Outcrop Pattern Research Articles

Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck–Isle of Wight Disturbance

The interpretation of a densely spaced and well-calibrated seismic grid sheds new light on the development and evolution of key regional and local structures in the Wessex Basin. The results help to resolve long-standing controversies concerning the tectonic significance of apparently anomalous outcrop patterns and the role of important, local ancillary structures with respect to the major monoclinal folds with which they are associated. Although the structures are entirely consistent with the effects of contractional reactivation (tectonic inversion) of normal faults, the subsurface data demonstrate the role that original extensional fault segmentation and associated relay ramps had on original depositional patterns, subsequent inversion geometries and resultant outcrop patterns. As well as illustrating regional controls on the formation of structures, the new seismic-based interpretations enable a reassessment of the Lulworth Crumple and the Ballard Down Fault. The Lulworth Crumple is interpreted as a parasitic fold complex generated by internal folding of the inverted, incompetent syn-rift fill in the immediate hanging wall to the Purbeck Fault, a reactivated major normal fault. The Ballard Down Fault’s origin is interpreted to result from the formation of a local, late-stage ‘out of the syncline’ reverse fault which propagated southwards and upwards through a Chalk succession. As the Chalk had already been rotated to form the northward-dipping steep limb of the Purbeck Monocline at Ballard Down, the structure cuts down stratigraphically. The results stress the importance of understanding the nature of original extensional fault geometries and the competence of the sedimentary units incorporated in folds in gaining a full understanding of the genesis and evolution of structural styles in inverted basins.

Fault-induced transgressive incised shoreface model for the Canadaway Group, Catskill Delta complex

ABSTRACT Submarine, fossiliferous conglomerates in the Upper Devonian Rushford Formation of the Canadaway Group crop out in Allegany County, western New York State. Characteristics of the conglomerates and an erosional disconformity at the base of the conglomerates suggest that the origin of the Rushford Formation conglomerates can be explained by the transgressive incised shoreface model of Bergman and Walker (1987). Evidence for the erosional disconformity separating the conglomerate from the underlying coarsening-upward sequence includes: (1) a sharp change in lithology, as evidenced by the fine-grained sandstone of the Rushford Formation overlain by a coarser unit that varies from a fossiliferous, quartz-pebble, coarse-grained quartzose arenite to a fossiliferous, quartz-pebble, quartzose paraconglomerate (hereafter referred to as conglomerate); (2) a sharp change in composition from feldspar-rich quartz sandstone below the disconformity to fossil-rich, feldspar- and mica-poor conglomerate above the disconformity; (3) a sharp change in average grain size from ~ 0.06 mm below the disconformity to ~ 0.2 mm above the disconformity. Trace fossils found at the contact between the fine-grained sandstone and the conglomerate indicate the presence of a Glossifungites firmground and a depositional hiatus. In the transgressive incised shoreface model, the erosional disconformity is formed during periods of stillstand in an overall transgression. The outcrop pattern of the conglomerates with respect to faults known to be active in the Late Devonian suggests that the erosion and emplacement of the lag may have developed as a result of interaction between local fault-block motion and a regional transgression. This relation between local faulting and the transgressive surface of erosion promotes a modification of Bergman and Walker's (1987) model for deposition of conglomerates during a stillstand induced by fault-block motion: a fault-influenced transgressive incised shoreface model.

Regional variation in modern radiocarbon ages and the hard-water effects in Lakes Michigan and Huron

Eight new radiocarbon ages, all determined by accelerator mass spectrometry, on modern (pre-bomb) mollusks have been added to similar data provided from three samples in the Lake Michigan and Huron basins. These data confirm the existence of a substantial hard water effect correction ranging from about 250 years to 500 years in these lakes. They also show that the magnitude of these corrections form a spatially coherent pattern that can be related to the pattern of outcrop of Paleozoic (radioactively inert) carbonates that surround the basins and the pattern of circulation within the basins.

