Showing papers on "Fault (geology) published in 1985"

Glacier surge mechanism: 1982-1983 surge of variegated glacier, alaska.

Abstract: The hundredfold speedup in glacier motion in a surge of the kind the kind that took place in Variegated Glacier in 1982-1983 is caused by the buildup of high water pressure in the basal passageway system, which is made possible by a fundamental and pervasive change in the geometry and water-transport characteristics of this system. The behavior of the glacier in surge has many remarkable features, which can provide clues to a detailed theory of the surging process. The surge mechanism is akin to a proposed mechanism of overthrust faulting.

Pericollisional Strike Slip Faults and Synorogenic Basins, Canadian Cordillera

Abstract: During large scale convergence of oceanic and arc type lithospheric fragments towards a cratonic promontory along western North America from Middle Jurassic through Paleogene time non subductable crust of the approaching Pacific realm was deflected dextrally northward or sinistrally southward from this reverse indenter in the California Nevada region Paleontologic and paleomag netic data suggest oblique dextral displacements on the order of 1 500 to 2 000 km for the accreted terranes in the western Cordillera of Canada These dextral displacements were first concentrated along closing sutures from Middle Jurassic to Early Cretaceous time later they were also taken up by peri collisional fault zones which propagated into the western parts of the Cordilleran thrust belt and involved the Coast Plutonic Complex mid Cretaceous to Paleocene A subduction in the thrust belt and inferred B subduction west of the Coast Plutonic Complex were thus accompanied by dextral displacements within the Omineca and Coast fault arrays respectively imparting northwest directed stretching fabrics onto ductile metamorphic or igneous rocks and discrete fault strands on high level crustal rocks The convergent strike slip fault motions in the Canadian Cordillera created mainly sedimentary source areas rather than subsiding basins Pericollisional basins that did receive clastic materials from zones of oblique convergence were I marginal basins in the process of closing 2 relict or tectonically overloaded depressions on accreting terranes 3 foreland basins created by thrust prop agation in the miogeoclinal succession and 4 small pull apart or restraining bend depressions near high angle strike slip faults Basins in the accreted terrane complexes west of the Cordilleran thrust belt received most of their detrital material from exposed volcanic plutonic and oceanic sedimentary rocks the predominantly turbiditic basin fill suffered repeated deformation high sustained heat flow and intrusive activity The foreland basin to the east of the thrust belt on the other hand received most of its detrital input from carbonate and quartz rich clastic rocks of the miogeocline and metamorphosed equivalents the predominantly shallow water clastic deposits of the foreland basin experienced considerably less deformation and thennal alteration than the varied sedimentary assemblages of the accreted belt

The Najd Fault System, Saudi Arabia and Egypt: A Late Precambrian rift‐related transform system?

Abstract: The Najd Fault System is a complex set of left-lateral strike-slip faults and ductile shear zones that strike NW-SE across the Precambrian of Arabia and Egypt. This system was developed during the interval 540–620 Ma. It is up to 400 km wide with an exposed length of 1100 km; inferred buried extensions of the Najd give it a total length of 2000 km. It is the best exposed and may be the largest pre-Mesozoic zone of transcurrent faulting on earth. Previous models for the Najd Fault System suggest it formed as a result of a major Late Precambrian continent-continent collision. This model is not preferred here because (1) the lack of evidence for a pre-Late Precambrian continent to the east of the Najd Fault System; (2) the difference between the orientation of the Najd Fault System and that predicted by slip-line theory; (3) the younger age of Najd movements compared with that of collisional sutures in the Arabian Shield; and (4) lack of evidence for wide-spread crustal uplift that would be expected to accompany collision. A new model for the origin of the Najd Fault System accounts for each of these objections: The Najd Fault System formed in response to a broad zone of NW-SE directed crustal extension that accompanied juvenile continental crustal formation in northernmost Afro-Arabia. This model also accounts for the following observations: (1) Strands of the Najd parallel the direction of extension in the North Eastern Desert of Egypt and Sinai; (2) the timing of the principal rifting movements (ca. 575–600 Ma) overlap with those of the Najd (ca. 560–620 Ma); (3) in spite of observation (2), the Najd Fault System is not recognized in northernmost Afro-Arabia; instead the Najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached. The Najd Fault System thus represents a set of continental transforms developed in response to a major episode of Late Precambrian extensional continental crust formation in northernmost Afro-Arabia.

