Zone Of Shortening Research Articles

Syn-tectonic Dipilto batholith (NW Nicaragua) linked to arc-continent collision: High- and room-temperature AMS evidence

The northern Central America region (Honduras and Nicaragua) is characterized by geotectonic units delimited by strike-slip fault systems and shortening zones of poorly constrained ages. The eastern portion of the Chortís block, named the “Dipilto micro-block” or “Patuca” is one of these units that besides with the Siuna Serpentinite Mélange, provides evidence for Early Cretaceous arc-continent collision. Nevertheless, within the Dipilto micro-block is the Early Cretaceous Dipilto batholith (NW Nicaragua), an Ilmenite-Series granitoid suites whose emplacement tectonic setting is unknown.We document the room- and high-temperature anisotropy of magnetic susceptibility (RT-AMS and HT-AMS) and hysteresis cycles from 31 sites in the Dipilto batholith and cross-cutting dikes as well as the oxide mineral microscopy of main lithologies. This is to investigate the magmatic fabric in the granitoids and dikes, and the relation to the emplacement dynamics. The RT-AMS and HT-AMS results show magnetic lineation and foliation (Kmax and Kmax–Kint plane) dominated by paramagnetic minerals (biotite and hornblende). The central parts of the intrusion show a NE-SW Kmax distribution and sites near to boundary of the country-rock a Kmax–Kint plane sub-parallel to the contact. These results and previous structural data in the surrounding Nueva Segovia Schist country-rock support an intrusion emplaced in a contractional setting concurrent with the batholith's NE-SW regional geometry. The Dipilto batholith age (119–112 Ma) implies a syn-collisional origin that together with our results indicate an Early Cretaceous progressive shortening in the region associated with the Siuna Intraoceanic Arc collision (~134–113 Ma). Furthermore, they indicate that the shortening azimuth (i.e., the direction of tectonic transport) is ~127°–137° in its current geographical position, but was 227°–237° in its Early Cretaceous paleo-position, probably related to the final stage of shortening and collision events of the southwestern Mexican paleocontinent.

Constraints on the Geometry and Frictional Properties of the Main Himalayan Thrust Using Coseismic, Postseismic, and Interseismic Deformation in Nepal

AbstractThe geometry and frictional properties of a fault system are key parameters required to understand its seismic behavior. The Main Himalayan Thrust in Nepal is the type example of a continental megathrust and forms part of a fault system which accommodates a significant fraction of India‐Eurasia convergence. Despite extensive study of this zone of shortening, the geometry of the fault system remains controversial. Here, we use interseismic, coseismic, and postseismic geodetic data in Nepal to investigate the proposed downdip geometries. We use interseismic and coseismic data from previous studies, acquired before and during the 2015 7.8 Gorkha earthquake. We then supplement these by processing our own postseismic deformation data, acquired following the Gorkha earthquake. We find that kinematic modeling of geodetic data alone cannot easily distinguish between the previously proposed geometries. We therefore develop a mechanical joint coseismic‐postseismic slip inversion which simultaneously solves for the distribution of coseismic slip and rate‐strengthening friction parameters. We run this inversion using the proposed geometries and find that they are all capable of explaining the majority of geodetic data. We find values for the rate parameter, , from the rate‐and‐state friction law that are between 0.8 and , depending on the geometry used. These values are in agreement with results from laboratory studies and those inferred from other earthquakes. We suggest that the limitations of earthquake cycle geodesy partly explain the continued controversy over the geometry and role of various faults in the Nepal Himalaya.