Controls on accretion of flysch and mélange belts at convergent margins: Evidence from the Chugach Bay thrust and Iceworm mélange, Chugach accretionary wedge, Alaska

Controls on accretion of flysch and mélange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic‐Cretaceous mélange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed mélange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm mélange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite‐pumpellyite facies prior to formation of the Iceworm mélange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian‐Maastrichtian depositional ages of the Valdez Group, and fault‐related fabrics in the Iceworm mélange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90–100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating mélanges like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style including the fold‐thrust structures that dominate the outcrop pattern in the Valdez belt. Rapid underplating and frontal accretion of the Valdez Group caused a critical taper adjustment of the accretionary wedge, including exhumation of the metamorphosed McHugh Complex, and its emplacement over the Valdez Group. The Iceworm mélange formed in a zone of focused fluid flow at the boundary between the McHugh Complex and Valdez Group during this critical taper adjustment of the wedge to these changing boundary conditions.

Chapter 17 Carboniferous-Permian history of Svalbard

Rocks formed in these two periods do not easily divide at their mutual boundary and it is convenient to treat them together. In doing so we are addressing perhaps the best known and most conspicuous formations of Svalbard. Few geologists have been to the archipelago without noticing fossils and making some observations on these rocks. We are therefore embarking on a substantial study. A three-fold division of Paleozoic rocks in Svalbard is convenient in which Silurian and Devonian or middle Paleozoic history, dominated by Caledonian events, is followed by a Late Paleozoic interval of increasingly stable conditions which show little impact from Variscan, Ellesmerian or Uralian events elsewhere. This contrast applies conspicuously in Permian western Arctic regions. The Carboniferous-Permian outcrops are shown on Fig. 17.1. These rocks are the lower element in the Post-Devonian cover sequence divided between the Spitsbergen Basin and the Eastern Platform and Bjørnøya. The outcrops are disposed in two main areas in Spitsbergen and one in Bjørnøya. The Spitsbergen Basin was at first divided into troughs by inherited N-S faults. These then coalesced and extended throughout Spitsbergen. The present outcrop pattern resulted (i) from Late Cretaceous tilting with loss by erosion to the north, burial to the south and wide E-W exposure across the middle. (ii) A linear belt along the west coast, brought to the surface by folding and uplift along the Cenozoic West Spitsbergen Orogenic Belt from Kongsfjorden to Hornsund. Outcrops are frequently controlled by overthrusting from the west. (iii) The Bjørnøya Carboniferous outcrop

The Mesozoic and Tertiary history of the Irish Sea

Abstract Uplift centred on the Irish Sea in latest Cretaceous times accompanied by a locally raised geothermal gradient is supported by apatite fission track analyses from the area, the outcrop patterns of the Mesozoic rocks of England and the positive gravity anomalies over the whole of the Irish Sea region. This conclusion allows more precise modelling of the history of the region and the lost Mesozoic cover can be reconstructed, with varying degrees of certainty, by combining the results of extrapolation from existing land and submarine outcrops and subcrops, with the amount of cover lost. Stratigraphical considerations suggest the occurrence of both Late Cimmerian and latest Cretaceous inversions of the Irish Sea area. Following rapid erosion of the central part of the domed area in the Palaeogene, alluvial plain sedimentation was established over the Irish Sea region, developing around valleys cut through the Mesozoic sediments of the area. Deflation of the dome and accompanying faulting allowed the sea to enter the area, initially primarily through St George’s Channel in the early Oligocene and into the whole of the basin in Neogene times.

The geology and hydrocarbon prospectivity of the North Channel Basin

Abstract The geology and hydrocarbon prospectivity of the offshore North Channel has been evaluated, integrating data from a number of onshore wells with seismic, gravity, magnetic and onshore outcrop studies. Interpretation of these data suggest that the North Channel can be divided into two sub-basins, separated by the offshore extension of the Southern Uplands Fault Zone (SUFZ). The narrow, southern, Portpatrick sub-basin has a NNW-SSE trend and consists of a series of simple tilted fault-blocks which dip east, towards the basin-bounding Portpatrick Fault. The northern sub-basin widens considerably into the Larne-Firth of Clyde basin complex, which has a predominantly Caledonide (NE-SW) orientation and is an extension of the Scottish Midland Valley. This sub-basin is characterised by westerly dipping fault blocks, the reversal in basin polarity apparently accommodated along the SUFZ. The primary reservoir interval is expected to occur in the Lower Triassic Sherwood Sandstone Group, although secondary targets may exist in the Permian and Carboniferous. Shallow coring and seismic facies analysis suggests that the Mercia Mudstone Group subcrops Quaternary-Recent deposits at sea bed and overlies the Sherwood Sandstone. Halites and shales within the Mercia Mudstone Group should provide an effective regional top-seal, except in areas where the Sherwood Sandstone is close to the surface. Large tilted-fault block and rollover anticline traps have been identified on seismic data, although the Carboniferous source rock potential and burial history are poorly understood and exploration risk is high. Regional outcrop patterns and the compaction state of the Mesozoic strata, together with vitrinite measurements suggest the basin was uplifted in the early Tertiary. In common with the East Irish Sea, maturity was probably reached prior to the end of the Cretaceous. This implies that source rocks within the basin will not currently be generating hydrocarbons, having effectively been ‘switched’ off by Tertiary uplift. Charge mechanics are uncertain but must involve early pre-uplift migration or remigration.