Geometry of propagating continental rifts

Abstract: Reconstruction of past plate configurations through palinspastic restoration of continental margins1–4 requires a detailed knowledge of the geometries of horizontal continental extension. Extension is often restricted to discrete, linear zones termed rifts5,6, and rift models that invoke propagating vertical dykes and cracks7–10 are useful for assessing the relative significance and timing of uplift, subsidence and extension in the tectonic evolution of rift basins. However, many models do not adequately account for the observed asymmetry of rifts. I describe here the general three-dimensional character of young and aborted continental rifts, which can be used to derive a structural model for the propagation of rifts in continental lithosphere. The rifts become asymmetric as a consequence of the role played by low-angle normal faults in the overall rift geometry.

Structural framework of the Gulf of Elat (AQABA), Northern Red Sea

Abstract: The Gulf of Elat (Aqaba) occupies the southern part of the Dead Sea rift. The rift is considered to be a plate boundary of the transform type (partially leaky) which connects seafloor spreading in the Red Sea with the Sagros-Taurus zone of continental collision. The deep water in the Gulf of Elat, up to 1850 m, provides a rare opportunity to examine the process of continental rifting by marine geophysical techniques. The bathymetry alone provides much information about fault patterns in this area. The fragmentation of the once continuous Arabian-African platform is a complicated process. It shapes the structure of the gulf which has developed through continuing tectonism, primarily consisting of faulting. Recent geophysical and geological studies of the Gulf of Elat including bathymetry, bottom photographs, continuous seismic profiles, seismic refraction, gravity, magnetics, heat flow, and coring provide new information about the shallow and deep crustal structure of this important segment of the world rift system. The shallow structure of the gulf is dominated by three elongated en echelon basins, which strike N20°–25°E. Undulations in the floors of the basins produce several distinct deeps. These basins are interpreted as pullaparts. The new data from the gulf suggest that classical models for the formation of these structures should be modified. Only one of the longitudinal faults of each depression is a strike-slip fault, while the other is predominantly a normal fault. The situation with the other two sides of the basin which are supposed to be composed of normal faults may also be more complex than previously thought. At least in one case, one such boundary is composed of a strike-slip fault while the other has no significant fault. Crustal models of the Gulf of Elat based on gravity data indicate that the basins are rather shallow and do not extend into the lower crust. The fill of the basins extends to about 5 km below the seafloor in. the northern and southern basins and less in the central basin. Most significant magnetic anomalies in the Gulf of Elat extend from land into the sea. None of them, however, extends from one coast to another across the gulf. This supports the geological evidence for a shear between the Arabia and Sinai plates. The magnetic field over the southern third of the gulf is rather smooth and markedly different from that of the other parts. The nature of the magnetics, the crustal structure of the western margin, and the heat flow values indicate a thinner crust and different tectonic processes in the southern part of the gulf. Overall, it seems that within the southern Gulf of Elat a transition occurs between crustal spreading that takes place in the Red Sea to a rifting without spreading that takes place along the Dead Sea transform. Spreading activity propagates from south to north. The most active place is the central basin which is being propagated northward into the shallow northern basin.

Geological manifestations of ridge collision: Evidence from the Golfo de Penas-Taitao Basin, southern Chile

Abstract: Recent geological and geophysical studies in the southern Andes adjacent to the intersection of the Chile Rise with the Peru-Chile Trench (ANT-NAZ-SAM triple junction) have revealed a number of features and a Neogene geologic history that are unique along the Pacific margin of South America. This history includes (1) development of a Tertiary-Quaternary marine basin with up to 3 km of sediment infill (Golfo de Penas-Taitao basin, GTB), (2) disruption of the region by a series of faults with both normal and strike slip movements, and (3) localization of silicic, near-trench volcanism and epizonal plutonism and related hydrothermal activity. The northern portion of the GTB began to subside in the Late Miocene (possibly earlier), and has subsequently been deformed, uplifted, and exposed. Gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today (Golfo de Penas). Subsidence and uplift have thus occurred diachronously in the region, although it is unclear when subsidence began in the Golfo de Penas. Tectonic disruption of the region is likely related to the Liquine-Ofqui fault (LOF), a major, NS-trending, crustal shear zone that curves westward and terminates in the Golfo de Penas. The LOF has both down-to-the-west and right lateral offset and separates the main Andean Cordillera on the east from a large crustal block (the Chiloe block) on the west. We hypothesize that the GTB has developed as a pull-apart basin in response to northward movement of the Chiloe block along the LOF. We propose a dynamic model whereby a stress gradient that decreases longitudinally away from the Chile Rise/Peru-Chile Trench intersection is set up because the youngest, most buoyant, oceanic lithosphere is being subducted at the triple junction. The Chile Rise is viewed as a type of indenter which is acting to drive the Chiloe block northward in front of the northward-migrating triple junction. This model explains the unique set of geologic features found in the region, and suggests that ridge-trench interactions may be an important factor in orogenesis at active continental margins.

