Major Thrust Zone Research Articles

Switching from the top-NW backthrusting to top-SE post-orogenic deformation in the Aegean domain: Insights from the calcite microfabric and siliciclastic rocks of the Amorgos Unit (SE Cyclades, Greece).

Detailed microstructural analysis was conducted in the imbricated Amorgos stratigraphy to decipher the complex deformation history of the calcitic mylonitic zones related to different tectonic events. Four major thrust zones were developed during syn-metamorphic tectonic burial by top-NW general shear deformation. Dynamic recrystallization is the dominant deformation mechanism, evolving from the SGR to the BLG field, characterizing a downward increase in strain from the highest Potamos to the lower Paradesia thrusts under low-T conditions. Mesoscopic kinematics within these thrusts reveal top-NW shearing. However, in microscale, both top-NW and top-SE kinematics are observed. In most cases, this top-SE reversal resulted from strain partitioning due to a strong pure shear component. However, the Mouros and Richti shear zones present dominant top-SE kinematics in all scales. Additionally, overprinting relationships within the metaflysch of the Mouros shear zone suggest that the top-SE kinematics of this zone post-dates the former top-NW ones. Therefore, we suggest that a switch from the top-NW general shear deformation during the syn-orogenic compressional phase to a late top-SE extensional deformation occurred in the early Miocene, resulting in the exhumation of the Amorgos Unit below 200 °C, via the Santorini Detachment System.

Geochemistry and origin of listwaenites within the Northern Branch of Neo-Tethyan ophiolites, Turkey

The study presents geological field observations, petrographic characteristics and bulk rock geochemical data of listwaenitic occurrences related to Jurassic–Cretaceous IAESZ ophiolites of the northern part of Turkey. Listwaenites of the Jurassic–Cretaceous Eldivan ophiolite and the Cretaceous Dağküplü ophiolite occur as mineralised or non-mineralised blocks and/or veins associated with serpentinised peridotites and ophiolitic mélanges along major thrust zones within the Izmir–Ankara–Erzincan Suture Zone (IAESZ) of the Northern Branch of the Neo-Tethys, Turkey. The silica–carbonate listwaenites (Type I) consist mainly of dolomite and minor calcite with quartz and minor phyllosilicates (chlorite, mica), serpentine and hematite. They are formed by circulation of meteoric hydrothermal fluids, along major décollement faults and thrust faults during the emplacement of serpentinised peridotites. The Kaymaz listwaenites (Type II) are composed mainly of fine-grained microcrystalline quartz with negligible dolomite, calcite, magnesite, serpentine and Fe-hydroxides. They display hard resistant masses, and are adjacent to the Eocene granitic bodies. Listwaenites are gentically related to the serpentinised peridotites, while the mineralisation appears to be related to the granite-related hydrothermal fluids.

Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling.

The architecture and mechanical properties of the subduction interface impact large‐scale subduction processes, including mass and volatile recycling, upper‐plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly understood, due to a lack of constraints on rock types, strain distribution, and interface thickness in this depth range. We characterized these factors in the Condrey Mountain Schist, a Late Jurassic to Early Cretaceous subduction complex in northern California that consists of an upper and lower unit. The Lower Condrey unit is predominantly pelagic and hemipelagic metasediment with m‐to km‐scale metamafic and metaserpentinitic ultramafic lenses all deformed at epidote blueschist facies (0.7–1.1 GPa, 450°C). Major and trace element geochemistry suggest tectonic erosion of the overriding plate sourced all ultramafic and some mafic lenses. We identified two major ductile thrust zones responsible for Lower Condrey unit assembly, with earlier strain distributed across the structural thickness between the ductile thrusts. The Lower Condrey unit records distributed deformation across a sediment‐dominated, 2+ km thick shear zone, possibly consistent with low velocity zones observed in modern subduction zones, despite subducting along a sediment poor, tectonically erosive margin. Periodic strain localization occurred when rheological heterogeneities (i.e., km‐scale ultramafic lenses) entered the interface, facilitating underplating that preserved 10%–60% of the incoming sediment. Modern mass and volatile budgets do not account for erosive margin underplating, so improved quantification is crucial for predicting mass and volatile net flux to Earth′s interior.

