East-northeast West-southwest Research Articles

Digital elevation model‐based lineament analysis of the Zindapir anticline, Sulaiman fold‐and‐thrust belt, Pakistan

The N‐S oriented Zindapir anticline is an eastward verging mega‐structure (~130 km along and ~40 km across the strike) in the eastern Sulaiman fold‐and‐thrust belt on the western margin of the Indian Plate in Pakistan. In this study, we carried out a lineament analysis of the anticline based on shuttle radar topographic mission (SRTM) images and digital elevation model (DEM) data to demonstrate the application of this technique for modelling of the fractured reservoirs. We used shaded relief images, variations in the vertical exaggeration, sun and azimuth angles to enhance the DEM data and structures in the area. Stereographic projections indicated that the anticline is an upright, asymmetrical, gentle, doubly plunging and east‐vergent fold. The synthesized lineament map of the fractures indicates a total of 402 fractures across the Zindapir anticline. A total of 204 and 198 fractures were observed in the eastern and western limbs, respectively. Statistical analysis portrayed that the lineaments of the majority of fractures followed northwest‐southeast (NW‐SE) and west‐northwest—east‐southeast (WNW‐ESE) trends. These are oblique or transverse to the fold axis and could be interpreted as shear and tensional fractures related to drag and compression along the western boundary of the Indian Plate. Our study provides an example of lineament analysis for structural characterization and modelling of fractured reservoirs in a collision‐mountain setting.

Structural geology of the Mount Meager Volcanic Complex, BC, Canada: implications for geothermal energy and geohazards

The Mount Meager Volcanic Complex ( Qwe̓ lqwe̓ lústen or Mt. Meager) coincides tectonically with the intra-arc to back-arc transition zone and exhibits the loci of strain partitioning in response to a rapid change in orientation of the Pemberton and Garibaldi Arc segments that are coeval with a shift in Pacific plate motion after 5 Ma. This strain partition is manifested through development of a transpressional deformation from 5 to 1.9 Ma at the latitude of Mt. Meager. Mt. Meager is an active volcanic system with at least two explosive eruptions in the last 25 000 years, the most recent occurring around 2360 BP. Additionally, it is the site of the largest landslide in Canadian history, which occurred during the summer of 2010, originating from the southeastern side the massif. During early exploration at Mt. Meager, geothermal boreholes drilled to 3 km reached 270 °C but did not find sufficient permeability to sustain self-flowing conditions. To understand the geological challenges in Mt. Meager’s geothermal exploration, we analyzed outcrop-scale faults and folds, incorporating structural mapping, volcanic rock paleomagnetism, and radiometric dating to establish kinematic history and kinematic compatibility of structural geology features, including faults and folds. Our findings suggest that stress partitioning during the last 5 Ma resulted in the formation of a transpressional structure exhibited as an elongate and rhomboidal structure at Mt. Meager with anomalously high topographic elevations which led to east northeast–west southwest crustal shortening and exhumation of crystalline basement. This new structural geology model improves our understanding of the geothermal reservoir and potentially significant geohazards.

An integrated approach to detect the structural setting of South Dyiur area, Central Western Desert, Egypt, via potential field data

The South Diyur exploration block of nearly 38,000 sq. km is located in the Farafra Oasis region in Egypt’s Western Desert. The North West-South East (NW-SE) trend is the regional structural surface features in the South Diyur District. The Bouguer gravity and magnetic data have been analyzed to provide new information about the tectonic setting and subsurface structures of the study area. The advanced filtering techniques were used to map structural as well as varying depths of the study area. The total intensity aeromagnetic map was first corrected by the application of the reduction to the pole technique (RTP). Then, the fast Fourier transform is applied on the gravity and magnetic maps to show the regional and residual sources and their depths, using the radial averaged power spectrum curve which divided into two segments; according to the slope, the maximum depth is 4,000 m that probably reflects the deep sources at this place. And the shallow sources (near-surface geological structures) showed depth of 600 m. The structure of the basement that influences the sedimentary cover thickness and the region’s hydrocarbon potentiality is interpreted as uplifts and basins resulted from parallel fault groups. It is found that there are strike slip faults trending NW-SE, regional faults divided the area into many blocks forming grabbens “Misawage graben and Dirrut basin” and horsts “Bahariya-Cairo uplift,” and the residual faults have many trends but the general trend from the rose diagram is west north west-east south east.

