Low-angle Faults Research Articles

Remembering First Oil in Nevada

In June 1954 Nevada became the twenty-ninth oil-producing state in the United States (Picard 1955). Interestingly, production was from volcanic rocks from the open-hole interval 6,450 to 6,730 ft (1,966 to 2,051 m) in the Oligocene Garrett Ranch volcanics, an unexpected reservoir in the kind of rocks rarely productive anywhere in the world. The pour-point (65-80° F) and gravity (26-29° API) of the crude were high, similar to oils found in the Eocene Green River Formation of the Uinta Basin, northeast Utah. Cumulative production in the field through September 1978 was 3.3 million barrels of oil. An early estimate of ultimate primary reserves was four million barrels of oil (Bortz and Murray, 1979). The trap is a faulted truncated wedge of Oligocene and Cretaceous-Eocene rocks with a top seal of impermeable valley fill, a bottom seal of Paleozoic rocks, and an east-side seal formed by a basin boundary fault and impermeable Paleozoic rocks. The new field in Railroad Valley of east-central Nevada, finally totaling fourteen producing wells, was called Eagle Springs after the locality and the name of the discovery well drilled by the Shell Oil Company. Twenty-two years after the Eagle Springs discovery a larger oil field, Trap Spring, was discovered by Northwest Exploration Company less than ten miles west of Eagle Springs, in Tertiary ash-flow tuffs. Two hundred dry holes had been drilled in Nevada between the two discoveries. In 1982, six years after the Trap Spring discovery, Amoco Production Company drilled the first well outside of Railroad Valley at Blackburn field on the east side of Pine Valley in Eureka County. Blackburn, a structural trap above a Tertiary low-angle extensional fault, produces from Devonian reservoirs. In 1983, Northwest Production brought in the Grant Canyon field about 10 mi (6 km) south of Eagle Springs. The oil reservoir of Devonian carbonates there is entrapped in a ‘buried-hill’. The discovery in 2004 of the Covenant field in Central Utah, because of similarities to large oil fields in the thrust belt of Wyoming and Utah and some resemblance to the Nevada fields of the Great Basin, ignited a frenzy of leasing which still goes on when land is available. Located along the thrust-belt (hingeline), Covenant produces oil from the Jurassic Navajo Sandstone that apparently originated in the Paleozoic.

Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the “rolling-hinge” model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.

Thrust faults and extensional detachment faults in Cretan tectono-stratigraphy: Implications for Middle Miocene extension

The revised tectono-stratigraphy of Crete and especially of the “Phyllites–Quartzites” complex demonstrated the distinction of the probable Paleozoic low-medium grade metamorphic rocks of the Arna unit from the underlying Permo–Triassic phyllites and associated carbonate sediments (Trypali facies) of Western Crete unit as well as the overlying Permo–Triassic phyllites and associated sediments of the Tyros/Ravdoucha Beds at the base of the Tripolis unit. The pre-existing mixture of the above tectono-stratigaphic units in a single complex created a number of misinterpretations as far as stratigraphy, metamorphism and interpretation of low angle faults as thrusts or detachments. Especially in cases where the inferred tectonic contact concerns the transition between the Tyros Beds and the base of the Tripolis carbonate platform there is no structural omission and therefore the contact represents a minor disharmonic sliding surface and not a detachment. Based on the revised tectono-stratigraphic analysis the determination of the structural omission for each tectonic contact was possible and several detachments were described for the first time. Footwall rocks of the detachments comprised several tectonic units usually from the lower nappes and hanging wall rocks comprised several tectonic units usually from the upper nappes. The detachment may separate not only metamorphosed units in the footwall (Mani, Western Crete, Arna) from non metamorphosed units in the hanging wall (Tripolis, Pindos and higher nappes) but also all other possible combinations from the Cretan nappe pile. Extension in Crete started in the Middle–Late Miocene with the formation of extensional detachment faults. The reported extensional structures of Oligocene to Early Miocene age do not correspond to crustal extension of Crete but to localized shear zones related to nappe stacking and the exhumation of metamorphic rocks. Extensional detachments in Crete form a tectonic horst through two oppositely dipping E–W-trending zones; one dipping north, related to the opening of the Cretan basin, and the other dipping south, related to the formation of the Messara supra-detachment basin. The deformation history of units within Crete can be summarized as: (i) compressional deformation producing arc-parallel east–west-trending south-directed thrust faults in Oligocene to Early Miocene time; (ii) extensional deformation along arc-parallel, east–west-trending detachment faults in Middle Miocene time, with hanging wall motion to the north and south; and (iii) Late Miocene–Quaternary transtensional deformation along high-angle normal and oblique normal faults that disrupt the older arc-parallel structures.

