Steep Normal Faults Research Articles

The intraplate Mw 7 Machaze earthquake in Mozambique: Improved point source model, stress drop, and geodynamic implications

The February 22nd 2006 Mw = 7 Machaze earthquake is one of the largest, if not the largest, earthquakes reported since 1900 within Continental Africa. This large continental intraplate event has important implications to our understanding of tectonics and strong ground motion prediction locally and in the global context. Thus, accurate estimates of source parameters of this earthquake are important. In this study, we inverted the complete azimuthally distributed high frequency (0.05–2 Hz) P waveform dataset available for a best-fitting point source model and obtained stress drop estimates assuming different theoretical rupture models from spectral fitting. Our best-fitting point source model confirms steep normal faulting, has strike = 173° (309°), dip = 73° (23°), rake = −72° (−132°), and shows a 12%–4% improvement in waveform fit compared to previous models, which translates into an error minimization. We attribute this improvement to higher order reverberations near the source region that we took in to account and the excellent azimuthal coverage of the dataset. Preferred stress drop estimates assuming a rupture velocity = 0.9 x shear wave velocity (Vs) are between 11 and 15 MPa though, even higher stress drop estimates are possible for rupture velocities lower than 0.9Vs. The estimated stress drop is significantly higher than the global stress drop average of intraplate earthquakes, but is consistent with stress drop estimated for some intra-continental earthquakes elsewhere. The detection of a new active structure that appears to terminate in Machaze, its step-like geometry, and lithospheric strength all favors a hypothesis of stress concentration in the source region, which is likely the cause of this event and the higher than average stress drop.

Asynchronous timing of extension and basin formation in the South Rhodope core complex, SW Bulgaria, and northern Greece

AbstractUpper crustal extensional structures range from steep normal faults to shallow‐dipping detachments. The relationship between extension and formation of synkinematic hanging wall basins including their relative timing is not well understood. The South Rhodope core complex, Southern Balkans, has experienced extension for >40 Ma leading to a number of extensional structures and Cenozoic sedimentary basins. We present new bedrock and basin detrital zircon and apatite (U‐Th‐Sm)/He ages from the Pirin and Rila Mountains and the Sandanski basin. Results identify three episodes of Cenozoic extension in SW Bulgaria accommodated by (1) the Eocene/Oligocene Mesta detachment; (2) the early to middle Miocene Gorno Spanchevo fault (circa 18–15 Ma), which is the northern prolongation of the Strymon low‐angle detachment; and (3) the late Miocene West Pirin fault (≤10 Ma). Detachment faulting on the Strymon fault accommodated tens of kilometers of ENE‐WSW extension and created ~1500 m topographic relief, but because the resulting hillslopes were gentle (≤10°), extension did not lead to enhanced footwall erosion or formation of a hanging wall basin. In contrast, the West Pirin normal fault resulted in mostly vertical motion of its footwall causing steep topography, rapid erosion, and formation of the synrift Sandanski basin. Digital topographic analysis of river channel profiles identifies the latest episodes of deformation including westward tilting of the Sandanski and Strymon basins and Quaternary N‐S extension. This study demonstrates that basin formation in the South Rhodope core complex is related to normal faulting postdating the main episode of crustal stretching by detachment faulting.

Characteristics and Numerical Simulation of Extensional Structure Since Late Mesozoic in Western Shandong, China

