of the Banda block within the Flores Trench and of eastward convergence of the Makassar Straits with the Banda block. The analysis made in this paper confirms that this deformation is compatible with the eastward motion of Sundaland with respect to Eurasia determined by the GEODYSSEA programme but is not compatible with the assumption that Sundaland belongs to Eurasia, as was often assumed prior to this study.

Quaternary normal faulting in southeastern Sicily (Italy): a seismic source for the 1693 large earthquake

Abstract: SUMMARY We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian oVshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is aVected by WNW‐ESE regional extension producing normal faulting of the southern edge of the Siculo‐Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by diVerent ages and related to distinct tectonic processes. The older NW‐SE-trending normal fault segments developed up to #400 kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW‐SSE-trending normal fault segments and extends along the Ionian oVshore zone following the NE‐SW-trending Avola and Rosolini‐Ispica normal faults. These faults are characterized by vertical slip rates of 0.7‐3.3 mm yr’1 and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes (M~7) could be related to a 45 km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7 mm yr’1 is calculated, and a recurrence interval of about 550±50 yr is proposed for large events similar to that of January 1693.

An Elusive Blind-Thrust Fault Beneath Metropolitan Los Angeles

Abstract: Seismic reflection profiles, petroleum wells, and relocated earthquakes reveal the presence of an active blind-thrust fault beneath metropolitan Los Angeles. A segment of this fault likely caused the 1987 Whittier Narrows (magnitude 6.0) earthquake. Mapped sizes of other fault segments suggest that the system is capable of much larger (magnitude 6.5 to 7) and more destructive earthquakes.

Tectonics of the Jurassic-Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22°-26°S): A story of crustal deformation along a convergent plate boundary

Abstract: The tectonic evolution of a continental magmatic arc that was active in the north Chilean Coastal Cordillera in Jurassic-Early Cretaceous times is described in order to show the relationship between arc deformation and plate convergence. During stage I (circa 195–155 Ma) a variety of structures formed at deep to shallow crustal levels, indicating sinistral arc-parallel strike-slip movements. From deep crustal levels a sequence of structures is described, starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions (Bolfin Complex). The high-grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike-slip and thrust kinematics with a NW–SE directed maximum horizontal shortening, i.e., parallel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds, thrusts, and high-angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160–150 Ma), strong arc-normal extension is revealed by brittle low-angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels. During stage III (155–147 Ma), two reversals in the stress regime took place indicated by two generations of dikes, an older one trending NE–SW and a younger one trending NW–SE. Sinistral strike-slip movements also prevailed during stage IV (until ∼125 Ma) when the Atacama Fault Zone originated as a sinistral trench-linked strike-slip fault. The tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates. The structures of stages I and IV suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations. However, decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages II and III.

Prehistoric dates of the most recent Alpine fault earthquakes, New Zealand

Abstract: We use four methods to investigate the earthquake history over the past 600 yr of the central Alpine fault, New Zealand. Trenches across the active fault trace at five locations identify two ruptures on the fault in the past 450 yr. Three other indirect indicators (episodes of landsliding and aggradation, dated either via 14 C or from forest establishment on the resulting surfaces, and tree-ring growth anomalies) provide further evidence for these two earthquakes and a greater resolution in estimates of their timing and extent. The most recent earthquake occurred in A.D. 1717 and the rupture extended over a section of fault between Milford and the Haupiri River, a distance of 375 km. About 1630, another earthquake occurred on the fault and this extended between at least the Paringa and Ahaura Rivers (250 km). Regional episodes of landsliding and forest establishment provide evidence for a third earthquake at about 1460 over the same section of fault. The pattern of earthquake recurrence is not regular, but averages ∼200 yr and varies from ∼100 yr to at least 280 yr, which is the lapsed time since the most recent rupture.

Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada

Abstract: New fission-track data together with 1:24 000-scale geologic mapping and analysis of Tertiary sedimentary deposits provide better constraints on the time and nature of motion along the Snake Range decollement, a classic Basin and Range metamorphic core complex detachment fault in east-central Nevada. Here, the fission-track method provides a particularly effective tool for dating faulting where bracketing or crosscutting relations are not available. These new data suggest that the Snake Range decollement forms part of a more extensive, 150-km-long north-south–trending fault system, the Snake Range–Deep Creek Range fault system. This fault system extends along the eastern flank of the northern and southern Snake Range, Kern Mountains, and Deep Creek Range, and accommodated at least 12–15 km of rapid slip in the Miocene, ca. 17 Ma. This component of motion is distinctly younger (by about 15–20 m.y.) than an earlier episode of slip and extension across the region bracketed stratigraphically and geochronologically as late Eocene–early Oligocene age. Apatite fission-track ages (n = 57) in most parts of the Snake Range and adjacent ranges cluster at 17 Ma, indicating rapid cooling from >125 to 310 to <50 °C. Formation of at least part of the pervasive mylonitic fabrics in the northern Snake Range may have occurred during this Miocene time interval, very late rather than early in the extensional history of the region. Coarse fanglomerate and rock-avalanche deposits in flanking Tertiary basins provide additional evidence for major tectonism at this time. Comparison of the timing of events in the northern Snake Range to that along strike of the fault system indicates that Miocene slip along the low-angle northern Snake Range decollement and exhumation of extensive footwall mylonites were coeval with more typical Basin and Range high-angle rotational faulting in the Deep Creek Range and Kern Mountains to the north and in the southern Snake Range to the south. This suggests that the two styles of faulting (low-angle detachment and high-angle rotational) can occur simultaneously along the length of a single normal fault system. Data from the northern Snake Range also underscore the importance of a vertical component of uplift of the range in Miocene time, leading to the present domal geometry of the northern Snake Range decollement. When considered together with footwall deformational fabrics, the new data are most simply explained as the consequence of higher local geothermal gradients and a shallower brittle-ductile transition zone along the northern Snake Range part of the fault system. It can be speculated that the Snake Range metamorphic core complex represents the top of a stretching welt of hotter, deeper level crust that rose during extension. This rising welt may have been localized by the presence of previously thickened crust beneath the region and could have been triggered by increased regional magmatism and heating accompanying rapid extension in Miocene time.

Continuous Deformation Versus Faulting Through the Continental Lithosphere of New Zealand

Abstract: Seismic anisotropy and P-wave delays in New Zealand imply widespread deformation in the underlying mantle, not slip on a narrow fault zone, which is characteristic of plate boundaries in oceanic regions. Large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault show that pervasive simple shear of the mantle lithosphere has accommodated the cumulative strike-slip plate motion. Variations in P-wave residuals across the Southern Alps rule out underthrusting of one slab of mantle lithosphere beneath another but permit continuous deformation of lithosphere shortened by about 100 kilometers since 6 to 7 million years ago.

Basin analysis of the Jurassic–Lower Cretaceous southwest Tarim basin, northwest China

Abstract: The Jurassic through Cretaceous southwest Tarim basin, northwest China, contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with Eurasia of the Changtang block during Late Triassic–Early Jurassic time and with the mega-Lhasa block during Late Jurassic–Early Cretaceous time. This tectonism is chronologically linked with the creation of a narrow lower Middle Jurassic transtensional basin with thick sedimentary infill, succeeded by a broader Upper Jurassic–Lower Cretaceous compressional(?) basin with thinner sedimentary infill. The older basin formed between a north-northwest–striking dextral fault on the eastern side of the southwest Tarim basin in the Tian Shan and a postulated strike-slip or normal fault on the western margin of the basin along the Kunlun Shan. The former fault is now the Talas-Ferghana fault; the latter may be a predecessor to the Main Pamir thrust. Subsidence analysis of the thickest sedimentary section suggests thermal subsidence, interpreted as the result of transtension between the two basin-bounding faults. The younger basin extends farther east and west and does not preserve evidence of activity along the Talas-Ferghana fault. The change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin-bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes.

Secular and tidal strain across the Main Ethiopian Rift

Abstract: Using a combination of laser ranging and GPS data acquired between 1969 and 1997 we derive a separation velocity for the Somali and Nubian plates in Ethiopia (4.5±1 mm/yr at N108±10E). This vector is orthogonal to the NNE-trending neotectonic axis (Wonji fault belt) of the Ethiopian rift axis. Current rifting is concentrated within a 33-km-wide zone that includes a 7-km-wide belt of late Quaternary faulting where maximum surface strain rates are comparable to those at active plate boundaries (0.1 μstrain/yr). The strain-field suggests that thin (<5 km) elastic crust separates thick continental lithosphere, a geometry quite different from oceanic rifting, and a mechanical configuration that favors the amplification of regional strain. Semidiurnal strain tides, however, as measured by kinematic GPS methods are not amplified along or across the rift, indicating that the rift zone's low rigidity applies only at periods of years.