Planar high-angle faulting in the basin and range: Geodetic analysis of the 1983 Borah Peak, Idaho, earthquake

Abstract: Geodetic elevation changes record the broad-scale deformation associated with the M -- 7.0 October 28, 1983, Borah Peak, Idaho, earthquake on the Lost River fault. The crest of the Lost River Range rose 0.2 m, and adjacent Thousand Springs Valley subsided 1.0 m, in relation to reference points 45 km from the main shock epicenter. The deformation was modeled by dislocations in an elastic half-space. A planar fault with uniform dip slip of 2.05 + 0.10 m, dipping 47 o + 2oSW and extending to a vertical depth of 13.3 + 1.2 km, fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution. The geodetic moment is 2.6 +_ 0.5 x 1019 N m (2.6 _+ 0.5 x 1026 dyn cm), and the estimated static stress drop is 2.9 _+ 0.4 MPa (29 _+ 4 bars). Tests for coseismic slip on listric faults (which flatten with depth) and on detachments (horizontal faults or shear zones) showed fits to the geodetic data that are inferior to those for planar high-angle faults. No detectable coseismic slip occurred on the Mesozoic White Knob thrust fault, although the low-angle thrust sheet intersects the south end of Lost River fault near the 1983 mainshock epicenter. If the high-angle Lost River fault abuts a flat-lying detachment fault or shear zone, such a structure must lie at depths of > 12 km, near the brittle-ductile transition, where stick-slip behavior gives way to creep. The depth and geometry of faulting at Borah Peak is similar to that inferred from seismic and geodetic evidence for the 1954 M = 7.2 Fairview Peak, Nevada, and the 1959 M = 7.3 Hebgen Lake, Montana, events, suggesting that if detachments are active at these localities, they are deep and most likely slip by creep.

Quaternary normal and reverse faulting and the state of stress in the central Andes of south Peru

Abstract: Field studies in the Andes of southern Peru show that in the High Andes and Pacific Lowlands, Quaternary and Recent faults are normal. This extensional tectonics postdates compressional deformations of Pliocene-early Quaternary age. In the sub-Andes the observed deformations are compressional; they affect early Quaternary deposits. Some of the faults separate Quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects. Striations on the fault planes indicate N–S trending extension in the High Andes and Pacific Lowlands. The total amount of crustal stretching is small, probably of the order of 1% during the last 1–2 m.y. In the sub-Andes, folds and faults affecting Neogene and early Quaternary deposits result from N–S shortening. Nevertheless, it is supposed that this N-S shortening is of early quaternary age. The present-day compression probably strikes E-W, judging from focal mechanisms in the sub-Andes of central Peru, southern Bolivia, and northwest Argentina. Data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the Andes of southern Peru. The High Andes and Pacific Lowlands, subjected to N-S trending extension, are bounded by two zones of E-W trending compression: the sub-Andes to the east, and the contact between the convergent Nazca and South America plates to the west. In our model the maximum horizontal compressive stress trajectory σ Hmax is roughly parallel with the E-W convergence between the two plates; σ Hmax corresponds to σ 1, in the sub-Andes and to σ 2 in the High Andes. The latter situation is caused by the elevated mass of the High Andes, where σ zz (the vertical stress) is inferred to be σ 1. Thus the third principal stress axis, being orthogonal to the other two axes, it is oriented N-S, allowing extension to occur in that direction. On the other hand, in the sub-Andes σ zz is σ 3, and horizontal E-W shortening occurs. The state of stress in the Andean continental crust above the 30° dipping slab appears to be different from that in the Andes of Central Peru situated above the flat subducting segment. In this region, compressional deformantion affect a wider part of the Cordillera.