Part 1: Chapter 3 Svalbard's geological frame

This chapter gives an outline of the geology of Svalbard, an introduction to the principles applied in this work and a preview of the interpretations and conclusions that have come out of these studies. The key position of Svalbard in relation to other Arctic lands is shown in a Polar projection (Fig. 3.1). 3.1 The space frame: Svalbard's structural frame 3.1.1 Solid outcrops The conventional geological map of Svalbard shows in outline the outcrop areas according to age. (Fig. 3.2). Although rock ages are matters of interpretation, the diagrammatic map is at least consistent with the conclusions in this work. It differs from most maps in the larger outcrop of Vendian rocks within the basement. The outcrop pattern ignores Quaternary deposits, i.e. it is a solid geology map. It is diagrammatic because detail is impossible to show on this scale. The larger ice masses which obscure the exposure are also diagrammatic. 3.1.2 Regional descriptive sectors Because the intention in this work is generally to describe rocks before interpreting their history and so not to begin with too many assumptions, the rocks are described in Part 2 of this work in the eight sectors or regions (Chapters 4 to 11) as shown in Fig. 3.3. Their boundaries are not fixed because it is convenient to treat some geological matters together in one chapter, even when they might overlap onto another. They are not terranes. Where strata are described the succession is presented as observed with the youngest at the top (as

Recognizing artifactually generated coordinated stasis: implications of numerical models and strategies for field tests

Abstract A null model that generates the first and last occurrences of species through time in a single stratigraphic section predicts several patterns startlingly similar to those described as coordinated stasis. The model is able to produce these patterns by assuming stochastically constant extinction and origination rates, a Gaussian form of facies control with respect to water depth, and cyclically varying water depths in accord with sequence stratigraphy. The model predicts that highstand and lowstand systems tracts will be characterized by faunal tracking with only minor amounts of faunal turnover. Sequence boundaries and flooding surfaces within the transgressive systems tract will be characterized by elevated rates of faunal turnover. These pulses of faunal turnover will exhibit ecological selectivity in terms of the preferred depths, facies tolerances, and abundance of species. Where outcrop patterns of first and last occurrences match these predictions, biologically-driven or environmentally-driven coordinated stasis may not be distinguishable from stratigraphic artifact. However, pulses of faunal turnover that occur within highstand and lowstand systems tracts are likely to be real turnover events. Multiple outcrop studies that span a significant distance along paleoslope are necessary to disprove artifactual turnover that occurs at a sequence boundary or within a transgressive systems tract.

