Wrench Fault Research Articles

Structure and Facies Development of the Dutch/North German Rotliegende Basin

The apparent east-west extension of the southern Rotliegende basin, stretching from southern England via the Netherlands and north Germany to Poland, developed from several subbasins running in a northwest-southeast direction. The orientation of the subbasins and the graben systems have largely been caused by a regional stress field, which existed in the Late Paleozic of northern central Europe. The maximum extension was in an east-west direction. The graben systems of northern Germany and the southern part of the North Sea are running roughly north-south and are connected via a parallel set of wrench faults. The subbasin with the largest Rotliegende thickness lies in the German part of the North Sea. It subsided in the region where the rift axis of the north-south-running north German graben system experienced left lateral displacments by northwest-southeast-running wrench faults. The active graben zone extended into the Horn-Bamle-Oslo graben. The initial Dutch subbasin was connected with the early central graben and merged with the north German subbasin in the course of the progressive sedimentation of the basin. In contrast to the north German subbasin, where the initial sedimentation was mainly determined by the north-south-directed graben tectonics, intensive northwest-southeast-directed step faults developed in the Dutch subbasin. Themore » initial subbasins were arranged in an en echelon pattern and merged during the main subsidence of the basin. The origin of the subbasins is linked to the Stephanian basins. Their development continued while several climate changes occurred up to the early Mesozoic. The development of the intracontinental sedimentation from the small initial subbasin to the widespread southern Rotliegende basin can therefore be divided into three main stages: initial stage-tectonics more effective than climate cycles, main stage-equal effect of tectonics and climate cycles, and late stage-climate cycles more effective than tectonics.« less

Divergent Wrench Faulting in the Belize Southern Lagoon: Implications for Tertiary Caribbean Plate Movements and Quaternary Reef Distribution

Divergent Wrench Faulting in the Belize Southern Lagoon: Implications for Tertiary Caribbean Plate Movements and Quaternary Reef Distribution

Satellite Imagery Interpretation and Integration for Hydrocarbon Exploration

A systematic approach to the analysis of sedimentary basin through the integration of satellite imagery data is presented. It begins with regional interpretation of geological structures that are exposed at the basin's margins and progresses toward the recognition of structural features that can be extended into the basin's interior where they manifest more subtle geomorphic expressions. Emphasis is placed on the recognition of structural elements that can be constrained with other conventional explorational tools such as gravity, magnetics and seismic data. The main steps in the analytical procedures are demonstrated with examples from various case studies. These include; regional structural interpretation, detailed analysis of key areas, development of structural/geomorphic models, delineation of prospective leads, prospect mapping and model testing. Examples cited include; detection of basement warp structures and related incised valley systems in the Western Canada Basin and the Powder River Basin, U.S.A. delineation of a fracture play related to reactivated wrench fault systems in the Swiss Molasse Basin and early detections of buried Triassic grabens in the Coastal Plain region of Eastern U.S.A.

Tertiary Tectonic in the Tehuantepec Isthmus, Mexico

A microplate model in the basement was proposed according to photointerpretation of satellite imagery and supported with microtectonic studies in the Tehuantepec's Isthmus. The microplate is located in the northwestern part of the [open quotes]Sierra de Chiapas,[close quotes] and structurally has lineaments that correspond with sinestral wrench faults oriented northeast-southwest and dextral faults oriented northwest-southeast. In the front of the microplate, these faults are joined in an arc form. The microplate began its movement forward to the north in the middle Tertiary. This movement originated in a regional compressional stress that was younger to the north. The stress changed the orientation of the anticline axis from northwest-southeast to west-east. In its western limit, the stress produces a sinestral shear stress that built a rotational deformation in the [open quotes]Sierra Atravesada,[close quotes] and represents a superimposed tectonic block over an ancient (laramide) orogeny. This system has also produced other secondary transtensional effects oriented northwest-southeast, represented along the [open quotes]Depression Central del Istmo.[close quotes] The microplate has formed a tensional system opening the [open quotes]Superior, Inferior, and Mar Muerto[close quotes] lagoons. The microplate is strongly related with the relief, seismic activity, and the tectonics of the salt of the Tehuantepec's Isthmus.