A refined seismic analysis and design of buried pipeline for fault movement

Abstract: Some lifelines, such as gas and oil transmission lines and water and sewer pipelines, have been damaged in recent earthquakes. The damages of these lifelines may cause major, catastrophic disruption of essential services for human needs. Large abrupt differential ground movements that result from an active fault present one of the most severe effects of an earthquake on a buried pipeline system. Although simplified analysis procedures for buried pipelines across strike-slip fault zones that cause tensile failure of the pipeline have been proposed, the results are not accurate enough because of several assumptions involved, such as the omission of flexural rigidity of the pipe, simplification of soil resistant characteristics, etc. Note that the omission of flexural rigidity cannot satisfy equilibrium conditions for pipelines across a ‘reverse’ strike-slip fault that causes compressions in the pipeline. This paper presents a refined analysis procedure for buried pipelines that is applicable to both strike-slip and reverse strikeslip faults after modifying some of the assumptions used previously. Based on the analytical results, this paper also discusses the design criteria for buried pipelines which are subjected to various fault movements. Parametric responses of buried pipeline for various fault movements, angles of crossing, buried depths and pipe diameters are presented.

Spreading structure of the Troodos ophiolite, Cyprus

Abstract: Orientations of dikes within the sheeted dike complex of the Troodos ophiolite reveal primary spreading structure produced at a complex ridge/transform intersection. Three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes. Faults flatten at depth into a detachment within the upper parts of the plutonic complex. Large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids. We suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north-trending (present coordinates), slow-spreading ridge crest. A simple model for ridge migration indicates eastward jumps of 8–13 km; changes in ridge orientation are suggested by changes in trends of dikes and graben axes. Comparison of the pattern of dikes near the Arakapas fault zone with the structure of active ridge/transform intersections suggests that the fault is a right-offset (sinistral) transform, in contrast to earlier models in which a ridge to the west of the exposed Troodos complex was proposed.

Evidence for transform faulting in the Ionian sea: The Cephalonia island earthquake sequence of 1983

Abstract: Accurate locations of aftershocks of the January 17, 1983 (Ms=70) main shock in the Ionian islands have been determined, as well as fault plane solutions for this main shock and its largest aftershock, which are interpreted as a right-lateral, strike-slip motion with a thrust component, on a fault striking in about a NE-SW direction

Neogene and Quaternary faulting in and along the Qinling Shan

Abstract: The structure of the Qinling Shan mountain range (Shaanxi, China) involves major east-south-east strike-slip faults. Field observations made during two expeditions in 1983 and 1984 show that the faults are active and may have undergone several tens of kilometres of post-Eocene left-lateral displacement. The faults cut the morphology sharply and offset left-laterally small channels. Their trace is marked by cataclastic to mylonitic non-coaxial deformation zones at least 50–100 m wide. The Qinling faults allow the eastward extrusion of south China and may be considered the easternmost continuation of the Altyn Tagh fault system. They link extensional tectonics in northeastern China to the Himalayan collision.

The evolution of the Gulf of Corinth (Greece): an aftershock study of the 1981 earthquakes

Abstract: Summary. A preliminary study of the aftershocks of three earthquakes that occurred near to Corinth (Greece) in 1981 is combined with observations of the morphology and faulting to understand the evolution of the Eastern Gulf of Corinth. The well located aftershocks form a zone 60km long and 20km wide. They do not lie on the main fault planes and are mostly located between the north-dipping faulting on which the first two earthquakes occurred and the south-dipping faulting associated with the third event. A cluster of aftershocks also lies in the footwall of the eastern end of the south-dipping fault of the third event. Morphologically, it is observed that in the evolution of the Eastern Gulf of Corinth, antithetic faulting apparently predates the appearance of the main faulting at the surface. This evolution can be explained by motion on a deep seated, shallow angle, aseismic normal fault. A model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes.

The types and role of stepovers in strike slip tectonics

Abstract: Stepovers are fundamental features along strike slip faults of various lengths Two types of stepover between strike slip faults are considered in this paper I along strike stepovers that are due to en echelon arrangement of faults in map view and 2 down dip stepovers that are due to en echelon arrangement of faults in cross section Along strike stepovers produce pull apart basins and push up ranges depending on the sense of stepover Down dip stepovers of both senses may produce strike slip faults in orientations different from the initial major strike slip faults that are arranged en echelon Some possible mechanisms that produce stepovers and control the sense of stepover are I bending of initially straight faults 2 faulting within a weak zone oriented slightly off a local failure plane 3 segmentation of faults to accommodate curved fault traces 4 horizontal slip across pre existing extensional fractures or dip slip faults that have steps 5 a change of physical parameters such as elastic moduli and pore pressure and 6 stress field resulting from fault interaction