Obstructed minibasins on a salt‐detached slope: An example from above the Sigsbee canopy, northern Gulf of Mexico

AbstractSalt‐detached gravity gliding/spreading systems having a rugose base‐of‐salt display complex strain patterns. However, little was previously known about how welding of supra‐salt minibasins to the sub‐salt may influence both the downslope translation of minibasins on salt‐detached slopes and the regional pattern of supra‐salt strain. Using a regional 3D seismic reflection data set, we examine a large salt‐stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra‐canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip of the obstructed minibasin. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g., caught in a feeder) to mild (e.g., welded to a flat or gently dipping base‐of‐salt). By mapping both the presence of obstructed minibasins and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3‐D strain fields on a salt‐detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin‐rich salt‐detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our example is detached on allochthonous salt and where the base‐of‐salt is rugose, with the findings applicable to other such systems worldwide (e.g., Gulf of Mexico; Scotian Margin, offshore eastern Canada). However, our findings are also applicable to systems where the salt is autochthonous but has significant local basal relief (e.g., Santos Basin, Brazil; Kwanza Basin, Angola).

Analogue model of gravity driven deformation in the salt tectonics zone of northeastern Mexico

In the deep seated gravity-driven deformation systems of the Gulf of Mexico contemporaneous extension and contraction of the overburden is favored by mechanical decoupling from the basement along thick salt sequences (up to 4 km). The updip extension is located inland, on the continental shelf of northeast Mexico, and is characterized by extensional listric faults and roll-overs; the downdip shortening zone is located at the deep waters and is characterized by a fold and thrust belt detached above the salt layer. Two physical experiments are used to discuss some aspects of these gravity-driven systems. The experimental setup includes a motor-driven experimental table, an inclined brittle basement (1°), a silicone layer simulating the salt sequences, and sand layers simulating the pre-kinematic Jurassic-Cretaceous strata before Laramide shortening. Deformation resulted in further tilting of the basement (3° to 4°). After the onset of deformation, thin sand layers were added at regular time intervals simulating the syntectonic sedimentation. The experiments reproduced the geometry of the deformation at the frontal ramp characterized by a seaward vergent thrust and its associated deformed region (the Perdido fold belt). The fold and thrust belt localization was favored by the change in basement inclination (a built-in slope change). Key elements interpreted in one available section of the area were reproduced in the model: a) the presence of an antithethic roll-over in the extensional zone and, b) the basinward vergence of folds and thrusts observed in the downdip shortening zone in the mexican Perdido fold belt.

Extension-related buttress-like folds, the western side of the Gulf of Suez rift, Egypt

The wedge-shaped St. Paul block, western side of the Gulf of Suez rift, exposes Late Cretaceous beds that are folded into transverse folds. The block is bounded from the east and west by east- and southeast-dipping rift-related normal faults, respectively. This study reveals that the transverse folds are extension-related, and formed due to the buttress-like effect that was created during the movement along the rift-related normal faults. The more competent Eocene hanging-wall block of the N–S-striking fault buttressed against the moving less competent Cretaceous hanging-wall block of the NE–SW-striking fault. A localized zone of shortening developed between the two fault trends causing the intervening wedge-shaped block to be crumpled and folded into a series of kilometer-scale, gentle SSE-plunging folds. The buttressing, within block internal localized strain caused by the zigzag geometry of the NE–SW-striking fault, and the effect of the minor cross fault explain the SSE-to-ESE plunges of the mesoscopic folds. This study turns the attention to the buttress-like effect as one of the mechanisms of extension-related folds during rifting.

The effects of base-salt relief on salt flow and suprasalt deformation patterns — Part 2: Application to the eastern Gulf of Mexico

In the eastern Gulf of Mexico, the pattern of early stage salt flow is complicated by basement topography consisting of a series of plunging arches that trend obliquely to the early flow direction. Seismic lines downdip of the Florida Middle Ground Arch reveal a puzzling array of structures. Sections trending roughly north–south (parallel to the regional dip) document predominantly extensional structures; however, east–west sections reveal shortening structures. Both sets of structures occur well updip of the downdip salt pinchout. We designed a physical-modeling study to investigate these puzzling relationships. Models presented in Part 1 of this paper indicate that localized shortening and extension can occur as salt passes over simple base-salt steps. Physical models were run with complex salt isopachs featuring plunging arches oblique to dip and salt flow. Models reveal the formation of shortening belts as the salt and its thin prekinematic overburden are translated across the arches. The complex salt isopachs deflect salt flow to produce convergent and divergent flow, which, along with flow-velocity gradients, results in the rotation of early formed thrust belts. Rotations of up to 70° were recorded in the most complex model, resulting in transported fold belts with trends that were close to dip parallel, similar to those observed on seismic data from the eastern Gulf of Mexico. Additional zones of shortening are found in and around complex salt pinchouts in the updip zones of the gravity-gliding system. The dynamic nature of these salt-related tectonic systems can result in the downdip translation of fold belts far from the basement topography over which they were created.