The extensional structure in western Shandong is composed of steep normal fault and decollement fault, which two together constitute a unique “extensional structure framework”. With the regional geological survey of West Shandong Rise and seismic data from Jiyang Depression, we make comprehensive researches on extensional structural features in western Shandong. A number of NW-trending and NWW-trending normal faults, together with several NE-trending and nearly EW-trending faults, constitute a typical “fault-block style” all over the research area. In the steep normal fault zone, there are dynamic breccia, fault clay and scraping trace making up signs of faults activity. Low-angle decollement faults are mainly found at unconformity / lithologically abrupt interfaces or disconformity surfaces, of which the most outstanding in western Shandong occurred along unconformity surface between the Archean and Cambrian (Ar/∈), and the disconformity surface between the Ordovician and Carboniferous (O/C). Based on the finite element method, we use the software of Ansys12.0 to carry out tectonic stress field numerical simulation of the evolution in research area since late Mesozoic. The result indicates that the strength of the tectonic stress field has experienced a process from strong to weak since late Mesozoic. The evolution of extensional structures of western Shandong commenced in late Jurassic, with the direction of NE-SW. And the research area entered its first massive extended fault-depression phase in Late Jurassic-Early Cretaceous (140 - 65 Ma). Then the extension continued in Paleocene-Early Eocene (65 - 53 Ma) which was a transforming transitional period with its extending orientation transformed from NE-SW to SN. In Eocene-Oligocene (53 - 23.3 Ma) the extension met its large-scale extensional activity with the direction of NW-SE. As can be seen in the study, the essential dynamic origin is mainly influenced by the structural changes since late Mesozoic, and the chief factors leading to the tectonic stress field change are the subduction of the Pacific Plate and the strike slip motion of the boundary faults. Key words: Numerical simulation; Extensional structures; Since late Mesozoic; Western Shandong

Relationship between syn-depositional faulting and carbonate growth in Central Luconia Province, Malaysia

Using 3D seismic and well data, detailed seismic interpretation has been conducted on two carbonate Platforms EX and FY located in the Central Luconia Province, Malaysia. The results provide an insight to understand the relationship of faulting with syn-depositional carbonate growth. Five geo-seismic units were interpreted from the Late Oligocene to present day sedimentation in the basin. Structural interpretation of both platforms shows that almost all the faults in the deeper part of the platforms are normal listric faults that resulted from the final rifting stage of the South China Sea. Small-scale, steep normal faults within the carbonate units behave relatively as syn-depositional faults that became the base and template for the reefs to grow as these faults created conjugate and branching faults systems. Apart from becoming templates for the reefs to grow, these syn-depositional faulting events had also interrupted the growth of reefs especially those that were located at the platform margin. Slight movement of the branching faults induced sub-marine landslide to create reefs collapse. The establishment of relationship between faulting and carbonate growth in Central Luconia Province might be useful for revisiting mature hydrocarbon reservoirs.

Basement characteristics along South West Indian Margin

Abstract Results from detailed analyses of multi-channel seismic reflection data in association with gravity anomalies and lithology data from drilled holes are used to find out the basement structure in the vicinity of the Laccadive Ridge and Laccadive basin along southwest Indian margin. Morphology and seismic interpretation results show the basement in the study area having faulted blocks with horst/graben pattern, and there are basement highs parallel to the coastal line. While the margin highs are sediment deprived, thick sediments are observed in surrounding areas of the margin highs, with thickness up to 2-3s (TWT), and these sedimentary layers usually pinch out suddenly at the lower wall of steep normal faults. A thin layer of sediments is observed on the Laccadive Ridge in this region which has several small ridge peaks (distributed along ridge extension direction), that sometimes outcrop the seafloor. The basement ridge peaks are usually several thousand meters wide, and show obvious high abnormal gravity anomaly. The basement in the Laccadive basin is highly faulted, and appears to has evolved as stretched margin.

New insights into the magmatic plumbing system of the South Australian Quaternary Basalt province from 3D seismic and geochemical data