Sedimentation and tectonics in the Scottish Dalradian

Abstract: Synopsis Sedimentological research on the Scottish Dalradian has progressed from the recognition of sedimentary structures in the 1930s, via the identification of sedimentary facies from the 1950s onwards, to the integration, in the 1970s, of sedimentological data with that from studies of stratigraphy, tectonics and volcanism. This has now led to an understanding of the pre-orogenic evolution of the Dalradian terrane in terms of progressive lithospheric stretching associated with the break-up of the Proterozoic Supercontinent. The Appin and Argyll Groups were deposited on the NW side of a late Precambrian marine gulf which developed over a complex zone of crustal thinning between the Laurentian and Baltic parts of the Super-continent. As extension accelerated, subsidence rates increased and the Dalradian area of the gulf evolved from a relatively shallow shelf into a series of turbidite basins. Thinning of the lithosphere gave rise, in Argyll Group times, to locally intense igneous activity. Subsequently, complete continental rupture along the gulf axis led to the birth of the Iapetus Ocean. By Southern Highland Group times the Dalradian terrane had become part of the new, thermally-subsiding, Laurentian continental margin. One can envisage the geometry and facies variations of many horizons within the Dalradian in terms of a pattern of numerous fault blocks defined by listric normal faults, dipping SE towards the site of continental rupture, and NW–SE trending transfer faults, which divided the gulf and subsequent margin into a series of compartments. It was movements on these faults that largely controlled Dalradian stratigraphic evolution. For example, pulses of rapid stretching, and consequent fault activity, produced basin-deepening sequences which mark the base of the Easdale and Crinan Subgroups.

Fault Offsets and Lateral Crustal Movement on Europa

Abstract: Structural evidence is presented for tension cracking associated with strike slip faulting and crustal movement in the bright ice covered Galilean satellite Europa. The structure and morphology of wedge shaped bands argues that they formed as a result of the rotation and lateral displacement of crustal units bounded by near vertical faults penetrating through the brittle crustal layer. The significant rotation and lateral motion of crustal blocks near the anti-jove point on Europa, without graben formation, also argues that the lithosphere in the fractured area is mechanically decoupled from the solid silicate interior, by either warm ice at depth or liquid water. Ice at depth and at a large fraction of its melting temperature is expected to behave as a fluid over geologically short time intervals due to its extremely low viscosity relative to the cold, brittle ice near the surface. One proposed convection mechanism is thus considered unlikely as it would be difficult to transmit internal stress through a decoupling layer to the surface.

Interpretación geodinámica de la vertiente centro-occidental surpirenaica (cuencas de Jaca-Tremp)

Abstract: A system of imbricated thrusts (piggy-back sequence), coeval with the sedimentary filling of the Jaca and Tremp basins, was generated during Eocene and Oligocene. The Eocene sedimentation was conditioned by ramp anticlines, thus creating syntectonic slope facies on the smooth or rear slope of each thrust (Burgui marls and limes tones). Separating these ramp anticlines are several troughs containing different turbiditic systems. Within these systems and channelized along the troughs, assembla,ges of carbonate megabeds are intercalated. These assemblages migrated southwards, while each individual megabed within every assemblage shifted eastwards. This arrangement reflects the general thrust sheets movements as they were emplaced earlier at the western pan of the Jaca basin, being progressively displaced towards South and East. The carbonate material that compose the megabeds would source from a northern Eocene platform, located on the active basin margin, as well as from erosion on the ramp anticlines simultaneously created. The imbricated thrust system affecting the cover is genetically related to and overlain a big duplex system developed on the basement. The latter dips NW, being of "hinterland dipping" type and has its root or "branch line" located on an intrapaleozoic fault within the axial zone. The tectonic configuration was generated by an NNW-SSE compression, probably due to the dextral relative movement between the Iberian and European plates.