Identification of landslide-prone slopes at Paglajhora area, Darjeeling Himalaya, India

Paglajhora area, located in the southeastern part of the Kurseong town in the Darjeeling Himalaya region, India, experiences frequent slope failure especially in monsoon season. Due to the intricate disposition of major thrust zones (MCT, MBT, etc.), heavy rainfall, neotectonic and anthropogenic activities, etc., the cause of slope failure is not similar all over the area, which makes difficult to categorize the landslides based on their mode of failure. The present work is aimed to demarcate the landslide-prone areas from the surface displacement (horizontal) map derived by optical image correlation (OIC) technique applying normalized cross-correlation (NCC) algorithm of two temporal Landsat 8 OLI (level 1TP) images, supported by the ground field survey. Results indicate that the overall range of surface displacement (horizontal) rate in the study area varies from 0.4 to 8 m/year out of which the range 0.5–1.5 m/year being dominant classifying the slide type as slow-to-very slow earth flow with a velocity class varying from 3 to 2. Places showing relatively higher displacement values (3 to 8 m/year) match accurately with the already reported slide zones and the large exposed scarp surfaces noticed during the field surveys or distinguished from the satellite imagery. Areas with lower displacement rates (< 1.5 m/year) experience very slow earth flow, which are confirmed by the presence of cracks and steps on the major roads. Depths of shallow subsurface slip planes are established from dipole-dipole geo-electric surveys in few locations showing moderate surface velocity.

Using amphibolite chemistry to assist structural and stratigraphic interpretation at Broken Hill, Australia

Multiple samples taken across the strike of conformable syn-depositional amphibolite (meta-dolerite) bodies in the Willyama Supergroup, Broken Hill, showed that all exhibit chemical variation, mostly consistent with igneous fractionation. The fractionation trends are characterised by very significant reduction in MgO, Ni and Cr from base to top of individual sills, increase in Zr, and less regular variation in Fe and Ti contents. This asymmetric chemical zonation can be used to determine the original ‘way up’ in amphibolite sills as thin as 12 m. Given that the amphibolite sills were emplaced prior to folding of the enclosing sequence, the way up direction of the sills is also the younging direction of the enclosing metasediments, making this a useful tool in stratigraphic and structural studies of folded rocks. Outcropping conformable amphibolite bodies were selected for across-strike sampling in variably well-interpreted stratigraphic positions. Despite uncertainties related to overly wide-spaced sampling and possible internal structural complexity, in most cases the fractionation trend agreed with the prior stratigraphic interpretation, providing an independent stratigraphic ‘way up’ or ‘younging’ direction and demonstrating the potential usefulness of the technique. Subsequent sampling and analysis of drill core through amphibolite sills in the Broken Hill mine area tested alternative interpretations of the enclosing stratigraphy and structure. Fractionation patterns in three intersections of the amphibolite immediately west of the orebody clearly show younging towards the orebody. Intersections of the thicker amphibolite immediately east of the orebody reveal complex and conflicting fractionation patterns indicating internal structural complexity, consistent with this amphibolite being involved in a major thrust zone. The results in the mine area are in agreement with those of a previous study based on graded-bedding observations in the enclosing metasediments. The identification of igneous fractionation patterns in the amphibolites confirms the interpretation that the amphibolite bodies were in fact sills rather than lavas. The extremely high iron contents of some of the amphibolite sills are the result of extreme fractionation of a tholeiitic magma, not metasomatism. KEY POINTS Most Broken Hill amphibolites retain relatively simple igneous fractionation patterns. The fractionation patterns can indicate the stratigraphic younging direction. The stratigraphy west of the Broken Hill orebody youngs eastward towards the orebody. The amphibolite east of the orebody shows complex deformation.

Miocene crustal extension following thrust tectonic in the Lower Sebtides units (internal Rif, Ceuta Peninsula, Spain): Implication for the geodynamic evolution of the Alboran domain

In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10kbar and 780–820°C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550°C and 1–3kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.