Subsurface fault geometries and crustal extension in the eastern Basin and Range Province, western U.S.

We provide the first synthesis of seismic reflection data and active present-day crustal deformation for the greater Wasatch fault zone. We analyzed a number of previously unpublished seismic reflection lines, horizontal and vertical crustal velocities from continuous GPS, and surface geology to investigate the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults across the eastern third of the northern Basin and Range Province. The seismic reflection data show recent activity along high-angle normal faults that become listric with depth and appear to sole into preexisting décollements, possibly reactivating them. We interpret these listric normal faults as reactivated Sevier-age structures that are connected at depth with a regionally extensive detachment horizon. These observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension. We modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space. The estimated model results include a low-angle dislocation (~ 8–20°) at a locking depth of ~ 7–10 km and slipping at 3.2 ± 0.2 mm/yr. Despite the model's relative simplicity, we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data, and suggests an active regionally extensive sub-horizontal surface in the eastern Basin and Range. This result may imply that this surface represents aseismic creep across a reactivated low-angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle–ductile transition zone under the present-day Basin and Range extensional regime. This result may also have implications for crustal rheology, and suggests that geodesy might, under some circumstances, serve as an appropriate tool for inferring deeper crustal structure.

Green Lake Landslide and other giant and very large postglacial landslides in Fiordland, New Zealand

Green Lake Landslide is an ancient giant rock slide in gneiss and granodiorite located in the deeply glaciated Fiordland region of New Zealand. The landslide covers an area of 45 km 2 and has a volume of about 27 km 3. It is believed to be New Zealand's largest landslide, and possibly the largest landslide of its type on Earth. It is one of 39 known very large (10 6–10 7 m 3) and giant (≥10 8 m 3) postglacial landslides in Fiordland discussed in the paper. Green Lake Landslide resulted in the collapse of a 9 km segment of the southern Hunter Mountains. Slide debris moved up to 2.5 km laterally and 700 m vertically, and formed a landslide dam about 800 m high, impounding a lake about 11 km long that was eventually infilled with sediments. Geomorphic evidence supported by radiocarbon dating indicates that Green Lake Landslide probably occurred 12 000–13 000 years ago, near the end of the last (Otira) glaciation. The landslide is described, and its geomorphic significance, age, failure mechanism, cause, and relevance in the region are discussed, in relation to other large landslides and recent earthquake-induced landslides in Fiordland. The slope failure occurred on a low-angle fault zone undercut by glacial erosion, and was probably triggered by strong shaking (MM IX–X) associated with a large (≥ M 7.5–8) earthquake, on the Alpine Fault c. 80 km to the northwest. Geology was a major factor that controlled the style and size of Green Lake landslide, and in that respect it is significantly different from most other gigantic landslides. Future large earthquakes on the Alpine Fault in Fiordland are likely to trigger more very large and giant landslides across the region, causing ground damage and devastation on a scale that has not occurred during the last 160 years, with potentially disastrous effects on towns, tourist centres, roads, and infrastructure. The probability of such an event occurring within the next 50 years may be as high as 45%.