Source parameters and faulting processes of the 1959 Hebgen Lake, Montana, earthquake sequence

Abstract: The August 1959 (Ms = 7.5) Hebgen Lake, Montana, earthquake is the largest earthquake to have occurred in the intermountain region in historic time. Studies of waveforms at regional and teleseismic distances indicate that the main shock of the sequence was a double event consisting of a shock of mb = 6.3 (Mo = 3 × 1018 N m) followed 5 s later by one of m = 7.0 (Mo = 1 × 1020 N m). Comparisons between fault plane solutions from short-period first motion data, seismic moment tensors determined from the inversion of long-period body wave data, and observed surface faulting indicate that rupture occurred along one or more fault planes with strike orientations slightly discordant with the trace of surface faulting. A close association between the surface scarps and Laramide thrust faults also suggests that the events may represent normal faulting along reactivated older thrust faults. Focal mechanisms from short-period first motion data for aftershocks with Ms > 5.5 in northwestern Yellow-stone National Park showed strike-slip, normal, and reverse mechanisms with a variety of nodal plane orientations that reflect the complex tectonics of the Yellowstone Plateau. Limited focal depth information suggests a decrease in the maximum focal depth of earthquakes from 15 km at Hebgen Lake to 6–10 km in northwestern Yellowstone National Park.

Block rotation and deformation by strike-slip faults: 2. The properties of a type of macroscopic discontinuous deformation

Abstract: Study of the general features of deformation by systems of subparallel strike-slip faults shows that the governing constraints are kinematic: the fault blocks must remain in contact with each other and the deformed area must fit with its surroundings. As a result, fault blocks that move laterally without significant internal deformation also rotate about vertical axes relative to boundaries of the fault domain by an amount that is quantitatively related to fault slip, spacing, and orientation. Areas where left-and right-lateral faults move simultaneously are usually divided into domains that contain faults of one kind only. Deformation of such multidomain areas is nonhomogeneous, but the differences between domains can compensate for each other, so that the regional deformation is quite simple. Strike-slip faulting combined with block rotation is an efficient mechanism of deformation because modest offsets and rotations change the linear dimensions of faulted areas by several tens of percents, which may often greatly exceed the results of other types of coeval deformation. Paleomagnetic data can provide an independent measure of block rotations and should be used to supplement structural data. Block rotation changes the initial angles between right- and left-lateral faults and also their orientation relative to the stress field; the faults probably tend to rotate away from the axis of maximum compression. Rotated faults continue to move because they are weak surfaces. Since the deformation depends on geometric factors such as fault orientation and domain geometry, it is not simply related to the stress field. These properties, including rotation of material units, characterize discontinuous deformation in general.

The Seismicity, Geomorphology and Structural Evolution of the Corinth Area of Greece

Abstract: In the eastern Gulf of Corinth, geological structure is closely reflected in the topography. In the 1981 Corinth earthquakes ground deformation accentuated existing geomorphological patterns. We explore the relationship between seismicity and landforms in space and time. The area examined extends beyond the epicentral region of the 1981 events and its evolution is traced over the last 40000 years. To understand the relation between deep and superficial structures we discuss processes that occurred in the 1981 earthquakes, and go on to propose that surface faulting appears only above linear faults at depth that move in earthquakes with magnitudes ( M s ) greater than six. Where faults at depth intersect or bend, the faulting becomes distributed and prim ary faulting does not reach the surface. Here the surface manifestation of faulting at depth is surface folding and secondary faulting. Such regions are identified in maps of the eastern Gulf of Corinth. Evidence for the evolution of the fault systems is provided by eyewitness accounts of shoreline changes following the recent earthquakes, historical and archaeological data pertaining to old sea levels, and 14 C dates for molluscs collected from fossil shorelines. The results of these studies broadly confirm an earlier view of J. A. Jackson et al . ( Earth planet. Sci. Lett . 57, 377—397 (1982)) that motion has shifted from faulting south of the city of Corinth to faulting in the Perachora peninsula. But the transfer is not complete: areas lying between the two fault systems have moved up and down in historical and archaeological times indicating that both systems are still active. The evidence for activity of the northern system points to a repeat time of 300 years for earthquakes com parable with those of 1981. The work discussed in this paper suggests that some of the features previously described as m arine terraces are fault-controlled continental surfaces. Furtherm ore, beach deposits at a num ber of different levels long regarded as successive shorelines are in fact contemporaneous. The critical ages were obtained by radiocarbon dating of shell carbonate, a technique which yields dependable results if applied to samples which have been carefully selected and pretreated. Palaeontological dating of the terraces, in contrast, is found to be misleading because the faunas are environm entally controlled and consequently diachronous.