Predicting sediment flux from fold and thrust belts

The rate of sediment influx to a basin exerts a first‐order control on stratal architecture. Despite its importance, however, little is known about how sediment flux varies as a function of morphotectonic processes in the source terrain, such as fold and thrust growth, variations in bedrock lithology, drainage pattern changes and temporary sediment storage in intermontane basins. In this study, these factors are explored with a mathematical model of topographic evolution which couples fluvial erosion with fold and thrust kinematics. The model is calibrated by comparing predicted topographic relief with relief measured from a DEM of the Central Zagros Mountains fold belt. The sediment‐flux curve produced by the Zagros fold belt simulation shows a delay between the onset of uplift and the ensuing sediment flux response. This delay is a combination of the natural response time of the geomorphic system and a time lag associated with filling, and then subsequently uplifting and re‐eroding, the proximal part of the basin. Because deformation typically propagates toward the foreland, the latter time lag may be common to many ancient foreland basins. Model results further suggest that the response time of the bedrock fluvial system is a function of rock resistance, of the width of the region subject to uplift and erosion, and, assuming a nonlinear dependence of fluvial erosion upon channel gradient, of uplift rate. The geomorphic response time for the calibrated Zagros model is on the order of a few million years, which is commensurate with, or somewhat larger than, typical recurrence intervals for episodes of thrusting. However, model experiments also highlight the potential for significant variations in both geomorphic response time and in sediment flux as a function of varying rock resistance. Given a reasonable erodibility contrast between resistant and erodible lithologies, model sediment flux curves show significant sediment flux variations that are related solely to changes in rock resistance as the outcrop pattern changes. An additional control on sediment flux to a basin is drainage diversion in response to folding or thrusting, which can produce major shifts in the location and magnitude of sediment source points. Finally, these models illustrate the potential for a significant mismatch between tectonic events and sediment influx to a basin in cases where sediment is temporarily ponded in an intermontane basin and later remobilized.

Tectono‐magmatic control on vertical dip slip basement faulting: An example from the Fennoscandian Shield

ABSTRACTVertical dip‐slip basement faults play an important role in the evolution and structuring of the Earth's crust. The Proterozoic anorogenic rapakivi‐anorthosite setting of the Fennoscandian Shield in southern Finland exhibits a widespread pattern of vertical dip‐slip basement faults that are deeply eroded. The Porkkala‐Mantsala (PM)‐fault, located c. 30 km W of Helsinki is part of a system of crustal lineaments that closely follows the outcrop pattern of Mid‐Late Proterozoic anorogenic crustal elements, such as basic dyke swarms, the outline of rapakivi granites and remnants of sediment‐filled grabens. These lineaments are formed by low‐grade dip‐slip faults that overprint Svecofennian shear zones. Structural analysis of the PM‐fault supports an interpretation in terms of reactivation of a high‐grade ductile wrench zone. Successive stages of brittle deformation visible in as well the PM‐fault and the Obbnäs granite demonstrate that brittle deformation in the PM‐fault is coeval with the intrusion of the anorogenic Obbnäs rapakivi granite. Based on the spatial and temporal relationship of anorogenic magmatism and block faulting, a genetic relationship is proposed.

Delamerian refolding of the Palaeoproterozoic Broken Hill Block

Dome and basin structures are defined by the outcrop patterns of stratigraphic sub‐units of the Palaeoproterozoic Willyama Supergroup within the Broken Hill Block in far western New South Wales. These structures probably reflect the refolding of the granulite‐grade metamorphic rocks and Olarian‐aged folds along north‐northwest‐south‐southeast axes in sympathy with the overlying Adelaidean sediments during the Delamerian Orogeny. The Broken Hill orebodies and associated Olarian folds were refolded by this event. Delamerian and reactivated Olarian shear zones have influenced the orientation of refolding.

Interfering folds in constrictional deformation

Experiments on buckle folding indicate that in pure constriction ( λ 2 = λ 3 < 1) a dome-and-basin pattern develops in initial stages. The domes and basins do not occur along distinct rows, and with progressive deformation they do not evolve into tubular or sheath folds. The pattern is progressively replaced by diversly oriented folds with curved hinge lines. In general constriction (1 > λ 2 > λ 3), there is an association of domes and basins with nonplane noncylindrical folds. The areas of domes and basins are reduced with increasing deformation. There is a dominant fold trend perpendicular to λ 3 with a less dominant trend perpendicular to λ 3. Folds at an angle to these two sets may also occur. The divergent folds link up in smooth arcs or join up at a dome or a basin. Over a relatively large domain the interference pattern produced by general constriction can be recognized by the association of domes and basins with nonplane noncylindrical folds, by occurrence of hair pin bends of hinge lines of open folds, by occurrence of amoeboid outcrop patterns and by absence of a consistent overprinting relation among different sets of folds. Layers subparallel to the λ 3 axis of general constriction may give rise to two or more sets of coaxial cylindrical folds with orthogonal axial surfaces.

Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, East Antarctica

AbstractMeta-sediments in the Larsemann Hills that preserve a coherent stratigraphy, form a cover sequence deposited upon basement of mafic–felsic granulite. Their outcrop pattern defines a 10 kilometre wide east–west trending synclinal trough structure in which basement–cover contacts differ in the north and the south, suggesting tectonic interleaving during a prograde, D1 thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D2 and D3, each defined by a unique lineation direction and shear sense. D2 structures which are associated with the dominant gneissic foliation in much of the Larsemann Hills, contain a moderately east-plunging lineation indicative of west-directed thrusting. D2 comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D3 structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S3 is the dominant gneissic layering and folds sequences resemble D2 folding. Outside the D3 high-strain zone occurs a low-strain D3 window, preserving low-strain D3 structures (minor shear bands and upright folds) that partly re-orient D2 structures. All structures are truncated by a series of planar pegmatites and parallel D4 mylonite zones, recording extensional dextral displacements.D2 assemblages include coexisting garnet–orthopyroxene pairs recording peak conditions of ∼ 7 kbar and ∼ 780°C. Subsequent retrograde decompression textures partly evolved during both D2 and D3 when conditions of ∼ 4–5 kbar and ∼ 750°C were attained. This is followed by D4 shear zones which formed around 3 kbar and ∼ 550°C.It is tempting to combine D2–4 structures in one tectonic cycle involving prograde thrusting and thickening followed by retrograde extension and uplift. The available geochronological data, however, present a number of interpretations. For example, D2 was possibly associated with a clockwise P–T path at medium pressures around ∼ 1000 Ma, by correlation with similar structures developed in the Rauer Group, whilst D3 and D4 events occurred in response to extension and heating at low pressures at ∼ 550 Ma, associated with the emplacement of numerous granitoid bodies. Thus, decompression textures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.

A latest Cretaceous hotspot and the southeasterly tilt of Britain

The southeasterly regional dip of England and Wales is usually ascribed to Miocene movements. Loss of up to 2 km of cover from northern England, shown by apatite fission track analysis, has been related to uplift and erosion induced by a mantle hotspot, but its effects were far more widespread than considered hitherto. Outcrop patterns throughout much of England, Wales and eastern Ireland can be related to a hotspot beneath the Irish Sea. Uplift occurred within the late Maastrichtian and erosion continued into the late Palaeogene. After post-Oligocene down-faulting of the Irish sea, the modern drainage was initiated on the eroded surface. Unconformities provide the best means of dating orogenic movements; in some localities the time interval between the youngest rocks preserved beneath the unconformity and the oldest preserved above may be small enough to enable the dating of fold movements to be determined with some precision. Thus in southeastern Britain the major E-W structures of the Hampshire Basin, the Weald and the London Basin can be dated as Miocene, as rocks of Oligocene age are involved in the folding in the Hampshire Basin and Pliocene sands rest in erosional pockets of the Chalk of SE England. Because fold movements tend to affect large areas of the crust at discrete intervals of time, the southeasterly tilt of Britain, evident from the fact that a NW-SE transect from Anglesey to London crosses outcrops of rocks getting progressively younger from the Precambrian of Anglesey to the Tertiary of the London Basin, has

Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and 40Ar/39Ar geochronology

Cordilleran ophiolites typically occur as basement for accreted terranes. In the Klamath Mountains, ophiolitic terranes were progressively accreted by underthrusting beneath North America. The Josephine ophiolite is the youngest of the Klamath ophiolites and forms the basement for a thick Late Jurassic flysch sequence (Galice Formation). This ophiolite‐flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the Klamath Mountains above and a coeval plutonic‐volcanic arc complex below. The outcrop pattern of the roof (Orleans) thrust indicates a minimum displacement of 40 km, and geophysical studies suggest &gt;110 km of displacement. The basal (Madstone Cabin) thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km. A rapid sequence of events, from ophiolite generation to thrust emplacement, has been determined using 40Ar/39Ar and Pb/U geochronology. Ophiolite generation occurred at 162–164 Ma, a thin hemipelagic sequence was deposited from 162 to 157 Ma, and flysch deposition took place between 157 and 150 Ma. Tight age constraints on thrusting and low‐grade metamorphism associated with ophiolite emplacement (Nevadan orogeny) are provided by abundant calc‐alkaline dikes and plutons ranging in age from 151 to 139 Ma. Deformation and metamorphism related to the Nevadan orogeny appears to have extended from ∼155 to 135 Ma. Most of the crustal shortening took place by thrusting, constrained to have occurred from ∼155 to 150 Ma on both the roof and basal thrusts. Minimum rates of displacement are 2.4 and 3.6 mm/year for the basal and roof thrusts, respectively, but correlations with coeval thrusts yield rates of 8.4 and 22 mm/year (within the range of plate velocities). The high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western North America and an active arc from ∼155 to 150 Ma. A steep thermal gradient was present in the Josephine‐Galice thrust sheet from ∼155 to 150 Ma, with amphibolite facies conditions developed along the basal thrust. After accretion of the ophiolite by underthrusting, the ophiolite and overlying flysch underwent low‐grade dynamothermal regional metamorphism from 150 to 135 Ma. The upper age limit is tightly constrained by a 135 Ma K‐feldspar cooling age, syntectonic plutons as young as 139 Ma, and a Lower Cretaceous angular unconformity. Very rapid exhumation is indicated by the late Valanginian to Hauterivian age (∼130 Ma) of the unconformably overlying strata, suggesting unroofing by extensional tectonics.

