Extensional Reactivation Research Articles

Active tectonics along the Nebrodi–Peloritani boundary in northeastern Sicily (Southern Italy)

In the epicentral area of the seismic swarm of the June–September 2011, at southern edge of the Calabrian arc in NE Sicily, very recent extensional motions remobilised two main NW–SE dextral faults. The extensional reactivation of strike–slip faults responded to a new regional dynamic, also evidenced by GPS and seismological data. The inverted structures are aligned at the margin of a wide crustal block that is moving apart from the rest of the island and is uplifting faster than the adjacent regions. The active faults terminate to the northwest at the intersection with a prominent NNE trending fault that represents the western boundary of the mobile block. The vertical displacement along this border exactly matches the difference in elevation of the marine terraces resting inside and outside the block, respectively. On the contrary, only part of differential displacement of the marine terraces was actually accommodated as cumulative motion along the two NW oriented inverted faults, across the southwestern boundary of the block. Amounts of the vertical displacement were distributed on distinct fault planes of the previous dextral shear belts. The widespread fracturing is also the best explanation for the seismic swarm of the 2011, whose epicenters spread on a discrete rock volume rather than concentrated along a single fault plane. The diffuse fracturing seems to represent a peculiar style of deformation, connected to the tectonic inversion of previous strike–slip shear zones. Seismic swarm also affects the northern termination of the Calabrian arc where active extensional deformation reactivated previous strike–slip faults. The similarity of the two regions suggests that seismic swarm can be peculiar of extensional belts developed on previous strike–slip shear zones, along which the pre-existing geometry favours the dispersion of the tectonic motion on a network of small linked fault planes.

Tectonic history and setting of a seismogenic intraplate fault system that lacks microseismicity: The Saline River fault system, southern United States

Abstract Although the northwest-striking Saline River fault system of southeastern Arkansas is not defined by microseismicity, it is associated with sand blows and shows evidence of Pleistocene and Holocene surface ruptures, suggesting a significant seismogenic potential. This fault system is within the northern Gulf of Mexico interior coastal plain, a region only recently recognized as containing seismogenic faults. To better characterize this active fault system, we reconstructed its post-Paleozoic history using petroleum and coal industry wire-line well log and seismic reflection subsurface data. The Saline river fault system initiated as a series of northwest-striking grabens during Triassic/Jurassic uplift and incipient Gulf of Mexico rifting along the basement Alabama–Oklahoma transform margin of the North American Proterozoic craton. During post-rift subsidence, these grabens were buried by Gulf sediments until mid-Cretaceous uplift and igneous activity resulted in minor extensional reactivation of graben faults. Faulting style changed from extension to transpression during the Late Cretaceous due to compression of eastern North America as the North Atlantic rapidly widened and due to thermal weakening of the Alabama–Oklahoma transform lithospheric discontinuity as it obliquely crossed a mantle hot spot. In the Late Cretaceous, graben faults experienced contractional reactivation and steep, deeply-rooted transpressional faults developed within and parallel to the graben system. These transpressional faults locally displace Eocene, Pleistocene, and Holocene sediments. Fault activity continues on the Saline River fault system due to thin crust along the Alabama–Oklahoma transform and to high heat flow, which act together to weaken the crust and promote seismogenic tectonism. The fault system may lack appreciable microseismicity because the aftershock sequence of the last large earthquake has had time to dissipate.

