Transtensional Faults Research Articles

Seismotectonics of the Pamir

AbstractBased on a 2 year seismic record from a local network, we characterize the deformation of the seismogenic crust of the Pamir in the northwestern part of the India‐Asia collision zone. We located more than 6000 upper crustal earthquakes in a regional 3‐D velocity model. For 132 of these events, we determined source mechanisms, mostly through full waveform moment tensor inversion of locally and regionally recorded seismograms. We also produced a new and comprehensive neotectonic map of the Pamir, which we relate to the seismic deformation. Along Pamir's northern margin, where GPS measurements show significant shortening, we find thrust and dextral strike‐slip faulting along west to northwest trending planes, indicating slip partitioning between northward thrusting and westward extrusion. An active, north‐northeast trending, sinistral transtensional fault system dissects the Pamir's interior, connecting the lakes Karakul and Sarez, and extends by distributed faulting into the Hindu Kush of Afghanistan. East of this lineament, the Pamir moves northward en bloc, showing little seismicity and internal deformation. The western Pamir exhibits a higher amount of seismic deformation; sinistral strike‐slip faulting on northeast trending or conjugate planes and normal faulting indicate east‐west extension and north‐south shortening. We explain this deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik‐Afghan depression, where it causes thin‐skinned shortening of basin sediments above an evaporitic décollement. Superposition of Pamir's bulk northward movement and collapse and westward extrusion of its western flank causes the gradual change of surface velocity orientations from north‐northwest to due west observed by GPS geodesy. The distributed shear deformation of the western Pamir and the activation of the Sarez‐Karakul fault system may ultimately be caused by the northeastward propagation of India's western transform margin into Asia, thereby linking deformation in the Pamir all the way to the Chaman fault in the south in Afghanistan.

Geology of the late Miocene south-eastern Volterra Basin (Northern Apennines, Italy)

We present a 1:10,000 scale geological map for the south-eastern sector of the Volterra Basin (Northern Apennines, Italy), together with supporting stratigraphic-structural data. The Volterra Basin consists of a major structural depression within the Northern Apennines hinterland, NNW-SSE-oriented and filled with more than 2000 m of late Miocene-Quaternary deposits. Its south-eastern sector is classically considered as a type area for late Tortonian non-marine strata, here mapped and refined in terms of internal stratigraphy adopting a scheme of depositional and lithostratigraphic units. Stratigraphic assessments helped in redefining the character of the lower boundary of the non-marine succession, as well as in mapping a newly recognized angular unconformity. Deformation structures affecting the basin fill include blind normal faults rooted to a deep detachment, outcrop-scale transtensional faults and clusters of gentle folds. Faults and folds appear to be kinematically linked. Our structural observations largely agree with those present in the literature, supporting a model of post-orogenic crustal stretching.

Left-lateral transtension along the Ethiopian Rift and constrains on the mantle-reference plate motions

We present the kinematics of the Ethiopian Rift, in the northern part of East African Rift System, derived from compilation of geodetic velocities, focal mechanism inversions, structural data analysis and geological profiles. In the central Ethiopian Rift, the GPS velocity field shows a systematic magnitude increase in ENE direction, and the incremental extensional strain axes recorded by earthquake focal mechanisms and fault slip inversion show ≈N100°E orientation. This deviation between direction of GPS velocity vectors and orientation of incremental extensional strain is developed due to left lateral transtensional deformation along the NE–SW trending segment of the rift. This interpretation is consistent with the en-échelon pattern of tensional and transtensional faults, plus the distribution of the volcanic centers, and the asymmetry of the rift itself. We analyzed the kinematics of the Ethiopian Rift also relative to the mantle comparing the results in the deep and shallow hotspot reference frames. While the oblique orientation of the rift was controlled by the pre-existing lithospheric fabric, the two reference frames predict different kinematics of Africa and Somalia plates along the rift itself, both in magnitude and direction, and with respect to the mantle. However, the observed kinematics and tectonics along the rift are more consistent with a faster WSW-ward motion of Africa than Somalia observed in the shallow hotspot framework. The faster WSW motion of Africa with respect to Somalia plate is inferred to be due to the lower viscosity in the top asthenosphere (LVZ—low-velocity zone) beneath Africa. These findings have significant implication for the evolution of continental rifting in transtensional settings and provide evidence for the kinematics of the Ethiopian Rift in the context of the Africa–Somalia plate interaction in the mantle reference frame.