Structural and Hydrological Parameters Modeling of Wilcox Group, Central Louisiana and Mississippi

ABSTRACT In a 900 square miles area between Louisiana and Mississippi 300 well logs were studies. Three major sand formations in middle Wilcox and five target log picks within upper and middle Wilcox were correlated. The lifthofacies variation appear to be related to the sequence and eustatic boundaries. The temperature and salinity values modeled were also obtained from E-log and BHT (Bottom Hole Temperature) data. The relationships among the temperature and salinity distributions, topography, stratigraphy and subsurface structure are provided. Several real images of 3-D topography, formation structure and stratigraphic boundary were reconstructed by using the well log data. This detailed study suggests that the productive oil fields along the east and west flank of Mississippi Embayment are related to the salinity anomalies, and are controlled by structural and stratigraphic features. From the 3-D maps confident interpretation about the structure and stratigraphy changes with time and basin growth history can be made. For instance, it appears that the Holocene Mississippi River course in the study area is nearly coincident with the main depositional axis of Middle and Late Wilcox time. The current Mississippi River channel, as well as the basin's depositional axis of 20-50 million years ago, appear to have been affected by some regional structural events. Also, a north-south salinity anomaly in the east part of study area is a possible indicator of a northeast trending wrench fault along the Mississippi river. End_of_Record - Last_Page 536-------

Field relations and movement history of the Coniston Fault, Cumbria

The Coniston Fault is one of the largest and most fundamental faults in the Lake District. It is most easily appreciated near to Coniston where the Coniston Limestone is displaced horizontally (north to south) by 1.5 km, but, although the fault complex is easily traced for more than 40 km from the Vale of St John to well south of Coniston Water, the displacements in other areas are not always easy to determine. It is unlikely that the fault is entirely strike-slip, but if this were the case a dextral displacement at Coniston would be theoretically wrong for a north-south fault formed in the NW-SE Caledonian stress field, although such anomalies are common in geology. A possible explanation would be for a shallow wedge between the Coniston Fault and Brathay Fault to have been pushed forward during the Acadian movements, giving rise to the Coniston Thrust where it was overridden by the Coniston Grits. In any case the field evidence shows that the Coniston Fault had a long history of movement which started during, or before, the Borrowdale Volcanic episode, with any strike-slip (wrench) component most probably belonging to the late Caledonian (Acadian) phase. To the east of Coniston the fault, together with the adjacent Brathay Fault, maps out as a composite structure, bending into an ENE-bedding thrust, and then bending back again into the Beacon Tarn wrench fault. To the north the Coniston Fault extends through the Borrowdale Volcanics to Grasmere where it splits, one branch following the line across Dunmail Raise to Thirlmere and the Vale of St John, and the others crossing the fells to Ullswater. One concern of this paper is to consider the relationship of the Coniston Fault and other structures to the Borrowdale Volcanic sequences, parts of which change abruptly at the fault. Much of this I believe to have been a result of fault movements during the volcanic episode, possibly related to the caldera formation further west. Regional folding was also influenced by the Coniston Fault since the well-defined NE-trending Wrynose Anticline and Fault, also likely to have been initiated during the volcanic episode, terminate at the fault. Renewed post-volcanic-pre-Coniston Limestone movement of the Coniston Fault, followed by erosion, resulted in the absence of the Tarn Hows Formation west of the fault, a clear demonstration of pre-Coniston Limestone movement. The Tarn Hows Formation is the highest part of the volcanic sequence in this area. The latest demonstrable movements were post-Bannisdale Slates and of presumed Acadian age, although by analogy with adjacent areas there may have been posthumous activity in post-Carboniferous and post-Triassic times.

