Graben Formation Research Articles

Large deformation finite-element modelling of progressive failure leading to spread in sensitive clay slopes

The occurrence of large landslides in sensitive clays, such as spreads, can be modelled by progressive development of large inelastic shear deformation zones (shear bands). The main objective of the present study is to perform large deformation finite-element modelling of sensitive clay slopes to simulate progressive failure and dislocation of failed soil mass using the coupled Eulerian Lagrangian (CEL) approach available in Abaqus finite-element software. The degradation of undrained shear strength with plastic shear strain (strain-softening) is implemented in Abaqus CEL, which is then used to simulate the initiation and propagation of shear bands due to river bank erosion. The formation of horsts and grabens and dislocation of soil masses that lead to large-scale landslides are simulated. This finite-element model explains the displacements of different blocks in the failed soil mass and also the remoulding of soil around the shear bands. The main advantages of the present finite-element model over other numerical models available in the literature are that it can simulate the whole process of progressive failure leading to spread. The finite-element results are consistent with previous conceptual models proposed from field observations. The parametric study shows that, depending upon geometry and soil properties, toe erosion could cause three types of shear band formation: (a) only a horizontal shear band without any global failure; (b) global failure of only one block of soil; (c) global failure of multiple blocks of soil in the form of horsts and grabens.

Thermal maturity and petroleum kitchen areas of Liassic Black Shales (Lower Jurassic) in the central Upper Rhine Graben, Germany

In the central Upper Rhine Graben (URG), several major oil fields have been sourced by Liassic Black Shales. In particular, the Posidonia Shale (Lias e, Lower Toarcian) acts as excellent and most prominent source rock in the central URG. This study is the first comprehensive synthesis of Liassic maturity data in the URG area and SW Germany. The thermal maturity of the Liassic Black Shales has been analysed by vitrinite reflectance (VRr) measurements, which have been verified with T max and spore coloration index (SCI) data. In outcrops and shallow wells (<600 m), the Liassic Black Shales reached maturities equivalent to the very early or early oil window (ca. 0.50–0.60 % VRr). This maturity is found in Liassic outcrops and shallow wells in the entire URG area and surrounding Swabian Jura Mountains. Maximum temperatures of the Posidonia Shale before graben formation are in the order of 80–90 °C. These values were likely reached during Late Cretaceous times due to significant Upper Jurassic and minor Cretaceous deposition and influenced by higher heat flows of the beginning rift event at about 70 Ma. In this regard, the consistent regional maturity data (VRr, T max, SCI) of 0.5−0.6 % VRr for the Posidonia Shale close to surface suggest a major burial-controlled maturation before graben formation. These consistent maturity data for Liassic outcrops and shallow wells imply no significant oil generation and expulsion from the Posidonia Shale before formation of the URG. A detailed VRr map has been created using VRr values of 31 wells and outcrops with a structure map of the Posidonia Shale as reference map for a depth-dependent gridding operation. Highest maturity levels occur in the area of the Rastatt Trough (ca. 1.5 % VRr) and along the graben axis with partly very high VRr gradients (e.g. well Scheibenhardt 2). In these deep graben areas, the maximum temperatures which were reached during upper Oligocene to Miocene times greatly exceed those during the Cretaceous.

PALEOSTRESS ANALYSIS OF KAROO SUPERGROUP OF THE TSHIPISE-PAFURI BASIN, SOUTH AFRICA

The Tshipise-Pafuri basin is located in the Limpopo Province, north-eastern South Africa. This paper presents the first paleostress-analysis results from fault-slip data on the sedimentary rocks of Karoo Supergroup of late Palaeozoic to early Mesozoic in Tshipise-Pafuri basin. Stress inversion of 251 fault-slip data was performed using an improved Right-Dihedral method, followed by rotational optimization (WINTENSOR Program, Delvaux, 2012). Fault-slip data including fault planes, striations and sense of movements, were obtained from rock exposures distributed in ten stations in the basin. The orientation of the principal stress axes (σ1, σ2 and σ3) and the ratio of the principal stress differences (R) show a paleostress field marking two main stress regimes. The main strain field in the Tshipise-Pafuri basin is characterised by west to east to east-northeast to west-southwest extension and the second strain field is a minor north to south to west-northwest to east-southeast reactivated extension. It includes two paleostress regimes, one with dominantly dip-slip normal faulting (extensional regime) and a minor normal faulting (extensional regime). The north-eastern margin of the Kaapvaal craton was down-faulted into a graben structure, in which the pre-Karoo Soutpansberg Group was deposited. This faulting, which controlled graben formation, continued during the deposition of the Karoo sediments and was reactivated in post-Karoo times, resulting in a very complex structural setting. The east-west trends of the stress fields in the basin follows the east-west elongated low lying Limpopo Mobile Belt.

Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean

The Karukera spur is an easternmost submerged rise of the Lesser Antilles fore-arc in the Guadeloupe archipelago, culminating about 4000 m above the fore-arc basin 150 km west of the deformation front. The analysis of 3500 km 2D high-resolution multichannel seismic reflection and multibeam bathymetric data, and the study of 14 core samples, allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution. Seven seismic units (US1 to US7) are evidenced, organised into four major depositional sequences separated by erosional surfaces. Units US1 to US4 thicken to the south-southeast on the spur. The sedimentary deposits rest upon a deeply eroded metamorphic basement of the Caribbean Plate as in La Desirade Island. Sequence 1 (units US1 and US2) remains unknown in its lower part because it does not outcrop. Its upper part comprises early Miocene shallow water carbonates. Sequence S2 (Unit US3) only comprises an upper Miocene transgressive systems tract deposited into basinal environment. It is bounded by major erosional surfaces SB2 and SB3. Sequence 3 (US4 to US6) is Zanclean to Pleistocene. It is dominantly composed of shallow water, reefal deposits. Sequence 4 corresponds to recent deposits. The Karukera spur experienced three main extensional episodes with alternations between uplift and subsidence periods. Uplifts are responsible for emersions and erosions. Subsidence severely drowned shallow water deposits. The first extensional episode led to the formation of horsts and grabens in the basement, prior to early Miocene deposits. The second extensional episode occurred during upper Miocene, in accordance with data in surrounding islands. The third tectonic episode developed since Calabrian. Noticeable is the constant reactivation of N130°–N150°, N150°–N180° and N40° trending faults since early Miocene and the neoformation of E–W trending faults since Calabrian. Our results suggest that at the latitude of Guadeloupean archipelago the Lesser Antilles subduction zone is an erosional margin because of the successive sweepings of the Santa Lucia then Tiburon Ridges.

Tectonic and sedimentary architecture of the Karukéra spur: A record of the Lesser Antilles fore-arc deformations since the Neogene

The Karukéra spur is an easternmost submerged rise of the Lesser Antilles fore-arc in the Guadeloupe archipelago, culminating about 4000 m above the fore-arc basin 150 km west of the deformation front. The analysis of 3500 km 2D high-resolution multichannel seismic reflection and multibeam bathymetric data, and the study of 14 core samples, allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution. Seven seismic units (US1 to US7) are evidenced, organised into four major depositional sequences separated by erosional surfaces. Units US1 to US4 thicken to the south-southeast on the spur. The sedimentary deposits rest upon a deeply eroded metamorphic basement of the Caribbean Plate as in La Désirade Island. Sequence 1 (units US1 and US2) remains unknown in its lower part because it does not outcrop. Its upper part comprises early Miocene shallow water carbonates. Sequence S2 (Unit US3) only comprises an upper Miocene transgressive systems tract deposited into basinal environment. It is bounded by major erosional surfaces SB2 and SB3. Sequence 3 (US4 to US6) is Zanclean to Pleistocene. It is dominantly composed of shallow water, reefal deposits. Sequence 4 corresponds to recent deposits. The Karukéra spur experienced three main extensional episodes with alternations between uplift and subsidence periods. Uplifts are responsible for emersions and erosions. Subsidence severely drowned shallow water deposits. The first extensional episode led to the formation of horsts and grabens in the basement, prior to early Miocene deposits. The second extensional episode occurred during upper Miocene, in accordance with data in surrounding islands. The third tectonic episode developed since Calabrian. Noticeable is the constant reactivation of N130°–N150°, N150°–N180° and N40° trending faults since early Miocene and the neoformation of E–W trending faults since Calabrian. Our results suggest that at the latitude of Guadeloupean archipelago the Lesser Antilles subduction zone is an erosional margin because of the successive sweepings of the Santa Lucia then Tiburon Ridges.

Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland.

Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M>5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.