Tectonics of slow spreading mid-ocean ridges and consequences of a variable depth to the brittle/ductile transition

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

Character and Regional Significance of Great Falls Tectonic Zone, East-Central Idaho and West-Central Montana

Abstract: The Great Falls tectonic zone, here named, is a belt of diverse northeast-trending geologic features that can be traced from the Idaho batholith in the Cordilleran miogeocline, across thrust-belt structures and basement rocks of west-central and southwestern Montana, through cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. Geologic mapping in east-central Idaho and west-central Montana has outlined a continuous zone of high-angle faults and shear zones. These structures (1) extend more than 150 km (93 mi) northeastward from near Salmon, Idaho, toward Anaconda, Montana, (2) had recurrent movement from middle Proterozoic to Holocene time, (3) controlled the intrusion and orientation of Late Cretaceous to early Tertiary dike swarms, and (4) ontrolled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent fault movement in this zone and strong structural control over igneous intrusion suggest a fundamental tectonic feature that has influenced the tectonic development of the Idaho-Montana area from at least middle Proterozoic time to the present.

Geometry and mechanism of faulting of the 1980 El Asnam, Algeria, earthquake from inversion of teleseismic body waves and comparison with field observations

Abstract: The El Asnam earthquake of October 10, 1980 (Ms = 7.3), provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models. The event produced extensive surface faulting. In addition to the main tectonic deformation, which is clearly a thrust, widespread secondary normal faulting was observed at the surface. In the southwest region the surface break of a thrust fault could be traced for 24 km. In the northeast part of the fault zone, normal faults with throws of several meters were observed. Although no clear thrust type surface breaks were observed in this region, geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation. In this paper the rupture process of the El Asnam earthquake is investigated by inversion of teleseismic P and SH waves. The effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood, north-eastern segment of the fault zone. During the earthquake, rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes. The seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters: seismic moment 3.9×1019 N m, depth 6 km, strike 220°, dip 46°, and rake 72°. The duration of rupture on this fault segment was approximately 10 s which, together with the observed fault length of 24 km, indicates a rupture velocity of 2.4 km/s. Unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region. The normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region. Guided by the evidence from aftershocks and geodetic measurements, a solution with a thrust-type mechanism was also sought. Using a priori constraints based on the field observations (the observed strike and slip angle), we obtain the following thrust source mechanism for the northeast segment: seismic moment 3.6×1019 N m, depth 6 km, strike 230°, dip 20°, and rake 91°. This solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest, suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth. Inversions are nonunique with respect to the seismic moment associated with normal faulting; the ambiguity being caused by the limited bandwidth of the observed body waves. The moment estimated from the surface waves at periods about 300 s, however, favors thrust faulting to be dominant in the northeast region.

Implications of the northwestwardly younger age of the volcanic rocks of west-central California

Abstract: Erosional remnants of volcanic fields in west-central California form a linear northwest-trending belt growing younger in age to the northwest. Major fields within the belt are represented by the Neenach Volcanics, Pinnacles Volcanic Formation, Quien Sabe Volcanics, volcanic rocks in the Berkeley Hills, Tolay Volcanics, Sonoma Volcanics, and Clear Lake Volcanics. Dispersion in the age-distance relation is reduced by restoration of inferred offsets on transecting right-lateral fault systems. The offsets include 115 km on the San Gregorio–Hosgri fault, 314 km on the San Andreas fault, 43 km on the Hayward-Rodgers Creek fault, and 28 km on the Carneros-Franklin-Sunol-Calaveras fault. On the basis of the age and restored position of the volcanic rocks, we judge that the locus of initial active volcanism migrated northwestward ∼3.75 cm/yr from 25 to 12 Ma, and ∼1.35 cm/yr from 12 Ma to the present. The volcanic rocks apparently formed south of the northwardly retreating edge of the subducted part of the Juan de Fuca plate, corroborating one corollary of a published model of an expanding hole in the subducted Farallon-Juan de Fuca-Cocos plate. The present position of the locus of melting at Clear Lake, California, requires substantial overthrusting of the Juan de Fuca plate by the Pacific plate, as was postulated on the basis of foreshortening of magnetic anomalies in the Gorda basin. The change in rate of northwestward migration ∼12 Ma reflects a change in spreading direction of the Juan de Fuca plate vis-a-vis the Pacific plate, previously recognized from changes in orientation of oceanic magnetic anomalies. From the migration rates, it can be inferred that the relative movement between the Pacific plate and the westernmost fringe of the North American plate averaged ∼3.5 cm/yr from 27 m.y. ago to the present.