REGIONAL SIGNIFICANCE OF THE ARQUEBUS- I WELL, BROWSE BASIN, NORTH WEST SHELF, AUSTRALIA

The Arquebus-1 well was drilled in 1991 on exploration permit WA-206-P located in the southern portion of the Browse Basin. The Browse Basin is one of the least explored offshore basins in Australia and although two major gas discoveries have been made, no significant oil accumulations have been found. The Arquebus-1 well tested Middle to Upper Jurassic sandstones in a large three-way dip closed structure along the main Jurassic basin margin fault system, which has subsequently been inverted by Tertiary wrench faulting. The well was plugged and abandoned despite the presence of numerous shows and pay indicated by wireline logs. Five formation tests were performed and despite a long 12-hour test, only mud filtrate was recovered, suggesting that significant fluid invasion had occurred. The pressure data indicate a good water gradient with a 51 m gross column of light oil and gas. This is supported by detailed analysis of fluid inclusions, capillary pressure analysis, wireline logs and sidewall cores. The total gross hydrocarbon column may be as great as 105 m. The extremely low clay content, the uniform pore throat size, the slow drilling rate and the overbalanced drill mud made the sandstones prone to the nearly complete flushing of formation fluids and associated formation damage. The presence of an inferred oil column in the Jurassic sandstones at Arquebus-1, indicates that oil has been generated in the Browse Basin and that three-way dip closed structures are potentially viable traps in the area.

The Lega Dembi gold mine: an example of shear zone-hosted mineralization in the Adola greenstone belt, Southern Ethiopia

In the Adola Gold District of Southern Ethiopia the outline of the contacts that separate the greenstone belt and the gneissic terrain defines regional scale ductile to brittle-ductile shear zones. Possibly economic mineralization is found to be controlled by these major shear zones. Mapping out and documenting the movement senses of these major shear zones in detail, therefore, was vital for the prediction of the nature and orientation of the minor structures that are likely to host gold deposits. Structural information combined with identification and mapping of suitable host rocks such as the Lega Dembi schists, can provide a powerful tool in exploration for gold in the Adola area. Such techniques are not only important in mineralized areas such as the Lega Dembi Gold Mine, where some, but not all, mineralized structures have been discovered, but also in unexplored terrains. Mapping at a scale of 1:25,000 resulted in detection of several superposed shearing events with both sinistral and dextral senses of displacement. These shear zones are expressed as ductile, brittle-ductile and brittle types. Study of geometric compatibilities between the structural elements viz., shear zones, tectonic lineations and folds helped in locating the Lega Dembi Gold Mine within a grossly N-S trending major ductile transcurrent sinistral shear zone. This has subsequently been reactivated and overlapped by N-S trending mesoscale shear zones with similar trend but of opposed sense of displacement. The formation of the later dextral shear zones took place under similar ductile conditions within a single but extended phase of deformation, D 2, that generated the reversed motion. Generally D 3 E-W wrench faults with minor WNW-ESE/WSW-ENE trending conjugate pairs and mesoscale D 4N-S/NE-SW trending conjugate systems of brittle to brittle-ductile shear zones have been recognized. Mineralization is hosted by quartz veinlets and stringers that propagated along mesoscale D 2 ductile to brittle-ductile shear zones.

A serpentinite ridge in a collisional paleomargin setting: the Beni Malek massif, External Rif, Morocco

