Discrete Fault Research Articles

Experiments on faulting in a two-layer cover sequence overlying a reactivated basement fault with oblique-slip

Two series of experiments were undertaken to investigate the development of faults in a cover sequence overlying a reactivated basement fault with oblique-slip. The experiments represent physical models properly scaled to account for gravitational forces, brittle behaviour of the upper cover and ductile behaviour of evaporites in the lower cover. Models were made with a Coulomb layer (sand) overlying a Newtonian ductile layer (silicone putty). In each series, the thickness of the silicone layer was varied from each experiment to the next. In all normal-wrench experiments, a domain of faults appeared in the brittle layer. Vertical offsets were mostly of normal sense, but some reverse faults occurred in experiments with thick silicone layers. The thicker the silicone, the wider the fault domain. The deformation zone was, in all cases, limited to the faulted domain. In the reverse-wrench experiments, discrete reverse Riedel faults appeared without a silicone layer. As the silicone layer became thicker, the importance of the reverse faults diminished. With the thickest layer of silicone (3 cm), no faults appeared in the overburden, but a monoclinal flexure accommodated the dip-slip component. In the presence of silicone, the deformation zone was not limited to a faulted domain, and the strike-slip component was accommodated by a bulk shearing of the overburden in the uplifted compartment. These experiments demonstrate that, in basins deformed in oblique-slip mode, any deformation that is localized along discrete faults at the basement level becomes distributed over a much wider zone of overburden when this overlies an interval of evaporites.

Centrifuge modelling of fold—thrust structures in a tripartite stratigraphic succession

Analog models measuring 127 × 76 mm in plan were deformed at 2500–4000 g in a centrifuge. Scaled stratigraphic sequences were constructed of anisotropic multilayers with individual layers of Plasticine and silicone putty as thin as 40 μm. The plasticine—silicone putty multilayers are analogs for interbedded competent carbonates and clastics, and incompetent pelites, given the model ratios of acceleration, 2500 g; length, 5 × 10 −6; specific gravity, 0.6; time 10 −10. Modelling of fold—thrust tectonics emphasizes the influence of stratigraphic succession on structural evolution. The models are constructed with a tripartite stratigraphic succession comprising basal and upper, well-laminated and incompetent units, and a middle, somewhat more isotropic and competent unit. The models deform by three mechanisms: layer-parallel shortening, folding and thrust faulting. They reproduce a number of fold—thrust relationships that have been observed in nature. Folds are typically periclinal, in en échelon arrays. Folds and thrusts are arcuate in plan, reflecting differential shortening. Fold attitudes grade from upright at high levels to overturned at deeper levels within a structural panel, reflecting drag against the basal décollement; fold axial surfaces and thrust faults are listric. While competent units may be offset by localized displacement on thrust faults, the discrete faults may die out both upwards and downwards into regions of ductile strain in less-competent units. Thrust faults appear to follow staircase trajectories through the strata, transecting incompetent units at shallow angles to bedding and competent units at steeper angles. However, the apparent staircase pattern results from propagation of a fault along a relatively straight trajectory through previously-folded strata. Foreland-verging thrusts are more common than back thrusts; the latter have steeper dips. The models suggest a mechanism of thrust-ramp nucleation following detachment folding: long-wavelength buckling of a competent unit can initiate localized strain (folding and layer-parallel shear) in an underlying incompetent unit, beneath the anticlines of the competent unit; thrust faults propagate up-section from these high-strain zones through the foreland-dipping limbs of buckle-folds in the competent unit. This mechanism may explain the commonly-observed spatial periodicity of thrust ramps. The model results bear similarities to natural fold—thrust belts in which the stratigraphic succession consists of three mechanical units, for example, the Asiak Foreland and Bear Creek Hills fold—thrust belts of the Slave Province, Northwest Territories, Canada.