Effects of Ninetyeast Ridge Magmatism and Pre India-Eurasia Collision Dynamics on Basement and Crust-lithospheric Structures of the Northeastern Indian Ocean

Abstract: The coexistence of multifaceted geodynamical situation in the Northeastern Indian Ocean (Bay of Bengal) lithosphere inspires to investigate the origin, present day orientation and gravitational stability of its basement, crust and lithospheric structures. The origin, evolution and the present day configuration of these structures such as the northward down dipping of the Bay of Bengal basement and its lithosphere, the Moho of the 85°E ridge, the formation of the Ninetyeast ridge median graben and ridge normal faults and their interaction with slow convergence and stress diffusion processes in the north Andaman trench, remain conspicuous. In this paper, we explain the above mentioned processes and the existence of the Bay of Bengal basement features through plausible tectonic scenarios that related with past time dynamics of the Indian Plate prior to the India-Eurasia collision and lithospheric thermal perturbation due to the Ninetyeast Ridge magmatism. Our analysis on the basis of satellite altimeter-derived geoid, gravity, sediment thickness, basement undulation, isostatic and prospecting geoid anomaly; suggests apart from the enhanced Oligocene Himalayan erosion and subsequent increased sediment load, the gravitational stability of the basin load (like formation of ridge graben) was also affected by altering the basin rheology to inelastic, owing to the enhanced thermal perturbation by the ninetyeast ridge magma. This made the Bay of Bengal lithosphere rheologically weak. Pre India-Eurasia collision, fast movement and fast convergence of the Indian plate at erstwhile north Andaman subduction zone made the lithosphere relatively stretchable due to the prevalence of intra-plate extensional stresses. The NE-SW orientation of the Central basin depression and the northward (NE-SW) dipping of the Bay of Bengal (BOB) lithosphere are pointing to such a stretchable lithosphere and its remnants are present at the northeast of the northern Ninetyeast ridge, as observed from the low prospecting geoid anomaly. The presence of such a thermally altered, weak lithosphere as a subducting slab at the north Andaman trench caused the present day observed low convergence rate, lack of seismicity, as well as the cessation of fault rupture followed by the Great Sumatra Andaman earthquake of Mw (9.1) on 26 December 2004. The Ninetyeast ridge oblique collision with the north Andaman trench was presumed around Pleistocene based on the timing of abandonment of the Nicobar fan from the Bengal head fan. The discontinuity of the ridge along the oblique convergence zone of the north Andaman and its isostatic stability are mainly controlled by the subduction zone dynamics, unlike the 85°E ridge.

Lucky Strike seamount: Implications for the emplacement and rifting of segment‐centered volcanoes at slow spreading mid‐ocean ridges

AbstractThe history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow spreading Mid‐Atlantic Ridge at the Lucky Strike Segment is deduced using near‐bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (&lt;1 Myr), associated with focused eruptions, and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction. This volcanic phase likely requires efficient melt pooling and a long‐lived crustal magma chamber as a melt source. A reduction in melt supply triggers formation of an axial graben rifting the central volcano, and the onset of seafloor spreading may eventually split it. At Lucky Strike, this results in two modes of crustal construction. Eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows, accumulating a thick layer of extrusives. Away from the volcano summit, deformation and volcanic emplacement is distributed throughout the rift valley floor, lacking a clear locus of accretion and deformation. Volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber. The mode of rapid volcano construction and subsequent rifting observed at the Lucky Strike seamount is common at other central volcanoes along the global mid‐ocean ridge system.

Directional flank spreading at Mount Cameroon volcano: Evidence from analogue modeling

AbstractMount Cameroon is characterized by an elongated summit plateau, steep flanks, and topographic terraces around its base. Although some of these features can be accounted for by intrusion‐induced deformation, we here focus on the contribution of edifice‐scale gravitational spreading in the structure of Mount Cameroon. We review the existing geological and geophysical data and morphostructural features of Mount Cameroon and surrounding sedimentary basins. Volcanic ridge gravitational spreading is then simulated by scaled analogue models on which fault formation is recorded using digital image correlation. Three sets of models are presented (i) models recorded in cross section (Type I), (ii) models recorded from above with a uniform (Type IIa), and (iii) nonuniform ductile layer (Type IIb). Type I models illustrate the formation of faults accommodating summit subsidence and lower flank spreading. Type IIa models favor displacement perpendicular to the long axis, with formation of a summit graben and basal folds, but fail to reproduce the steep flanks. Type IIb models investigate the effect of spatial variations in sediment thickness and/or properties consistent with geological evidence. Directional spreading of the volcano's central part perpendicular to the long axis is accounted for by a sediment layer with restricted lateral extent and increasing thickness away from the volcano axis. The later model closely reproduces key features observed at Mount Cameroon: steep upper flanks are accounted for by enhanced lateral spreading of the lower flanks relative to the summit. The relevance of these findings for understanding flank instabilities at large oceanic volcanoes is finally highlighted.