The active fault pattern based on morphotectonic and structural data in the south ofUlubat Lake and the Susurluk valley along the southern branch of North Anatolian FaultZone: A criticism of the bend model in northwest Anatolia

Field observations and morphotectonic evaluations on the south of Ulubat Lake and the Susurluk valley show that the dominant active fault pattern consists of northeast-southwest and east northeast-west southwest right-lateral faults and northwestsoutheast normal faults developing between these strike-slip faults. We redefined the active faults in the south of Ulubat Lake and discovered that these northeast-southwest right-lateral faults control the route of Susurluk River. Based on these discoveries and evaluations, we can claim that the "bend model" previously proposed for the region is not valid, as seen in the Mustafakemalpaşa Fault and Balıkesir-Kepsut Fault examples. Instead, we suggest that the faults belonging to the southern branch of the North Anatolian Fault Zone extend to Akhisar and İzmir via Bursa, Susurluk, and Balıkesir.

Halokinetic analysis of the Frade field area, Campos Basin, Brazil: Salt tectonics within an offshore strike-slip setting

The Frade field, located within the Campos Basin in the southeastern Brazilian margin, is a key oil field that produces from Oligo-Miocene turbidite reservoirs that derived their structural positioning due to the presence of an underlying salt diapir. The evolution of the Frade salt structure was examined using well data, selected 2D lines, and a 3D volume that were interpreted in detail focusing on the Aptian evaporite interval and its influence on the overburden. Analysis of the salt-sediment interaction indicated a complex deformation history that included five main stages of deformation, some assisted by tectonic reactivation episodes. (1) Post-Albian reactivation of a nearby north–northwest-south–southeast basement fault caused the Albian carbonate interval to fault, forming a west–northwest-east–southeast shear zone with a dextral strike-slip component. This movement initiated thin-skinned tectonics that offset the Albian carbonates and formed a pull-apart basin that accommodated a thick Late Cretaceous interval, which weakened the overburden and allowed for the initial formation of the Frade salt diapir. (2) Renewed diapir growth thickened and redistributed the Cenomanian-Maastrichtian sedimentary package proximal to the Frade salt anticline. (3) An initial and localized collapse of the Frade salt anticline occurred during early Paleogene extension. (4) Paleogene shortening caused the salt to flow, resulting in salt withdrawal in the southeast and diapir rejuvenation near its present-day apex, forming several inversion structures. In addition, the Paleogene shortening resulted in a low-relief anticlinal structure that rotated the turbidites into geometries favoring hydrocarbon accumulation. (5) A return to an extensional regime occurred during the late Oligocene/early Miocene. The results of this study provide a new insight into the development of strike-slip salt tectonic structures and show for the first time within the Campos Basin an Albian-level pull-apart basin that formed in association with salt tectonics.

Post-orogenic structural style and reactivation in the northern Bowen Basin, eastern Australia

The northern Bowen Basin in eastern Australia was affected by the middle Permian–Late Triassic Hunter--Bowen Orogeny, but the effect of the post-orogenic deformation events is less well understood. 2D seismic surveys and field observations were utilised to the east of Moranbah to understand structural style and reactivation in the northern Bowen Basin. Large-scale faults include the Daunia and New Chum thrusts, and the Daunia Graben bounding faults. These faults displace the upper Permian–Triassic lithological units with throws larger than 30 m. Large-scale thrusts are parts of the Late Triassic north-northwest-striking Jellinbah Thrust Zone in the northern Bowen Basin. The Daunia Graben comprises normal faults, parallel to the thrusts, with a maximum 80 m throw. Small-scale faults affect one or two lithological units with throws less than 30 m. Small-scale reverse faults mainly strike north-northwest, whereas small-scale normal faults strike northeast–southwest, east-northeast–west-southwest and north-northeast–south-southeast. The Daunia Graben and some small-scale normal faults developed during a regional extensional regime either in the Late Triassic or in the mid-Cretaceous to early Cenozoic. A series of small-scale faults probably developed during the Hunter--Bowen Orogeny as subsidiary structures associated with the Jellinbah Thrust Zone, whereas some small-scale reverse faults propagated during the Late Jurassic–Early Cretaceous and middle–late Cenozoic. Post-orogenic reactivation phases affected east of Moranbah through: (1) the development of the Daunia Graben within the hanging wall of the New Chum Thrusts, possibly owing to the negative inversion of pre-existing structures; (2) its subsequent partial inversion associated with small-scale reverse faults; and (3) the reactivation of small-scale normal and strike-slip faults. The driving mechanisms of these reactivation phases are related to far-field stress fields associated with episodic Late Triassic to late Cenozoic extensional and contractional regimes at Australian Plate boundaries. In contrast to the middle Permian–Late Triassic Hunter--Bowen Orogeny phases, post-orogenic deformation events in the Bowen Basin were not robust enough to produce a large number of large-scale faults. This probably reflects the retreat of the active subduction zone from eastern Australia since the Late Triassic. KEY POINTS Large-scale faults in the northern Bowen Basin affected upper Permian–Triassic units with throws >30 m, whereas small-scale faults displaced one or two lithological units with throws <30 m. Some large-scale and small-scale faults are part of the Late Triassic north-northwest-striking Jellinbah Thrust Zone, which deformed the northern Bowen Basin in response to the Hunter--Bowen Orogeny. Some large-scale and small-scale reverse and normal faults developed during post-orogenic reactivation phases from the Late Triassic to late Cenozoic.