Delineating the network of brittle structures with geotechnical, structural and reflection seismic data, Kevitsa open pit, northern Finland

This study aims at developing an internally consistent 3D network of brittle structures for the Kevitsa open pit Ni–Cu sulphide mine hosted by a Palaeoproterozoic mafic-ultramafic layered intrusion. The used approach involves compilation of depth and data-dependent three-dimensional geological sub-models at smaller depths, and their subsequent correlation with rock quality data, and reflection seismic structural models at greater depths. The work aims at bridging the characteristic gaps within and between the used geological–geophysical datasets, caused by the variable scales, coverages and resolutions. The second aim of the work is to provide a tectonic model explaining the generation of the major brittle structures within the area. The used data include digital elevation models, fracture and rock quality data from drill holes and 3D-photogrammetry, and seismic interpretations from previous investigations. The results show that the most distinct fracture zones dip gently towards WNW, steeply towards SE and sub-vertically, with NNW-SSE strikes. The gently WNW-dipping fracture zone shows the largest spatial extent and is clearly recognisable in all the source data sets. A kinematic model containing the gently WNW-dipping structures as major thrust zones, and smaller structures as second order structures is inferred. This work shows how surface and sub-surface data from various sources and depths are successfully integrated into high confidence structural models, which can be used as input data in future slope stability studies of the Kevitsa open pit.

Geometry and Kinematic Evolution of the Hotan-Tiklik Segment of the Western Kunlun Thrust Belt: Constrained by Structural Analyses and Apatite Fission Track Thermochronology

The western Kunlun thrust belt defines the boundary between the stable Tarim Basin in the north and the intensely deformed Cenozoic Tibetan Plateau in the south. Because of its important tectonic position, understanding its tectonic evolution should have important implications for propagation of deformation from Tibet to its neighboring cratonal regions during India-Eurasia convergence. We here present new structural analyses based on field investigations and seismic reflection profiles across the Hotan-Tiklik segment of the western Kunlun thrust belt. The results indicate that the structural section crosses two major thrust zones: the Tiklik zone in the hinterland to the south and the Hotan zone in the foreland to the north. Within these, the Hotan thrust zone is thin skinned, with its deformation characterized by fault-bend folding and fault slipping along detachment layers, whereas the Tiklik thrust zone involves basement, with its deformation driven by the currently steeply dipping Tiklik fault. Results from apatite fission track thermochronology in combination with growth strata and balanced cross section indicate that the Hotan-Tiklik segment underwent two-stage deformation: (1) development of the Tiklik thrust during the late Oligocene–early Miocene and again since the mid- to late Miocene and (2) activity of the Hotan thrust since the mid- to late Miocene as a result of basinward propagation of thrusting. The balanced cross section, combined with the apatite fission track results, suggests that the Hotan-Tiklik segment contributes a total shortening magnitude of more than ca. 34 ± 6 km. Within this, ca. 4 ± 2 and ca. 23 ± 1 km of the shortenings were absorbed by the Hotan anticline and the Hotan detachment fault, respectively, both of which were related to detachment layers. This suggests that detachment layers played an efficient role in propagating deformation from the western Tibetan Plateau into the Tarim Basin.

Fold-to-Fault Progression of a Major Thrust Zone Revealed in Horses of the North Mountain Fault Zone, Virginia and West Virginia, USA

The method of emplacement and sequential deformation of major thrust zones may be deciphered by detailed geologic mapping of these important structures. Thrust fault zones may have added complexity when horse blocks are contained within them. However, these horses can be an important indicator of the fault development holding information on fault-propagation folding or fold-to-fault progression. The North Mountain fault zone of the Central Appalachians, USA, was studied in order to better understand the relationships of horse blocks to hanging wall and footwall structures. The North Mountain fault zone in northwestern Virginia and eastern panhandle of West Virginia is the Late Mississippian to Permian Alleghanian structure that developed after regional-scale folding. Evidence for this deformation sequence is a consistent progression of right-side up to overturned strata in horses within the fault zone. Rocks on the southeast side (hinterland) of the zone are almost exclusively right-side up, whereas rocks on the northwest side (foreland) of the zone are almost exclusively overturned. This suggests that the fault zone developed along the overturned southeast limb of a syncline to the northwest and the adjacent upright limb of a faulted anticline to the southeast.