Transpression and juxtaposition of middle crust over upper crust forming a crustal scale flower structure: Insight from structural, fabric, and kinematic studies from the Rengali Province, eastern India

Deformational, metamorphic, monazite age and fabric data from Rengali Province, eastern India converge towards a multi-scale transpressional deformational episode at ca. 498–521 Ma which is linked with the latest phase of tectonic processes operative at proto-India-Antarctica join. Detailed sector wise study on mutual overprinting relationships of macro-to microstructural elements suggest that deformation was regionally partitioned into fold-thrust dominated shortening zones alternating with zones of dominant transcurrent deformation bounded between the thrust sense Barkot Shear Zone in the north and the dextral Kerajang Fault Zone in the south. The strain partitioned zones are further restricted between two regional transverse shear zones, the sinistral Riamol Shear Zone in the west and the dextral Akul Fault Zone in the east which are interpreted as synthetic R and antithetic R' Riedel shear plane, respectively. The overall structural disposition has been interpreted as a positive flower structure bounded between the longitudinal and transverse faults with vertical extrusion and symmetric juxtaposition of mid-crustal amphibolite grade basement gneisses over low-grade upper crustal rocks emanating from the central axis of the transpressional belt.

The role of the granite emplacement and structural setting on the genesis of gold mineralization in Egypt

The Eastern Desert of Egypt is well known as a gold-mining district since ancient times. Gold mineralization is closely associated with the granitic rocks in such way that the mineralization is either hosted by or occurs immediately adjacent to the granite intrusions. Granitic rocks accompanying gold mineralization in the Eastern Desert can be grouped into three categories i.e. syn-late tectonic calc-alkaline granites, calc-alkaline to mildly alkaline granites of the transitional stage and post-tectonic alkaline granites.Tectonically, gold mineralization is linked with the tectonothermal stages that were operative during the evolution of the Arabian–Nubian Shield (ANS). During the primitive stages of the island-arc formation, pre-orogenic gold mineralization (auriferous exhalites) was formed by hot brines accompanying submarine volcanic activity. No role for the granite is observed in this stage. Syn-orogenic gold mineralization (i.e. gold hosted in altered ophiolitic serpentinites along thrust faults and in sutures, quartz veins hosted in the metavolcano-sedimentary assemblage and/or the I-type granitic rocks surrounding them) connected with the collision and accretion stage is characterized by emplacement of calc-alkaline (I-type) older granite batholiths. Shear fractures reflected in brittle–ductile shear zones and amphibolite-green schist facies regional metamorphism were broadly contemporaneous with this intense compressional tectonic regime. Available fluid inclusion microthermometry and isotopic studies reveal that both metamorphic and magmatic fluids related to the syn-late tectonic calc-alkaline granites were operative. A further indication for the role of the granites is indicated by the presence of some concentrations of Antimony, Bismuth, Molybdenum, Tungsten, Rubidium, Beryllium, Tin, Yttrium, Ytterbium, Tantalum and Niobium in some auriferous quartz veins in the Egyptian gold mines.In the cratonal development of the (ANS), the land underwent a transitional stage between the major subduction-related calc-alkaline magmatic activity and the subsequent post-tectonic plutonism represented by the alkaline granites. This transitional stage is dominated by the eruption of Dokhan volcanics and deposition of molass-type Hammamat sediments. At ~590–530Ma, the Arabian–Nubian Shield was deformed by post-accretionary structures, in the form of N-trending shortening zones such as the Hamisana shear zone and NW-trending strike-slip faults such as the Najd fault system. The regional NNW–SSE directed extension opened spaces that were progressively sealed with different magmatic phases including among them a considerable proportion of rocks referred to as “younger granites” in the Egyptian literature. Late-orogenic gold mineralization connected with the transitional stage is represented principally by the gold-bearing quartz veins traversing Hammamat molasse sediments, quartz veins traversing syn-extensional younger granites and generally quartz veins in ductile to brittle shears related to the Najd fault system and within Hamisana shear zone and its splays.By the end of Pan African orogeny until the Tertiary, the basement was intermittently intruded by a number of sub-alkaline to per alkaline granite bodies that host Mo, Sn, W, Nb–Ta and U mineralization in the Eastern Desert of Egypt. Anorogenic gold mineralization connected with post-orogenic granites is represented by small amounts of the element in disseminations, stockworks and quartz veins of Sn–W–Ta–U mineralization.The present review shows that gold mineralization in Egypt is an expression of two major cycles with distinct magmatic and tectonic characteristics, and the two cycles were separated by a transitional stage. The emplacement of granites in the compressional cycle played an important role in metamorphosing the country rocks by producing the heat energy required for the regional metamorphism and the providing of the magmatic fluids. The H2O–CO2 fluids enriched in volatiles were released at the greenschist–amphibolite facies transition at 450°–500°C and mixed with the I-type calc-alkaline granite related fluids and both moved down a temperature gradient away from the amphibolite-green schist transition at depth to a lower temperature regime in the upper levels where it is deposited in brittle–ductile shear zones. With the extensional cycle, the syn-extensional granite intrusions acted as heat engine in such way that the heat of the granite drove the convective cells to circulate through the auriferous host-granite contacts, leaching gold and other elements and depositing it in structurally favorable sites. In addition, the contrasts in competency between the granites with brittle deformational characteristics and the surrounding country rocks with a ductile response to stress, led to a generation of extensive fracture pattern within the more competent unit.