Mud volcanism and fluid geochemistry in the Cheleken peninsula, western Turkmenistan

A geological and geochemical study has been carried out to investigate the relationships between major mud volcano structures and deep fluid migration in the Cheleken peninsula, in the South Caspian Basin. The fluid geochemistry allowed the origin and migration of the saline waters and the hydrocarbons to be deduced along with the regional source and reservoir rocks. The emitted waters formed by the mixing of deep highly saline water from the main source rocks of the Maykop Fm with the Caspian-like pore water contained in the Pliocene reservoirs. The water composition is very similar to that emitted by the mud volcanoes in Azerbaijan, allowing comparisons to be done between the reservoirs in the western and eastern sides of the South Caspian Basin. The associated oil is derived from a mixed type II/III kerogen deposited in a sub-oxic marine environment and generated during the early oil window. The oil biomarkers indicate that the source rock is the Maykop Fm., as previously determined for the other areas of the South Caspian Basin.The spontaneous emissions, showing different morphologies, are mainly aligned along normal and transtensive fault systems, which provide effective pathways for rapid fluid ascent from deep reservoirs to the surface.

Submarine volcanic mounds in the Pearl River Mouth Basin, northern South China Sea

A number of enigmatic, km-scale mound structures have been discovered in the Baiyun Sag of the Pearl River Mouth Basin using high-resolution 2D and 3D seismic data combined with multi-beam bathymetry. Based on detailed seismic characterization, we interpret these as sill-fed volcanic mounds. Associated structures include igneous sills, trans-tensional faults, lava flows and compactional drape folds. The igneous intrusions and associated mounds were emplaced in the late Early Miocene (c. 18.5 Ma or shortly thereafter), constrained by the onlap relationship between the mounds and their overburden. The volcanic mounds are preferentially developed above or adjacent to basement highs, indicating a structural control on the igneous plumbing system. We propose that the fractures and tectonic faults above the raised basement follow pre-existing zones of weakness and may have acted as feeding conduits for the magmatic material. The discovery of the Early Miocene igneous province has important implications for the understanding of the evolution of the Pearl River Mouth Basin and for regional hydrocarbon prospectivity. (C) 2014 Elsevier B.V. All rights reserved.

Construction of a sheeted dyke complex: evidence from the northern margin of the troodos ophiolite and its southern margin adjacent to the arakapas fault zone

The structure of a sheeted dyke complex in relation to intrusion is important for an understanding of sea-floor spreading. Here, we provide data for a previously undescribed ~ E-W cross-strike section along the northern margin of the Troodos massif. For comparison, we also provide new information for a ~NS section along the southern margin of the Troodos massif, adjacent to the Arakapas Fault Zone, an inferred oceanic transform fault. The N-Troodos margin section is divided into two main segments with opposing dip directions, corresponding to the Solea graben in the east and the Stravros graben in the west. High-temperature, hydrothermally altered dykes occur relatively early in a relative intrusive chronology of both grabens, followed by gabbroic bodies and ubiquitous grey sheeted dykes; plagiogranite dykes intruded at relatively later stages. Chemical analysis reveals a trend towards a boninite-type composition, both in the sheeted complex as a whole and in several specific sections for which crosscutting relations were documented in detail. The relatively steeply dipping, late-stage, boninitic-type dykes are interpreted as feeders to the Upper Pillow Lavas. To the N of the Arakapas Fault Zone, large plagiogranite bodies intruded relatively early and were in turn transected by grey dykes, small gabbroic intrusions and late-stage isolated brown dykes. Chemical analyses show that both early-stage grey dykes and the later-stage brown dykes were fed by chemically similar magma. Highly depleted boninite-type dykes occur close to the Arakapas Fault Zone. The relatively late-stage brown dykes show a swing in orientation towards ~ E-W near the transform zone, reflecting an inferred up to ~ 90° relatively clockwise, vertical-axis rotation. The earlier-stage grey dykes are indicative of lesser amounts of vertical-axis rotation but show complex rotations about low-angle axes. Small-scale faults are mainly of normal and left-lateral strike-slip type. Taking alternatives into account, we infer that the faulting in the dyke section took place near a left-offset spreading axis within an overall right-slipping, transtensional transform fault zone. In addition, depleted mantle harzburgites were exposed and eroded on the seafloor within an inferred southward extension of the transform zone (now within the Late Cretaceous Moni Melange), proving additional evidence of a transtensional setting near or within the Arakapas transform. The combined evidence suggests that the Troodos sheeted dyke complex as a whole developed in response to unstable, asymmetrical spreading above a subduction zone in which relatively late-stage amagmatic extension is likely to have played an important role. The spreading crust was bounded to the south (in present coordinates) by a relatively broad transtensional transform fault zone, along which mantle rocks were locally exposed and eroded on the seafloor.