Microstructural evolution of a deformation zone in the upper ocean crust: Evidence from DSDP hole 504B

The deepest reference section through in-situ ocean crust lies at DSDP site 504B. Structural relations combined with existing detailed lithostratigraphy and alteration studies have been used to constrain the nature of deformation histories within a zone between 840 and 958.5 m. This zone lies close to the lithological transition zone between pillows and dykes. It is one of the most deformed intervals in the 540B core. Several discrete fault planes have been identified within this interval. Microstructures from these faults provide evidence for polyphase deformation persisting through one or more stages of alteration. A wide range of structures were generated during slip including cataclastic zones, isoclinal microfolds, crystallographic preferred orientation in quartz and preferred orientation of phyllosilicates. Repeated sequences of fracturing overprinted by non-coaxial (brittle and plastic) deformation are observed. The deformation zone is interpreted as a long-lived fault zone which may have nucleated as a result of the mechanical and transient thermal boundary at the top of the transition zone. Stockwork-like alteration may have developed in a dilational site generated during slip on a fault. Alteration histories imply that the fault was active during motion out of the main rift zone and local rotations of the minimum principal stress directions may have been related to flexure during this process. Reactivation of faults with a change in shear sense as material moved out of the rift may also account for the structures. Alternatively, the local stress field could have been modified during the evolution of a process zone associated with fault propagation.

Contrasting structural styles in siliciclastic and carbonate rocks of an offscraped sequence: The Peralta accretionary prism, Hispaniola

The Peralta accretionary prism of Hispaniola, which developed in the late Eocene epoch during a period of convergence or oblique convergence, constitutes an off-scraped sequence containing both siliciclastic and carbonate rocks. These different rock types responded to accretionary deformation in markedly different ways, with most strain being accommodated in the mudstone-rich siliciclastic unit. Deformation in the siliciclastic unit is concentrated in thick zones (1 to 450 m) of intense stratal disruption, which we interpret as major accretionary thrust faults. The disrupted zones are characterized by boudinaged sandstone beds, common tight to isoclinal folds, numerous thrust faults, and a pervasive scaly clay fabric in mudstone interbeds. Microstructural studies of the sand-stones indicate that deformation began when these sediments were unlithified, and it continued through complete lithification. The sandstones are characterized by a microstructural progression from possible particulate flow, to distributed cataclastic grain breakage, to grain breakage confined to zones (web structure), to discrete planes of slip. Pressure solution occurred concurrently with development of the web structure and the discrete faults. In contrast, the carbonate units are generally stratally coherent. Microfabrics in the carbonate units indicate that they were relatively well lithified during deformation, with strain being accommodated primarily by brittle faulting and pressure solution. In addition to enhanced lithification of the limestones, we suggest that the presence or absence of highly ductile mudstone interbeds represented the dominant control on the location of the stratally disrupted thrust fault zones within the accretionary prism. The presence of these mechanically weak layers in the siliciclastic unit facilitated the development of the pervasive scaly clay fabric in the stratally disrupted zones, and slip along the numerous scaly fault planes probably accommodated a large component of the total strain during accretion. The absence of such mudstone layers in the limestone outcrops resulted in more coherent behavior of these rocks during accretion.

Model for late Neogene deformation in Panama

Research Article| June 01, 1990 Model for late Neogene deformation in Panama Paul Mann; Paul Mann 1Institute for Geophysics, University of Texas at Austin, Austin, Texas 78759 Search for other works by this author on: GSW Google Scholar Jeff Corrigan Jeff Corrigan 2Department of Geological Sciences, University of Texas, Austin, Texas 78713 Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (6): 558–562. https://doi.org/10.1130/0091-7613(1990)018<0558:MFLNDI>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Paul Mann, Jeff Corrigan; Model for late Neogene deformation in Panama. Geology 1990;; 18 (6): 558–562. doi: https://doi.org/10.1130/0091-7613(1990)018<0558:MFLNDI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Previous workers, mostly using earthquake data, have postulated both discrete faults and complex, diffuse zones for the Caribbean-Nazca plate boundary in Panama. We have better resolved the location and character of late Neogene plate-boundary structures by using satellite imagery and a compilation of existing geologic map data. On the basis of these results, we propose that Panama is moving northwestward away from a zone of active convergence along the continental margin of South America toward more easily subducted areas of the Colombian Basin. Movement is accommodated across a diffuse zone by a complex interaction of oroclinal bending, left-lateral strike-slip faulting, and subduction. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Transfer Zones in the East African Rift System and Their Relevance to Hydrocarbon Exploration in Rifts (1)