Moho, crustal architecture and deep deformation under the North Marmara Trough, from the SEISMARMARA Leg 1 offshore–onshore reflection–refraction survey

Understanding further the nature and evolution at lithospheric scale of the Sea of Marmara on the North Anatolian Fault needs constraints on the deep crustal and Moho spatial variation. This has been probed here with offshore–onshore and OBS, Ocean Bottom Seismometer refraction seismics, in addition to coincident MCS, marine multichannel reflection seismic profiles over the whole North Marmara Trough. The diverse strikes of MCS profiles in a dense grid allow to avoid misinterpretation of late echoes in the deep basin as Moho reflections and attribute them to sidesweeps. Moho is instead positively identified from reversed observations of first-arrival head and refracted waves at the top of the mantle obtained at large offset by land stations. A significant and sharp reduction in its depth, on the order of 5 km occurs beneath both the eastern and western rims of the North Marmara Trough, with a more progressive crustal thinning from the south. The wide-angle reflections on OBS and land stations document in addition to Moho the top of a lower crustal reflective layer, which is also sampled by MCS profiles, and appears to follow Moho topography. The dense grid of MCS profiles along the southwestern margin of the North Marmara Trough reveals a dipping reflector through the upper crust with tilted basement blocks on top. This low-angle fault is suggested as a normal sense detachment extending in depth towards the reflective lower crust. The upwarp of the Moho and lower crustal layer towards the North Marmara Trough suggests that crustal thinning occurs mostly in the upper crustal part, with lateral transport of the material towards WSW in the footwall of the detachment, and possibly other features to the south, in the motion of Anatolia with respect to stable Eurasia oblique to the North Marmara Trough. Thinning can be accommodated in an asymmetric partitioning of the displacement on several branching faults at lithospheric scale.

Recognizing the seismic cycle along ancient faults: CO 2-induced fluidization of breccias in the footwall of a sealing low-angle normal fault

The Zuccale low-angle normal fault exposed on the island of Elba, Italy, is a crustal-scale structure which contains a strongly foliated fault core. In the immediate footwall of the Zuccale fault, cohesive fault-related breccias which were initially deformed by typical frictional deformation mechanisms experienced fluidization over areas of at least 10 −2–10 −3 km 2. Three internal variants of fluidized breccia are recognized, with each related to a separate fluidization event. Characteristics of the fluidized breccias include: (1) a highly irregular ‘intrusive’ boundary with the overlying fault core; (2) no grain-scale evidence for typical frictional deformation mechanisms; (3) an association with carbonate cements indicating that fluids contained CO 2; and (4) a clast-preferred orientation suggesting that fluids were moving vertically and spreading laterally as they encountered the foliated fault core. Our observations suggest that the fluidized breccias are representative of the interseismic period along the Zuccale low-angle fault, and developed across small fault patches during build-ups in fluid overpressure. Attainment of a critical fluid overpressure triggered embrittlement and the formation of low-angle slip surfaces and sub-vertical tensile veins within the overlying fault core, which may account for the presence and the dimensions (10 −1–10 −3 km 2) of rupture surfaces which produce microseismicity along active low-angle normal faults in central Italy.

Kinematics of Franciscan Complex exhumation: New insights from the geology of Mount Diablo, California

Three major lithotectonic units of an ancient arc-trench system are present in western California. In ascending structural order, they include: the Franciscan Complex, a remnant accretionary prism partly including high-pressure (blueschist-facies) metamorphic rocks in a subduction melange; the relatively unmetamorphosed Coast Range ophiolite; and the Great Valley Group forearc sediments. In the Mount Diablo study area east of San Francisco, the subduction complex and ophiolite are juxtaposed across the Coast Range fault, and together they represent a structurally attenuated section through the ancestral forearc crust exposed by uplift and folding of the late Cenozoic Mount Diablo anticline. The Coast Range fault is locally associated with a sheared zone in the lower part of the attenuated ophiolite. Shear-sense indicators in the serpentinite, such as S-C fabrics, rotated clasts, and slick-enfibers, record the deformation kinematics and consistently indicate that the ophiolite moved down in a normal sense relative to the Franciscan rocks in the modern reference frame. Based on these kinematic relations and metamorphic facies contrasts between the Franciscan and the Coast Range Ophiolite, as much as 6–18 km structural attenuation has occurred along the Coast Range fault. After restoration of vertical axis rotation and late Cenozoic fold deformation, the Coast Range fault dips at a low angle toward the northeast, and shear-sense data restore to top-to-the-northeast motion on the Coast Range fault, implying normal displacement of the ophiolite with respect to the structurally underlying Franciscan Complex. These relations are consistent with the hypothesis that the Franciscan rocks were exhumed by crustal attenuation along a low-angle fault system. These data suggest that the Coast Range fault dominantly accommodated normal displacement and ductile thinning, and that these processes were primarily responsible for the structural exhumation of high-pressure assemblages in the Franciscan Complex relative to the structurally overlying forearc crust.