Post-rift uplift and focused fluid flow in the passive margin of northern South China Sea

An important post-rifting tectonic event with extensive uplift and fluid flow took place at the northern margin of the South China Sea (SCS) during the late Miocene. Based on an analysis of two- and three-dimensional seismic data plus drilling well data, the tectonic event mainly affected the Dongsha Rise and the adjacent depressions. Tectonic deformation likely occurred in the Dongsha Rise, the eastern part of the Panyu Swell and several depressions of the Zhu I Sag. The post-rifting deformation in the study area was characterized by faulting, erosion, igneous activity and hydrothermal fluid flow. The tectonic movement caused abundant secondary NW and WNW tensional and transtensional faults, with lengths of 1 to 10 km. It also created a clear angular unconformity which marks the time of the tectonic movement Late Neogene igneous rocks are found in the strata in the Dongsha Rise only close to the continent-ocean transition zone. Geothermal activity occurred in the Liuhua carbonate platform and adjacent depressions. Deformation is more intense in the eastern area than in the western area. According to sequence stratigraphy, we infer that the tectonic event mainly occurred in the late period of Late Miocene and ended at the Mio/Pliocene boundary (5.5 Ma). The dynamic mechanism for tectonic movement could be associated with subduction of the South China Sea slab beneath the Philippine Sea Plate at the Manila trench. When subduction of lower density crust within the continental-ocean transition zone was initiated, large resistance stresses may have led to lithosphere bending and crustal uplift in the Dongsha area. (C) 2014 Elsevier B.V. All rights reserved.

Exploring for unconventional hydrocarbon plays in the Glyde Basin, Northern Territory, using FALCON® airborne gravity gradiometry (AGG) data

A FALCON® AGG and magnetic survey in the Glyde Sub-basin aims to define the structural pattern to identify unconventional hydrocarbon plays. The survey area consists of variable thick fluvial and lacustrine to shallow marginal marine carbonate-siliciclastic sequences and lesser volcanic rocks. The dominant tectonic feature identified on the AGG data is the Emu Fault Zone: a major structure of the central-southern part of the McArthur Basin. Seismic and surface geology suggest its overall sub-vertical strike-slip nature with positive flower structure geometry. In the north, a regional size pop-up structure, reverse, transpressional and strike-slip faults, and abundant synclines and anticlines identified in the AGG data indicate sinistral transpression. In the south, a transtensional segment of the Emu Fault Zone formed several regularly oriented, fault controlled depocentres. A geological cross-section across the Glyde Sub-basin supported by gravity modelling indicates a system of inverted transtensional faults. Offsets and bifurcations are common, forming local-scale transtensional or transpressional areas. The relationship between dolomitic carbonaceous siltstone, fault-related hydrothermal dolomite (HTD), and the tectonic pattern strongly suggests that the Emu Fault Zone controlled fluid migration and fault-related HTD deposition. Brecciated HTD reservoirs are best developed where a combination of strike-slip movement and extension allowed dolomitising and porosity-generating fluids to migrate along fracture networks, especially in transtensional pull-apart structures, and along the principal faults bounding elevated parts of the basement. This is consistent with the results of the Glyde–1 ST1 exploration well, which drilled 122 m of gas charged dolomitic breccia.