Late Mesozoic and possible early Tertiary accretion in western Washington State: The Helena-Haystack mélange and the Darrington-Devils Mountain fault zone

The Helena-Haystack melange (HH melange) and coincident Darrington-Devils Mountain fault zone (DDMFZ) in northwestern Washington separate two terranes, the Northwest Cascade System (NWCS) and the western and eastern melange belts (WEMB). The two terranes of Paleozoic and Mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history. The HH melange is a serpentinite-matrix melange containing blocks of adjacent terranes but also exotic blocks of schistose metavolcanic rocks and Jurassic tonalite and associated amphibolite. The HH melange must have formed between early Cretaceous and late middle Eocene time, because it contains tectonic clasts of early Cretaceous Shuksan Greenschist and is overlain by late middle Eocene sedimentary and volcanic rocks. Less certain constraints on its age are a tectonic clast of metarhyolite that yields 90 Ma metamorphic ages and the presumption that the melange was emplaced before the outboard Olympic terrane arrived at about 50 Ma. The apparent continuity of the HH melange and the Decatur terrane of the San Juan Islands suggests that the melange is the strongly tectonized equivalent of the Fidalgo ophiolite. The out-crop pattern suggests that the HH melange overlies rocks of the NWCS and it may have formed when the WEMB terranes were thrust over rocks of the NWCS. Much of the exposed belt of the HH melange is overlain by late middle Eocene feldspathic sandstone and volcanic rocks of the Barlow Pass Volcanics of Vance (1957a), which are cut by numerous faults of the DDMFZ paralleling the melange. The Barlow Pass Volcanics appear to overlie the Straight Creek fault without large offset, but a displaced exotic block of amphibolite with attached early or early middle Eocene(?) sandstone in the melange suggests that strike-slip movement along the DDMFZ was synchronous with movement on the Straight Creek fault, and stretched cobbles in the conglomerates of the Barlow Pass Volcanics suggest post-Straight Creek movement. The possible continuation of the DDMFZ to the northwest as the San Juan and the West Coast faults on Vancouver Island suggests That the structure has had a major role in the emplacement of all the westernmost terranes in the Pacific Northwest. This major suture is strongly bowed to the northeast opposite the great oroclinal bend of the Olympic terrane, suggesting that the emplacement of that terrane may have deformed a once straighter strike-slip zone.

Geophysical Investigations of Volcanic Terrains: A Case History from the Gawler Range Volcanic Province, South Australia