Postglacial rebound and fault instability in Fennoscandia

Abstract: SUMMARY The best available rebound model is used to investigate the role that postglacial rebound plays in triggering seismicity in Fennoscandia. The salient features of the model include tectonic stress due to spreading at the North Atlantic Ridge, overburden pressure, gravitationally self-consistent ocean loading, and the realistic deglaciation history and compressible earth model which best fits the sea-level and ice data in Fennoscandia. The model predicts the spatio-temporal evolution of the state of stress, the magnitude of fault instability, the timing of the onset of this instability, and the mode of failure of lateglacial and postglacial seismicity. The consistency of the predictions with the observations suggests that postglacial rebound is probably the cause of the large postglacial thrust faults observed in Fennoscandia. The model also predicts a uniform stress field and instability in central Fennoscandia for the present, with thrust faulting as the predicted mode of failure. However, the lack of spatial correlation of the present seismicity with the region of uplift, and the existence of strike-slip and normal modes of current seismicity are inconsistent with this model. Further unmodelled factors such as the presence of high-angle faults in the central region of uplift along the Baltic coast would be required in order to explain the pattern of seismicity today in terms of postglacial rebound stress. The sensitivity of the model predictions to the eVects of compressibility, tectonic stress, viscosity and ice model is also investigated. For sites outside the ice margin, it is found that the mode of failure is sensitive to the presence of tectonic stress and that the onset timing is also dependent on compressibility. For sites within the ice margin, the eVect of Earth rheology is shown to be small. However, ice load history is shown to have larger eVects on the onset time of earthquakes and the magnitude of fault instability.

Mechanics of low‐stress forearcs: Nankai and Cascadia

Abstract: The Nankai and Cascadia subduction zones have low-stress forearcs, where the margin-normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms. In the absence of active back-arc spreading, the forearc margin-normal stress is controlled mainly by the total shear force along the subduction fault (or the plate coupling force) and the gravitational force. The plate coupling force creates compression, and the gravitational force, in the presence of margin topography, creates lateral tension. We give a simple expression that relates the forearc stress to topography and average fault strength, with the latter represented by an effective coefficient of friction μ′. We also use a finite element model of two converging plates in frictional contact to model the forearc stresses. The analyses indicate that a low-stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth. For Nankai and Cascadia, where the coupled fault areas are narrow and shallow because of the young and hot subducting plates, μ′ is about 0.05 or less and in any case less than 0.09. The low fault strength is probably not a unique property of low-stress forearcs, since the same low μ′ values combined with wider and deeper coupled areas can be shown to create large margin-normal compression. In addition, there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces. Great earthquakes occur at subduction zones having low-stress forearcs such as Nankai and Cascadia.

The 1997 May 10 Zirkuh (Qa'enat) earthquake (Mw 7.2): faulting along the Sistan suture zone of eastern Iran

Abstract: Summary The destructive Zirkuh-e-Qa’enat earthquake of 1997 May 10 (Mw 7.2, Ms 7.3, mb 6.3) produced 125 km of NNW–SSE right-lateral strike-slip surface faulting on the Abiz fault in the Sistan suture zone of eastern Iran: the longest known surface rupture associated with an Iranian earthquake. Analysis of the body-wave seismograms from the main shock shows that rupture occurred in four main subevents, propagating in a sequence from north to south. Although predominantly strike-slip, the orientation of the faulting in each subevent varies, with appreciable reverse components in the north-central part and at the southern end of the Abiz fault. This change in fault style along the Abiz fault inferred from the seismograms is also seen in the coseismic surface ruptures and the geomorphology. Average coseismic surface displacements were approximately 2 m, implying a static stress drop of only 5 bar (0.5 MPa). The 1997 surface ruptures followed clear traces of late Quaternary slip on the Abiz fault, and for its northern 50 km re-ruptured fault segments that had slipped in previous earthquakes of Ms 6.0–6.6 in 1936 and 1979. The 1997 earthquake ruptured the northern end of the N–S right-lateral strike-slip system of the Sistan suture zone, ending where it abuts a system of E–W left-lateral strike-slip faults which have also slipped in large earthquakes during the last 30 years. The earthquakes on this conjugate system of strike-slip faults form a sequence that may have been triggered by the enhancement of stress on one fault as a result of slip on a neighbouring fault. Together, these faults achieve N–S right-lateral shear of deforming Iran against stable western Afghanistan by N–S slip on the right-lateral faults and clockwise rotation of the E–W left-lateral faults.