Relationships between tectonics and magmatism in a transtensive/transform setting: An example from Faial Island (Azores, Portugal)

The relationships between tectonics and hotspot-related magmatism in transform/transtensive settings are poorly known. The Azores archipelago, lying where the transform plate boundary between the Nubian and Eurasian plates meets the Mid-Atlantic Ridge, is a rare site to investigate these relationships. The distinct tectono-magmatic features of Faial make it the ideal island to focus the study. Here, we analyze the relationships between tectonics and magmatism, using remote-sensing, field, and paleomagnetic analyses. Dominant WNW-ESE–trending lineaments correspond to major faults with transtensive dextral motion and NE-SW opening, probably related to the nearby transtensive Terceira Rift. Most (∼60%) dikes, vent elongations, and alignments are parallel to this system. The lavas forming the basement of the distinctive WNW-ESE–trending Pedro Miguel graben strike parallel to the graben axis, dipping outward with a tilt consistent with that of paleomagnetic data. These lavas are dissected by the WNW-ESE–trending transtensive faults. Therefore, the graben consists of outward-tilted fault-bounded blocks, forming two opposite-verging dominoes. Its estimated stretching factor (β = 1.35) and mean extension rate (between 3.4 and 8.2 mm/yr) are similar or slightly larger than those of Terceira Rift, to the east. Part of the graben extension may be magma induced, via diking. We suggest that: (1) Faial, along with the nearby Pico Island, is a major locus of extension within the Azores, directly above the imaged hotspot; and (2) the Faial-Pico magmatic segment constitutes the offset, westward magmatic continuation of the Terceira Rift. The segment opening highlights the importance of extensional strain along magmatic transtensive plate boundaries.

GPS constraints on active deformation in the Isparta Angle region of SW Turkey

SUMMARY We use survey and continuous GPS observations during the period 1997‐2010 to investigate active deformation in the Isparta Angle region of SW Anatolia, Turkey. This region, bordered bytheFethiyeBurdurFaultZone(FBFZ)inthewestandtheSEextensionoftheAksehirSimav Fault Zone (AKSFZ) in the east, accommodates a part of the active deformation of W Turkey. Our results show that the Isparta Angle region rotates counter-clockwise (CCW) with respect to Anatolia. Both the FBFZ and the AKSFZ are predominantly transtensional boundaries that accommodate southward motion of the Isparta region with respect to Anatolia. The FBFZ has left-lateral strike-slip behaviour along its SW segment that changes to right-lateral strike-slip along its NE extension. This change in the sense of strike-slip motion is accommodated by extension on a NW‐SE striking normal fault system that is associated with the Menderes Graben system. Transtensional fault systems along the boundaries of the Isparta Angle with Anatoliaareinconsistentwithextrusionmodelsforpresent-daysouthwardmotion.Anincrease in motion rates towards the Hellenic and Cyprus arc subduction systems, and CCW of the Isparta region, supports dynamic models involving active rollback of the subducting African Plate, toroidal mantle flow around the eastern edge of the subducting African slab near the Hellenic arc/Florence Rise junction, and/or gravitational collapse of the overriding Anatolia Plate.

Seismotectonics of the Armutlu peninsula (Marmara Sea, NW Turkey) from geological field observation and regional moment tensor inversion