Precambrian basement reservoirs: Case study from the northern Bongor Basin, the Republic of Chad

Bongor Basin in the south of Chad is a Cretaceous–Paleogene rift basin developed on Precambrian crystalline basement and is part of the central African rift system (CARS). High oil flows have been tested from five basement buried hills, indicating that the Precambrian basement is a good reservoir for hydrocarbon accumulation. Core analyses from 10 wells indicate that the basement rocks are composed of plutonic rocks (including granite, syenite, monzonite, quartz monzonite, and diorite) and orthometamorphites (including migmatitic granite, migmatitic gneiss, and granulite). Structural mapping of the top basement shows that faults in basement have a principal strike orientation of west–northwest-east–southeast and a secondary orientation of northeast–southwest. A combination of mud losses, well testing results, cores, wire-line logs, Formation MicroImager, sonic scanner logs, and three-dimensional seismic data were used to identify and detect the basement reservoirs. Two main sets of high-angle fractures, striking west–northwest-east–southeast and west–southwest-east–northeast, were identified. Three facies are divided based on the degree of weathering, intensity of fracturing, and sonic anisotropy. The basement can be divided vertically into four zones: weathered and leached, fractured, semifilled fractured, and tight zones. The weathered and leached and fractured zones are the main basement reservoirs in the study area. The effective porosities range from 3% to 10%, in places up to 25%. The porosity and permeability of basement reservoirs depend on the structural location and hydrothermal and authigenic precipitates developed during and after the basin formation. The discovery of basement reservoirs in the Bongor Basin will aid the hydrocarbon exploration of a new play in the CARS.

Characteristic and Mixing Mechanisms of Thermal Fluid at the Tampomas Volcano, West Java, Using Hydrogeochemistry, Stable Isotope and 222Rn Analyses

The Tampomas Volcano is a Quaternary volcano located on Java Island and controlled by a west-northwest–east-southeast (WNW-ESE) regional fault trend. This regional structure acts as conduits for the hydrothermal fluids to ascend from a deeper system toward the surface and, in the end, mix with groundwater. In this research, water geochemistry, gas chemistry and isotopes 2H, 18O and 13C were used to explore the subsurface fluid characteristics and mixing mechanisms of the hydrothermal fluids with groundwater. In addition to those geochemical methods, soil-gas and dissolved 222Rn observations were performed to understand the geological control of fluid chemistry. Based on the analytical results, the hydrothermal system of Tampomas is only developed at the northeastern flank of the volcano, which is mainly controlled by NE-SW structures as deep fluid conduits, while the Cimalaka Caldera Rim around Sekarwangi act as the boundary flow of the system. This system is also categorized as an “intermediate temperature system” wherein fluid is derived from the interaction between the volcanic host-rock at 170 ± 10 °C mixed with trace organic gas input from sedimentary formation; afterwards, the fluid flows laterally and is diluted with groundwater near the surface. Soil-gas and dissolved 222Rn confirm that these permeable zones are effective conduits for the ascending thermal fluids. It is found that NE faults carry higher trace elements from the deeper system, while the circular feature of the Caldera Rim acts as the boundary of the hydrothermal system.