Sediment-hosted Zn–Pb–Cu deposits in the Central African Copperbelt

Stratabound epigenetic sulphide Zn–Pb–Cu ore deposits of the Central African Copperbelt in the Democratic Republic of Congo and Zambia are mostly hosted in deformed shallow marine platform carbonates and associated sedimentary rocks of the Neoproterozoic Katanga Supergroup. Economic orebodies, that also contain variable amounts of minor Cd, Co, Ge, Ag, Re, As, Mo, Ga, and V, occur mainly as irregular pipe-like bodies associated with collapse breccias and faults as well as lenticular bodies subparallel to bedding. Kipushi and Kabwe in the Democratic Republic of the Congo and Zambia, respectively, are the major examples of carbonate-hosted Zn–Pb–Cu mined deposits with important by-products of Ge, Cd, Ag and V in the Lufilian Arc, a major metallogenic province famous for its world-class sediment-hosted stratiform Cu–Co deposits. The carbonate-hosted deposits range in age from Neoproterozoic to early Palaeozoic (680 to 450 Ma). The formation of the relatively older Neoproterozoic deposits is probably related to early collision events during the Lufilian Orogeny, whereas the younger Palaeozoic deposits may be related to post-collisional processes of ore formation. Fluid inclusion and stable isotope data indicate that hydrothermal metal-bearing fluids evolved from formation brines during basin evolution and later tectonogenesis. Ore fluid migration occurred mainly along major thrust zones and other structural discontinuities such as karsts, breccias and faults within the Katangan cover rocks, resulting in ore deposition within favourable structures and reactive carbonates of the Katangan Supergroup.

New data on the Indus Kohistan seismic zone and its extension into the Hazara-Kashmir Syntaxis, NW Himalayas of Pakistan

This paper deals with the data obtained from local networks in northern Pakistan for 251 earthquakes of magnitude ≥4.0 for October 8, 2005 to December 31, 2006 period. The study presents focal mechanism solutions (FMS) of 12 pre- (1904–2005) and 17 post- (October 8, 2005–December, 2005) Muzaffarabad Earthquake, their detailed tectonic interpretation, and correlation with surface evidence of co-seismic rupture with published synthetic aperture radar data. Distribution of landslides obtained from National Engineering Services of Pakistan and the earthquake damages are also discussed. Aftershock distribution, which is more prominent in the crystalline zone (northwest of Muzaffarabad), defines a 50-km-wide NW–SE trending zone that extends for 200 km from the main mantle thrust to the center of the Hazara–Kashmir Syntaxis. The FMS of the main shock and 16 aftershocks having magnitude ≥4.0 indicate thrusting to be the dominant mechanism with rupture planes having NW–SE trend and NE dip. In addition, 12 FMS of pre-Muzaffarabad Earthquake (1904–2004) from the same area have been determined and results are compared. This leads to the conclusion that the wedge-shaped NW–SE trending blind zone, referred to by earlier workers as the Indus Kohistan Seismic Zone (IKSZ), has been activated during the Muzaffarabad earthquake. The right-lateral component in all FMS, supported by the surface evidences, suggests the involvement of Balakot–Bagh Fault (BBF). We propose that the IKSZ is the source of the October 8, 2005 Muzaffarabad earthquake that reactivated the BBF. Furthermore, the IKSZ does not end at the nose of the syntaxis but extends further southeast of it. Tectonic complexity seems to be due to a variety of factors. Also, thrust and reverse solutions near the northern collisional boundary (main mantle thrust) have mostly NE/SW-directed P-axis orientations. From the detailed FMS analysis, three conclusions have been drawn: (1) Shallow events (depth ≤10 km) with prominent strike slip solutions (earlier earthquakes) are associated with the surface strike slip faults (e.g., Muzaffarabad Fault) and/or the Besham domal structure; (2) moderate depth events (depth 10–25 km) with thrust/reverse solutions but having minor right-lateral strike slip component (all Muzaffarabad earthquakes and two earlier) are associated with the IKSZ; (3) deeper earthquakes (depth below IKSZ) with pure thrust/reverse solutions may be related to the under-thrusting of the Indian plate beneath the IKSZ, which represents a major thrust zone. Imbricate thrusting and breaking and thickening of the crust are considered to be caused by steep bending of the under-thrusting plate at the collisional boundary.