Superposed deformation and inherited structures in an ancient dilational step-over zone: Post-mortem of the Rengali Province, India

In the eastern Indian shield, a dextral strike-slip system juxtaposed the Archaean Singhbhum Province against the Proterozoic Eastern Ghats Belt at ∼490–470 Ma. Two WNW–ESE trending strands of the strike-slip system enclose a multiply deformed (D1 to D3) intervening domain called the Rengali Province, with D3 representing dextral shearing. In a granulite lens within the province, an early fabric (Sgr) was deformed by an amphibolite facies D1–D2 deformation continuum in the late Archaean time, forming cylindrical folds. In the surrounding quartzofeldspathic gneisses, quartzites and mica schists of the province, superimposition of syn-D3 shortening on D1-D2 folds generated complex non-cylindrical geometries; the granulites escaped D3 strain. Microstructures in the province-bounding shear zones confirm that D3 deformation was associated with mylonitization, dynamic recrystallization and greenschist facies metamorphism. In the quartzites, syn-D3 folds can be correlated with rotation of D1–D2 structures through the shortening zone of bounding dextral shears. Since the province-bounding shears form a step-over zone, the structural complexity within the Rengali Province arises from superposition of syn-D3 shortening structures on initially asympathetically oriented inherited cylindrical D1-D2 folds. Hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied D3 as the step-over zone was dilational in nature.

Offshore fault modeling using late Quaternary marine terrace records in the crustal shortening zone of Northeast Japan back-arc

Offshore fault modeling using late Quaternary marine terrace records in the crustal shortening zone of Northeast Japan back-arc

Internal forces, tension and energy density in tethered cellular membranes

We analyze tethered cellular membranes by considering the membrane resultants, tension and densities of two modes of energy, bending and adhesion. These characteristics are determined based on a computational (finite-difference) analysis of membrane shape. We analyze the relative contribution and distribution of the membrane characteristics in four typical zones of the membrane surface. Using an axisymmetric model, we found that the meridional and circumferential components of the resultant are different near the tether body and they converge to the value of membrane tension farther from the tether. At the beginning of the area of membrane detachment from the cytoskeleton, the density of bending energy is on the same order of magnitude as membrane tension (resultant). Away from the tether, the bending energy density quickly decreases and becomes of the same order as that of the adhesion energy in the membrane–cytoskeleton attachment area. In that area, both modes of energy are significantly smaller than the membrane tension. We also consider the effect of the membrane bending modulus on the distribution of the membrane characteristics. An increase in the bending modulus results in changing the length and position on the membrane surface of zone 1 characterized by significant evolution of the resultant components. It also results in shortening zone 2 that covers the rest of the area of membrane detachment. The obtained results can help in a better interpretation of the measurements of membrane mechanical properties as well as in analyses of proteins and channels in curved membranes.