Origin, evolution and sedimentary processes associated with a late Miocene submarine landslide, southeast Spain

A submarine landslide, the Alhama de Almería Slide, influenced late Tortonian and early Messinian (late Miocene) sedimentary processes in the vicinity of Alhama de Almería in southeast Spain. Its 220-m-high headscarp and deposits are now subaerially exposed. The landslide occurred at the northern slope of the antecedent relief of the present-day Sierra de Gádor mountain range. This is a large antiform trending east–west to east-northeast–west-southwest, which has been uplifting since the late Miocene due to convergence of the African and Eurasian plates. During the Tortonian, this relief was an island separated from the Iberian Peninsula mainland by the Alpujarra corridor, a small and narrow intermontane basin of the Betic Cordillera in the western Mediterranean Sea. The materials involved in the slope failure were Triassic dolostones and phyllites from the metamorphic Alpujárride Complex and Tortonian marine conglomerates, sandstones, and marls that formed an initial sedimentary cover on the basement rocks. Coherent large masses of metamorphic rocks and Miocene deposits at the base of the headscarp distally change to chaotic deposits of blocks of different lithologies embedded in upper Tortonian marine marls, and high-strength cohesive debrites. During downslope sliding, coherent carbonate blocks brecciated due to their greater strength. Phyllites disintegrated, forming a cohesive matrix that engulfed and/or sustained the carbonate blocks. Resedimented, channelized breccias were formed by continuing clast collision, bed fragmentation, and disaggregation of the failed mass. The conditions leading to rock/sediment failure were favoured by steep slopes and weak planes at the contact between the basement carbonates and phyllites. Displacement of collapsed rocks created a canyon-like depression at the southeast edge of the landslide. This depression funnelled sediment gravity flows that were generated upslope, promoting local thick accumulations of sediments during the latest Tortonian-earliest Messinian. The insights from this exposed outcrop have implications for understanding the mechanisms and products of mass-transport deposits on the modern seafloor and the recognition of past failures from subsurface records.

Characterization of Basement Fracture Reservoir In Field ‘X’, South Sumatera Basin, Based On The Analysis of Core And FMI Log

Basement reservoir is a reservoir that is located in the basement rock, comprised of either igneous rock or metamorphic rock that has secondary porosity, resulting in its capability to store oil and gas. The research was conducted at field 'X' that is located at South Sumatra basin and it is a part of Jambi Sub-Basin. The study was focused on discussing hydrocarbon potential in Fields 'X', particularly at the basement metamorphic rock. The study was conducted at two wells in the field. The secondary porosity system of the basement is fracture porosity. Fracture analysis as secondary porosity system was performed on two wells, HA-1 and HA-2, by using FMI log interpretation. Based on the analysis of fracture on HA-1 well, the trend of fracture system is Northeast - Southwest (NE-SW) with a fracture porosity of 1.49%. On a different note, the trend of fracture system on HA-2 wells is East Northeast - West Southwest (ENE-WSW) with a fracture porosity of 0.888%. The effect of rock properties itself has little influence on the number of fractures as opposed to the effect of surrounding tectonic forces. The fractures are controlled by geological structures following Jambi pattern that has an orientation of Northeast - Southwest (NE-SW). Although the fracture porosity is relatively small, it is enough to storing hydrocarbons in economical quantity.

Demarcating saline water intrusion pathways using remote sensing, GIS and geophysical techniques in structurally controlled coastal aquifers in Southern India

Application of remote sensing and geographical information system (GIS) with the help of ground truth studies like vertical electrical soundings (VES) and water quality measurements gave a comprehensive picture on saline water intrusion pathways in the structural controlled aquifers of Uppanar and Cauvery River systems located in Sirkazhi and Mannanpandal coastal areas of Southern India. The detailed geomorphic units and lineaments were mapped using IRS LISS-III (1:50,000 Scale) and SRTM Elevation Digital data under GIS environment. The area is mainly alluvial deposits of Holocene Age and classified into three major geomorphic units (i.e., fluvial, fluviomarine, and marine geomorphic units). The identified lineaments are mostly trending North-northeast (NNE)–South-southwest (SSW), Northeast (NE)–Southwest (SW), Northwest (NW)–Southeast (SE), East–West (EW) and East-Northeast (ENE)–West-Southwest (WSW) directions. Some of them are potential for fresh groundwater occurrence and movement. The lineaments trending NE–SW, NNE–SSW, and ENE–WSW act as pathways for saline water intrusion to the inland. The lineament L1 (NNE–SSW trending) conformed as a sinisterly shifted fault played a significant role in saline water intrusion. The area within the parallel lineaments L2 and L3 oriented ENE–WSW direction indicates low topography, beach ridges, paleo-channel sinuosity, large aerial extent of tidal flats, and backwater inundation. All the above geomorphic units gave indirect evidence of land subsidence, which was confirmed with the help of sub-surface geo-electrical signals obtained through VES and in situ groundwater quality measurements. Integrating all the above results, the saline water spread and fresh water pockets were identified.