Surface processes and tectonics: Forcing of continental subduction and deep processes

It is now well accepted that surface processes provide a critical feedback on the surface tectonic deformation, whatever it is, orogenic building or basin evolution. However, the idea that the influence of these processes may go below the crustal levels, is less common. In this preliminary study, we use coupled thermo-mechanical numerical models to investigate the possible influence of surface processes on the styles of continental collision, in particular, continental subduction. For that, we further exploit the recent successful model of continental subduction of the early stages of India–Asia collision by Toussaint et al. [Toussaint G., Burov, E., and J.-P. Avouac, Tectonic evolution of a continental collision zone: a thermo-mechanical numerical model, Tectonics, 23, TC6003, doi:10.1029/2003TC001604, 2004b.]. On the example of India–Asia-like settings, we show that not only the surface topography but also the total amount of subduction may largely vary as function of denudation rate (controlled by the coefficient of erosion, k). Erosion provides a dynamic discharge of the hanging wall of the major thrust zone, whereas the sedimentation increases loading on the footwall and this helps down-thrusting of the lower plate. Both processes reduce the resistance of the major thrust and subduction channel to subduction. However, very strong or very slow erosion/sedimentation enhance the possibility of plate coupling and promote whole-scale thickening or buckling. The maximal amount of subduction is thus achieved for some intermediate values of erosion rates when the tectonic uplift rate is fine-balanced by the denudation rate. In our case the optimal balance is reached for the values of k on the order of 3000 m2/yr. We then extended our model beyond the conditions of India–Asia collision, in terms of the tested range of k and convergence rates. The experiments suggest that for provided settings, both extra slow (k<50–100 m2/yr) and extra rapid erosion (k>6000–8000 m2/yr) limit, by up to 50%, the total amount of subduction, if not totally prevent it. The model demonstrates the large capability of surface processes to adopt to different deformation styles: the orogenic building and subduction successfully develop (subduction number, S>0.5) in the range of k between 500 m2/yr and 6000 m2/yr at convergence rates ranging from 1 cm/yr to 6 cm/yr. Within this range, some peculiar features of orogenic style such as the geometry of the accretion prism, amount of upper crustal subduction, horizontal progression of the mountain range/thrust fault and the amount of exhumation of metamorphic facies are sometimes quite different. We conclude that surface processes may control deep, mantle level tectonic evolution.

Deformation pattern in the underthrust carbonate-rich sequence of the Sibillini Thrust (central Italy): insights for shear zone evolution in modern subduction complexes

Modern convergent zones at tropical latitudes are characterized by subduction of carbonate sediment. Although carbonate response to deformation is different from clay, they are commonly treated as having a similar rheology. This approximation, though, is inadequate since carbonate behavior is complicated by cementation and pressure solution. Our goal, here, is to focus on the deformation of carbonate sediment in the footwall of a major fossil thrust zone and compare it with a possible scenario in a modern environment. The focus area is the Monti Sibillini Thrust, in the Umbria-Marche sector of the Northern Apennines where the pelagic, Late Eocene-Oligocene carbonates of the Scaglia Cinerea Formation are underthrusted beneath the late Cretaceous-Middle Eocene Scaglia Rossa Formation. The data collected during this meso-structural study allowed for an interpretation of the temporal and spatial relationships between the observed deformation structures (S-C deformation bands, shear veins and stylolites). These results allow the development of a dynamic model for the deformation. The proposed model explains the observed features through cycles of fluid-pressure build up, hydraulic fracturing, and consecutive collapse, producing pressure-solution and stylolitization.