Syn- and post-accretionary structures in the Neoproterozoic Central Allaqi-Heiani suture zone, Southeastern Egypt

The Allaqi-Heiani suture (AHS) is the western part of the main Allaqi-Heiani-Gerf-Onib-Sol Hamed-Yanbu suture and represents one of the Neoproterozoic, arc–arc sutures in the Arabian-Nubian Shield (ANS). It separates the ca. 750Ma South Eastern Desert terrane in the north from the ca. 830–720Ma Gabgaba terrane in the south. The AHS is a deformed belt of ophiolitic rocks, syn-tectonic granitoids and metasediments. The central AHS zone is divided into three structural domains. The western domain (I) is characterized by NNE dipping thrusts and SSW-vergent folds. The central domain (II) includes upright tight to isoclinal NNW–SSE oriented folds and transpressional faults. The eastern domain (III) shows NNW–SSE oriented open folds. Structural analysis indicates that the area has a poly-phase deformation history involving at least two events. Event D1 was an N–S to NNE–SSW regional shortening generating the SSW-verging folds and the NNE dipping thrusts. Event D2 was an ENE–WSW shortening producing NNW–SSE oriented folds in the central and eastern parts of the study area and reactivating older thrusts with oblique-slip reverse fault movement. The tectonic evolution of the area involves two episodes of collision: an early collision between the South Eastern Desert terrane and the Gabgaba terrane along the AHS after the consumption of a basin floored by oceanic crust above a north-dipping subduction zone; and a later collision between East- and West-Gondwanas at ca. 750–650Ma, leading to the closure of the Mozambique Ocean. This collision deformed the AHS along N–S trending shortening zones and produced NW–SE and NE–SW oriented sinistral and dextral transpressional faults, respectively. The early collision episode is related to the terrane accretion during the early Pan-African orogen, while the later phase is related to a late Pan-African or Najd orogen.

Quantifying 3D post-accretionary tectonic strain in the Arabian–Nubian Shield: Superimposition of the Oko Shear Zone on the Nakasib Suture, Red Sea Hills, Sudan

Deformed conglomeratic clasts exposed along the Neoproterozoic Nakasib Suture and the Oko Shear Zone are used to calculate three-dimensional (3D) tectonic strain associated with the latter to quantify strain associated with post-accretionary deformational belts in the Arabian–Nubian Shield. The Nakasib Suture is a NE-trending fold and thrust belt that is sinistrally offset (∼10 km) by the cross-cutting NNW- to NW-trending strike-slip faults of the Oko Shear Zone. The Nakasib Suture was formed as a result of collision between the Haya terrane and the Gebeit terrane at ∼750 Ma ago. The Oko Shear Zone was subsequently formed as a result of an E–W directed shortening of the Arabian–Nubian Shield due to collision between East and West Gondwana at ∼670–610 Ma ago. This analysis indicates the following: (1) The Nakasib Suture is dominated by flattening strain with the flattening plane of the associated strain ellipsoid oriented at 21°/77°SE. This flattening deformation is interpreted to be associated with nappe emplacement from north to south. (2) Some regions along the Nakasib Suture are characterized by constriction strain that might be due to refolding of the early nappes about NE-trending axes. (3) The Oko Shear Zone is characterized by constriction strain, with the XY plane of the strain ellipsoid oriented at 171°/68°E. The strain ellipsoid associated with the Oko Shear Zone manifests superimposition of E–W shortening on the NE-trending fold and thrust belt associated with the Nakasib Suture. (4) The tectonic strain of the Oko Shear Zone, superimposed over the structures of the Nakasib Suture, is characterized by a strain ellipsoid whose flattening plane is oriented at 21°/49°W. The strain ellipsoid of the tectonic strain has a major axis with a quadratic elongation of 3.6 and an orientation of 357°/25°, an intermediate axis with a quadratic elongation of 1.2 and an orientation of 231°/30°, and a minor axis with a quadratic elongation of 0.25 and an orientation of 115°/18°. This suggests that the post-accretionary deformation of the Arabian–Nubian Shield was superimposed as a NW–SE directed shortening that created early N–S shortening zones and late NW-trending sinistral strike-slip faults.

Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling

We present lithospheric‐scale analog models, investigating how the absolute plates' motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long‐term process (∼10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at ∼7 Ma following the beginning of buoyant slab segments' subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges' subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity.

Structural and remote sensing studies of the southern Betsimisaraka Suture, Madagascar

An assessment of the southern Betsimisaraka Suture (B.S.) of southeastern Madagascar using remote sensing and field investigation reveals a complex deformation history. Image processing of Landsat ETM+data and JERS-I Synthetic Aperture Radar (SAR) imagery was integrated with field observations of structural geology and field petrography. The southern B.S. divides the Precambrian basement rocks of Madagascar in two parts. The western part includes Proterozoic rocks whereas the eastern part is an Archean block, named the Masora block. The southern part of the B.S. includes high-grade metamorphic rocks, recording strong deformation and has mineral deposits including chromite, nickel, and emerald, characteristic of oceanic material that is compatible with a suture zone. Large-scale structural features indicate ductile deformation including three generations of folding (F1, F2, and F3) associated with dextral shearing. The first folding event (F1) shows a succession of folds with NE striking axial planes. The second folding event (F2) mainly has north–south striking axial planes and the last event (F3) is represented by mega folds that have ENE–WSW axial plane directions and have NNW and SSE contractional strain patterns. Closure of the Mozambique Ocean between two components of Gondwana sandwiched rocks of the B.S. and formed upright folds and shortening zones which produced N–S trending lineaments. Later dextral movements followed the contraction and formed NW–SE trending lineaments and N–S trending normal faults associated with dextral strike slip faults and fractures.

Lithospheric structure and dynamic processes of the Tianshan orogenic belt and the Junggar basin

Located at the center of the Eurasian continent and accommodating as much as 44% of the present crustal shortening between India and Siberia, the Tianshan orogenic belt (TOB) is one of the youngest (<20 Ma) and highest (elevation>7000 m) orogenic belts in the world. It provides a natural laboratory for examining the processes of intracontinental deformation. In recent years, wide angle seismic reflection/refraction profiling and magnetotelluric sounding surveys have been carried out along a geoscience transect which extends northeastward from Xayar at the northern margin of the Tarim basin (TB), through the Tianshan orogenic belt and the Junggar basin (JB), to Burjing at the southern piedmont of the Altay Mountain. We have also obtained the 2D density structure of the crust and upper mantle of this area by using the Bouguer anomaly data of Northwestern Xinjiang. With these surveys, we attempt to image the 2D velocity and the 2D electric structure of the crust and upper mantle beneath the Tianshan orogenic belt and the Junggar basin. In order to obtain the small-scale structure of the crust–mantle transitional zone of the study area, the wavelet transform method is applied to the seismic wide angle reflection/refraction data. Combining our survey results with heat flow and other geological data, we propose a model that interprets the deep processes beneath the Tianshan orogenic belt and the Junggar basin. Located between the Tarim basin and the Junggar basin, the Tianshan orogenic belt is a block with relatively low velocity, low density, and partially high resistivity. It is tectonically a shortening zone under lateral compression. A detachment exists in the upper crust at the northern margin of the Tarim basin. Its lower part of the upper crust intruded into the lower part of the upper and the middle crust of the Tianshan, near the Korla fault; its middle crust intruded into the lower crust of the Tianshan; and its lower crust and lithospheric mantle subducted into the upper mantle of the Tianshan. In these processes, the mass of the lower crust of the Tarim basin was carried down to the upper mantle beneath the Tianshan, forming a ∼20-km-thick complex crust–mantle transitional zone composed of seven thin layers with a lower than average velocity. The thrusting and folding of the sedimentary cover, the intrusive layer in the upper and middle crust, and the mass added by the subduction of the Tarim basin into the upper mantle of the Tianshan are probably responsible for the crustal thickening of the Tianshan. Due to the important mass deficiency in the crust and the upper mantle of the Tianshan, buoyancy must occur and lead to rapid ascent of the Tianshan. The episodic tectonic uplift of the Tianshan and tectonic subsidence of the Junggar basin are closely related to the evolution of the Paleozoic, Mesozoic, and Cenozoic Tethys.