The Armutlu peninsula, located in the eastern Marmara Sea, coincides with the western end of the rupture of the 17 August 1999, İzmit MW 7.6 earthquake which is the penultimate event of an apparently westward migrating series of strong and disastrous earthquakes along the NAFZ during the past century. We present new seismotectonic data of this key region in order to evaluate previous seismotectonic models and their implications for seismic hazard assessment in the eastern Marmara Sea. Long term kinematics were investigated by performing paleo strain reconstruction from geological field investigations by morphotectonic and kinematic analysis of exposed brittle faults. Short term kinematics were investigated by inverting for the moment tensor of 13 small to moderate recent earthquakes using surface wave amplitude spectra. Our results confirm previous models interpreting the eastern Marmara Sea Region as an active transtensional pull-apart environment associated with significant NNE–SSW extension and vertical displacement. At the northern peninsula, long term deformation pattern did not change significantly since Pliocene times contradicting regional tectonic models which postulate a newly formed single dextral strike slip fault in the Marmara Sea Region. This area is interpreted as a horsetail splay fault structure associated with a major normal fault segment that we call the Waterfall Fault. Apart from the Waterfall Fault, the stress strain relation appears complex associated with a complicated internal fault geometry, strain partitioning, and reactivation of pre-existing plane structures. At the southern peninsula, recent deformation indicates active pull-apart tectonics constituted by NE–SW trending dextral strike slip faults. Earthquakes generated by stress release along large rupture zones seem to be less probable at the northern, but more probable at the southern peninsula. Additionally, regional seismicity appears predominantly driven by plate boundary stresses as transtensional faulting is consistent with the southwest directed far field deformation of the Anatolian plate.

Basin seismic stratigraphy and aspects of prospectivity in the NE Baffin Bay, Northwest Greenland

Subsurface mapping in the northeastern part of Baffin Bay, offshore Northwest Greenland indicates the presence of large basement structures and sedimentary basins. We identify eight seismic-stratigraphic units that can be genetically related to the regional geology and tectonic development. The seismic interpretation includes the top of Paleocene to Eocene volcanic rocks, which cover most parts of the southern and western study area. Using this horizon as a time marker, the seismic units are divided into pre-Cenozoic and Palaeogene to Neogene strata based on comparison to equivalent units exposed onshore West Greenland and Canada and sampled by exploration wells farther south in the Disko West/offshore Nuuk region. Our interpretation of the faults and tectonic elements suggest that the main extensional faults and basin formation took place mainly during the Cretaceous and into the Paleocene. Subsequent compressional tectonics and inversion were accompanied by strike-slip faulting and local transtensional faults and flexures. This last phase of deformation corresponds to the well documented Eurekan Orogeny in Ellesmere Island and northern Greenland that developed as a result of northward motion and possible counterclockwise rotation of Greenland relative to the North American plate. We surmise that these tectonic phases exerted a strong control on basin infill by activation of hinterland sediment sources during Cretaceous and Palaeogene times. The deep basins may include Cretaceous and Palaeogene organic-rich sediments with hydrocarbon source potential. We estimate that maturation could have started during the Eocene.

Processing and interpretation of vintage 2D marine seismic data from the outer Hanö Bay area, Baltic Sea

A grid of previously unpublished, vintage 2D marine seismic lines has been processed and interpreted to the east of Hanö Bay, SW Baltic Sea. The 3200 km 2 study area lies on the transition between the Hanö Bay Basin to the West and Baltic Synelcise to the East, NE of the Tornquist intra shield tectonic zone. Data from the NA79, NA80 and RW84 surveys were selected for this study from the extensive Oljeprospektering AB (OPAB) Baltic Sea dataset. New processing workflows have been developed for the data which focus on suppressing two significant forms of noise, namely multiple and side scattered noise. Deconvolution in the tau-p domain, parabolic radon demultiple and post stack deconvolution are shown to be effective at attenuating multiple noise, while FK filtering in shot and receiver gathers is effective at removing side scattered noise. The newly processed data were interpreted and a series of maps detailing the structure of the basement, Cambrian and Silurian/Paleozoic horizons were constructed. These maps differ significantly to previously published interpretations of the area. Within the study area, a region of significant Late Carboniferous/Early Permian transtensional faulting and Late Cretaceous inversion is mapped in detail. This structure would have exhibited normal offsets of up to 600 m before inversion with later inverted displacements of up to approximately 200 m in places. This feature appears to extend some 20 km to the SW of the study area to the major fault bounding the Christiansø High. Based on the seismic interpretation, the area appears to have had a similar overall geological history as the adjacent Tornquist Zone to the SW. • A grid of 2D lines is processed and interpreted from the outer Hanö Bay area. • Key data issues are multiple and side scattered noise and large receiver spacing. • Parabolic radon demultiple, tau-p deconvolution and FK filtering are used. • A zone of major late Palaeozoic faulting and late Cretaceous inversion is mapped.