In the Eastern Rif Belt of northern Morocco the Beni Malek serpentinised peridotite massif is included in a low-grade metasedimentary tectonic pile, which was thrust southward onto the Atlas foreland, and which belongs to the external zones of the Maghrebides belt. We describe serpentinite clasts within calcareous deposits that are transgressive onto the ultramafic (lherzolitic) massif and detrital serpentinite layers that occur within greenschist formations. Reworking of the serpentinite clasts occurred at the beginning of the Late Jurassic-Early Cretaceous accumulation of the overlying, marginal sedimentary prism. We connect the ultramafic outcrop with a regional, NE-SW trending magnetic anomaly (the Temsamane anomaly), and conclude that the source of the serpentinite clasts extended roughly parallel to the local, NE-SW trend of the North African margin. By comparison with the submarine serpentinite ridge of the Galician margin, we suggest that the Beni Malek ultramafics were part of a similar ridge of serpentinised mantle rocks exhumed by the Tethyan rifting off northwestern Africa. The extensional emplacement of this ridge is correlated with the early stage of uplift of the Ronda-Beni Bousera lherzolite massifs under the Alboran thinned continental block, and with ocean-floor spreading on both sides(?) of the Alboran block. The seismic Nekor fault parallels the Temsamane anomaly to the southeast and is interpreted as a former extensional fault of the North African paleomargin that was reactivated as a sinistral wrench fault during the Miocene by the Iberian-African collision, which caused the obduction of the Beni Malek massif itself.

Evidence for cenozoic crustal extension in the Bering Sea region

Geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the Bering Sea have been created or altered by crustal extension and back‐arc spreading. These processes have occurred during and since early Eocene time when the Aleutian Ridge developed and isolated oceanic crust within parts of the Bering Sea. The crust in the Aleutian Basin, previously noted as presumably Early Cretaceous in age (M1–M13 anomalies), is still uncertain. Some crust may be younger. Vitus arch, a buried 100‐ to 200‐km‐wide extensionally deformed zone with linear basement structures and geophysical anomalies, crosses the entire west central Aleutian Basin. We suggest that the arch and the inferred fracture zones in the Aleutian Basin are early Cenozoic structures related to the early entrapment history of the Bering Sea. These structures lie on trend with known early Cenozoic structures near the Bowers‐Shirshov‐Aleutian ridge junction and on the Beringian continental margin (with possible continuation into Alaska); the structures may have coeval and cogenetic(?) histories for early Cenozoic and possibly younger times. Cenozoic deformation within parts of the Bering Sea region is principally extensional, although the total amount of extension is not known. As examples, the Komandorsky basin formed by back‐arc seafloor spreading, the Aleutian Ridge has been extensively sheared, and extensional block faulting is common. Sedimentary basins of the Bering shelf have formed by extension associated with wrench faulting. The Cenozoic deformation throughout the Bering Sea region probably results from the interaction of major lithospheric plates and associated regional strike‐slip faults. We present models for the Bering Sea over the past 55 m.y. that show oceanic plate entrapment, back‐arc faulting and spreading along Vitus arch, breakup of the oceanic crust in the Aleutian Basin at fracture zones, and back‐arc spreading in Bowers Basin.

Mesozoic and early Tertiary rift tectonics in East Africa

A complex history of crustal extension occurred in east and central Africa during the Mesozoic and early Tertiary. Beginning in the Late Jurassic, this resulted in a large system of rifts, the Central African rift system, that spanned from central Sudan to southern Kenya. Late Jurassic rifting is best documented in the White and Blue Nile rifts of the Sudan, and records east-west extension in half-graben that were connected by large-scale shear zones and pull-apart basins. Early Cretaceous rifting re-activated Jurassic basins and spread to the large South Sudan rifts and Anza rift in Kenya. By the Late Cretaceous, the extension direction shifted to the NE-SW, and the presently observed large-scale rift geometry was established. In the early Tertiary, some Mesozoic basins were again reactivated, while other regions underwent wrench faulting and basin inversion. The large number of basins preserved in the Central African rift system can be used to construct an evolutionary model of continental rift tectonics. Early phases of extension at low strains produced alternating half-graben/accommodation zone geometries similar to those observed in most young and active continental rifts. At higher strains, some border faults were abandoned so that through-going, simpler active fault systems could evolve. This is interpreted as representing a switch from complex, oppositely dipping detachment structures, with strike dimensions of 50–150 km, to regional detachment structures that continue for hundreds of kilometers parallel to the rift. This change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate. Non-rotational, high angle, normal faulting dominates in the development of these late basin geometries. Deciphering similar rift basin histories from passive continental margins may, in many cases, exceed the limits of available reflection seismic data. East-west striking shear zones, including the Umm Hani and South Sudan shears, served as basin-terminating structures in the Late Jurassic rift system. These shear zones were later reactivated during the Cretaceous and early Tertiary, controlling the positions of some accommodation zones and “jogs” (apparent offsets) in rift trends. The South Sudan shear may have continued to play a role in Neogene extensional tectonics, as it connects “jogs” in both the Abu Gabra/Anza rift (Jurassic? to Early Tertiary) and Turkana/Chew Bahir rift (Neogene to Recent).