Investigation of the tectonics of the Main Marmara Fault by means of deep-towed seismic data

We report a study of the active principal deformation zone (PDZ) of the Main Marmara Fault (MMF) in the sub-basins of the Sea of Marmara by means of deep-towed seismic (Pasisar) as well as multi-beam bathymetry data collected by Ifremer's r/v Le Suroı̂t in September 2000. Our main objective is to investigate the active deformation within the uppermost sedimentary layers with a much higher resolution than hitherto has been available. To the west of the Sea of Marmara, the PDZ is located along the southern flank of the Tekirdağ Basin where the sediments are affected by steep reverse faults dipping toward the north. E–W strike–slip faults are also observed in the central part of the Tekirdağ Basin and thrusting occurs along its N50°-trending margin. We interpret these structures in terms of a slight clockwise rotation in the basin north of the PDZ. To the east, the PDZ enters the Central Basin and follows a steep scarp along the southern flank of a tectonic depression. The scarp consists of an en échelon fault system with a normal component. These faults are combined with small parallel anticlines and synclines that extend along the southern portion of the depression. The northern scarp of the depression is made of a mixed system of faults with both normal and reverse components associated with anticlines and synclines. These faults are best interpreted as right lateral strike–slip faults with a vertical component that is dominantly normal. These faults and the sigmoid shape of the depression are compatible with a clockwise rotation above the PDZ. This recent tectonic structure appears to be superimposed over a pre-existing graben that is now essentially deactivated. The PDZ continues eastwards, out of the Central Basin, as a N50°E-trending NW-verging thrust system toward the Kumburgaz Basin that is located on a restraining bend of the PDZ. This shortening zone consists of two main N60°E-trending branches. The northern one is more pronounced and composed of two successive restraining bends. The southern branch is smoother and forms a gently curved connection between the two segments of the PDZ. This suggests that the PDZ migrates southward to cut through this restraining bend. Further east, the PDZ enters the Çınarcık Basin along its northern scarp. Active deformation observed on the Pasisar profiles along the 290°-trending eastern portion of this scarp consists of N110±5°-trending dextral strike–slip faults connecting short segments of active N130–140° normal faults that control elongated depocenters. Along the southern flank of the Çınarcık Basin, the E–W İzmit fault enters the basin from the east. Pasisar data confirm the extension of the İzmit strike–slip fault into the Çınarcık Basin and the large development of normal faulting along the southern flank of the basin. Some of the normal faulting observed here may be related to horse tail termination of the İzmit fault, while most of it is the expression of strain partitioning.

Interaction of metamorphism, deformation and exhumation in large convergent orogens

AbstractCoupled thermal‐mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr−1, and a slope‐dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m−3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m−3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro‐side of the system, and is driven by a depth‐dependent pressure gradient that is facilitated by the development of a temperature‐dependent low viscosity horizon in the mid‐crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post‐convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust‐related exhumation on the flanks. Velocities in the pro‐side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro‐side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal‐sense offset at its upper boundary, reverse‐sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.