Structure and development of the Andean system between 36° and 39°S

One of the main morphological changes along the Southern Central Andes occurs from 36° to 39°S. The northern portion is characterized by prominent basement structures and a thick-skinned orogenic front with relief of over 2000m with a deep level of exhumation where more than 4km of section has been eroded. Contrastingly, the southern part is formed by mildly inverted basement structures restricted mainly to the hinterland zone, which reaches only 1500–1700m relief. We quantify the variable contributions of two main contractional stages through the construction of three regionally balanced sections across the Andes, constrained by field and geophysical data. Extensional re-activation described for this segment in late Oligocene-early Miocene and Pliocene to Quaternary times, after the two main contractional episodes, suggests only 3km of stretching that represents 30–10% of the original longitude. We, therefore, conclude that while initial Late Cretaceous to Eocene compression was similar along strike (∼10–7km), it is the contrasting degrees of Neogene shortening (∼16–6km) that have played the largest role in the along strike differences in structure and morphology along this portion of the southern Andes. Variable Neogene arc expansion could be responsible for the contrasting contractional deformation: In the north, late Miocene arc-related rocks cover most of the retroarc zone (>200km with respect to the late Miocene arc front in the south), presumably driven by a shallow subduction episode in the area, whereas to the south they remain restricted to the continental drainage divide. Other factors involving architecture of previous rift structures, are proposed as additional mechanisms that accommodated variable shortening magnitudes through inversion.

Fluvial architecture variations linked to changes in accommodation space: Río Chico Formation (Late Paleocene), Golfo San Jorge basin, Argentina

The Upper Paleocene Río Chico Formation is a 50–180m thick fluvial succession developed in a passive-margin setting, Golfo San Jorge basin, Central Patagonia, Argentina. A detailed description and interpretation of outcrops was carried out, analyzing exposures from the northern basin margin to the most complete successions at the southern depocenter. The unit is characterized by a regional fluvial system that flowed to the south-east. Five main lithofacies associations were defined: (I) active fluvial channels, with three sub-types: braided, meandering and low-sinuosity, (II) sheet-flood deposits, (III) proximal floodplain (natural levee and crevasse-splay), (IV) distal floodplain, and (V) abandoned channels.Lateral/vertical changes in fluvial architecture of the Río Chico Formation were recognized by variations in preserved thickness, fluvial styles, geometry of fluvial channels, regional paleoflow directions, and channel/floodplain ratios. Close to the northern basin margin, the fluvial succession is 50–60m thick, composed of braided channels, sheet-flow deposits, and high channel/floodplain ratio. In a basinward direction, the alluvial succession increases to 180m in thickness, the dominant fluvial styles change to low-sinuosity and meandering channels and channel/floodplain ratio reduces.The fluvial architecture of the Río Chico Formation shows two main depositional trends that resulted from changes in accommodation space across the basin. The interpreted break-point coincides with the underlying Cretaceous basin-boundary, thus the synsedimentary extensional reactivation of the pre-existing tectonic lineament generated differential subsidence, delimiting two different accommodation settings.

Cenozoic tectonics of the Tuz Gölü Basin (Central Anatolia Plateau, Turkey)

We present a new 3D geologic model for the architecture and Cenozoic tectonic evolution of the Tuz Gölü Basin, a major sedimentary basin in the Central Anatolian orogenic plateau. This model is grounded on 7 depth-converted seismic reflection profiles in combination with the analysis of backstripped subsidence curves, isochore maps, and a palinspastically restored cross-section. Two stages of basin formation are detected during Cenozoic times. During the Palaeogene, around 2 km of basement subsidence led to the development of a sag basin broader than the present basin in the absence of bounding faults. After a period of uplift and erosion, sedimentation restarted by Tortonian times. Up to 3.5 km of post-Palaeogene sediments were deposited in relation to this second regional subsidence phase, which continued possibly well into the Pliocene. During this time, the 2 main fault systems found in the area, the Tuz Gölü and the Sultanhanı faults, developed as south-west dipping, NW-SE striking, normal faults. At some time in the Late Miocene-Early Pliocene, during regional subsidence, a previously unreported phase of contraction occurred, which led to the development of a north-east-vergent thrust sheet, the culmination of which forms the morphologic ridge to the east of the Tuz Gölü Lake. This structure presently divides the previously continuous Tuz Gölü Basin. Finally, minor extensional reactivation occurred. At the regional scale, the pre-Late Miocene subsidence is coeval with the initiation of volcanism in the Central Anatolian Volcanic Province and marine carbonate deposition in southern Turkey, and the latest Miocene shortening is (partly) contemporaneous with the onset of uplift in the same region.