Recent seismic reflection studies of large-volume, anorogenic basaltic provinces at passive continental margins have challenged the traditional viewpoint that erupted magmas predominantly ascend through the lithosphere via dykes that exploit high-angle faults. However, such seismic-based methods are yet to be applied to identify the magmatic plumbing systems of low-volume basaltic provinces, such as the Cretaceous–Cenozoic Newer Volcanic Province (NVP) in southeastern Australia (total volume <20 000 km3). The South Australian Quaternary Basalt (SAQB) represents the most recent phase of activity both within the NVP and on the Australian continent. This province is located within the Otway Basin, and a large amount of seismic data from gas exploration is available for this region. Consequently, the SAQB represents a superb natural laboratory in which geochemical and seismic data can be combined in order to study the magmatic plumbing system of a low-volume continental basalt province and discriminate between the competing hypotheses for magma transport through the Earth's crust. Geochemical analyses and thermodynamic modelling suggests that the magma that fed the SAQB was generated by adiabatic decompressional melting of a secondary mantle plume. These models imply that melt segregation took place at successively lower pressures from about ∼4000 to 3000 MPa during the ca 1 Ma SAQB eruptive history that culminated in the Northern Group, Mt Schank and Mt Gambier eruptions. During ascent, the magma underwent 34–41% fractional crystallisation and cooled ∼200 °C, while residing in the crust for a time period on the order of days to weeks. Interpretation of a 3D seismic survey that overlaps with the northeastern part of the SAQB reveals a saucer-shaped sill with an unusual morphology, exhibiting a series of vertical concentric steps towards its outer rim. This sill appears to be fed by magma that intruded along steep normal faults from a feeder dyke that is hosted by the regional, NW–SE-trending Hungerford-Kalangadoo Fault. Our results suggest that the melt that fed the SAQB rose through the crust via dykes and high-angle normal faults, with less evidence for significant horizontal transport of magma than observed in large-volume basaltic provinces in sedimentary basins at rifted continental margins, possibly highlighting a correlation between the extent, volume and magma supply rate within a basaltic province and the nature of magma ascent.

Structural control of basement denudation during rifting revealed by low-temperature (U–Th–Sm)/He thermochronology of the Socotra Island basement—Southern Gulf of Aden margin

Abstract In the Gulf of Aden, different types of fracture zones (F.Z.) have been defined and potential links with continental transfer zones have been proposed (Bellahsen et al., 2013-this volume-a). In this study, we investigate the denudation history of the south-eastern continental margin of the Gulf of Aden on the Socotra Island, in order to highlight the interplay of normal and transfer/transform tectonic structures in the course of rift evolution. Samples belong from two distinct East and West domains of the Socotra Island separated by the continental Hadibo Transfer Zone (HTZ). Tectonic denudation started during the Priabonian–Rupelian along low-angle normal faults and removed part of the overlying sedimentary formations allowing basement exhumation toward the surface (~ 1.1–1.5 km of exhumation). Forward t–T modelling of the data requires a slightly earlier date for initiation of rifting in the E-Socotra domain (~ 38 ± 2 Ma), compared to the W-Socotra domain (~ 32 ± 2 Ma), which suggests that the HTZ was already active at that time. A second major event of basement cooling and exhumation (additional ~ 0.7–1 km), starting at about ~ 20 ± 2 Ma, has only been recorded on the E-Socotra domain. This second denudation phase significantly post-dates local rifting period but appears synchronous with Ocean Continent Transition formation (OCT: 20–17.6 Ma). This late syn-OCT uplift is maximum close to the HTZ, in the wedge of footwall delimited by this transfer system and the steep north-dipping normal faults that accommodated the vertical motion. This particular pattern of uplift and denudation during the OCT reorganisation suggests that the late uplift of the margin can be strongly differential from a segment to another, depending on the amplitude of thinning experienced by each of the adjoining segments.