The North Anatolian Fault within the Sea of Marmara: a new interpretation based on multi-channel seismic and multi-beam bathymetry data

We analyze 2200 km of multi-channel seismic reflection profiles that have become recently available in the Sea of Marmara. This analysis benefits from the recent acquisition of multi-beam bathymetric data covering the axial portion of the northern basins. We conclude that the northern Sea of Marmara is at present cut by an active continuous strike–slip fault system, that we call the Marmara Fault. It links the 270° İzmit portion of the northern branch of the North Anatolian Fault to the east of the sea to the 245° Ganos Fault to the west. The Marmara Fault itself consists of two main parts. The western one is a single 80 km long fault. It follows a 265° direction that differs by 20° from the direction of the Ganos Fault. At its northeastern extremity, it turns toward the northern margin of the Çınarcık Basin. This western part of the fault is seismically quite active. The eastern part is more complex. It extends over 65 km in a 280° direction. There is thus a 10° clockwise rotation with respect to the 270° İzmit Fault strand that should result in a slight extensional component. Within the eastern Çınarcık Basin, this part of the Marmara Fault has two parallel branches, 10 km apart, the northern one near the foot of the northern slope, the southern one near the foot of the southern slope. A shortening zone occupies the northwestern corner of the Çınarcık Basin and much of the adjacent eastern Central High and separates the northern branch from the western one. The southern branch on the other hand may cross the eastern Central High along a narrow 280° valley. The data we have provide no definitive evidence on which of these two branches carries most of the motion. The establishment of the present trace of the fault through the Marmara Basin is contemporaneous with a recent phase of deposition of deep giant ripples, parallel to the bathymetric slope contours and adjacent to the turbidites basin deposits. The base of these deep basins sedimentary deposits is marked by a widespread erosional unconformity below which lies a gently folded sedimentary sequence that may consist of shallow Thrace Basin deposits.

Plate‐boundary strain partitioning along the sinistral collision suture of the Philippine and Eurasian plates: Analysis of geodetic data and geological observation in southeastern Taiwan

Crustal deformation and strain partitioning of oblique convergence between the Philippine Sea plate and the Eurasian plate in the southern Longitudinal Valley of eastern Taiwan were characterized, based on geodetic analysis of trilateration network and geological field investigation. The Longitudinal Valley fault, one of the most active faults on Taiwan, branches into two individual faults in the southern Longitudinal Valley. These two active faults bound the Plio‐Pleistocene Pinanshan Conglomerate massif between the Coastal Range (the Luzon island arc belonging to the Philippine Sea plate) and the Central Range (the metamorphic basement of the Eurasian plate). A geodetic trilateration network near the southern end of the valley shows a stable rate of the annual length changes during 1983–1990. The strain tensors for polygonal regions (including triangular regions) of the Taitung trilateration network reveal that there are two distinct zones of deformation: a zone of shortening (thrusting) between the Pinanshan massif and the Central Range on the west and a strike‐slip movement between the Pinanshan massif and the Coastal Range on the east. The analysis of a discontinuity model consisting of three rigid blocks separated by two discontinuities has been carried out. The results show that the deformation in this region can be characterized by two major faults. A reverse fault is located between the Plio‐Pleistocene Pinanshan massif and the metamorphic basement of the Central Range, with a shortening rate of about 12 mm/yr in the direction N280°E. A strike‐slip fault is located principally along the river between the Pinanshan massif and island arc system of the Coastal Range with a purely strike‐slip component of about 22 mm/yr in the direction N353°E. The analysis of the geodetic data further suggests that substantial deformation (probably strike slip in type) occurs within the Pinanshan massif. Geological evidence of deformation in the Plio‐Pliestocene Pinanshan Conglomerate includes regional folding, a conjugate set of strike‐slip fractures at the outcrop scale, and morphological lineaments related to fracturing, all indicating that the Pinanshan massif is being deformed within a transpressive stress regime. Regional kinematic data indicate that a significant portion of the 82 mm/yr of motion between the Eurasian plate and the Philippine Sea plate is absorbed in the southern Longitudinal Valley by the decoupling of two distinct major faults. The geometry of the oblique convergence and the rheology of the rock units (the well‐consolidated Plio‐Pleistocene conglomerate and the sheared mélange formation) play the two important roles in the partitioning of crust deformation.