Along-strike changes in fault array and rift basin geometry of the Carboniferous Billefjorden Trough, Svalbard, Norway

The Billefjorden fault zone (BFZ) of Svalbard, Norway, is a long-lived major tectonic lineament with significant influence on the structural style of the northwestern Barents Shelf. The fault zone can be traced for around 2–300km in an N-S direction, where it is made up of three major fault strands, all dipping 50–70° to the east. Field and seismic data delineate gradual shifts in displacement between master faults (northern Odelfjellet, Balliolbreen, and southern Drønbreen faults) and suggest that two c. 20km long, 2–3km wide relay zones exist along strike. The western fault strand consistently has a pre-Carboniferous reverse character; placing basement on top of Devonian sedimentary units with a throw estimated at 10km. Subsequent Carboniferous extensional reactivation of segments of the BFZ resulted in formation of the Billefjorden Trough, with a rift basin showing a fairly consistent polarity for its length. This suggests a profound control by the older, Devonian reverse faults on the extensional structural style.The Billefjorden Trough extends for a minimum of 110km along strike, and is 20–30km wide at the most. A gentle north plunge of the basin axis in the south is mirrored by an opposite plunge in the north, offering a depth/thickness of around 2000m in the central realm, diminishing to 500m in the south. Geometries of the trough suggest sedimentary response to fault-growth, with weak lithologies such as evaporites promoting fault-monocline formation and associated lenticular basin geometries. On the contrary, wedged shaped fill geometries toward major faults is consistent with significant fault movement and growth sequences. Subsequent mild Tertiary reactivation of fault segments of the BFZ is suggested by local reverse faults and deep-seated anticline-syncline pairs.

Upper Cretaceous-Paleocene extensional phase in the Golfo San Jorge basin (Argentina): Growth-fault model, paleoseismicity and paleostress analysis

Abstract A total of 170 synsedimentary normal faults preserved in the marine Salamanca Formation (Early Paleocene of the Golfo San Jorge basin) were described from exposures of the Northern Flank of the basin. The result of the paleostress analysis of those normal faults indicates an NE-SW (49°) extensional direction during Early Paleocene times. Synsedimentary normal faults and seismites in the unit demonstrate the occurrence of an extensional tectonic event coeval with the deposition of the Salamanca Formation. Available 3D seismic surveys of the subsurface of the basin allowed recognizing asymmetric, fault-bounded Upper Cretaceous-Paleocene depocentres, identifying at least two main tectonic pulses in this extensional phase. The mapping of normal faults from seismic attributes (e.g. time slices) is WNW-ESE (278°–98°), a trend that apart significantly from outcrop results. The major faults in the subsurface that affect the Early Paleocene succession are the result of the extensional reactivation of Lower Cretaceous normal faults, which are mainly related to pre-existing structures in the basement of the basin. The D/L ratio of normal faults in the subsurface give smaller values than those based on theoretical relationships, being considered as sub-displaced normal faults. These low D-L values imply that the length of the major Paleocene faults was reached earlier by inheritance of previous faults. In addition, the subsurface lengths of faults allowed estimating paleoearthquake magnitudes greater to M ≈ 5 from empirical relations, matching with previous results obtained from the analysis of soft-sediment deformational structures preserved in the Salamanca Formation. This study displays at several scales the nature, origin and tectonic products of the Upper Cretaceous-Paleocene extensional phase in the Golfo San Jorge basin.