Circumferential graben and the structural evolution of Alba Mons, Mars

Alba Mons is a unique, very extensive but shallow volcanotectonic construct in northern Tharsis, Mars. Numerous models have been presented to explain the formation of Alba Mons and its most characteristic feature, a wristwatch-like pattern of radial and circumferential graben. We used a wide selection of topographic datasets to characterize the fault throw variation on nine topographic transects across the circumferential graben in order to provide observational constraints for the different formation models, and to gain further insight into the evolution of Alba Mons. In most of the transects, summed throws from outwards-facing (away from the center of the volcano) faults are larger than from the inwards-facing (towards the center) ones. Only the very gently sloping western transects show the opposite, emphasizing the east–west-asymmetry of Alba Mons. 10–40% of the observed topographic relief of Alba Mons along the nine transects can be accounted for by this throw difference between the inwards- and outwards-facing faults. These results are consistent with predictions of models suggesting an uplift mechanism to explain the formation of the circumferential graben, but not with models invoking central subsidence. Horizontal extensional strain along the transects varies between 0.5% and 2%, consistent with strain predictions of the late-stage sill complex inflation model of McGovern et al. (McGovern, P.J., Solomon, S.C., Head J.W. III, Smith, D.E., Zuber M.T., Neumann, G.A. [2001]. J. Geophys. Res. 106(E10), 23769–23809).

A Revised Paleo-Earthquake Chronology on the Southeast Reelfoot Rift Margin near Memphis, Tennessee

A trench was excavated across the southeastern Reelfoot rift margin for paleoseismic research purposes and for the 2011 Seismological Society of America national meeting field trip. The trench was parallel to and 6 m southwest of the Oldham trench described in 2006. In this 2011 trench, faulted alluvial fan stratigraphy and liquefaction deposits less than 4000 yr old were exposed. The trench revealed three tectonic deformation events. The first event (graben formation) and the second event (sand blow, minor faulting, and injection of sand dikes) both post-date a paleosol circa 4000 yr B.P. and pre-date a surficial colluvial soil deposit circa 2000 yr B.P. The third event (minor shallow liquefaction and surface deformation) post-dates a 2000-year-old surface colluvial soil. These age constraints allow this third event to be attributed to the 1811-1812 New Madrid earthquakes or a remotely triggered earthquake related to the 1811-1812 earthquake sequence. Although we cannot rule out that the deformation revealed in this trench was caused by earthquakes on other faults in the New Madrid seismic zone, our favored interpretation is that the deformation was caused by rupture on an underlying fault at the base of the Mississippi River bluff that was previously imaged in a shallow reflection profile near this site.

The origin of graben and ridges in Rachmaninoff, Raditladi, and Mozart basins, Mercury

The Rachmaninoff, Raditladi, and Mozart peak‐ring impact basins on Mercury display a distinctive pattern of tectonic features consisting of a central zone that is either devoid of tectonic landforms or contains small ridges, a medial annulus of prominent and predominantly circumferentially oriented graben, and a distal zone displaying graben that occur in a mix of orientations and that are less evident toward the peak ring. Here we use finite element models to explore three candidate scenarios for the formation of these tectonic features: (1) thermal contraction of the interior smooth plains, (2) isostatic uplift of the basin floor, and (3) subsidence following volcanic loading. Our results suggest that only thermal contraction can account for the observed pattern of graben, whereas some combination of subsidence and global contraction is the most likely explanation for the central ridges in Rachmaninoff and Mozart. Thermal contraction models, however, predict the formation of graben in the centermost region of each basin, where no graben are observed. We hypothesize that graben in this region were buried by a thin, late‐stage flow of plains material, and images of partially filled graben provide evidence of such late‐stage plains emplacement. These results suggest that the smooth plains units in these three basins are volcanic in origin. The thermal contraction models also imply a cooling unit ~1 km thick near the basin center, further supporting the view that plains‐forming lavas on Mercury were often of sufficiently high volume and low viscosity to pool to substantial thicknesses within basins and craters.