The geodynamic evolution of the eastern Eurasian margin in Mesozoic times

The Cenozoic features of Eurasia, such as marginal seas and intracontinental deformation, are unraveled using available paleomagnetic and field data. The Late Cretaceous shape of Eurasia is then reconstructed. From the examples of SW Japan and South Russian Far East, the major features of the Mesozoic Eurasian margin—suture zones marked by ophiolites or high-pressure metamorphic rocks, olistostromes characterized by the lithology of the olistoliths, calc-alkaline magmatism and strike-slip faults—are outlined. Correlations of the ophiolitic belts and the olistostromes along the eastern Eurasian margin from Borneo to Siberia allow the definition of four microcontinents; the West Philippines, South Japan, and the Anuy and Okhotsk Sea microblocks from south to north. The Mesozoic structure of the Eurasian margin is due to three processes: 1. (1) oceanic subduction and the accretion of subduction complexes; 2. (2) the collision of these microcontinents with Eurasia between the Late Jurassic and the Late Cretaceous; and 3. (3) the reactivation of the ophiolitic sutures by left-lateral wrench faulting.

Baikal rift zone: Structure and geodynamics

The Cenozoic Baikal rift zone is superimposed on the Caledonian Baikal fold belt, representing the suture between the Precambrian Siberian craton and several microcontinents. The Baikal rift zone consists of a system of disconnected fault-bounded basins and extensional and wrench faults that straddles a major arch, having a topographic relief of 2–3 km. Rifting activity commenced during the Oligocène and is still active as evident from high seismicity of the Baikal rift zone. Across the Baikal rift zone, upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8–11 km across the Baikal basins as compared to the adjacent unextended areas. The discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir. Indeed, geophysical data indicate that the Sayan-Baikal dome coincides with such a diapir, the top of which is located at the crust/mantle boundary. This intrusion is held responsible for Oligocene and Quaternary doming of the rift zone. Development of the intra-continental Baikal rift system is thought to be related to asthenospheric diapirism (active rifting); however, intra-plate stresses in conjunction with the Himalayan collision may have played some role.

Morphology of the Blanco Transform Fault Zone-NE Pacific: Implications for its tectonic evolution

The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to ‘kill’ the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion.

Evidence for a Panafrican volcanic arc and wrench fault tectonics in the Jbel Saghro, Anti-Atlas, Morocco

The Saghro hills constitute the northern branch of the Panafrican mobile belt in the eastern part of the Moroccan Anti-Atlas. The Precambrian terranes are predominantly composed of volcaniclastic and volcanic series witnessing a mainly explosive volcanism, intruded by diorites and granites. Their age attribution is problematic, and has been reconsidered in the present study based on structural evidence and Rb/Sr data of quartzdiorites: penetratively deformed volcano-sedimentaries intruded by these plutons aged between 754 and 722 Ma are now considered as Lower Precambrian II. The Upper PII series have been reattributed, and only formations overlying a basal conglomerate and angular disconformity are considered PIII.