Impact of lithospheric flexure on the evolution of shallow faults in the Timor foreland system

Re-evaluation of structural evolution on the Laminaria High (Timor Sea, Australia), shows that lithosphere flexure, associated with the Tertiary collision between the Australian north-west margin and the Banda volcanic arc, is an important mechanism for Neogene fault development and the reactivation of Jurassic structures. 2D elastic models of the lithosphere flexure show that increasing flexural extension is likely to affect the Laminaria High from the Late Miocene onwards. The evaluation of the Neogene fault slip history, based on instantaneous throw estimates, suggests an initiation of faulting during the Late Miocene when the Laminaria High entered the flexed area (forebulge). Increasing fault growth is recorded during the Pliocene and correlates with the Laminaria High reaching the hinge of the forebulge structure. Maximum episodes of fault growth are recorded between the Late Pliocene and Early Pleistocene when the Laminaria High was located near the forebulge hinge or in the slope where curvature and flexural stress are likely to be maximal. NNW extension associated with plate flexure allowed the oblique extensional reactivation of buried reservoir-bounding Jurassic structures. The orientation of the Jurassic structures relative to the flexural extensional front appears as a key factor controlling the type of segment linkage between the reactivated Jurassic faults and the newly developed Neogene faults. Observations from the Laminaria High suggest that with an angle > 20° between σ Hmax and strike orientation of the Jurassic faults, soft-linkage is likely to occur between Jurassic and Neogene segments. An angle < 20° is more likely to trigger hard-linkages between these segments. The development of continuous fault zones results in a higher up-fault migration risk for hydrocarbon leaking from fault-bounded reservoirs.

A continuum mechanics approach to quantify brittle strain on weak faults: application to the extensional reactivation of shallow dipping discontinuities

A number of field observations suggest that sliding on fault planes may occur at very shallow dip in the brittle field. The existence of active low angle normal faults is much debated because (1) the classical theory of fault mechanics implies that faults are locked when the dip is less than 30° and (2) shallow dipping fault planes do not produce large earthquakes (M > 5.5). To reconcile observations and theory, we propose a new model for fault reactivation by introducing an elasto-plastic frictional fault gouge as an alternative to the classical dislocation models with frictional properties. Contrary to the classical model which implies that the dilation angle ψ equals the friction angle φ, our model accounts for ψ < φ and permits ψ < 0 in the fault gouge as deduced from laboratory and field observations. Whilst the predicted locking angles differ in most cases by less than 10° from the classical model, a significant amount of plastic strain (strain occurring in elasto-plastic regime) is predicted to occur on badly oriented faults prior to locking in a strain-hardening regime. We describe four modes of reactivation which include complete/partial reactivation with/without tensile failure in the surrounding medium. This paper presents analytical solutions to separate those four different modes as well as numerically computed estimates of the amount of plastic strain accumulated before locking. We conclude that plastic strain on badly oriented faults is favoured by compaction of the fault gouge and that those faults are probably aseismic because this strain always occurs in a hardening regime.

Neogene to Quaternary extensional reactivation of a fold and thrust belt: The Agrio belt in the Southern Central Andes and its relation to the Loncopué trough (38°–39°S)

The Andean orogenic front between 38 and 39°S is formed by extensional structures delimiting the Loncopué trough. These structures are superimposed to Late Cretaceous–Late Miocene compressive structures that formed the Agrio fold and thrust belt. Gravity data, and limited borehole and seismic data complimented with previous field studies were used to construct two structural cross sections across the area of Neogene extensional deformation and remnants of the previous compressive structure. One of them was restituted for Late Cretaceous times previously to extensional relaxation of the fold and thrust belt and Late Oligocene–Early Miocene and Pliocene to Quaternary stages. Different modeling techniques and filters for gravity anomalies show that i) a series of deeper and larger depocenters, where Late Oligocene to Early Miocene sequences are buried, were located particularly at the Present trough boundaries; while ii) smaller and probably shallower depocenters were concentrated at the axial -eastern trough following structures at surface with young morphological expression. The former are interpreted as representative of the initial extensional stage when Late Cretaceous compressional basement structures were relaxated, while the latter could be the product of incipient-young extensional reactivation of the Agrio fold and thrust belt. General coincidence between both areas of extensional relaxation at the western Agrio fold and thrust belt implies that a common basement discontinuity is recurrently being reactivated through time, probably in coincidence with the eastern main Loncopué fault boundary.

Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

Field mapping and structural analysis have allowed us to characterise the fault geometry and the post-metamorphic tectonics of an area located in the Northern Cottian Alps (inner Western Alps). Two main faulting stages were distinguished here. The first (Oligocene?-Early Miocene) is related to the development of an E–W-striking left-normal shear zone. This shear zone is interpreted as an antithetical of two regional, N–S right-lateral structures: the Col del Lis-Trana Deformation Zone (LTZ) and the Colle delle Finestre Deformation Zone (CFZ). The second faulting stage (post-Early Miocene) is related mainly to the development of N–S normal faults, coeval with the extensional reactivation of the LTZ and the CFZ. We discuss this kinematic evolution in the framework of the geodynamic evolution of the Western Alps.

Evaluating hydrocarbon trap integrity during fault reactivation using geomechanical three-dimensional modeling: An example from the Timor Sea, Australia

Three-dimensional (3-D) coupled deformation and fluid-flow numerical modeling are used to simulate the response of a relatively complex set of trap-bounding faults to extensional reactivation and to investigate hydrocarbon preservation risk for structural traps in the offshore Bonaparte Basin (Laminaria High, the Timor Sea, Australian North West Shelf). The model results show that the distributions of shear strain and dilation as well as fluid flux are heterogeneous along fault planes inferring lateral variability of fault seal effectiveness. The distribution of high shear strain is seen as the main control on structural permeability and is primarily influenced by the structural architecture. Prereactivation fault size and distribution within the modeled fault population as well as fault corrugations driven by growth processes represent key elements driving the partitioning of strain and up-fault fluid flow. These factors are critical in determining oil preservation during the late reactivation phase on the Laminaria High. Testing of the model against leakage indicators defined on 3-D seismic data correlates with the numerical prediction of fault seal effectiveness and explains the complex distribution of paleo- and preserved oil columns in the study area.

Gravity and magnetic survey of the Oaxaca city region: Cenozoic horst-and-graben structure superimposed on the Oaxaca–Juarez terrane boundary, southern Mexico

Abstract A geophysical survey of the Oaxaca Fault along the north-trending Etla and Zaachila valleys area, southern Mexico, shows a series of NNW–SSE Bouguer and magnetic anomalies with steeper gradients towards the east. The Oaxaca Fault represents Tertiary extensional reactivation of the Juarez shear zone that constitutes the boundary between the Oaxaca and Juarez terranes. Cooperative interpretation of six combined gravity and magnetic NE–SW profiles perpendicular to the valleys indicates the presence of a composite depression comprising three N–S sub-basins: the northern Etla and southern Zaachila sub-basins separated by the Atzompa sub-basin. The Etla sub-basin is bounded by the moderately E-dipping, Etla Fault and the more steeply W-dipping Oaxaca Fault, which together constitute a graben that continues southwards into the Atzompa graben. The deeper Zaachila sub-basin, south of Oaxaca city, is a wide V-shaped graben with a horst in the middle. The new geophysical data suggest that the Oaxaca–Juarez terrane boundary is displaced sinistrally ca. 20 km along the E–W Donaji Fault, which defines the northern boundary of the Zaachila sub-basin. On the other hand, the Oaxaca Fault may either continue unbroken southwards along the western margin of the horst in the Zaachila sub-basin or be offset along with the terrane boundary. The sinistral movement may have taken place either during the Late Mesozoic-Early Cenozoic, Laramide Orogeny as a lateral ramp in the thrust plane or under Miocene–Pliocene, NE–SW extension. The former suggests that the Donaji Fault is a transcurrent fault, whereas the latter implies that it is a transfer fault. The models imply that originally the suture was continuous south of the Donaji Fault and provide a constraint for the accretion of the Oaxaca and Juarez terranes.