The effect of changing regional tectonics on an arc volcano: Methana, Greece

The peninsula of Methana has the longest recorded volcanic history of any volcanic centre in the South Aegean Arc. Regional fault patterns in the arc changed during the Pliocene–Quaternary, with E–W-striking listric faulting increasingly important through the Quaternary, as recorded in well-dated sedimentary basins. This study investigates how the geochemistry and eruptive style of volcanic products is influenced by regional tectonics. The volcanic stratigraphy of Methana was refined using radiometric dating, lithogeochemistry and field observations that included recording deformational structures and enclave abundance. Small N–S-striking Pliocene domes and a central volcano of uncertain type (phase A) were eroded to produce a widespread volcaniclastic apron (phase B). In the early Quaternary, an explosive central volcano with flank eruptions of andesite developed (phase C). Dacite domes and small andesitic stratovolcanoes formed throughout the mid and late Quaternary (phases D–H).Basaltic andesite and andesite of phase C are the least evolved rocks, characterised by high TiO2. Rocks that have experienced important assimilation and fractional crystallisation in the crust have a high abundance of enclaves, Th, U and alkalies (Na, K). Ni and Cr are abundant in phase A andesites, due to crystallisation and entrainment of olivine and pyroxene, whereas phase H andesites have the highest relative abundance of Ba, Rb and Sr from crystallisation and entrainment of hornblende and biotite.Pliocene domes of phase A were emplaced on N–S-striking listric faults during regional E–W extension. Onset of NE–SW faulting, arguably crustal scale and strike slip in character, led to the eruption of the least evolved rocks of phase C. Thereafter, E–W-striking faults controlled the location of volcanism. Volcanism in phases F and G was particularly voluminous and was synchronous with the onset of steep normal faulting in the Gulf of Corinth, 150km west of Methana. These steep faults cross-cut older, lower angle, listric faults. It is times of changes in regional fault patterns, as in phases C, F and G, that have the greatest influence on the eruptive style and abundance of volcanic products.

Extensional fault-propagation folding in mechanically layered rocks: The case against the frictional drag mechanism

“Fault drag” (deflection of beds or other markers into folds that are convex in the direction of relative slip) is often interpreted as the product of frictional sliding along a fault and progressive tilting of beds with increased amount of displacement along a fault. We analyze two sets of normal faults, with throws ranging from 0.5m to 5m, and associated fault-related folds in mechanically layered upper Cretaceous carbonate, anhydrite, and shale in central Texas. For each fault set, we interpret the fault displacement and fault-related folds exposed in outcrop to represent different stages in the developmental sequence. In both fault sets, faults in dolostone or limestone lose displacement and tip into less competent anhydrite or shale where deformation is accommodated by folding and smaller scale deformation. Fold wavelength is established early and at small displacement (<1m throw). With increasing displacement the monoclinal fold limb steepens and is extended parallel to bedding, locally producing boudinage in the most competent bed between incompetent beds. Clay smear is well developed where a 0.35m thick clay shale is locally thinned to 0.1m associated with fault throws of 0.5 to 5m. Bed tilting and the development of apparent drag is not the product of frictional sliding but instead folding at the tip of an arrested, in this case upwardly, propagating normal fault. We conclude that synthetic dip associated with steep normal faults (i.e., fault drag) should not be assumed to be the product of frictional drag, but must be considered in the context of the mechanical stratigraphy. Instead, fault-tip folding in mechanically layered rocks produces synthetic dip (drag) early in the fault development history prior to propagation of the fault tip through the folded layer.

Basal and Frontal Accretion Processes versus BSR Characteristics along the Chilean Margin

Multichannel seismic reflection data recorded between Itata (36°S) and Coyhaique offshores (43°S) were processed to obtain seismic images. Analysis of the seismic profiles revealed that weak and discontinuous bottom simulating reflectors were associated to basal accretion processes, while strong and continuous bottom simulating reflectors were associated to frontal accretion processes. This can be explained considering that during basal accretion processes, extensional tectonic movements due to uplifting can favour fluid escapes giving origin to weaker and most discontinuous bottom simulating reflectors. During frontal accretion processes (folding and thrusting), high fluid circulation and stable tectonic conditions however can be responsible of stronger and most continuous bottom simulating reflectors. Along the Arauco-Valdivia offshores, steep accretionary prisms, normal faults, slope basins, and thicker underplated sediment bed were associated to basal accretion, while along the Itata, Chiloe and Coyhaique offshores, small accretionary prisms, folding, and thinner underplated sediment bed were associated to frontal accretion.