Near-Surface Location, Geometry, and Velocities of the Santa Monica Fault Zone, Los Angeles, California

High-resolution seismic-reflection and seismic-refraction imaging, combined with existing borehole, earthquake, and paleoseismic trenching data, suggest that the Santa Monica fault zone in Los Angeles consists of multiple strands from several kilometers depth to the near surface. We interpret our seismic data as showing two shallow-depth low-angle fault strands and multiple near-vertical (∼85°) faults in the upper 100 m. One of the low-angle faults dips northward at about 28° and approaches the surface at the base of a topographic scarp on the grounds of the Wadsworth VA Hospital (WVAH). The other principal low-angle fault dips northward at about 20° and projects toward the surface about 200 m south of the topographic scarp, near the northernmost areas of the Los Angeles Basin that experienced strong shaking during the 1994 Northridge earthquake. The 20° north-dipping low-angle fault is also apparent on a previously published seismic-reflection image by Pratt et al. (1998) and appears to extend northward to at least Wilshire Boulevard, where the fault may be about 450 m below the surface. Slip rates determined at the WVAH site could be significantly underestimated if it is assumed that slip occurs only on a single strand of the Santa Monica fault or if it is assumed that the near-surface faults dip at angles greater than 20–28°. At the WVAH, tomographic velocity modeling shows a significant decrease in velocity across near-surface strands of the Santa Monica fault. P -wave velocities range from about 500 m/sec at the surface to about 4500 m/sec within the upper 50 m on the north side of the fault zone at WVAH, but maximum measured velocities on the south side of the low-angle fault zone at WVAH are about 3500 m/sec. These refraction velocities compare favorably with velocities measured in nearby boreholes by Gibbs et al. (2000). This study illustrates the utility of combined seismic-reflection and seismic-refraction methods, which allow more accurate reflection imaging and compositional estimations across areas with highly variable velocities, a property that is characteristic of most fault zones.

Slip Partition of the 26 December 2006 Pingtung, Taiwan (M 6.9, M 6.8) Earthquake Doublet Determined from TeleseismicWaveforms

Teleseicmic vertical P waves of an earthquake doublet occurring on 26 December 2006 offshore southwestern Taiwan are analyzed to recover spatial slip distribution. Ground-displacement records for period 0.01-0.5 Hz are inverted using linear inversion for finite fault analysis. Waveform inversion results show that a dual segment fault of different strike is necessary for the first event to explain teleseismic waveforms. The trend in fault geometry, thus, inferred was coincidentally consistent with the bathymetry in the fault rupture area, suggesting the possible tectonic structure controlling fault propagation. Both earthquakes show larger slip in the asperities near the hypocenter and there are two main asperities of similar moments. The first event ruptured along a high angle eastern dipping plane while the second event ruptured along a low-angle western dipping fault plane. The spatial distribution of the Pingtung earthquake doublet shows a compensative and conjugate relationship, and the seismic moments of the earthquake doublet were 2.7×10^26 and 1.8×10^26 dyne-cm, respectively. The stress drops corresponding to the asperities of the two earthquakes have values of 3 to 4 bars, individually, similar to the values observed in other subduction zones.