A multidisciplinary approach to the study of the fault networkin the internal Cottian Alps (Western Alps)

The architecture of the fault network and the late- to post-metamorphic structural evolution in the internal Cottian Alps have been studied by an integrated approach combining morphostructural analysis, field mapping and detailed mesostructural analysis between the Susa and Pellice Valleys. Available apatite fission track data indicate that this sector of the Western Alps reached shallow crustal levels, where brittle deformation mechanisms prevail, by Late Oligocene times. The analysed area comprises two main N-S striking regional, polyphasic semi-brittle to brittle deformation zones: the Colle delle Finestre Fault Zone (CFFZ) and the Col del Lis-Trana Deformation Zone (LTZ). The activity of those long-lived structures evolved, respectively, from dextral and dextral-reverse to extensional. In the block bounded by these two regional structures, the late- to post-metamorphic tectonic evolution is characterized by two faulting stages: (a) the development of an E-W left-lateral fault zone with a minor normal component of movement; at the regional scale, this fault zone may be considered as an antithetical shear of the major LTZ and the CFFZ; (b) the extensional reactivation and development of N-S normal faults, coeval with those recorded both by the LTZ and the CFFZ. The geometry of the fault and fracture systems has been compared with the remote sensed lineaments pattern, characterized by four main systems: Ln1 (N0°-30°E), Ln2 (N45°-70°E), Ln3 (N80°-100°E) and Ln4 (N100°-120°E). The lineament systems display similarities in terms of orientation, geometry and length with the fault systems observed at different scales. These similarities allowed us to extrapolate the structural model outlined in the mapped area to the adjoining sectors. Comparison between the different data suggests that the pattern of the lineaments detected between the LTZ and the CFFZ agrees with the architecture of a strike-slip fault network, which accommodated the internal deformation of the block bounded by these two major N-S transcurrent deformation zones. The transcurrent activity of these faults is here interpreted as coeval with the late dextral movements along the Canavese Line and with the propagation, in the Late Oligocene-Early Miocene, of the roughly E-vergent South-Alpine Thrusts beneath the Tertiary Piemonte Basin. The extensional event that followed may be related to the uplift with the subsequent isostatic readjustment of the chain which may have also induced the normal reactivation of the Penninic Frontal Thrust. This extensional regime is also in agreement with focal mechanisms of recent earthquakes.

The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation

The SW England Rhenohercynian passive margin initiated with rift-related non-marine sedimentation and bimodal magmatism (Late Lockhovian). Continued lithospheric extension resulted in the exhumation of mantle peridotites and limited seafloor spreading (Emsian-Eifelian). Variscan convergence commenced during the Late Eifelian and was coeval with rifting further north. Collision was marked by the Early Carboniferous emergence of deep marine sedimentary/volcanic rocks from the distal continental margin, oceanic lithosphere, pre-rift basement and upper plate gneisses (correlated with the Mid-German Crystalline High of the Saxothuringian Zone). Progressive inversion of the passive margin was strongly influenced by rift basin geometry. Convergence ceased in the Late Carboniferous and was replaced by an extensional regime that reactivated basin controlling/thrust faults and reorientated earlier fabrics (Start-Perranporth Zone). The resultant exhumation of the lower plate was accompanied by emplacement of the Early Permian SW England granites and was contemporaneous with upper plate sedimentary basin formation above the reactivated Rhenohercynian suture. The Rhenohercynian passive margin probably developed in a marginal basin north of the Rheic Ocean or, possibly, a successor basin following its closure. The Lizard ophiolite is unlikely to represent Rheic Ocean floor or associated forearc (SSZ) crust. The Rheic and Rhenohercynian sutures may be coincident or the Rheic suture may be located further south in the Léon Domain.