Fractured Smackover Limestone in Northeast Louisiana; Implications for Hydrocarbon Exploitation

ABSTRACT Highly fractured Jurassic Smackover carbonates are present in northeast Louisiana in an area covering more than 2,000 sq mi (5,120 sq km). Vertically, more than 1,000 feet (330 m) of fractured section has been recovered in continuous conventional cores through the Smackover Formation. The most extensively fractured section extends eastward from Union and Ouachita Parishes in Louisiana into west central Mississippi, and northward from the northern part of Richland and Madison Parishes into southeast Arkansas. Wrench faulting together with normal faulting are likely causes of the shear and extension fractures found in the Smackover. The intensity of the fracturing appears to be extremely high when compared to the level observed elsewhere in the Gulf Coast Basin's productive Smackover trend. Regionally, wrench faulting and its related fracturing also may have provided a process for generating vertical and long distance hydrocarbon migration pathways from Middle and Lower Smackover source beds. These source rocks combined with: 1) a subsequent favorable thermal history, 2) an overlying upper Smackover reservoir (developed by depositional, secondary and structural processes), and 3) an effective overlying seal, have yielded a favorable hydrocarbon system. The fractured Smackover has been sparsely drilled and currently the area has no production from these rocks.

TECTONIC CONTROL OF WORLD OIL RESERVES: AUSTRALIA'S POSITION

Although simple extensional clay models may be representative of grabens tens of kilometres in length, rotational divergence of continents on a sphere produces very different structures. Repeated periods of compression during separation result in wrench faults and compressional anticlines developing along major crustal fractures as a consequence of changes in momentum between a continent and adjacent smaller continental blocks along its rifted margin.The global distribution of rotationally divergent continental margins can be accounted for by asymmetric expansion of the earth. The southern bulge caused by expansion has emphasised non-marine deposition on southern continents with marine deposition more common in the northern hemisphere.Phanerozoic source rocks of the northern hemisphere account for 97 per cent of the world's produced and current reserves of oil. Australia's share of this extreme distribution asymmetry is less than one half of one per cent, yet the country covers five per cent of the Earth's continental crust.The proportion of undiscovered oil reserves outside OPEC and the former USSR is approximately 30 per cent, or some 12 per cent of the world's estimated ultimately recoverable reserve of 2 trillion (Tera) barrels. The majority of Australia's undiscovered reserves lie on the North West Shelf where about 12 per cent of the country's estimated ultimately recoverable reserve could be found.Although Australia is politically stable, lower petroleum taxes would attract exploration for smaller, structurally complex oil fields. While such taxes may be considered politically difficult at present, a by-product of concerted oil exploration would be an enormous increase in Australia's gas reserves to feed the national pipeline grid for the 21st century. Industry can assist increased success rates by greater attention to current technical deficiencies, such as the structural interpretation of seismic lines.

Subsurface Geodynamics and Petroleum Geology of Transform Margin Basins in the Sahel of Mahdia and El Jem (Eastern Tunisia) (1)

Geophysical, structural, and seismic-stratigraphic analysis of the Mahdia sahel of Tunisia, based on seismic data and sedimentologic and microtectonic field measurements, reveals a dynamic basin formation from the Aptian to the Quaternary. Seismic-structural and isopach mapping of seismic horizons of Cretaceous and Cenozoic megasequences shows the partition of the mobile graben depocenter and carbonate and siliciclastic platform basins. These basins are limited by synsedimentary restrictions along strike-slip faults oriented east-west and north-south. The interplay of these wrench faults, in regional transtension and transpression during the Cretaceous and Cenozoic, induced the synchroneous opening and closing of the graben and platform basins, producing large accumulatio s of black shales and turbidites, and reef, oolite, and carbonate platform deposits. This was associated with the dragging, rotation, and blocking of dekakilometer-size blocks assimilated to lithospheric microplates. These movements were accompanied by volcanism and Triassic salt intrusions along the bordering active faults in the east-west-trending and north-south-trending corridors. In these basins, sequential inversion of basin subsidence and the superposition of rhombic basins and wrench depocenters created migrating depocenters and provided the conditions in rapidly subsiding areas for source rock maturation and, later, hydrocarbon migration into stratigraphic and tectonic traps. The Tunisian Sahel domain at the northern border of the African plate is characterized by tectonic mecha isms which provide a model for tectono-sedimentary basin evolution in a wrench context.