Transfer zones in extensional regions display a wide range of geometries from discrete fault zones to zones of broad warping. The classification of extensional fault displacement transfer zones developed in this paper includes three main criteria: (1) primary subdivision of transfer zones by relative attitude and direction of throw of the major faults (synthetic and conjugate); (2) secondary subdivision of conjugate transfer zones into transfer zones where the normal faults dip toward each other (convergent) and where the transfer zones occur between faults that dip away from each other (divergent); (3) the tertiary subdivision of conjugate relationships of transfer zones and secondary subdivision of synthetic transfer zones are by the fault terminations in plan view; fau t tips approach, or they overlap, completely overlap (termed collateral), or are in line (termed collinear). A high abundance of overlapping transfer zones occur in the East African rift where extension is low and large-scale cross faults (proto-transform faults?) are uncommon. Commonly, transfer zones in this region are relatively high areas with complex internal fault geometries flanked by deeps. Transfer zones in rifts contain complex but somewhat predictable structural geometries that make them optimum locations for structural hydrocarbon traps. The classification presented herein can help define and delineate those zones and to some degree predict their internal structural geometry.

Earthquakes associated with diffuse zones of deformation in the oceanic lithosphere: some examples

We review the results of source parameters of earthquakes associated with the eastern end of the Azores-Gibraltar plate boundary, the Davie ridge near Madagascar, and the Horizon bank in the north Fiji basin to characterize the nature of present-day tectonics in diffuse zones of deformation in the oceanic lithosphere. In comparison with typical plate boundaries, diffuse zones of deformation are in general characterized by complex morphological expressions and scattered seismicity of up to several hundred kilometers width. Although the average rate of displacement across these regions is not resolvable by current models of global plate reconstruction, the deformation is not truly intraplate in nature because strain is concentrated and often distinct tectonic boundaries can be identified. Earthquakes with seismic moments as large as 8 × 10 20 Nm have occurred in bands of scattered seismicity that are associated with diffuse zones of deformation in the oceanic lithosphere. In two cases, the zones of diffuse deformation continue into the continental lithosphere east of Gibraltar and north of the Davie ridge. However, the largest earthquakes occur in the oceanic part of these two zones. In contrast to previous reports, the depth of earthquakes is quite shallow (< 15 km) near the Horizon bank (also known as the Hazel-Holme fracture zone) where the predominant mode of deformation is strike-slip motion. This zone is an important tectonic feature in the southwestern Pacific because the rate of seismic strain release along it in recent years is several orders of magnitude higher than that of a proposed nascent plate boundary north of the north Fiji basin. The maximum focal depth reaches about 50 km in the zone of ocean-ocean collision on the eastern end of the Azores-Gibraltar plate boundary where most of the mechanical lithosphere must have been broken during large earthquakes. Focal depths of up to 35 km were observed beneath ocean floors of Mesozoic age at the zone of extension along the Davie ridge. The current deformation is apparently unrelated to the past motion along the Davie ridge and this region seems to be part of a wide zone of diffuse extension which marks the southern termination of the Nubian-Somalian plate boundary. In regions where a mixture of focal mechanisms occur, the consistent parameter is usually the orientation of the P-or the T- axes , instead of that of the nodal planes or slip vectors. Thus, as in the continental lithosphere, zones of diffuse deformation in the oceanic lithosphere seem to be better described by a regional compression or extension field rather than by displacement on discrete faults.