On the formation and evolution of the Pannonian Basin: Constraints derived from the structure of the junction area between the Carpathians and Dinarides

The large number and distribution of rollback systems in Mediterranean orogens infer the possibility of interacting extensional back‐arc deformation driven by different slabs. The formation of the Pannonian back‐arc basin is generally related to the rapid Miocene rollback of a slab attached to the European continent. A key area of the entire system that is neglected by kinematic studies is the connection between the South Carpathians and Dinarides. In order to derive an evolutionary model, we interpreted regional seismic lines traversing the entire Serbian part of the Pannonian Basin. The observed deformation is dominantly expressed by the formation of Miocene extensional detachments and (half) grabens. The extensional geometries and associated synkinematic sedimentation that migrated in time and space allow the definition of a continuous and essentially asymmetric early to late Miocene extensional evolution. This evolution was followed by the formation of few uplifted areas during the subsequent latest Miocene–Quaternary inversion. The present‐day extensional geometry changing the strike across the basin is an effect of the clockwise rotation of the South Carpathians and Apuseni Mountains in respect to the Dinarides. Our study infers that the Carpathian rollback is not the only mechanism responsible for the formation of the Pannonian Basin; an additional middle Miocene rollback of a Dinaridic slab is required to explain the observed structures. Furthermore, the study provides constraints for the pre‐Neogene orogenic evolution of this junction zone, including the affinity of major crustal blocks, obducted ophiolitic sequences and the Sava suture zone.

Melange: A viscoelastic lattice‐particle model applicable to the lithosphere

This article introduces the software Melange, a 3D lattice‐particle hybrid model. The software was specifically designed in order to simulate ductile visco‐elasto‐plastic deformation and can be used to study tectonic processes in the lithosphere from the micro to the macro scale. Melange is under an open source license. The code takes both relevant yield mechanisms for the deformation of lithospheric material into account: dynamic brittle failure and ductile creep, where ductile creep is modeled as viscoelasticity. The software considers effects of the local geology, of the inherent disorder of geomaterials, of rheological layering of the lithosphere and applies repulsion when the material fractures. Driving forces are the externally applied strain and the gravitational load. Melange applies an elastically isotropic regular 3D lattice with HCP geometry and next‐nearest neighbor interactions. Young's modulus, viscosity, material density and system size can be freely chosen. Poisson's ratio is restricted to values &lt;0.25. The most important innovation of the software is a physically consistent scheme to model viscoelasticity in a lattice‐particle model. The scheme couples the local lattice‐geometry to the time‐dependent volume‐conservative viscoelastic deformation of particles. The procedure achieves the typical viscoelastic stress‐strain relationship. The article incorporates a number of benchmark simulations, where the model output is being tested and evaluated. The results obtained with Melange show strong similarities with the benchmark values, e.g., stress fields in crack vicinity. Geologically relevant examples include structures associated with the brittle‐ductile transition, growth of wing cracks and brittle graben formation.

New geochronological results and structural evolution of the Pataz gold mining district: Implications for the timing and origin of the batholith-hosted veins