AVO analysis of a shallow reverse polarity in Sydney Basin

A vast shallow reverse polarity which is extended across the PEP11 block in offshore Sydney basin and the lack of any wells in the area has wondered the interpreters about the mechanisms which has caused such a seismic signature in the area. This study aims to characterize this reverse polarity using the AVO analysis and pre stack inversion techniques in a pilot study. Baleen 2004 2-D towed streamer data are the only recent seismic data acquired in PEP11 block. Shallow depth water has left significant amount of multiple energy in shot records. The presence of a very high velocity sequence of layers which terminate at the shore line cause a strong ray bending letting small amount of the energy penetrate to deeper parts. The reverse polarity reflector is part of a sequence of progradational sediments which are accommodated at the top of the high velocity layer.To characterize the reverse polarity, we have processed 1 second TWT of two perpendicular lines by preserving the relative amplitude up to pre stack time migration. Assuming clastic environment, AVO analysis of the reverse polarity over the lines shows different signatures across a steep normal fault. The shallow parts at the up-thrown part of the fault indicate the wet trend despite different reflectivity strength across the line. However, the deeper parts correspond to down-thrown shows the AVO class III. Pre-stack inversion confirms low impedance in areas with class III and higher impedance in areas which show the wet trend.

Extreme crustal thinning in the Bay of Biscay and the Western Pyrenees: From observations to modeling

Recent observations and models of continental rifting in magma‐poor environments have led to the concept of multiphase stages of lithospheric extension. In these concepts it is shown that extreme crustal thinning of the crust predates exhumation of lower crustal and subcontinental mantle rocks during final rifting. The Bay of Biscay is a V‐shaped ocean basin that opened in Aptian‐Albian time. In front of this propagating ocean, several rift basins formed that show evidence for extreme crustal thinning and locally also mantle exhumation (the Parentis, Arzacq‐Mauleon, and Cantabrian basins). In this paper we propose, based on geological and geophysical observations and using numerical modeling, a model that can explain the extreme crustal thinning observed in the Arzacq‐Mauléon and Parentis basins. Our results show that rifting in the Bay of Biscay was initiated by distributed oblique stretching (latest Jurassic to Early Aptian) before it underwent an more orthogonal asymmetric thinning and exhumation phase from Late Aptian to Albian time. These last two stages of deformation are similar to those observed in orthogonal rift systems. We show that thinning is accomplished by the formation of a semibrittle shear zone that allows for the transfer of middle to lower crustal material from the side of the rift collocated with the hanging wall to the side of the rift collocated with the footwall of the detachment system. The main difference with an orthogonal rift system appears to be generated by the formation of flower structures during the distributed oblique phase and the capacity of localizing the deformation in the subsequent stages. These oblique slip faults form very steep normal faults that induce the development of strongly localized, compartmentalized, and asymmetric rift basins. In the case of the Parentis and Arzacq‐Mauleon basins, these strike‐slip faults separate upper plate sag basins to the north from lower plate sag basins to the south. While the northern sag basins do not show any evidence for exhumation, the southern ones are more complex and floored by detachment faults, as indicated by the occurrence of syntectonic and posttectonic sediments onlapping directly onto exhumed lower crustal and mantle rocks.

Geometry and kinematics of Mykonos detachment, Cyclades, Greece: Evidence for slip at shallow dip

[1] The eastern part of Mykonos island (Cyclades, Greece) shows the detailed internal structure of a two-branch shallow-dipping extensional detachment system of Miocene age. This paper shows that the last stage of slip along the detachment, during the deposition of syn-rift sediments, occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension. Extensional deformation during and after a 13 Ma old granite intrusion is taken up by two main shallow-dipping shear zones: (1) The lower Livada detachment, which extends laterally across Tinos island, is a ductile structure located at the interface between the granite and the Upper Cycladic Nappe metabasites. A pervasive top-to-the-NE shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact. Later brittle faults, shallowly and steeply dipping, rework the ductile deformation with the same overall shear sense. (2) The upper Mykonos detachment is brittle and separates the metabasites from late Miocene sandstones and conglomerates. The detachment fault dips 12–15° toward the NNE, and its sense of shear is consistent with that of the Livada detachment. Soft-sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation. Rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone, indicating that the overall direction of extension is NNE/SSW with a sense of shear toward the NNE and the dip of the fault is throughout very low. The two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the Aegean Sea, which emphasizes the importance of low-angle extensional faults and shear zones in extensional tectonics.