Pliocene–Quaternary tectonic evolution of the Gulf of Gökova, southwest Turkey

Evolution of the east–west-trending Gökova Graben structure is related to the north–south extension of the Aegean segment of the Aegean–Anatolian Microplate. The Pliocene–Quaternary successions surrounding the onland portion of the Gökova Graben as well as coeval successions within the marine portion of the graben are cut by at least three families of faults that strike northwest–southeast, east–west and east-northeast–west-southwest. These orientations are inconsistent with a simple north–south extensional regime. Interpretation of seismic reflection profiles, multibeam bathymetry and GPS vectors indicates that the Gökova Graben developed as a lazy-S-shaped graben in the back-arc setting north of the Hellenic Arc. A counterclockwise rotation of the Aegean segment of the Aegean–Anatolian Microplate is the suggested mechanism for this geometry, as subduction zone rolled back occurred during the Pliocene–Quaternary. The Gulf of Gökova is the youngest of a series of basins that developed within this large back-arc system, including the Nisyros, Karpathos, and Kamilonisi basins, collectively named to as the Gökova–Nisyros–Karpathos Graben. It is proposed that this graben experienced a scissor-like opening initiating from the west during the Pliocene and progressing eastward during the Quaternary. Faults that are seemingly un-correlated onland and in the marine areas become remarkably aligned when the Marmaris-Rhodes Block is progressively rotated by 6° counterclockwise relative to the Muğla Block using a pole position located within the Gulf of Gökova. The scissor-like opening of the westernmost regions of the Gökova Graben likely occurred during the late Pliocene, whereas the central and eastern portions of the graben developed during the early–middle Pleistocene and late Pleistocene–Holocene, respectively. The onset of the opening of the westernmost segment of the Gökova–Nisyros–Karpathos Graben occurred at a time earlier than the late Pliocene.

Pull-apart development at overlapping fault tips: Oblique rifting of a Cenozoic continental margin, northern Mergui Basin, Andaman Sea

The northern Mergui Basin (Andaman Sea) contains east-northeast–west-southwest– to northeast-southwest–striking normal fault–bound basins, and north-northwest–south-southeast–trending strike-slip faults. The two largest strike-slip faults (Manora and Mergui) pass into extensional or transtensional basins at their tips, consistent with dextral offset. The faults provide examples of early stage pull-apart basin development at fault tips instead of the more common model for development at releasing bends. Offset of isochron markers for the Ranong Formation indicate that ∼8 km of dextral offset has occurred along the Mergui fault and 4.5 km of dextral offset has occurred on the Manora fault. The strike-slip faults and associated extensional faults formed relatively late for the history of the entire Mergui Basin during the Early Miocene. The northern part of the Mergui Basin developed after a phase of west-northwest-east-southeast extension during the Oligocene in the Mergui Basin to the south, indicating a rotation in the extension direction toward the north-northwest–south-southeast with time. The basin is part of a major transtensional system involving the Sumatra, West Andaman, and Sagaing faults that accommodated the northern motion of western Myanmar as India moved north relative to Southeast Asia. Fault activity in the northern Mergui Basin decreased significantly when the broad zone of Early Miocene transtension became focused on the Alcock and Sewell Rises during the Middle Miocene, and the West Andaman and Sagaing faults began to develop and interacted in a large pull-apart geometry with the Shan Scarp Fault, and later (Late Miocene or Pliocene) with the Sagaing Fault.

The Use of Microseismicity To Understand Subsurface-Fracture Systems and To Increase the Effectiveness of Completions: Eagle Ford Shale, Texas