The Paleozoic Pataz–Parcoy gold mining area is located in a right-stepping jog on the regional Cordillera Blanca fault, in northern Peru. Most of the 8million ounces of gold production from this area has come from quartz–carbonate–sulfide veins hosted by the Pataz batholith. Despite a subduction zone setting since at least the Cambrian, the area records several periods of extension and its present structure is that of a rift and graben terrain. The Pataz district (the northern part of the Pataz–Parcoy area) is dominated structurally by northwest to north northwest-striking (NW–NNW) faults and northeast to east northeast-striking (NE–ENE) lineaments, both of which have been active periodically since at least the Mississippian (Early Carboniferous). NW–NNW faults control the margins of a central horst that exposes basement schist and the Pataz batholith, and step across NE–ENE lineaments. The Lavasen graben, to the east of the central horst, contains the Lavasen Volcanics, and the Chagual graben, to the west, contains an allochthonous sedimentary sequence derived from the Western Andean Cordillera.New SHRIMP zircon geochronological data indicate emplacement of the Pataz batholith during the Middle Mississippian, at around 338–336Ma, approximately 10Ma earlier than previous estimates based on 40Ar/39Ar geochronology. The calc-alkaline, I-type batholith comprises diorite and granodiorite, the latter being the major component of the batholith, and was emplaced as a sill complex within the moderately NE-dipping sequence of the Eastern Andean Cordillera. Moderate- to high-temperature ductile deformation took place on the batholith contacts during or shortly after emplacement. Following emplacement of the batholith, differential uplift occurred along NW–NNW faults forming the Lavasen graben, into which the Lavasen Volcanics were deposited. SHRIMP U–Pb in zircon ages for the Lavasen Volcanics and the Esperanza subvolcanic complex, which was intruded into the western margin of the graben, are within error of one another at ca 334Ma. The ductile batholith contacts were cut by renewed movement on NW–NNW faults such that the margins of the batholith are now controlled by these steep brittle-ductile faults. The NW–NNW faults were oriented normal to the principal axis of regional shortening (ENE–WSW) during formation of the batholith-hosted, gold-bearing quartz–carbonate–sulfide veins. The misoriented faults were unable to accommodate significant displacement, leading to high fluid pressures, vertical extension in the competent batholith and formation of gold-bearing veins. Brittle failure of the batholith was most extensive in the northern Pataz district where the fault-controlled western contact of the batholith is offset by a swarm of NE–ENE lineaments.The timing of vein formation is not established, despite published 40Ar/39Ar ages of 312 to 314Ma for metasomatic white mica, which are interpreted as minimum ages of formation. Gold-bearing veins formed during or shortly after uplift of the Pataz batholith and formation of the Lavasen graben; they were therefore broadly coeval with deposition of the Lavasen Volcanics and emplacement of the Esperanza subvolcanic complex. These K-rich, weakly alkalic, ferroan (A-type) magmas may provide a viable source for the ore fluid that deposited gold in the Pataz batholith.

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.

Tectonic setting of the Ohře/Eger Graben between the central part of the České středohoří Mts. and the Most Basin, a regional study

*Corresponding author The paper presents the results of a new detailed survey in the SW part of the Ceske středohoři Mts. (a section of the Ohře/Eger Graben/Rift), focused on its tectonic setting. Courses of the SE marginal graben faults are traced in detail. This limiting fault zone is described as a horst, with the graben fill being structured into rhomboidal blocks; older known faults are specified, and new faults are depicted. Respecting the age ranges of the preserved stratigraphic units, fault movements are classified into those older than 36 Ma, younger than 24 Ma and younger than 16 Ma (with a possible age less than 9 Ma), based on different lithologies and abrupt lithological changes (i.e., the rock boundaries). The faults are compared with the earlier defined paleostress fields. Fault reversals were detected on several faults. An important role of crystalline basement ridges during the graben formation was identified: three complex tectonic structures are spatially associated with these ridges. Two areas with distinct tectonic and geological development were defined within the Ohře/Eger Graben. The Saxothuringian – Tepla–Barrandian terrane boundary was found to be less important for graben formation during the Cenozoic. This disagrees with simplified riftogenic model of the Ohře/Eger Graben. The supposed “Litoměřice Deep-seated Fault”, possibly representing a first-order terrane boundary, could not be confirmed in the post-Paleozoic rocks – this structure did not function as a fault during Cenozoic times.

Supradetachment basin evolution during continental extension: The Aegean province of western Anatolia, Turkey