Application of mineral deposit models in exploration: a case study of sediment-hosted gold deposits, Great Basin, Western United States

Abstract This paper emphasizes the development of mineral deposit models for exploration using as a case study the sediment-hosted, structure-controlled, disseminated gold deposits in the Great Basin of the Western United States. More than 60 deposits have been found in this area since the discovery of the Carlin deposit in 1962. Most deposits were discovered by direct prospecting and sampling of outcropping silicified zones (jasperoids). Similar poorly exposed and unexposed deposits remain to be discovered, but future exploration will require improved models that indirectly predict locations of these deposits. Many of these sediment-hosted gold deposits occur within silicified and structurally complex zones in calcareous rocks below regional east-directed thrusts. Geological characteristics that have been successfully employed in exploration include: erosional windows (fensters) through the thrusts; silty to arenaceous calcareous host rocks below the thrusts; silicified zones; anticlines and domes of the lower plate calcareous sediments; alignment of deposits along linear trends; and geochemical anomalies (Au, Ag, As, Sb, Tl, etc.). Although the use of these criteria will discover new deposits, exploration now requires additional criteria that are reliably, but not so directly, associated with the deposits. Identification of these criteria will require the development of genetic models that tie more regional geological characteristics to ore formation and localization. Development of successful mineral deposit models first requires a working definition of the sediment-hosted gold deposit class and its geological characteristics. The characteristics of each deposit included in this class are then documented and validated. Next, the geological processes most likely responsible for the shared characteristics are identified. Finally, the most reliable, informative and accessible criteria, reflective of each ore-forming process, are selected. From these criteria are chosen the most efficient and effective geological features for continued exploration. Processes currently suggested to be important in the formation of sediment-hosted gold deposits include: regional wrench fault tectonics; plutonism; hydrothermal fluid flow driven by intrusions and/or thrusting; fluid flow along regional unconformities; gold precipitation following carbonate dissolution, or fluid mixing; and dissolution of synsedimentary gold from the allochthonous western assemblage sediments and reprecipitation of that gold in the deposits. A general approach for identifying additional possible processes, testing alternative processes and finally selecting exploration criteria is presented.

Tectonic development of the Drummond Basin, eastern Australia: backarc extension and inversion in a Late Palaeozoic active margin setting

AbstractDetailed seismic reflection data combined with regional magnetic, gravity and geological data indicate that the Drummond Basin originated as a backare extensional basin associated with Late Devonian and Early Carboniferous active margin tectonism in the northern New England Fold Belt. Seismic reflection data have been used to generate a two‐way time map of seismic basement, providing a clear view of the basinal geometry and structural development. Broadscale structural asymmetry of the basin implies that simple shear along a deep, upper‐crustal detachment provided the extensional mechanism and generated an inter‐related set of listric normal faults and associated transfer faults, as well as steeply‐dipping planat normal faults. The orientation of normal faults near the basin margins appears to have been controlled by regional basement structural trends. Transfer‐fault trends were approximarely orthogonal to the line of plate convergence as assessed from the orientation of coeval are, forcare and subduction complex stratorectonic elements. Three distinct phases of infill are represented in the basinal stratigraphic succession. The first consists largely of volcanics and volcaniclastics, indicating that effusive magmatism and extension were closely associated in space and time. The second is quartzose and of basement derivation, but was not derived from footwall blocks at the faulted basinal margins to the east and north. Uplifted hanging‐wall crust beyond the western basinal margin, a product of west‐directed simple shear detachment, was the likely source terrain. The final infill phase consisted of volcaniclastics considered to have been derived from a coeval volcanic are to the east. Major faults at the basin margins provided conduits for magmatism during extensional basin development, and long after the basinal history was complete. During the Late Carboniferous and mid‐Triassic, the basin was affected by two discrete episodes of compressional deformation. This led to inversion with the development of folds, and reverse and wrench faults now seen at the surface.