SummaryExisting natural fractures often have a significant impact on both the stimulation and the production of oil and gas wells. The effective exploitation of unconventional reservoirs requires understanding of the local tectonic history and the present-day stress regime. Signal strength; high-quality reflection seismic, microseismic imaging; and the moderate structural complexity of the liquids-rich gas and tight oil Eagle Ford Shale make it an excellent place to study hydraulic fracturing in tight rocks. Microseismic-monitoring results showed clear structural trends relating to the reactivation of existing faults and fractures, and rock-failure mechanisms determined through source mechanism inversion of events. These results provided critical information to the operator for optimizing the hydraulic-fracture design.Microseismic data collected by use of a surface array allowed the full geometry of the result to be viewed with no directional bias. The geometry of the microseismicity trends related to fracturing developed during the stimulation treatment was representative of the true geometry of the structure. The large aperture and wide azimuth of the monitoring array facilitated the determination of source mechanisms from every event detected, which provided full coverage of the focal sphere of each source mechanism. The events identified two different source mechanisms, indicating a failure mechanism for fractures that is different from that for reactivated faults.Microseismicity with a northeast/southwest (NE/SW) orientation is interpreted to be related to either induced or reactivated faults or fractures. Microseismicity also formed trends that are contiguous across more than one wellbore in an east-northeast/ west-southwest (ENE/WSW) direction. These trends are interpreted to have formed as a result of fault reactivation. Source mechanisms from fracturing parallel to SHmax have failure planes that strike NE/SW with normal dip-slip failure on steeply dipping planes. Those from fault reactivation have strike-slip failure on ENE/WSW-striking failure planes. The NE/SW-striking, dip-slip fractures are parallel to extensional Gulf of Mexico (GOM) growth faulting, and the ENE/WSW-striking, strike-slip faults are at an angle of approximately 25° to the dominant fracturing trends.Microseismicity trends associated with faults are used to project where faults will intersect adjacent wells. The identification of these faults in the reservoir by means of microseismic mapping allows operators to modify their treatment parameters and stage spacing to avoid geologic hazards. The operator combines the treatment-pump parameters for the wells with the additional structural understanding gained from the analysis of fracture trends and source mechanisms to identify zones that should be avoided in subsequent treatments. In addition, the mapped microseismicity provides critical information that was used to modify well spacing for subsequent wells, thereby optimizing the completion plan and dramatically cutting costs.

Architecture and geodynamic evolution of the St Ives Goldfield, eastern Yilgarn Craton, Western Australia

A new structural evolution consisting of both extensional and contractional events has been defined for the St Ives Goldfield in the south-central Kalgoorlie Terrane of the eastern Yilgarn Craton in Western Australia. These events shaped the development of the fault architecture, which controlled the location of the regional anticlines, the magmatic centres, and the deposition of the Archaean greenstone successions. The fundamental grain of the St Ives Goldfield is north-northwest-trending. This trend is marked by faults which developed during D 1 extension, which was oriented east-northeast–west-southwest. Across these faults we map major stratigraphic changes in the thickness and composition of units, especially of the previously undivided Black Flag Group volcaniclastic rocks. The centre of the St Ives Goldfield is dominated by the Kambalda Anticline. This north-northwest-trending regional fold was likely established early during the D 1 extensional history, and was fully established during subsequent east-northeast-oriented D 2 contraction. The regional anticline is an important architectural element because (1) magmatism and gold mineralising fluids were focussed into this domed region, and (2) deformation was partitioned across the limbs and crest of this structure. The D 3 event involved regional uplift and extension, resulting in the formation of late basins (Merougil Conglomerate locally) and the emplacement of granitoids sourced from a metasomatised mantle wedge (Mafic-type porphyries). The most significant gold event in terms of endowment occurred during D 4b sinistral strike-slip shearing and associated thrusting (e.g., Tramways and Republican thrusts). These thrusts were previously interpreted as the first contractional structures to deform the area (‘D 1’), but are here reinterpreted as relatively late (D 4b). In this D 4b event, the north-northwest-trending faults underwent sinistral strike-slip shearing and were linked across the Kambalda Anticline by accommodation structures represented by generally east- to east-northeast-trending thrusts. Reactivation of D 1 transfer structures may have influenced the location of these later accommodation structures. Late-stage mineralisation during D 5 was the result of dextral strike-slip brittle shearing.

Deep structural control on distribution of raised beaches and Quaternary sedimentation in Brittany? A review