We present a detailed structural and stratigraphic record of a Neogene–Quaternary supradetachment sedimentary succession in the Aegean extensional province of western Anatolia, and we compare its tectonic features and evolution to those of other well-documented supradetachment basins around the world. The sedimentary fill of the Alasehir basin records the uplift and exhumation of a core complex in the footwall of a detachment fault within the Central Menderes Massif. Accumulation of footwall-derived clastic sediments in this basin started ca. 20 Ma, shortly after the initiation of the approximately E-W–trending Alasehir detachment and its shear zone, and continued until ca. 2 Ma. Major sedimentary facies types include fluvial and alluvial-fan deposits, debris-flow and mass-flow deposits, and locally developed lacustrine rocks. These sedimentary units were accumulated largely in distal depocenters within the extending basin, as the low-angle (15°–28°) detachment faulting created little accommodation space near the basin margins while producing high back-shed topography in the uplifted Menderes core complex. The drainage system was dominated mainly by extension-parallel transverse streams during the main phases of basin evolution. Extension-parallel, scissor (hinge) faulting produced differential uplift and subsidence in the adjacent fault blocks, changed the direction of sediment transport and drainage patterns over short distances, and resulted in the local uplift of the older basin strata. These processes led to the development of subbasins with lateral variations in basement topography, strata thickness, and sedimentary facies distribution, and generated a segmented basin architecture. High-angle synthetic and antithetic faults that formed extensively after 3 Ma caused back-tilting of the sedimentary strata, formation of half grabens with their own axial drainage systems, and development of angular unconformities. With the onset of this crustal-scale block faulting, the detachment fault ceased to operate, and the Quaternary Gediz graben started to develop at the northern end of the Alasehir supradetachment basin. Our comparative evaluation of select basins shows a maximum sediment thickness of 3 km, average extension rates of 6 to 8–9 mm yr −1 , and accumulation rates of 0.1–0.2 mm yr −1 (uncorrected for compaction) in supradetachment basins in general. The rates and amounts of extension, the geometry of detachment faulting, the rates of footwall uplift, and the kinematics and interplay of different fault systems are the most important factors controlling three-dimensional structural architecture and evolution of supradetachment basins.

The role of oroclinal bending in the structural evolution of the Central Anatolian Plateau: evidence of a regional changeover from shortening to extension

The role of oroclinal bending in the structural evolution of the Central Anatolian Plateau: evidence of a regional changeover from shortening to extensionThe NW-SE striking extensional Inönü-Eskişehir Fault System is one of the most important active shear zones in Central Anatolia. This shear zone is comprised of semi-independent fault segments that constitute an integral array of crustal-scale faults that transverse the interior of the Anatolian plateau region. The WNW striking Eskişehir Fault Zone constitutes the western to central part of the system. Toward the southeast, this system splays into three fault zones. The NW striking Ilıca Fault Zone defines the northern branch of this splay. The middle and southern branches are the Yeniceoba and Cihanbeyli Fault Zones, which also constitute the western boundary of the tectonically active extensional Tuzgölü Basin. The Sultanhanı Fault Zone is the southeastern part of the system and also controls the southewestern margin of the Tuzgölü Basin. Structural observations and kinematic analysis of mesoscale faults in the Yeniceoba and Cihanbeyli Fault Zones clearly indicate a two-stage deformation history and kinematic changeover from contraction to extension. N-S compression was responsible for the development of the dextral Yeniceoba Fault Zone. Activity along this structure was superseded by normal faulting driven by NNE-SSW oriented tension that was accompanied by the reactivation of the Yeniceoba Fault Zone and the formation of the Cihanbeyli Fault Zone. The branching of the Inönü-Eskişehir Fault System into three fault zones (aligned with the apex of the Isparta Angle) and the formation of graben and halfgraben in the southeastern part of this system suggest ongoing asymmetric extension in the Anatolian Plateau. This extension is compatible with a clockwise rotation of the area, which may be associated with the eastern sector of the Isparta Angle, an oroclinal structure in the western central part of the plateau. As the initiation of extension in the central to southeastern part of the Inönü-Eskişehir Fault System has similarities with structures associated with the Isparta Angle, there may be a possible relationship between the active deformation and bending of the orocline and adjacent areas.

Comparison of dike intrusions in an incipient seafloor-spreading segment in Afar, Ethiopia: Seismicity perspectives

[1] Oceanic crust is accreted through the emplacement of dikes at spreading ridges, but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood. Between 2005 and 2009 the ∼70 km long Dabbahu-Manda Hararo rift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which were recorded on temporary seismic arrays. A detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems. All of the migrating swarms of earthquakes started from a <5 km radius zone at the middle of the Dabbahu-Manda Hararo segment, and traveled northward and southward along the rift axis. Small magnitude earthquakes associated with the margins of the propagating dike tips are followed by the largest magnitude, primarily low-frequency earthquakes. The seismic moment distributions show >80% of energy is released during the propagation phase, with minimal seismic energy release after the dike propagation ceases. We interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low-frequency earthquakes. Dike lengths show no systematic reduction in length with time, suggesting that topographic loading and stress barriers influence dike length, as well as changes in tectonic stress. The propagation velocities of all the dikes follow a decaying exponential. Northward propagating dikes had faster average velocities than those that propagated southward, suggesting preconditioning by the 2005 megadike, or ongoing heating from a subcrustal magma source north of the midsegment.