Geometry, segmentation pattern and displacement variations along a major Apennine thrust zone, central Italy

Hundreds of kilometres-long, arc-shaped, continuous thrust faults commonly imply very large displacements by detachment-dominated thrusting. Therefore, their occurrence has a large impact on fold–thrust belt structure. One of the major thrust faults of the central-northern Apennines, the Umbria–Marche–Sabina Thrust Zone, is traditionally believed to represent one such feature, characterised by displacements of several tens of kilometres. However, detailed studies of this structure revealed that it is actually composed of a series of partially overlapping fault segments, rather than consisting of a single, continuous thrust. Cross-section balancing and restoration, carried out by the integration of surface geological data with available sub-surface information, points out relatively limited amounts of thrust displacement (<10 km). Displacement–distance profiles show moderate displacement gradients well compatible with those reported for coherent thrust sheets. They also suggest that individual thrust segments were originally isolated and then grew by lateral propagation, leading to overlap and variable fault interaction within relay zones. The relationship between maximum displacement and fault trace length tends to follow a power-law distribution, as it commonly occurs for fault populations. However, a better correlation could be obtained by further segmentation of the northernmost (blind) fault in an area of no seismic data, thus confirming that fault scaling relationships may be useful for pointing out possible problems with the structural interpretation of poorly constrained areas and for exploring viable alternative solutions.

Active tectonics and erosional unloading at the eastern margin of the Tibetan Plateau

The eastern margin of the Tibetan Plateau is marked by an extremely steep mountain front with relief of over 5 km. This topography, coupled with abundant Mesozoic thrusts within the margin, explains why tectonic maps of the India-Asia collision typically show the eastern margin as a major thrust zone. Actually, it does not like that. Field observations suggest that the margin is better characterized as a zone of NNE-directed dextral shear with extensive strike-slip faulting and secondary thrusting. The high relief and steep gradients are partially explained by erosional unloading of an elastic lithosphere; the pre-erosion inherited topography may be the inherited Mesozoic thrust belt landscape modified by a component of Cenozoic tectonic shortening.

Caledonide development offshore–onshore Svalbard based on ocean bottom seismometer, conventional seismic, and potential field data

The western Barents Sea and the Svalbard archipelago share a common history of Caledonian basement formation and subsequent sedimentary deposition. Rock formations from the period are accessible to field study on Svalbard, but studies of the near offshore areas rely on seismic data and shallowdrilling. Offshore mapping is reliable down to the Permian sequence, but multichannel reflection seismic data do not give a coherent picture of older stratigraphy. A survey of 10 Ocean Bottom Seismometer profiles was collected around Svalbard in 1998. Results show a highly variable thickness of pre-Permian sedimentary strata, and a heterogeneous crystalline crust tied to candidates for continental sutures or major thrust zones. The data shown in this paper establish that the observed gravity in some parts of the platform can be directly related to velocity variations in the crystalline crust, but not necessarily to basement or Moho depth. The results from three new models are incorporated with a previously published profile, to produce depth-to-basement and -Moho maps south of Svalbard. There is a 14 km deep basement located approximately below the gently structured Upper Paleozoic Sørkapp Basin, bordered by a 7 km deep basement high to the west, and 7–9 km depths to the north. Continental Moho-depth range from 28 to 35 km, the thickest crust is found near the island of Hopen, and in a NNW trending narrow crustal root located between ∼19°E and 20°E, the latter is interpreted as a relic of westward dipping Caledonian continental collision or major thrusting. There is also a basement high on this trend. Across this zone, there is an eastward increase in the V P, V P/ V S ratio, and density, indicating a change towards a more mafic average crustal composition. The northward basement/Moho trend projects onto the Billefjorden Fault Zone (BFZ) on Spitsbergen. The eastern side of the BFZ correlates closely with coincident linear positive gravity and magnetic anomalies on western Ny Friesland, apparently originating from an antiform with high-grade metamorphic Caledonian terrane. A double linear magnetic anomaly appears on the BFZ trend south of Spitsbergen, sub-parallel to and located 10–50 km west of the crustal root. Based on this correlation, it is proposed that the suture or major thrust zone seen south of Svalbard correlates to the BFZ. The preservation of the relationship between the crustal suture, the crustal root, and upper mantle reflectivity, challenges the large-offset, post-collision sinistral transcurrent movement on the BFZ and other trends proposed in the literature. In particular, neither the wide-angle seismic data, nor conventional deep seismic reflection data south of Svalbard show clear signs of major lateral offsets, as seen in similar data around the British Isles.