Meso-Cenozoic extensional structures in the Northern Tarim Basin, NW China

Meso-Cenozoic extensional structures are important for understanding the tectonics of the Chinese Central Asia. This paper presents a systematic investigation on the Meso-Cenozoic extensional structures in the Northern Tarim Basin. Close interpretations of seismic data reveal that the Meso-Cenozoic extensional structures were widely developed in the Northern Tarim Basin. These extensional structures are regionally composed of many small normal faults, which usually group into left- or right-step en echelon and form several transtensional fault zones. Combinations of normal faults in profile become small graben-horst or staircase-like cross-sections. Based on the areal distribution, structural style, combination relationship, formation and evolution time, and formation mechanism of the extensional structures, we found that the Meso-Cenozoic extensional structures in Northern Tarim Basin can be classified into two conjugate normal fault systems, which were formed separately in Jurassic-Early Cretaceous and Late Cretaceous-Neogene. The former is likely associated with the stress relaxation after a collisional orogeny accompanied with a certain degree of anticlockwise rotation of the Tarim block relative to the South Tianshan; the latter is possibly induced by the east by south tectonic escape of the Tarim block with a certain degree of clockwise rotation relative to the South Tianshan triggered by the far-field effect of the Himalayan orogeny.

Field, geochemistry and Sr-Nd isotopes of the Pan-African granitoids from the Tifnoute Valley (Sirwa, Anti-Atlas, Morocco): a post-collisional event in a metacratonic setting

In the Tifnoute Valley, three plutonic units have been defined: the Askaoun intrusion, the Imourkhssen intrusion and the Ougougane group of small intrusions. They are made of quartz diorite, granodiorite and granite and all contain abundant mafic microgranular enclaves (MME). The Askaoun granodi- orite and the Imourkhssen granite have been dated by LA-ICP- MS on zircon at 558±2 Ma and 561±3 Ma, respectively. These granitic intrusions are subcontemporaneous to the widespread volcanic and volcano-detrital rocks from the Ouarzazate Group (580-545 Ma), marking the post-collisional transtensional period in the Anti-Atlas and which evolved towards alkaline and tholeiitic lavas in minor volume at the beginning of the Cambrian anorogenic intraplate extensional period. Geochemically, the Tifnoute Valley granitoids belong to an alkali-calcic series (high-K calc-alkaline) with typical Nb-Ta negative anomalies and no alkaline affinities. Granitoids and enclaves display positive eNd-560Ma (+0.8 to +3.5) with young Nd-TDMbetween800and1200Maandrelativelylow 87 Sr/ 86 Sr initial ratios (Sri: 0.7034 and 0.7065). These values indicate a mainly juvenile source corresponding to a Pan-African meta- somatized lithospheric mantle pa rtly mixed with an old crustal component from the underlying West African Craton (WAC). Preservation in the Anti-Atlas of pre-Pan-African lithologies (c. 2.03 Ga basement, c. 800 Ma passive margin greenschist-facies sediments, allochthonous 750-700 Ma ophiolitic sequences) indicates that the Anti-Atlas lithosphere has not been thickened and was never an active margin during the Neoproterozoic. After a transpressive period, the late Ediacaran period (580- 545 Ma) is marked by movement on near vertical transtensional faults, synchronous with the emplacement of the huge Ouarzazate Group and the Tifnoute Valley granitoids. We pro- pose here a geodynamical model where the Tifnoute Valley granitoids as well as the Ouarzazate Group were generated during the post-collisional metacratonic evolution of the north- ern boundary of the West African craton. The convergence with the peri-Gondwanan active margin produced brittle fracturing of the cratonic boundary without thickening, allowing rising of magmas. The Tifnoute Valley granitoids display a metasomat- ized lithospheric mantle source mixed with a minor ancient (2 Ga) continental crust component from the underlying WAC.