Multistage melange formation within an accretionary complex, Diego Ramirez Islands, southern Chile

Accretionary complex rocks on the Diego Ramirez Islands of southern Chile formed during subduction along the convergent Pacific margin of Gondwanaland. Tectonic melange was produced during a sequence of deformation phases that included early (prelithification?) strata) disruption; isoclinal folding and ductile shearing associated with dynamothermal metamorphism; and postmetamorphic cataclastic flow and brittle faulting. Stratal disruption and melange formation are interpreted to have begun during initial underthrusting beneath the accretionary prism, and to have continued by ductile processes at deeper levels during progressive underthrusting-underplating. Pervasive cataclastic shearing to produce a later generation of melange, followed by slip along two generations of discrete fault zones, occurred as underplated material was tectonically transported to progressively higher levels within the accretionary prism. The multiple stages of melange formation reflect strata) disruption in distinct deformation regimes associated with different structural levels of the accretionary prism.

Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley Caldera, California

Detailed surface mapping, subsurface drill hole data, and geophysical modeling are the basis of a structural and hydrothermal model for the western part of Long Valley caldera. Six fault zones are recognized in the western caldera with dominant orientations of north, northwest, and northeast, sub‐parallel to regional fault trends in the surrounding Sierran basement. The internal structural geometry of the cores of 12 exogenous domes inside and outside the caldera suggests that the domes erupted at the intersections of these principal fault trends rather than along the axis of a single dike. Gravity modeling and subsurface data from deep geothermal wells indicate that the floor of the caldera is segmented into a number of discrete fault blocks with varying offsets. One of the northeast trending fault zones, designated Discovery fault in this paper, appears to be part of the original Sierran embayment that existed before caldera collapse. Recent hydrothermal alteration occurs along Discovery fault strands and composite vertical offset of intracaldera volcanic units across the entire fault zone may be as much as 400 m. Field relationships, geophysical interpretations, and interpretaions of drill hole data suggest that this fault is a fundamental flaw in the western caldera. The preexisting tectonic framework of the basement rocks controlled the configuration of the western caldera floor and, through it, the location of postcollapse eruptive centers. These deep basement structures may also provide the high fracture density which controls circulation in the present geothermal system of Long Valley.

The brittle-plastic transition and the depth of seismic faulting

A simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow. The two are separated by an abrupt brittle-plastic transition, which is assumed to be indicated by the lower limit of seismicity. Experimental studies, however, as well as the deformation structures of mylonites, indicate that a broad transitional field of semi-brittle behavior lies between these extremes. This is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws. For quartzofeldspathic rocks the semi-brittle field lies between T1, the onset of quartz plasticity at about 300 °C and T2, feldspar plasticity at about 450 °C. A model is presented in which the transition T1 does not correspond to a transition to bulk flow but to a change from unstable, velocity-weakening friction to stable, velocity-strengthening friction. T1 thus marks the depth limit of earthquake nucleation, but large earthquakes can propagate to a greater depth, T3, (T3<T2) which corresponds to the lower limit of dynamic frictional behavior in the semi-brittle field and approximately to the peak in strength. The zone between T1 and T3 is one of alternating behavior, with flow occurring m the interseismic period and with co-seismic dynamic slip occurring during large earthquakes. This zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting. The transition T1 is also marked by a change in the generation mechanism of fault rocks, from abrasive wear above which produces cataclastites, to adhesive wear below, which is proposed as an important generation mechanism of mylonites in the upper part of the semi-brittle field.