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

The lowermost section of the continental superdeep drill hole German Continental Deep Drilling Program (KTB) (south Germany) has been investigated for the first time by vertical seismic profiling (VSP). The new VSP samples the still accessible range of 6–8.5 km depth. Between 7 and 8.5 km depth, the drill hole intersects a major cataclastic fault zone which can be traced back to the Earth's surface where it forms a lineament of regional importance, the Franconian line. To determine the seismic properties of the crust in situ, in particular within and around this deep fault zone, was one of the major goals of the VSP. For the measurements a newly developed high‐pressure/high‐temperature borehole geophone was used that was capable of withstanding temperatures and pressures up to 260°C and 140 MPa, respectively. The velocity‐depth profiles and reflection images resulting from the VSP are of high spatial resolution due to a small geophone spacing of 12.5 m and a broad seismic signal spectrum. Compared to the upper part of the borehole, we found more than 10% decrease of the P wave velocity in the deep, fractured metamorphic rock formations. P wave velocity is ∼5.5 km/s at 8.5 km depth compared to 6.0–6.5 km/s at more shallow levels above 7 km. In addition, seismic anisotropy was observed to increase significantly within the deep fracture zone showing more than 10% shear wave splitting and azimuthal variation of S wave polarization. In order to quantify the effect of fractures on the seismic velocity in situ we compared lithologically identical rock units at shallow and large depths: Combining seismic velocity and structural logs, we could determine the elastic tensors for three gneiss sections. The analysis of these tensors showed that we need fracture porosity in the percent range in order to explain seismic velocity and anisotropy observed within the fault zone. The opening of significant pore space around 8 km depth can only be maintained by differential tectonic stress combined with intense macroscopic fracturing. VSP reflection imaging based on PP and PS converted reflected waves showed that the major fault system at the KTB site is wider and more complex than previously known. The so‐called SE1 reflection previously found in two‐ and three‐dimensional surface seismic surveys corresponds to the top of an ∼1 km wide fault system. Its lower portion was not illuminated by surface seismic acquisition geometry. VSP imaging shows that the fault zone comprises two major and a number of smaller SE dipping fault planes and several conjugate fracture planes. The previously recognized upper fault plane is not associated with a strong velocity anomaly but indicates the depth below which the dramatic velocity decrease starts. Regarding the complexly faulted crustal section of the KTB site as a whole, we found that fluctuation spectra of rock composition and seismic velocity show similar patterns. We could verify that a significant amount of P wave energy is continuously converted into shear energy by forward scattering and that multipathing plays an important role in signal formation. The media behaves effectively smoothly only at wavelength larger than 150 m. It was shown by moving source profiling that the media is orthorhombic on a regional scale. The tilt of the symmetry axes of anisotropy varies with depth following the dip of the geological structure.

Exhumation of early Tertiary, coesite-bearing eclogites from the Pakistan Himalaya

Early Tertiary, coesite-bearing eclogites crop out in the internal zones of the Indian Plate in the Kaghan Valley of North Pakistan. Field and geochronological data suggest that UHP metamorphism, at c . 46 Ma, accompanied subduction of the leading edge of continental India beneath Kohistan. A new U–Pb rutile age of 44.1 ± 1.1 Ma can be interpreted as either a growth or a cooling age, given the uncertainty of the closure temperature of rutile for Pb, but clearly documents a very short-lived metamorphic peak, with cooling occurring between 44 and 40 Ma. Eclogitic assemblages are preserved in Permo-Triassic mafic rocks in both cover and basement sequences, with the best-preserved eclogites hosted in the anhydrous basement. The eclogite-facies rocks lie on the hanging wall of a major south-directed ductile thrust zone, which placed basement rocks that contain coesite eclogites southward onto lower-pressure cover metasediments. Shear criteria show that those sequences that contain eclogite-facies rocks are flanked above by a zone of pervasive north-directed extensional shearing. Mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite- to greenschist-facies transition. Amphibole Ar–Ar cooling history data suggest that this was at c . 40–42 Ma. As in the Alps this implies exhumation from the metamorphic peak to greenschist-facies conditions within a few million years. This short time scale has important implications, as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at UHP and brought the UHP rocks back to ‘mid’-crustal levels. This late Eocene extensional shearing, and associated exhumation of HP or UHP rocks, has not previously been recorded in the Pakistan Himalaya. The thrusts and shears were later folded by major east-trending folds, themselves cut by Miocene-aged, north-vergent extensional faults that currently separate the Indian Plate from the structurally overlying Kohistan island arc.