Decoupling of deformation in the Upper Rhine Graben sediments. Seismic reflection and diffraction on 3-component Vertical Seismic Profiling (Soultz-sous-Forêts area)

A contribution to the definition of the structural pattern of the Soultz-sous-Forêts EGS (Enhanced Geothermal System) is presented here. After reprocessing, the PHN84J seismic reflection profile highlights the tilted blocks of the Merkwiller-Péchelbronn oilfield. In the Soultz-sous-Forêts horst, complex fault patterns are observed: the Hermerswiller normal fault flattens at depth and is rooted in decollements occurring in Triassic salt or clay series, while other steep normal faults affect underlying sedimentary formations and basement. Some methods for the exploitation of a seismic diffraction recorded by multi-component Vertical Seismic Profiling (VSP) are also illustrated to locate the diffractor without specific data processing. Polarisation and travel time analysis of a diffraction event recorded in the GPK1 borehole are analysed, and its exploitation combined with seismic reflection helps defining a tilted block geometry.

Geomechanical modeling of an extensional fault-propagation fold: Big Brushy Canyon monocline, Sierra Del Carmen, Texas

Field structural data from the Big Brushy Canyon monocline developed in Cretaceous strata of west Texas are combined with nonlinear finite element modeling to help bridge the gap between geometric, kinematic, and mechanical analysis techniques for understanding the deformation history of reservoir-scale geologic structures. The massive Santa Elena Limestone is offset along a steep normal fault, and fault displacement is accommodated upward by the folding of the Buda Limestone and Boquillas Formation and the thinning in the intervening Del Rio Clay. Mesostructures within competent Buda Limestone beds are concentrated in the monocline limb instead of the hinge and include bed-perpendicular veins that accommodate bed-parallel extension and bedding-plane slip surfaces that offset the veins and accommodate flexural slip. Finite element models were constructed to reproduce the monocline geometry and deformation distribution as well as to assess the effect of material properties and boundary conditions on structural evolution. The initial model configuration replicated the assumed predeformational geometry, included frictional sliding surfaces to allow for bedding-parallel slip, and used a displacement boundary condition at the base of the Santa Elena footwall to simulate fault motion. Geometry and strain evolution were tracked so that (1) fold shape, (2) cumulative extension, and (3) layer-parallel shear strain could be compared to field observations. Iterative model runs successfully matched field data and revealed the importance of benchmarking the model results against monocline geometry, layer-parallel extensional strain, and bedding slip in the natural example. Our results illustrate the potential use of this modeling approach whereby calibration is performed using available data and is followed by strain measurement throughout the model domain to aid in prediction of subseismic faults and fractures. This geomechanical modeling approach provides a powerful tool for site-specific subsurface deformation prediction in hydrocarbon reservoirs that incorporates details of the local mechanical stratigraphy and structural setting.

Rifting and shallow-dipping detachments, clues from the Corinth Rift and the Aegean

The Corinth Rift is superimposed on the Hellenic nappe stack that formed at the expense of the Apulian continental crust above the subducting African slab. Extension started in the Pliocene and the major steep normal faults that control the geometry of the present-day rift were born very recently, some 600 kyr ago only. They root into a shallow-dipping zone of microseismicity recorded near the base of the upper crust. The significance of this seismogenic zone is debated. Considering the northward dip of the zone of microseismicity, the depth of microearthquakes and their focal mechanisms, we observe a strong similarity with the northern Cycladic detachments in terms of expected pressure, temperature conditions and kinematics. We herein show (1) that the formation of the Corinth Rift can be considered a part of a continuum of extension that started some 30–35 Ma in the Aegean and that was recently localised in a more restricted area, (2) that the present-day structure and kinematics of the Corinth Rift can be explained with a series of decollements relayed by steeper ramps that altogether formed a mechanically weak, crustal-scale detachment, and (3) that the deformation, fluid behaviour and metamorphic features seen in the northern Cycladic metamorphic core complexes can be good analogues of the processes at work below the Corinth Rift.