Stratigraphic Play Types in Syn-Rift Sequences: Examples from the Gulf of Corinth Asymmetric Grabens, Greece: ABSTRACT

Since the late Miocene, the Gulf of Corinth system is central Greece has extended approximately north-south within a back-arc environment. Major east-southeast-west-northwest-trending normal faults dissect prerift sequences, creating asymmetric grabens on kilometers to tens of kilometers scales. Episodic activity on discrete fault segments controls rates of footwall uplift and basinal subsidence. These together with regional uplift across north Peloponnesus generate a complex sedimentary response. Syn-rift sequences range from continental to deep marine facies. Facies architecture reflects competing lateral and axial supply contributions, each creating distinct reservoir geometry and stratigraphic trap styles.

SEISMICITY OF THE GALAPAGOS 95.5°W PROPAGATING RIFT

An array of ocean bottom seismometers was deployed for 21 days about the 95.5°W Galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation. Seismicity was fairly constant (10–17 events per day) with no indication of swarm activity. Hypocenters were determined for 304 earthquakes, the largest of which is estimated to be mb = 2.3, with a seismic moment of 2.9 × 1020 dyn cm. The cumulative seismic moment released by all local earthquakes is estimated to be 6.6 × 1020 dyn cm. A b value of 0.77 ± 0.08 was estimated for the region, implying a stress regime dominated by a broad tectonic stress field. The distribution of earthquakes is consistent with the propagating rift hypothesis. Most earthquakes occurred south of the propagating rift within the De Steiguer Deep. Seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time. The seismicity observed along the propagating rift is diffuse. Earthquakes were observed at the propagating rift tip, but not in front of it. Almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip; that is, earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift. Six composite focal mechanisms are consistent with the propagating rift hypothesis. First motions from earthquakes located on the propagating rift indicate extension, whereas motions associated with the failing rift area indicate compression. A compressional regime is also indicated west of the transform zone. Twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right‐lateral strike‐slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere.

Geometry of Basement-Cored Structures Using Reasonable Rheological Models for Basement Deformation in Wyoming Province: ABSTRACT

Direct observations of the material behavior of both basement and sedimentary-cover rocks reveal that we may be able to relax the assumption that basement rocks deform rigidly and that cover rocks behave passively. Basement rocks, which lack mechanical anisotropy (e.g., no foliation, layering), may deform by faulting along narrow, discrete faults or along zones of faulting, which may be several kilometers wide. Both high-angle and low-angle faults are compatible with a general model of regional horizontal compression. Low-angle faults, in particular, may require internal deformation of the basement rocks. Based on limited data from surface mapping and/or subsurface data, it is possible to construct a number of plausible geometric models for the configuration of the basement. The resultant cross sections must balance either locally or regionally using both volume and line length in the sedimentary-cover rocks and using volume of the basement rocks and line length for the upper basement surface. Any of these geometric models has the possibility of depicting the true basement configuration and can be tested with selected additional data. Sequential restoration of the cross sections provides useful information about the relative timing for development of structures in both the cover rocks and the basement. Knowledge ofmore » timing (e.g., early out-of-syncline crowd vs. late mountain-flank thrusts) can be used in forward modeling and aids in exploration by accepting or rejecting certain specific cross sections without the need for additional subsurface data.« less

A seismological study of normal faulting in the Demirci, Ala ehir and Gediz earthquakes of 1969-70 in western Turkey: implications for the nature and geometry of deformation in the continental crust

In this study, seismological techniques are combined with surface observations to investigate the faulting associated with three large earthquakes in western Turkey. All involved normal faulting that nucleated at 6–10 km depth with dips in the range 30–50°. The two largest earthquakes, at Alaşehir (1969.3.28) and Gediz (1970.3.28), were clearly multiple events and their seismograms indicate that at least two discrete subevents were involved in producing the observed surface faulting. In addition, their seismograms contain later, longer-period signals that are likely to represent source, not structure or propagation, complexities. These later signals can be modelled by subevents with long time functions on almost flat detachment-type faults. As a result of these observations, we propose a model for the deformation of the lower crust, in which brittle failure of the top part occurs when high strain rates are imposed during an earthquake that ruptures right through the upper, brittle crust. Under these special circumstances, seismic motion occurs on discrete faults in the lower crust, which otherwise normally deforms by distributed creep. In the case of the normal faults studied here, motion in the uppermost lower crust takes place on shallow dipping faults that are downward continuations of the steeper faults that break to the surface. The faults thus have an overall listric geometry, flattening into a weak zone below the brittle layer at a depth that is probably dependent on the termperature gradient. This interpretation explains why detachment-type mechanisms are not seen in first motion fault plane solutions of normal faulting earthquakes, and suggests an origin for the Metamorphic Core Complexes seen in the Basin and Range Province, which probably represent flat lower crustal faults, analogous to those postulated at Alaşehir and Gediz, that have been uplifted to the surface.