Geometry of Cenozoic extensional faulting: Dixie Valley, Nevada

Abstract: Precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of Basin and Range Cenozoic extensional faulting at depth. Previous studies have attempted to identify simple horsts and grabens, tilted crustal blocks with planar faulting, or tilted crustal blocks with listric faulting in the shallow crust. Normal faults defining these crustal blocks may root (1) individually in the ductile lower crust, (2) in regional or local low-angle detachment faults, or (3) in igneous intrusions or decoupling surfaces produced by the intrusions. The present study, in Dixie Valley, west-central Nevada, makes use of a seismic reflection survey, gravity models, seismograms from earthquakes occurring on December 16, 1954, and geometrical block models. These data show a structurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step-faulted floor to the southeast. The northwest bounding fault is moderately dipping (50°) and planar to a depth of 3 km. The southeast boundary is step-faulted, and altogether the faults indicate an extension of 20% across the valley at the rate of 0.38 mm/y for the last 8 my. Synthetic earthquake seismograms confirm a focal depth of 15 km and fault dip of 62° for the Fairview Peak earthquake and suggest that the focal depth of the Dixie Valley earthquake was also 15 km instead of the previously reported 40 km. Local microearthquakes cluster around 10–15 km. The geometrical block models indicate that crustal horst-graben faulting and planar, high-angle normal faults rooted in a low-angle detachment surface do not readily account for development of the subsidiary (step) faults found in Dixie Valley. Extension of the crust by intrusion may develop high-angle faults and, with further intrusion, may develop the subsidiary faults and produce a complex, sagged, asymmetric graben like Dixie Valley.

Quaternary tectonic setting of the 1983 Borah Peak earthquake, central Idaho

Abstract: The 1983 Borah Peak earthquake was accompanied by extensive surface faulting along a part of the Lost River fault that has abundant evidence of latest Quaternary (last 15,000 yr) offset. This fault and two similar range-front normal faults along the Lemhi Range and Beaverhead Mountains lie in an area of basinand-range structure in central Idaho that is part of a roughly V-shaped belt of latest Quaternary surface faulting that extends from the Wasatch fault, through the Yellowstone area, to the Lost River fault. The position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late Cenozoic silicic volcanism along the YellowstoneSnake River Plain axis. The central segments of the Lost River, Lemhi, and Beaverhead faults have been active more recently, and probably more active throughout Quaternary time, than the southern and northern segments. The main 1983 surface faulting occurred in an area of high structural relief along a central segment of the Lost River fault that has ruptured in latest Quaternary time, which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting. Other perspective of fault behavior suggest additional possible sites, and all segments of the range-front faults are regarded as capable of surface faulting.

Earthquake focal mechanisms in the Eastern Transverse Ranges and San Emigdio Mountains, Southern California and evidence for a regional decollement

Abstract: Earthquake focal mechanisms obtained from P-wave first motions are presented for the Eastern Transverse Ranges and the San Emigdio Mountains in Southern California. The former region shows a predominance of strike-slip faulting whereas Quaternary faults in the region show thrust motion. We suggest that the observed strike-slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting. Fault deformation in the San Emigdio Mountains inferred from focal mechanisms is in accord with displacements across Quaternary faults in the area. This study and a search of the literature has yielded 19 mechanisms with shallow-dipping nodal planes. Previous workers have interpreted such mechanisms as evidence for a regional decollement. If such a regional decollement exists, these data give some indication of its regional extent. Slip directions inferred from the focal mechanisms with shallow-dipping nodal planes show some regional consistency, but this pattern cannot be entirely explained with current tectonic models. A comparison of the stress drop of an event having a shallow-dipping nodal plane with an event with steeper planes gave inconclusive results.

Sawtooth segmentation and deformation processes on the southern San Andreas Fault, California

Abstract: Five contiguous 12-13 km fault segments form a sawtooth geometry on the southernmost San Andreas fault. The kinematic and morphologic properties of each segment depend on fault strike, despite differences of strike between segments of as little as 3 degrees. Oblique slip (transpression) of fault segments within the Indio Hills, Mecca Hills and Durmid Hill results from an inferred 8:1 ratio of dextral slip to convergence across the fault zone. Triggered slip and creep are confined almost entirely to transpressive segments of the fault. Durmid Hill has been formed in the last 28 + or - 6 ka by uplift at an average rate of 3 + or - 1 mm/a.