Northward migration of the southern diagonal fault of the Hula pull-apart basin, Dead Sea Transform, northern Israel

The Hula Valley, located in northeastern Israel, is a 4 million-year old pull-apart basin bounded by two left stepped segments of the Dead Sea Transform. Basic models of pull-apart basins suggest four faults at all sides of the basin, a situation which is not always the case. In the Hula pull-apart basin the longitudinal (N–S) faults are clearly evident while the transversal (E–W) faults are less explicit. An NW oriented lineament, expressed as a north-facing continuous scarp 1–3 m high, was recognized in the southern part of the valley. Morphological, seismic and sedimentary considerations suggest that this scarp is the surface expression of a deep-seated fault. This fault, termed the Yesod HaMa'ala fault, forms the present southern boundary of the Hula pull-apart basin, and appears to be the southern diagonal fault, which links the Jordan fault in the east with the Yammunneh fault in the west. It is suggested that this fault scarp formed a shallow morphological barrier that partly blocked the southern outlet of the valley, enabling a permanent lake in the southern part of the Hula Valley. Thus, the uplifted southern block determined the SW shoreline of the lake. It is proposed that the Hula basin is a version of the classic pull-apart model. It is also suggested that in the absence of upwelling mantle material, and extension along the length of the basin, a crustal thinning and subsidence might have occurred through normal faulting of the upper brittle crust. The thick depositional sequence within the southern part of the basin shows that the present margins have been a major depositional site for a long time, and that the Hula Valley, has thus developed as an asymmetric, pull-apart (rhomb-shaped) basin, bounded by steep north-striking boundary normal (as well as strike-slip) faults, W–NW striking and northward-dipping normal faults, and a south-facing bending flexure. As activity within the basin continued, and northward migration of the faulting occurred, a new fault developed. According to this perception, the present active fault is the Yesod HaMa'ala fault, while the future fault will be located northward, as can be presumed from the present micro-seismic activity in the valley.

Higher Himalayan Shear Zone, Zanskar Indian Himalaya: microstructural studies and extrusion mechanism by a combination of simple shear and channel flow

Thin-section studies of the Zanskar Shear Zone (ZSZ) rocks reveal a top-to-SW and subsequent primary and secondary top-to-NE ductile shearing; brittle–ductile and brittle extensions; top-to-SW brittle shear, steep normal faulting and fracturing. In the proposed two-phase model of ductile extrusion of the Higher Himalayan Shear Zone (HHSZ), a top-to-SW simple shearing during 22–18 Ma was followed by a combination of top-to-SW simple shear and channel flow at 18–16 Ma. The second phase simulates a thin ZSZ characterized by a top-to-NE shearing. The channel flow component ceased around 16 Ma, the extruding HHSZ entered the brittle regime but the top-to-SW shearing continued until perturbed by faults and fractures. Variation in the extrusion parameters led to variable thickness of the ZSZ. Shear strain after the extrusion is presumably maximum at the boundaries of the HHSZ and falls towards the base of the ZSZ, which crudely matches with the existing data. The other predictions: (1) spatially uniform shear strain after the first stage, (2) fastest extrusion rate at the base of the ZSZ, and (3) a lack of continuation of the ZSZ along the Himalayan trend are not possible to validate due to paucity of suitable data. Non-parabolic shear fabrics of the ZSZ indicate their heterogeneous deformation.