信濃川左岸，時水背斜東翼の露頭に現れたタイプを異にする活断層群とその解釈

The area at the middle to lower reaches of the Shinano River is a well-known major Neogene thrust and fold belt in Japan. Deformed fluvial terraces, such as anticlinal ridges, synclinal valleys, and fault scarps along the Shinano River, provide a good record of recent tectonic activity in this belt. A large exposure (ca. 150 m long, and up to 10 m deep) was excavated by construction work on the eastern limb of the Tokimizu anticline, giving us an opportunity to observe various types of fault geometry. Four faults—F1, F2, F3, and F4—cut terrace deposits of ca. 130-150 ka (Koshijippara terrace) and underlying early Pleistocene Uonuma Formation. The westernmost fault, F1 is represented as a remarkable flexure dipping westward, suggesting the presence of a low angle thrust underneath. We found a very low angle fault dipping eastward from an additional 2 m deep excavation. The vertical slip at F1, judged from the height difference with the top of the gravel bed (Bed V), is 12 m. In contrast, faults F2 and F3 to the east of F1 follow the bedding plane of the steeply dipping Uonuma Formation, and are high angle reverse faults with the upthrown side to the east. The vertical slip is 3-4 m for F2 and 7.5 m for F3. Profiling across these faults shows that F1 is clearly expressed as a deformed terrace, but the topographical expression of F2 and F3 is not necessarily obvious. Similar faults to F2 are recognized in the study area from observations of the other three large exposures. We classify the faults in the study area into three types: Type 1 is a blind fault assumed at the base of the eastern limb of the Tokimizu anticline. This fault might be the most important contributor to the formation of the major tectonic relief in the study area, although we have no data to prove the nature of the fault plane itself from this study. F1 fault, demonstrated by Type 2, was found for the first time in this study, and is a low angle reverse fault truncating the structure of the Uonuma Formation with a vertical slip rate of 0.1 m/ka. The Type 3 fault is represented by F2, F3, and F4, and these are interpreted to be flexural slip faults along the bedding plane of the Uonuma Formation. Repeated faulting is confirmed from the progressive deformation of different beds not only for the F1 fault (Type 2) but also for the fold-related secondary faults, F2 and F3. No faulting has occurred since ca. 7,500 years BP, however.

Using footwall structures to constrain the evolution of low-angle normal faults

Controversy exists over the presence of normal faults that initiate and slip at low angles (<30°). Evidence from the Zuccale Fault, which dips c . 15° east on the island of Elba, Italy, indicates that the early stages of low-angle faulting were synchronous with development of subsidiary high-angle footwall faults. These footwall structures controlled fault rock distribution and caused the development of flow folds similar to those produced in analogue studies of synchronous high- and low-angle faulting. The current configuration of footwall structures indicates that the Zuccale Fault cannot have originally dipped greater than c . 20° east, and represents a primary low-angle normal fault.

Integrated gravity and seismic interpretation in the Norman Range, Northwest Territories, Canada

We integrate the interpretation of gravity data acquired across the Norman Range near Norman Wells, Northwest Territories, Canada, with geologic mapping and the processing and interpretation of a 2D reflection seismic line. Our purpose is to determine which of two contrasting structural models of deformation is supported by both gravity and seismic data. Interpretation of the gravity data implies that the more likely structural model is that of thin-skinned deformation with a low-angle thrust fault having a décollement within Upper Cambrian evaporites. We use this structural model to guide the development of a velocity model for prestack depth migration of the seismic line. Interpretation of the processed seismic line supports the thin-skinned deformation model.

Late Pleistocene slip on a low-angle normal fault, Searles Valley, California

The mechanical feasibility of normal-sense slip on low-angle faults remains a conundrum in extensional tectonics. The rarity of demonstrably active low-angle normal faults may imply that very specific criteria must be satisfied for significant fault displacement. We present new geologic observations, geomorphic mapping, and structural analysis for a low-angle fault zone along the eastern margin of Searles Valley, California. Our observations indicate that Pleistocene displacement along the range-front fault scarps is the near-surface expression of slip on a bedrock-rooted low-angle normal fault. Along the central portion of the range front in Searles Valley, high-angle faults offset late Pleistocene alluvial and lacustrine surfaces. These faults merge downward into a west-dipping, low-angle fault, but do not displace the low-angle surface. These geometric relations are satisfied only when displacement on the high-angle faults is accommodated by slip on the basal low-angle fault. We use displaced alluvial fan surfaces to determine slip rates across the fault system over late Pleistocene to Holocene time. Combining radiocarbon ages of lacustrine tufa deposits with high-precision topographic surveys of fault scarps yields average slip rates of 0.21–0.35 m/k.y. Additional mapping of faults within the Slate Range at the northern end of Searles Valley suggests that slip is transferred northward to the Manly Pass fault, a bedrock normal fault that trends northeast into Panamint Valley. Thus, although displacement along the range-front fault system dies out northward, we infer that active deformation occurs within the range and likely links extension in Searles Valley with deformation in Panamint Valley.