This paper is a review of the possible influences of deep structures on Quaternary sedimentation and the distribution of raised beaches, a problem which has never been raised before in any paper. The distribution of the Quaternary outcrops is very patchy in Brittany and seems to be organized in a random way, partly because the control by the short wavelength basement structures is not easy to recognize but also because the most efficient control is associated with long wavelength structures which are far beyond the scale of the usual geological maps and the immediate observations. Gravity data and terrain-levelling give far better results than the other geophysical techniques when the Moho undulations are compared with the surficial data. The two main domains involved in the long wavelength deformations are (a) the Alpine domain where all the Moho undulations are concentric with the Alps; and (b) the Armorican domain in which all the undulations are west–northwest–east–southeast oriented, parallel with the axis of the initial opening of the Bay of Biscay. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from compression (buckling). These associated with the Armorican Domain result from extension (boudinage). Both phenomena show a typical wavelength of 200–250 km, which is in agreement with the results of the physical and numerical modeling currently available. These two types of deformations are crosscutting in Brittany. The general uplift of Western and Central Brittany is a well-controlled geological phenomenon. This is shown by the incision of the rivers which increases towards the west, suggesting larger uplift in the west than in the east. This uplift has been also demonstrated by two different terrain-levelling techniques. The distribution of the Pleistocene/Holocene sediments is compared with the deep structures. The westernmost Alpine undulation controlled the north–south deposition of the recent loess and swamps in eastern Brittany. The Biscay Bay related structure controlled the deposition of loess along the northern coast of Brittany, and also the location of the Pleistocene raised beaches. The gradient in beach age results from the preferential uplift of the westernmost part of Brittany.

Extensional fault segmentation and linkages, Bonaparte Basin, outer North West Shelf, Australia

A detailed structural analysis of the Zone of Cooperation A-2 (ZOCA-2) three-dimensional (3-D) seismic survey on the outer northwestern edge of the Sahul Platform, northern Bonaparte Basin, outer North West Shelf, Australia, has identified three major populations of extensional faults. From oldest to youngest, these are (1) Jurassic north-south–trending extensional synrift faults, (2) Jurassic–Cretaceous east-west– to east-northeast–west-southwest–trending extensional faults, and (3) Neogene to present-day northeast-southwest–striking, right-stepping en-echelon faults. Seismic attribute analyses combined with fault-displacement analyses have illustrated the initial segmentation of all of the fault systems and characterized both the horizontal and vertical linkages formed by soft-linkage relay structures. The strongly segmented Neogene to present-day northeast-southwest–striking faults are interpreted to be a result of Neogene reactivation of underlying zones of weakness generated by preexisting east-northeast–trending Jurassic to Early Cretaceous faults. The Neogene to present-day northwest-southeast–directed extension was oblique to the underlying zones of weakness and thus formed characteristic strongly segmented en-echelon fault arrays. The Jurassic to Early Cretaceous extensional faults together with the overlying northeast-southwest–striking Neogene to present-day fault systems form a pseudoconjugate fault system separated by a Cretaceous sequence that acted as a decoupling horizon. Within this Cretaceous interval, which is characterized by polygonal fault systems, vertically segmented, isolated, and overlapping extensional fault arrays form a zone of soft linkage between the underlying Jurassic to Early Cretaceous rift faults and the overlying Neogene to present-day fault systems. This study shows that extension oblique to preexisting deeper fault systems produces en-echelon segmented extensional faults in the overlying sequences. Such en-echelon segmentation does not indicate major strike-slip deformation. The results of this research have implications for understanding the distribution, segmentation, linkages, and ages of extensional faults in many other rift basins as well as for the northern Bonaparte Basin. In addition, the complex 3-D linkage patterns shown in this study have significant implications for understanding trap geometries and fault-seal characteristics in other extensionally faulted basins.

Stratigraphic controls on production from a basin-centered gas system: Lower Cretaceous Cadotte Member, Deep Basin, Alberta, Canada

In this article, we examine the controls on gas production from Cretaceous clastic strand-plain deposits in an area (the Deep Basin of Alberta) that was instrumental in the development of the basin-centered gas concept. By integrating core, wireline logs, three-dimensional seismic data, petrophysical measurements, pressure, and production data, we show that the best gas production comes from stratigraphic sweet spots consisting of chert-rich shoreface and beachface deposits that alternate between tight quartzose sandstone. The physical properties of the chert-rich and quartzose units are sufficiently different that they can be distinguished seismically. Seismic modeling and observations show that high-amplitude areas correspond to porous (and productive) chert-rich shoreface and beachface deposits. A map of seismic amplitudes for the Cadotte Member shows narrow (∼0.5 km [0.3 mi] width), curvilinear, high-amplitude anomalies (corresponding to cherty shoreface and beachface deposits) that are parallel to depositional strike (east-northeast–west-southwest) and correlate precisely with production trends. Wells drilled outside the high-amplitude anomalies do not have sufficient permeability to produce gas at economic rates. The shore-parallel stratigraphic sweet spots are locally compartmentalized by shale-filled channels that cut at high angles through the strand plain. Like other Cretaceous units in this area, the Cadotte Member is wet structurally updip from gas-charged rocks. The contact between these two phases traversing through our study area and its location is controlled by the interplay of regional structural dip and depositional processes that controlled the lithology and stratigraphic architecture of the rocks.