EXTENSIONAL BASIN — FORMING STRUCTURES IN BASS STRAIT AND THEIR IMPORTANCE FOR HYDROCARBON EXPLORATION

The Bass, Gippsland and Otway Basins of southeastern Australia were initiated by north-northeast to south- southwest lithospheric extension, largely during the Early Cretaceous. The extensional stage was followed by a Late Cretaceous to Pliocene thermal subsidence stage and a late stage of compressional tectonic overprinting.The extensional stage was dominated by two orthogonal fault sets - shallow to moderately dipping, rotational, normal faults and steeply dipping, transfer (transform) faults. Thermal subsidence involved vertical rather than horizontal movements, and consequently generated a discrete fault geometry, comprising steep, down-to-basin, normal faults with small displacements. The major extensional structures exerted a range of controls on both sedimentation and structuring during the subsidence stage. Likewise, the location and style of late Tertiary compressional structures overprinted on the Gippsland and, to a lesser extent, Bass and Otway Basins are controlled by reactivation of major early normal and transfer faults. In particular, the Kingfish, Mackerel, Halibut, Flounder and Tuna fields in the Gippsland Basin overlie a single Early Cretaceous transfer fault zone that was a basinwide structural boundary during extension. These fields occupy en echelon compressional structures generated by left-lateral wrench reactivation of the transfer zone during late Tertiary northwest-southeast compression. The major extensional structures have had an important influence on all stages of the evolution of these basins. It is contended that a thorough understanding of their extensional framework is an important factor in hydrocarbon exploration of these and other basins.

Foreland deformation in the Appalachian Plateau, central New York: the role of small-scale detachment structures in regional overthrusting

Northwest-directed Acadian or Alleghenian overthrusting occurred through décollement along the black, carbonaceous Union Springs Shale in central New York. Detachment was accommodated at: (1) discrete faults marked by striated and polished shale bedding planes and fibrous calcite crystal growths between stepped surfaces: (2) duplex structures defined by upper and lower bounding faults 3–10 m apart with internal faults that developed both early and late in the movement history of the bounding faults: and (3) 5–20-cm wide shear zones also possessing an internal duplex or imbricate structure. Ramps through carbonate units adjacent to the black shale have displacements of less than one meter. Total displacement above the décollement horizon is not well-constrained, but is inferred to be on the order of a few tens or hundreds of meters. Fabric of the undeformed shale has been locally modified by a widely spaced cleavage (average 5-mm intervals) with no apparent shear offsets. This microstructure is most evident near ramps through overlying carbonate and in the vicinity of shear zone terminations. An intensely anastomosing and complex shear fabric is present within the shear zones. These structures appear to represent the initial stages of overthrust propagation in a region dominated mechanically by the presence of alternating thick, stiff sedimentary layers and weak shale horizons. Further southwest, more interior to the orogen, displacement was accommodated through décollement along deeper salt horizons. Detachment in the stratigraphically higher black shale was generally restricted to an area where the salt is not present, and may represent a regional stepping-up of structure in the Appalachian Foreland.