Effects of initial weakness on rift architecture

Abstract Much work has been conducted investigating the primary factors controlling rift architecture, using both computational and laboratory methods. Here, we examine the effects and relative importance that different types of initial weaknesses have on extensional lithospheric deformation style. We find that the type of initial weakness included in the model plays a primary role in determining the subsequent rift mode. Models using single localized weak seeds produce symmetric narrow rifts, irrespective of whether or not strain softening is included, while models that include regions of diffuse weaknesses produce a wide rift mode. Models that include an initial weak fault tend to produce more asymmetric rifting leading to a core complex mode. By distributing the strain laterally over large areas, the ductile lower crust tends to oppose the narrow rift mode forced by the initial weakness. These results suggest that initial weaknesses may play a major role in determining the mode of rifting. Our numerical experiments confirm that low-angle faults can form as the result of rotation of initially high-angle faults. While previous studies have suggested that the rheological and thermal profiles of the lithosphere play the most important role in rift mode determination, our results illustrate that initial weaknesses could play a major role in rift mode determination, highlighting the need to make initial weaknesses a primary consideration when modelling the extensional deformation of the lithosphere.

Paleomagnetic evidence of large footwall rotations associated with low-angle faults at the Mid-Atlantic Ridge

Exposures of gabbros and mantle-derived peridotites at slow-spreading oceanic ridges have been attributed to extension on long-lived, low-angle detachment faults, similar to those described in continental metamorphic core complexes. In continental settings, such detachments have been interpreted as having originated and remained active at shallow dips. Alternatively, currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift. While the latter interpretation would be more con sistent with Andersonian faulting theory, it predicts large footwall tilts that have not been observed in continental detachment faults. Here we use the magnetization of oceanic gabbro and peridotite samples exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge to demonstrate that substantial footwall rotations have occurred. Widespread rotations ranging from 50° to 80° indicate that original fault orientations dipped steeply toward the spreading axis.

Late orogenic intramontane basin development: the Granada basin, Betics (southern Spain)

ABSTRACTThe quantitative study of subsidence in the Granada basin, using decompaction and backstripping techniques, and contemporaneous relief development in the surrounding areas, especially in the Sierra Nevada, provides a good case example of the development of an intramontane basin. In the Granada basin, according to the interpretation of the seismic profiles and results of the backstripping analysis, subsidence and sedimentation rates were at a maximum in the late Tortonian and decreased progressively; meanwhile, the neighbouring areas were uplifted forming important relief. Chronostratigraphical revisions of the marine sediments show that the marine incursion that deposited sediments in the Granada basin lasted only 1.3 Ma, between 8.5 and 7.2 Ma. The gradual retreat of the sea in the Granada basin is not attributable to global eustatic fluctuations, but rather to uplift in the Sierra Nevada and its adjacent areas. From latest Tortonian to early Messinian times, the region became continental and the Granada basin acquired its present physiography and was differentiated as such. From the late Tortonian onwards, NNW–SSE compression combined with ENE–WSW extension affected the cordillera. In the Granada basin, extension controlled fault movements. There are two well‐defined fault sets: the first trends 70°N–90°E, with low angle faults (less than 30°) dipping towards the north and south, defining the subsiding areas which have approximately E–W direction; whereas the second set has a NW–SE direction, and cuts and displaces the previous ones, defining the main subsiding areas in the eastern part of the basin. The reinterpretation of seismic profiles reveals that the subsiding axes within the Granada basin persisted from the Tortonian to the present because of continued displacements of the main faults.