Triassic and Jurassic Tectonics in the Eastern Yanshan Belt, North China: Insights from the Controversial Dengzhangzi Formation and Its Neighboring Units

The Jurassic geology of the Niuyingzi–Dengzhangzi area of western Liaoning province south of Lingyuan has challenged geologists for many years. Its most controversial structural element is a narrow (0.1–1.5 km), subvertical, fault-bounded panel of Middle Proterozoic Changzhougou quartzite and, locally, higher Changcheng System units. The 35 km-long, NNE-striking panel is bordered on the west by the subvertical Daqizi fault (new name) and the steep, largely east-overturned Late Triassic–Early Jurassic Dengzhangzi Formation. It is bordered on the east by the subvertical Zhuzhangzi fault (new name) and west-dipping Middle Jurassic Guojiadian Formation strata. Previous structural interpretations have favored ideas that the Daqizi fault is a steep normal fault with Dengzhangzi side down, and that the Zhuzhangzi fault is a steep reverse fault with Guojiadian side down. We disagree with both interpretations and present evidence that both faults have formed during previously unrecognized Jurassic tectonic events. The Daqizi fault is interpreted as a late Early Jurassic to early Middle Jurassic west-directed thrust fault placing Changcheng strata across lower-plate Dengzhangzi strata. Both upper and lower plates were then folded. The Zhuzhangzi fault was originally a normal fault bounding a Guojiadian half-graben that formed during an episode of previously unrecognized Middle Jurassic extension. The fault was localized along the folded, east-dipping contact between Changzhougou quartzites and overlying shales of the Chuanlinggou Formation. As the fault developed, detached hanging wall Chuanlinggou and higher Changcheng strata above the Changzhougou quartzite were dropped down and buried beneath the half-graben fill. The geometry of the Dengzhangzi–Changzhougou–Guojiadian structural assemblage was subsequently modified by late Middle Jurassic or early Late Jurassic contraction. Among its effects were (1) reverse fault inversion of the Zhuzhangzi normal fault and (2) eastward Yangzhangzi–Wafangdian thrusting of Jixian strata across the Changcheng fault-bounded panel and its neighboring Jurassic units. Steepening and eastward overturning of Dengzhangzi strata occurred at this time. The results of our study indicate that the Jurassic tectonic history of the western Liaoning area was considerably more complicated than believed earlier.

The Itea–Amfissa detachment: a pre-Corinth rift Miocene extensional structure in central Greece

Abstract The Itea–Amfissa valley, separating Giona Mountain to the west from Parnassos Mountain to the east, is related to an extensional detachment observed along the eastern slopes of Giona. The detachment is traced for 30 km north of the Corinth Gulf and dips 25°–40° to the east, showing an east–west extension parallel to the Hellenic arc. The lower nappes of Pindos, Penteoria, Vardoussia and mainly the basal thrust of the Parnassos unit form part of the footwall, whereas the upper thrusts of the Parnassos unit and the Western Thessaly–Beotia nappe form part of the hanging wall. The eastern slopes of Giona are controlled by the detachment and several hundred metres of syn-tectonic breccia-conglomerates are observed at the top of the hanging wall rocks and are back-tilt towards the detachment plane. Two conglomeratic sequences are distinguished: the lower one consists of argillaceous matrix and abundant ophiolite detritus whereas the upper one bears carbonate matrix with carbonate detritus together with large olistholites of Mesozoic limestones. Based on calcareous nannofossils a middle Miocene age has been determined for the lower formation and a middle–upper Miocene age is probable for the upper. Planation surfaces cut on top of the sediments rise from south to north starting from sea level at Galaxidi to about 1400 m at Prosilio. The throw of the detachment is about 2.5–4.2 km measured mainly from the structural omission of the Alpine tectono-stratigraphic units. A contrast between the footwall and the hanging wall structure is described, with monoclinic sequence of the Parnassos nappe dipping to the west in the footwall but a complex synsedimentary horst and graben structure of sliding blocks of Alpine formations within the Miocene clastic sequences in the hanging wall. The detachment has been deformed by the east–west-trending steep normal faults that have created the Corinth rift during late Pliocene–Quaternary time showing a north–south extension. The Itea–Amfissa detachment forms the northern tip of the broader East Peloponnesus detachment, observed south of the Corinth rift structure from Feneos to Kyparissi. Similar geodynamic phenomena with large olistholites and breccia conglomerates are known from the Serravalian of Crete, related to the activity of the Cretan detachment.