Numerical simulation of extensional fault reactivation and fluid flow: Generic models related to the Timor Sea

Structural analyses and interpretation of seismic data from the Timor Sea indicate that most of the faults in the region have been reactivated during the Late Tertiary extensional event. These reactivated faults and the associated fluid flow represent a major risk to trap integrity and hydrocarbon preservation. To explore the controls of extensional fault reactivation on fluid flow in petroleum basins, this numerical modelling study has investigated a group of 3D coupled mechanical deformation and fluid flow models characterized by various simple fault geometries and reservoir stratigraphic architecture. The models simulate a regional-scale block (50×50 km in planview and 10 km in depth) with a larger fault and a small fault (either parallel or cross-cutting). The stratigraphic sequences defined for these models include four rock units. They are, from top to bottom, a carbonate layer, a thin shale layer, a sandstone unit and a relatively soft basal rock unit providing a more ductile domain for fault movement and for reducing the edge effects at the bottom model boundary. Such fault and stratigraphic architecture is a simplified representation of regional structures and stratigraphy in the Timor Sea. Model mechanical and fluid flow properties are based on rock property data for the region, in combination with the general rock property data from literature. To consider rock permeability enhancement with faulting, we have incorporated a simple scheme that allows fault permeabilities to increase with accumulation of shear strain, from their initial values (identical to those of host rocks) to various maximum values depending upon host rock type. Greater shearinduced permeability increase is allowed for the shale unit. Initial fluid pore pressures are important boundary conditions for fluid flow modelling. The current models have investigated the effects of four different initial pore pressure (or hydraulic head) distributions, based on data from the Timor Sea: 1) a slightly over-pressured system in the rocks below the shale; 2) a higher over-pressured system in the rocks below the shale; 3) a slightly over-pressured system but with a small hydraulic head difference between the left and right hand boundaries

Numerical modelling of strain localisation and fluid flow during extensional fault reactivation: Implications for hydrocarbon preservation

A numerical modelling approach is used to investigate the main controls of strain localisation on faults and fluid flow in fault-bound reservoir rocks during extensional reactivation. A series of 3D coupled mechanical deformation and fluid flow models were performed to simulate the response of simple fault and reservoir-seal geometries to extensional reactivation. The model results demonstrate that initial fault length is a primary control on strain distribution between faults and on fluid transport from the reservoir through the top seal. During extensional reactivation, longer faults in the population tended to accommodate greater shear strain and accumulate greater throw than smaller faults. With greater length contrast, there is an increased throw contrast for the same amount of extension. This behaviour can be attributed to the mechanism of strain sharing among fault elements. Spacing between adjacent faults is a secondary factor affecting strain partitioning. For a given fault length contrast, increasing the spacing increases the freedom of adjacent faults to move and leads to a more even distribution of strain. Fault strike overlap styles also affect strain distribution along faults. This is mainly expressed as the reduction of throws in the overlapping fault segments and partially overlapped or adjacent fault ends. Our models further demonstrate that fault lengths are likely to control fluid flow during extensional reactivation. Large faults in the models are the main conduit for fluid transport, expressed as strong upward fluid flow along these faults from the sandstone reservoir horizon through the shale seal.