P−T conditions during emplacement of the Bay of Islands ophiolite complex

A high-temperature contact is described between the basal pargasite-bearing spinel-lherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P−T conditions of 7–11 kbar, 750–850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle. Age data shows that the Bay of Islands Complex was 30–40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged (> 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (≥ 1 kbar, 10 cm/yr) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole. The observed pressures at the ophiolite-aureole contact are 3–7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite.

Algebraic reconstruction of a two‐dimensional velocity inhomogeneity in the High Hazles seam of Thoresby colliery

A high proportion of faces started in mechanized coal mines run into underground faults. The faults take many forms, from the splitting of a seam through a hidden stress pattern caused by subsidence or folding, to a washout or a vertical throw. All faults reduce face output. A throw of only 1.5 m can lead to a face being abandoned. Faults of this order cannot be mapped reliably from the surface. They may be mapped in an underground seismic survey. Roadways of a mine may give access to fast refracting horizons above and below a coal seam. Waveguiding in the plane of the seam, if it occurs, simplifies migration of wave trains reflected from discrete faults. The reduction problem involved in a fault mapping is only two‐dimensional. Given guiding, whether leaky or not, it is possible to map distributed faults of low reflectivity by shooting in transmission across a panel of coal. Algebraic reconstruction techniques are used here to reduce first break times‐of‐flight through a 425 × 950 m rectangular block of coal into the profile of a velocity inhomogeneity. Input data are derived by static correction from hand‐picked arrival times. The reduction itself is effected using an algorithm which accommodates underground site access restrictions. In back projecting first break velocities, a truncated cosine is used to weight the relative contributions of rays passing at different distances from any given mapping point. The reconstructed velocity field suggests that the coal panel is bisected by a ridge of higher velocity. The suspicion of a ridge is reinforced by results of an aberration test based on a standard Huygens‐Kirchhoff migration. The ridge is found to follow the general line of a system of pillars left in place during the mining of a lower horizon. It is concluded that channel waves may be used to map subsidence into old workings underground. Coal seams apparently share, with other sedimentary rocks, the property of a pressure‐sensitive seismic velocity.

The accommodation of relative motion at depth on the San Andreas Fault System in California

Plate motion below the seismogenic layer along the San Andreas fault system in California is generally assumed to occur by aseismic slip along a deeper extension of the fault. It is also possible that below the seismogenic layer, deformation is distributed laterally over a zone. Several observed features of the San Andreas fault in California have implications about the mode of accommodation of relative motion along the plate boundary beneath the seismogenic zone: the shallow depth of all earthquakes in California, the depth to which coseismic slip occurred during the 1906 San Francisco earthquake, the broad zone of strain accumulation, the broad heat flow anomaly, and the existence of widely separated parallel faults. The observations strongly imply that below the seismogenic zone, relative motion is distributed over a zone and occurs by inelastic flow rather than by aseismic slip on discrete fault planes. The existence of multiple faults further suggests that tractions at the base of the brittle layer are significant over time periods of years to hundreds of years.

The 3D propagation of décollement in the Jura

Summary The sequence of instabilizations leading to the formation of shallow nappes may be illustrated by the kinematics of the embryonic Jura Nappe. On the first order arc, due to regional boundary conditions, are superposed second and third order arcs that developed around more local irregularities in the distribution of strength. Successive instabilization of the Rheintal part of the Jura Décollement Nappe generally proceeds from S to N. Location of ramps where décollement climbs to the surface is largely influenced by pre-existing Rhinegraben structures. These also convey a certain anisotropy to the sedimentary cover which seems to be responsible, at least partly, for a different aspect of sinistral and dextral shear zones. Whereas sinistral shears are usually along discrete faults, mostly reactivated Rhinegraben faults, the dextral shears commonly are diffuse zones. Arcuate elements, convex to the N, connect the sinistral and dextral shear zones frontally. Regionally, the shear zones accommodate the divergence of movement, with lateral elongation, within the overall Jura Arc. More locally, they bound arcuate segments of ever smaller dimensions, and also seem to ease diverging movements in these smaller arcs.