Fault Reactivation Potential During CO2 Injection in the Gippsland Basin, Australia

The risk of fault reactivation in the Gippsland Basin was calculated using the FAST (Fault Analysis Seal Technology) technique, which determines fault reactivation risk by estimating the increase in pore pressure required to cause reactivation within the present-day stress field. The stress regime in the Gippsland Basin is on the boundary between strike-slip and reverse faulting: maximum horizontal stress (~ 40.5 MPa/km) > vertical stress (21 MPa/km) ~ minimum horizontal stress (20 MPa/km). Pore pressure is hydrostatic above the Campanian Volcanics of the Golden Beach Subgroup. The NW-SE maximum horizontal stress orientation (139°N) determined herein is broadly consistent with previous estimates, and verifies a NW-SE maximum horizontal stress orientation in the Gippsland Basin. Fault reactivation risk in the Gippsland Basin was calculated using two fault strength scenarios; cohesionless faults (C = 0; μ = 0.65) and healed faults (C = 5.4; μ = 0.78). The orientations of faults with relatively high and relatively low reactivation potential are almost identical for healed and cohesionless fault strength scenarios. High-angle faults striking NE-SW are unlikely to reactivate in the current stress regime. High-angle faults oriented SSE-NNW and ENE-WSW have the highest fault reactivation risk. Additionally, low-angle faults (thrust faults) striking NE-SW have a relatively high risk of reactivation. The highest reactivation risk for optimally oriented faults corresponds to an estimated pore pressure increase (Delta- P) of 3.8 MPa (~548 psi) for cohesionless faults and 15.6 MPa (~2262 psi) for healed faults. The absolute values of pore pressure increase obtained from fault reactivation analysis presented in this paper are subject to large errors because of uncertainties in the geomechanical model (in situ stress and rock strength data). In particular, the maximum horizontal stress magnitude and fault strength data are poorly constrained. Therefore, fault reactivation analysis cannot be used to directly measure the maximum allowable pore pressure increase within a reservoir. We argue that fault reactivation analysis of this type can only be used for assessing the relative risk of fault reactivation and not to determine the maximum allowable pore pressure increase a fault can withstand prior to reactivation.

Tectonic framework and extensional pattern of the Malaguide Complex from Sierra Espuña (Internal Betic Zone) during Jurassic–Cretaceous: implications for the Westernmost Tethys geodynamic evolution

Mapping, lithostratigraphic, biostratigraphic and structural detailed analyses in Sierra Espuna area (Internal Betic Zone, SE Spain) have allowed us to reconstruct the Jurassic–Cretaceous evolution of the Westernmost Mesomediterranean Microplate palaeomargin and, by correlation with other sectors (Northern Rift, central and western Internal Betic Zone), to propose a geodynamic evolution for the Westernmost Tethys. Extension began from Late Toarcian, when listric normal faults activated; these faults are arranged in three categories: large-scale faults, separating hectometric cortical blocks; main faults, dividing the former blocks into some kilometre-length blocks; and secondary faults, affecting the kilometric blocks. This fault ensemble, actually outcropping, in the Sierra Espuna area, broke the palaeomargin allowing the westerly Tethyan Oceanic aperture with an extension at about 17.2%. Extension was not homogeneous in time, being the Late Toarcian to the Dogger–Malm boundary the period when blocks underwent the greatest movement (rifting phase), leading to the drowning of the area (8.2% extension). During the Malm (drifting phase) extension followed (5.7%), while during the Cretaceous a change to pelagic facies is recorded with an extension of about 3.3% (post-drift stage). This evolution in the Westernmost Tethys seems to be related to areas out of the limit of significant crustal extension in the hanging wall block of the main cortical low-angle fault of the rifting.

Geophysical evidence for thin-skinned structural deformation in the Norman Range, Northwest Territories

Abstract Interpretation of a gravity survey acquired across the Norman Range, near Norman Wells, NWT, Canada, was integrated with geological and reflection seismic data to determine the most likely structural model for the subsurface. A thin-skinned structural interpretation of a low-angle thrust fault with a decollement within the Cambrian Saline River Formation yielded the best match between the calculated and observed gravity data and was supported by processed reflection seismic data. The interpretation predicts that potential reservoir rocks of the Devonian Ramparts Formation extend only a short distance below the Norman Range in the footwall of the Norman Range Thrust.