Cenozoic kinematic history of the Kohistan fault in the Pakistan Himalaya

Detailed mapping suggests a new scenario for the kinematic history of the Late Cretaceous Kohistan arc complex and for the cause of Cenozoic metamorphism and exhumation along the Indian continental margin in Pakistan. The earliest deformational phases resulted in early Cenozoic amphibolite-facies metamorphism, superposed tight to isoclinal F1/F2 folds, and development of strong penetrative rock fabric within the Indian plate in response to southwestward, and then southeastward, overthrusting of ophi-olitic melange in the Indus suture zone and West Pakistan fold belt, respectively. This deformation included the development of the Banna and Malakand thrust faults. Prograde metamorphism ended following this deformational phase, and the early structures were deformed along N-trending, open, upright, F3 folds and domes. This was followed by ESE- to E-directed thrusting of the Kohistan arc complex onto the Indian continent along the dominantly brittle Kohistan fault. Thrusting ended in the late Oligocene–early Miocene when the Kohistan arc assumed its present position along the northern margin of India. South-verging structures began to form on the Indian plate for the first time in the late Oligocene–early Miocene. Deformation had expanded to the foreland by the middle to late Miocene with development of the Panjal-Khairabad thrust. Neogene structures in the metamorphic zone include N-trending, east-side-up, high-angle fault zones that deform or reactivate the Kohistan fault, and NW-trending, east-side-up reverse faults, both of which are seismically active. In the tectonic scenario outlined here, shortening along the Kohistan fault occurred subsequent to metamorphism, and was directed toward the ESE-E throughout its history. The southern termination of the fault is considered to be a right-slip sidewall ramp such that the Kohistan arc never advanced much father south on the Indian plate than its present position. Broad flexure, folding, and erosion in front of and south of the actively advancing Kohistan arc complex contributed to exhumation of the Indian plate. This differs from interpretations that state that Kohistan was thrust in a southward direction; that underthrusting beneath Kohistan was the primary cause of early Cenozoic metamorphism on the Indian plate; and that top-side-north extensional reactivation of the Kohistan fault contributed to exhumation.

Provenance history of the Bangemall Supergroup and implications for the Mesoproterozoic paleogeography of the West Australian Craton

The 4–10 km-thick Bangemall Supergroup, comprising the Edmund and Collier Groups, was deposited between 1620 and 1070 Ma in response to intracratonic extensional reactivation of the Paleoproterozoic compressional Capricorn Orogen. The supergroup can be further divided into six depositional packages bounded by unconformities or major marine flooding surfaces. U–Pb dating of over 1200 detrital zircon grains from 19 samples representative of each of the major sandstone units within these packages has failed to identify any zircon populations attributable to syndepositional magmatism. However, this extensive dataset provides a provenance history of the Bangemall Supergroup, which is here integrated with paleocurrent data which indicates that all source areas were located within the Mesoproterozoic West Australian Craton, with the main source area for the northern Bangemall Supergroup being the Gascoyne Complex and southern Pilbara Craton. All samples have prominent age modes in the 1850–1600 Ma range, indicating significant contribution from the northern Gascoyne Complex and coeval sedimentary basins. Some samples also display prominent modes in the 2780–2450 Ma range, consistent with derivation from the Fortescue and Hamersley Groups of the southern Pilbara Craton. The Edmund Group has age-spectra in which the dominant modes become older upwards, recording unroofing of the underlying basement from the Gascoyne Complex to the Archean granites and greenstones of the Pilbara Craton. In contrast, the Collier Group records unroofing of the underlying Edmund Group, with possible additional contribution from the Pilbara Craton and Paterson Orogen, and is characterized by age-spectra in which the dominant modes become younger upwards. These data imply that the West Australian Craton remained intact throughout the Mesoproterozoic assembly of Rodinia, and was the only source of detritus for the Bangemall Supergroup.

Thermochronological evidence for Mio‐Pliocene late orogenic extension in the north‐eastern Albanides (Albania)

AbstractNew apatite and zircon (U–Th)/He and apatite fission‐track (FT) data allow constraining the timing of Miocene–Pliocene extensional exhumation that affected the central part of the Dinarides‐Albanides‐Hellenides orogen. Apatite (U–Th)/He ages in the northern and western Internal Albanides range from 57 to 17 Ma, contrasting to younger ages of 5.2–9.3 Ma in the eastern Internal Albanides. Eastward younging is also reflected in zircon (U–Th)/He ages varying from 101 Ma in the north‐western Internal Albanides to 19–50 Ma in the east, as well as in recently published apatite FT ages. Thermal history predictions with the new data point to a phase of rapid exhumation of the eastern Internal Albanides around 6–4 Ma, while the western Internal Albanides record slower continuous exhumation since the Eocene. This asymmetric exhumation pattern is most likely linked to extensional reactivation of NE–SW‐trending thrusts east of the Mirdita zone and within the Korabi zone of the eastern Internal Albanides.