Volcano-tectonic controls and emplacement kinematics of the Iriga debris avalanches (Philippines)

Mt Iriga in southeastern Luzon is known for its spectacular collapse scar that was possibly created in 1628 ad by a 1.5-km3 debris avalanche. The debris avalanche deposit (DAD) covered 70 km2 and dammed the Barit River to form Lake Buhi. The collapse has been ascribed to a non-volcanic trigger related to a major strike-slip fault under the volcano. Using a combination of fieldwork and remote sensing, we have identified a similar size, older DAD to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits cover wide areas of low, waterlogged plains, to a distance of about 16 km for the oldest and 12 km for the youngest. Hundreds of metre-wide and up to 50-m-high hummocks of intact conglomerate, sand and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata. In addition, large proportions of both DAD consist of ring-plain sediments that were incorporated by soft-sediment bulking and extensive bulldozing. An ignimbrite unit incorporated into the younger DAD forms small (less than 5 m high) discrete hummocks between the larger ones. Both debris avalanches slid over water-saturated soft sediment or ignimbrite and spread out on a basal shear zone, accommodated by horst and graben formation and strike-slip faults in the main mass. The faults are listric and flatten into a well-developed basal shear zone. This shear zone contains components from the substrate and has a diffuse contact with the intact substrata. Long, transport-normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing. The collapse orientation and structure on both sectors and DAD constituents are consistent with collapse being a response to combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand in sedimentary basins undergoing transtensional strike-slip faulting.

Structural records of the Late Cretaceous–Cenozoic extension in Eastern China and the kinematics of the Southern Tan-Lu and Qinling Fault Zone (Anhui and Shaanxi provinces, PR China)

Our study of the Late Cretaceous–Cenozoic extension in Eastern China is a kinematic analysis of faults (≈1500 striated fault data, 175 stress solutions at 90 sites) that has permitted us to separate stress fields belonging to six tectonic events. A WNW–ESE extension (1) has affected Eastern China from the Campanian to the Late Palaeocene–Early Ypresian. Under this extension, the Qinling (QLFZ) and Southern Tan-Lu (STLFZ) Fault Zones have been dextral transtensional fault zones. A first transpressional event (2) (NNE–SSW shortening) has occurred during the Late Ypresian–Lower Lutetian; the QLFZ and the STLFZ have been dextral transpressional fault zones. A NE–SW extension (3) has affected the Weihe graben during the Palaeogene; after a second transpressional event (4), the NE–SW extension (5) has been rejuvenated during the Late Miocene and the QLFZ has been a normal fault zone again and the STLFZ a sinistral transtensional fault zone; synrift subsidence has occurred in the Weihe graben. The second transpressional event (4) (WNW–ESE shortening) has occurred during the Late Oligocene–Early Miocene, during a period of uplifting and exhumation. At that time, the STLFZ and the QLFZ have become sinistral transpressional fault zones. A period of extension (6) has taken place during the Late Pliocene–Quaternary; the extensional direction has trended ≈NNW–SSE; during this event, the QLFZ has been a sinistral transtensional fault zone and the STLFZ a dextral transtensional fault zone. These results are compared with those previously published in order to evaluate the regional significance of these tectonic events.

Large-scale deformational systems in the South Polar Layered Deposits (Promethei Lingula, Mars): “Soft-sediment” and Deep-Seated Gravitational Slope Deformations Mechanisms