New structural model to explain geophysical features in northwestern New South Wales: implications for the tectonic framework of the Tasmanides

Paleozoic and older basement rocks in northwestern New South Wales and southern and central Queensland are largely obscured by sedimentary rocks of the Mesozoic Eromanga Basin and younger Cenozoic sediments. Interpretations of the basement geology for that area have been heavily reliant upon interpretations of the regional aeromagnetic and gravity data. Geophysically, northwestern New South Wales is characterised by an east–west-trending belt of magnetic features which trend northwest in the western part and northeast to east-northeast in the eastern part; and by an east–west-trending gravity ridge. These east–west-trending features have previously been interpreted to represent the southern part of the Thomson Orogen, a separate tectonic domain to the Lachlan Orogen to the south. However, apart from these east–west geophysical trends there is sufficient geophysical and lithological evidence to suggest continuity of the Lachlan Orogen into southern and central Queensland. A new model is herein proposed whereby the geophysical features are attributed to structures resulting from crustal thickening of a Cambro-Ordovician backarc package, west of a volcanic arc, which occurred during the Late Ordovician to Early Silurian Benambran Orogeny. While the principal stress direction was oriented east-northeast–west-southwest, the structures formed via a process of tectonic escape or extrusion due to differential strain caused by the shape of the Precambrian margin in western New South Wales. The model is consistent with field observations, geophysical imagery, borehole data and seismic data. Analogue models using sand and gelatine were constructed to illustrate the concept and demonstrate that it is feasible. The model permits the extension of the Delamerian and Lachlan Orogens into southern and central Queensland to join with similar rock packages in northern Queensland, consistent with some previous tectonic models for the Tasmanides. This brings into question the need to delineate a separate Thomson Orogen.

Late Paleo–Mesoproterozoic plate margin deformation in the southern Gawler Craton: Insights from structural and aeromagnetic analysis

The integration of structural analysis of outcrop and aeromagnetic interpretation has revealed a complex tectonic evolution for the southern Gawler Craton that reflects the regional tectonic signal during the late Paleo- to early Mesoproterozoic. Results suggest that exposures at Yarlbrinda Hill comprise the margin of a highly deformed St Peter Suite pluton emplaced within the southwestern Gawler Craton at ca. 1611 Ma. D 1 at outcrop records east- to northeast-directed ductile thrusting. Regionally, this deformation overprints north–south trending dextral Yarlbrinda Shear Zone fabrics within ca. 1622 Ma St Peter Suite granite and ca. 1690–1670 Ma Tunkillia Suite granite. Progressive east to northeast directed thrust movements led to exhumation of the deformed ca. 1611 Ma granite. D 2 deformation is recorded by the brittle reactivation of first generation mylonitic fabrics, accompanied by cataclasis, alteration and pseudotachylite development. D 3 records a switch to ∼northwest–southeast shortening, which led to the folding and tear faulting of the D 1–2 shear zone. A switch to extension and subsequent rotations of the principle stress axes led to Hiltaba Suite granite emplacement and contemporaneous faulting under ∼east–west extension during D 4 and west–northwest–east–southeast extension during D 5. Deformation of ca. 1622 Ma St Peter Suite granite is consistent with movement on the Yarlbrinda Shear Zone and requires the St Peter Suite to have been emplaced into or adjacent to the ca. 1690–1670 Ma Tunkillia Suite prior to shear zone activity and on the overriding plate during subduction. D 1–D 3 deformation is constrained by the emplacement of ca. 1580 Ma Hiltaba Suite granite and correlates with the Wartakan Orogeny in the southeastern Gawler Craton and the Olarian Orogeny in the Curnamona Province. A ∼52° disparity between the shortening directions in these respective areas suggests different degrees of Mesoproterozoic rotation, which likely aided the formation of an intervening basin province, the remnants of which are presently preserved in the Mount Painter Inlier. The record of convergent deformation is geochronologically constrained to a ∼30 Myr period and places this region of the central Gawler Craton near a late Paleoproterozoic convergent plate margin prior to a period of transient extension beginning at ca. 1580 Ma. The continental scale signal of tectonic events can be recognized at the outcrop scale highlighting the importance of detailed outcrop studies in sparsely exposed areas.