Relationships Among Eclogite Bodies and Host Rocks in the Lotru Metamorphic Suite (South Carpathians, Romania): Petrological Evidence for Multistage Tectonic Emplacement of Eclogites in a Medium-Pressure Terrain

The spatially most extensive unit containing pre-Alpine basement rocks in the South Carpathians, the Lotru Metamorphic Suite, hosts numerous eclogite bodies appearing in two different settings. Type I eclogites are mainly metamorphosed MORBs hosted in mica-rich rocks along a major synmetamorphic thrust zone. Calculated P-T conditions are located along an early retrograde array slightly exceeding 700°C at pressures between 1.8 and 2.7 GPa. Type II eclogites occur in a high-grade felsic complex structurally beneath the thrust zone. Peak P-T conditions were 800-900°C, 2.4-2.7 GPa for Mg-rich type IIa and ~600°C, 2.1-2.2 GPa in type IIb eclogites of Fe-Tirich basaltic composition. Differences in peak pressures as high as 1 GPa are documented between eclogites and their host rocks, which have undergone a polymetamorphic history, never reaching eclogitic conditions. The derived P-T histories imply a model of polystage tectonic emplacement of eclogites originating from both subducted oceanic crust and eclogitized mantle cumulates during a subduction-collision event. Eclogite emplacement in upper-crustal rocks resulted from a succession of syn-subduction exhumation of detached slab fragments, ascent of continental mantle fragments in mélanges with a felsic matrix, and eduction of lower-crustal slivers already containing accreted eclogites. During the collisional stage, the passive margin and the eclogite-bearing units were thrust far over the hotter active margin in a flake-tectonics style triggered by horizontal decoupling in the structurally weakened active margin. Weakening resulted from partial melting due to a thermal gradient increase in the back-arc zone. Metamorphic re-equilibration following the tectonic stacking occurred at 0.6 GPa and 600-700°C. The emplacement of the tectonic flakes without significant crustal thickening prevented rapid exhumation of the structure, thereby allowing a delayed unroofing that occurred only during subsequent events.

The Ntui-Betamba high-grade gneisses: a northward extension of the Pan-African Yaoundé gneisses in Cameroon

The Ntui-Bétamba area (southern Cameroon) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (kyanite-biotite-garnet gneisses, biotite-muscovite-garnet gneisses, calc-silicate rocks and quartzites) interpreted as a continental margin sedimentary series; and (ii) meta-igneous rocks comprising alkaline ultramafic to mafic pyroxene gneisses and amphibolites and amphibole-bearing alkaline orthogneisses. These units recrystallised under HP-HT conditions (T=750–800 °C, P≥0.9–1.3 GPa) and were deformed in relation to a major tangential tectonic event with a north-northeast-south-southwest kinematic direction. This lithological association and its tectono-metamorphic evolution show striking similarities with the Yaoundé gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther north, towards the Adamawa Shear Zone (Lorn series). The contrasted metamorphic evolution between areas located to the south of the Adamawa (high pressure: Yaoundé, Ntui-Betamba), and those located to the north (low pressure: Banyo), along with widespread remains of a Palaeoproterozoic crust, suggest important crustal thickening during tangential tectonics in southern Cameroon. As a consequence, the Adamawa Shear Zone is not simply a late Pan-African transcurrent or transpressive shear zone but appears to have been formerly a major (possibly intracontinental) thrust zone.