The present study is the first attempt at a detailed structural and kinematic analysis of large-scale deformational systems observed in the South Polar Layered Deposits (SPLDs) in the Promethei Lingula (PL) margins (Mars). By systematically collecting attitude data referable to previously unknown deformational structures and defining the cross-cut relationships of the structures, we reconstructed a deformational history consisting of two superimposed, well-defined stages. The first stage is dominated by large-scale strike–slip and transtensional faults arranged into conjugate systems and delimiting shear zones that show a wide range of subsidiary structures, including normal and reverse faults, drag folds, boudins, S–C tectonites and sub-horizontal interstratal shear planes marked by sygmoidal boudins. Other typical structures referable to this event are ductile folds (locally true convolute folds) and lobes (ball-and-pillow structures) affecting certain marker beds of the succession. We suggest that the structural assemblage might be the expression of a shallow soft-sediment tectonics that possibly occurred during warm periods of the South Pole climate.The second stage seems to affect the weaker and in certain cases pre-deformed stratigraphic levels of the SPLD succession. This stage is mainly characterized by extensional deformations caused by gravity. The consequence of the deformations is the nucleation of Deep-Seated Gravitational Slope Deformations (DSGSDs) marked by typical morphostructures, such as scarps, trenches and bulging basal contractant zones. These phenomena were never observed within an ice cap. According to terrestrial modeling, these slow collapses were caused by (1) the presence of detachment levels (i.e., subhorizontal bedding planes) along which the ice-sheet margins can slide and (2) the development of listric faults within the glacial mass, which merge with sub-horizontal shear planes in the subsurface.The presence of complex deformational systems in the SPLD necessarily implies that a large-scale dynamics of the ice-sheet occurred in the past. The relatively fast internal creep and basal/internal sliding, inferable from the structure assemblage, can be due to partial melting of the ice possibly caused by climatic changes in the Promethei Lingula region. In this manner, we believe that climate heating (which, according to the literature, is likely caused by orbital variations) softened some of the SPLD layers, triggering or accelerating the ice sheet’s outward movement.The evidence of a marked disharmonic deformational style through the SPLD succession suggests the possibility of local periodic compositional variations in the sequence.

Quantifying submarine fluid seep activity along the North Anatolian Fault Zone in the Sea of Marmara

Abstract The Sea of Marmara presents the rare case where active seafloor venting sites are found on the surface trace of a major plate boundary fault: the North Anatolian Fault Zone. The objective of the 2007 MarNaut project was to quantify the level of activity of these venting sites, and the source of the fluids emitted, with the goal of understanding the processes involved and setting a baseline for long-term studies of the relationship between seismic activity and fluid migration/expulsion processes. Sites for flow meter and fluid sampler deployment and coring included basin bounding transtensional faults and strike–slip faults cutting through the topographic highs. Significant fluid flow appears to be primarily an episodic phenomenon at all sites, with background rates on the order of mm/yr to cm/yr except at or very near rare focused vents. Basin bounding faults expel primarily shallow sourced fluid with a strong influence of brackish Pleistocene Lake Marmara water, with the notable exception of the main active fault along the northern edge of the Cinarcik Basin. Expulsion sites where the main fault crosses topographic highs are more complex with evidence for deep-sourced fluids including thermogenic gas. One site on the Western High displayed two mound structures that appear to be chemoherms atop a deep-seated fluid conduit. The fluids being expelled are brines with an exotic fluid chemistry along with thermogenic gas and oil. Our work shows that submerged continental transform plate boundaries can be hydrologically active and exhibit a diversity of sources and processes.

Structural and geomorphic features accommodating groundwater of Al-Madinah City, Saudi Arabia

Al-Madinah City is located in the western part of Saudi Arabia on the Arabian Shield. The area underwent several tectonic events that developed its structural and geomorphic features, such as the Infracambrian Najd strike-slip faults, development of the Cenozoic basaltic flows of Northern Harrat Rahat, and Cenozoic N–S and E–W transtensional faults, related to the Red Sea rifting. These successive events formed a deltaic-shaped basin of Al-Madinah. The Al-Madinah basin is part of a 400 × 150-km2 Wadi Qanah–Al-Hamd watershed, which exhibits mainly parallel drainage pattern. Sub-basins, within the main basin, exhibit trellised and radial drainage patterns. The trellised drainage pattern reflects control of the Cenozoic faults, whereas the radial drainage pattern reflects volcanic-related system. Rotation of the Arabian Plate after several extensional events that lead to the opening of the Red Sea influenced the drainage flow to be going from east to west. This geological history that include eruption, normal faulting, and erosion prior to and during the Red Sea rifting formed relief inversion geomorphology of Tertiary basalts that cap Precambrian rocks of the Ayr and Jammah Mountains in western Al-Madinah. The groundwater in the central area is part of the northern Harrat Rahat basaltic aquifer in which the groundwater level rises up in the central area due to the blocking of groundwater flow by constructions below the central area and due to reduced groundwater abstraction. Building a dam 60 km northwest of Al-Madinah would preserve more surface water than the Al-Bayda dam, in which all main valleys join in at the suggested location.