Flat-lying magnetic cover rocks, especially when they comprise thick volcanic sequences, present a difficult interpretation problem. Magnetic maps of such regions essentially reflect the exposed volcanic rocks and sources within and beneath the volcanic rocks cannot be easily resolved. Yet such areas are often of economic interest. The Gawler Range Volcanic Province in South Australia represents an example of one such volcanic terrain. The Volcanics are distinctly bimodal in composition: the 'older' units are mafic and the 'younger' units dominantly felsic, with few rocks of intermediate composition. The exposed Volcanics cover approximately 25 000 km2 and drill-hole intersections confirm their existence well beyond the present limits. Magnetite is present in both mafic and felsic volcanic rocks. The province has been identified as a target area for possible epithermal gold mineralisation and volcanogenic-hosted base metal mineralisation. Regional gravity and aeromagnetic data are available over the whole province, while part is covered by a recent high-resolution aeromagnetic and aeroradiometric survey. The Gawler Range Volcanic Province is associated with a large gravity high caused probably by mantle underplating or the emplacement of mantle derivatives prior to the extrusion of the volcanics. This is consistent with isotope data which were previously interpreted to indicate a mantle component for the Volcanics. Two major gravity trends (NE and ESE) intersect within the study area. The ESE-striking trend has been modelled as a deep crustal fault with the southern block down-faulted. These two trends are also reflected in the high-resolution magnetic maps once the high frequencies have been removed from the data. Initial images of the high-resolution aeroradiometric and aeromagnetic data correlated very closely with the outcrop and joint patterns. Areas of radiating fracture patterns may represent volcanic centres. The image of the K:U Ratio reflected minor variations in the otherwise homogeneous geochemistry of the Volcanics. Through image processing and filtering, and combining the interpretation of available geophysical data, an inferred geological map for the area has been suggested. The igneous province is interpreted as extending further eastwards beneath part of the Stuart Shelfing consisting of three well-defined units. The first corresponds to the main Gawler Range Volcanic Province, the second to the south-west gabbroic extension and the third to an eastward extension consisting of a felsic core circled by a mafic ring.

Paleomagnetism and tectonics of the Crescent Formation, northern Olympic Mountains, Washington

A stable prefolding magnetization has been discovered in pillow basalts of the Eocene lower Crescent Formation of the northern Olympic Mountains. The curved outcrop pattern of the Crescent Formation has been the target of several unsuccessful studies to test for oroclinal bending. The success of this study is due, in part, to the development of a small‐diameter electric core drill for sampling the fractured rims of basalt pillows. Thermal demagnetization produced stable endpoints by 580°C in 12 of the 34 sites sampled (large within‐site scatter was common in the remaining sites). Among the accepted sites, within‐site scatter was small and correction for bedding tilt significantly reduced the scatter between sites. The mean paleomagnetic pole for newly sampled pillow basalts (86.4° north latitude, 170.0° east longitude, A95=16.5°) is indistinguishable from the early to middle Eocene pole expected for North America. Including previous results from sites in subaerial basalt of the upper Crescent Formation in and near the easterm Olympic Mountains results in a more regional paleomagnetic pole (80.7° north latitude, 192.0° east longitude, A95 =8.0°, N=46) that shows no significant rotation (0.8° ± 14.4°) or poleward displacement (−3.6 ± 8.5). Analysis of the magnetic mineralogy suggests that the remanence is early, probably primary. The pole, therefore, should be valid for tectonic interpretation of the region. A circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern Crescent Formation experienced deformation due to the rise, in a domelike fashion, of the sediments of the Olympic Core terrane. Erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the Crescent Formation. The lack of significant rotation of the northernmost Coast Ranges contrasts with the net clockwise rotation seen to the south. The difference could be that irrotational northward translation (paleomagnetically insignificant yet geologically important), driven by oblique convergence, was accommodated by the north‐south trending strike‐slip faults to the east of the Olympic Mountains.

Southwest Virginia Underground Coal Mine Map Database and Base Maps: Synopsis of an Ongoing Coalfield Project

In September 1991, the Department of Mines, Minerals, and Energy of the Commonwealth of Virginia entered into an agreement with the office of Surface Mining to prepare a coal mine map database and to produce 1:24,000 scale individual coal-bed base maps showing documented underground mined areas throughout the Southwest Virginia coal field. The project results are to provide public, industry, and all levels of government a much-needed means of initial evaluation of many coalfield related concerns. The completed maps will be incorporated into an integrated geographic information system (GIS). Evaluating the entire coalfield involved a preliminary review of 48 quadrangles. Ongoing detailed, accurate information gathering of extensive underground mine map files was necessary to provide a needed organized map database. Construction of coalfield index maps of information gathered to date provide insight into coalfield-wide outcrop patterns, mine distributions, and coal-bed trends. A completed set of individual maps, referenced to the underground mine map database, showing the types of mining applicable per coal bed quadrangle is the designated project output.