West-vergent Thrust Research Articles

Structure and Holocene evolution of an active creeping thrust fault: The Chihshang fault at Chinyuan (Taiwan)

We conducted a variety of measurements and analyses at Chihshang Active Fault Observatory in eastern Taiwan, including surface-rupture mapping, three shallow borehole core analysis and kinematic analysis of geodetic measurements. We found that the Chihshang fault exhibits a three-branch fault system in the Chinyuan alluvial fan, which is composed of at least 100 m thick gravel deposits. Outside of the Chinyuan River, the Chihshang fault shows a single fault system with a sharp lithological contact. Combining the leveling results and trench excavation, we interpret that the three fault branches are coupled with a 50−60-m-wide pop-up structure in the hangingwall. Based on the ratio between vertical and horizontal displacements, we obtained dip angles of 38°, 62° and 16° for two west-vergent thrusts and an east-vergent backthrust, respectively. This pop-up was estimated to develop at the uppermost 30–40 m unconsolidated gravels during the last few thousand years above the main fault with a dip angle of 42°. By compiling the available ages data, we obtained an uplift rate of 2.3 cm/yr of the Chihshang fault and an alluvial sedimentation rate of 1.1 cm/yr during the past thousands years. Consequently, the individual uplift rate for each fault branch at the Chihshang Observatory was slightly less than the deposition rate of the Chinyuan River. No geomorphic fault scarp can thus be observed in the three-branch fault system area.

The Wyoming carbon underground storage project: Geologic characterization of the Moxa Arch and Rock Springs Uplift

The Wyoming carbon underground storage project: Geologic characterization of the Moxa Arch and Rock Springs Uplift

Structural controls on the formation of BSR over a diapiric anticline from a dense MCS survey offshore southwestern Taiwan

A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.

Architecture and Neogene to Recent evolution of the western Calabrian continental margin: An upper plate perspective to the Ionian subduction system, central Mediterranean

The western Calabria continental margin forms the transition between the Pliocene to Recent Marsili spreading center and continental Calabria, all parts of the upper plate of the Ionian subduction zone. Integrating high‐resolution and crustal seismic images constrained by gravity modeling, we provide a detailed reconstruction of the architecture of the margin and develop a new scheme for its Miocene to present evolution. This time span encompasses the continent‐continent collision between Africa and Eurasia, subsequent orogenic collapse and rifting apart between the two continental masses, and the Pliocene to Recent emplacement of oceanic crust in the Vavilov and Marsili basins. The crust of the margin thins from the Calabria coast (∼25 km) to the Marsili continent‐ocean transition (∼12 km). On the whole, upper and lower crusts thin proportionally with pure shear geometry. The continental margin is covered by an Oligocene(?) to present sedimentary succession reaching a maximum thickness of ∼6.0 km in the Paola Basin. During the Miocene the continental margin experienced regional shortening accommodated by a large number of mainly west vergent thrusts possibly associated with the late stages of the Kabilo‐Calabrian chain. Shortening continued through Pliocene to Recent but was accommodated by a limited number of west vergent thrust faults located in the western part of the margin and by a few tens of kilometers wide syncline located in the eastern part of the profile. The accommodation space created in the syncline core hosted a ∼4.5 km thick, Plio‐Quaternary sedimentary succession, the Paola Basin. No significant extensional fault is observed along the profile. Miocene to Recent subsidence was controlled by (1) a short‐wavelength component related to shortening and responsible for the formation of the Paola Basin syncline and, possibly, contributing to the uplift of onshore Calabria and (2) a long‐wavelength component responsible for the regional subsidence and oceanward tilting of the Calabria margin. Short‐wavelength subsidence ended in the late Pliocene, but long‐wavelength downward movements persisted and even accelerated during late Pliocene(?) to Quaternary times when the present‐day bathymetry was achieved. Both horizontal deformations and vertical movements are difficult to explain in the context of a normal back‐arc basin without taking into consideration patterns of secondary mantle flow generated by the retreat of the subducting slab.

Geophysical evidence for an extensive Pie de Palo Complex mafic–ultramafic belt, San Juan, Argentina

The recent completion of a high-resolution aeromagnetic survey over the Pie de Palo uplift of the western Sierras Pampeanas has revealed an area of large magnetic anomalies associated with the Pie de Palo Complex. The Las Pirquitas thrust, which has transported and uplifted the Pie de Palo Complex, is recognized for at least 30 km in a roughly NE direction along the western boundary of the Pie de Palo Complex, beyond its limited outcrop. The type of sediments of the Caucete Group in the footwall of the Las Pirquitas thrust, which are regarded as the leading edge of the Precordillera terrane, are associated with much less pronounced magnetic anomalies. In addition, a conspicuous, NNE trending, broad magnetic high stands out in the survey, several kilometers to the east of the main outcrops of the Pie de Palo Complex; this broad magnetic anomaly bisects the Pie de Palo basement block, and continues further south at least as far as 32°S, the southern boundary of the latest aeromagnetic survey. This magnetic anomaly is interpreted to represent a structure corresponding to the Grenvillian Precordillera–Pie de Palo tectonic boundary zone, and would comprise the buried largest part of the mafic–ultramafic belt. The geophysical model of the magnetic data indicates that the boundary zone dips to the east, possibly suggesting the existence of a set of synthetic east dipping, west-verging thrusts, of which only one major structure (Las Pirquitas thrust) is exposed; the possibility of other slivers of upthrust boundary zone material cannot be excluded. It is considered that the Pie de Palo Complex represents a small sliver upthrust from the unexposed boundary zone material (containing highly magnetic mafic–ultramafic rocks). The east-dipping, west verging structures associated with the Pie de Palo Complex are suggested to represent an Ordovician reactivation of a Grenvillian suture zone developed when the Precordillera basement and Pie de Palo terrane docked; this reactivation probably resulted from the collision of the Cuyania terrane onto the western margin of Gondwana.

Geometria e evolução do feixe de zonas de cisalhamento Manhuaçu - Santa Margarida, Orógeno Araçuaí, MG

Feixe de Zonas de Cisalhamento Manhuaçu-Santa Margarida de orientação geral NS estende-se por, aproximadamente, 300 km na região leste de Minas Gerais. Como uma das principais estruturas do núcleo cristalino do Orógeno Araçuaí, é constituído por um conjunto de zonas de cisalhamento dúcteis, que, em mapa, mostram traços sigmoidais. O seu segmento norte é composto por zonas de cisalhamento reversas que promovem o transporte de material em direção a oeste e, subordinadamente, por zonas transcorrentes dextrais. Os segmentos central e sul são dominados por zonas transcorrentes dextrais. O desenvolvimento do feixe deu-se em quatro fases deformacionais neoproterozóicas. A primeira fase promoveu a nucleação do conjunto de zonas reversas, que ficaram parcialmente preservadas no segmento norte do feixe. Durante a segunda fase, as zonas preexistentes experimentaram, de norte para sul, uma rotação progressiva para a vertical e para a direção NE-SW, além de intensa reativação transcorrente dextral. A terceira fase gerou zonas de cisalhamento normais de ocorrência restrita ao segmento norte. Na quarta fase, formaram-se falhas transversais e juntas. Em sua terminação sul, o feixe, dominado por zonas transcorrentes dextrais, rotaciona-se até confundir-se com as estruturas da porção NW da Faixa Ribeira. Tal fato implica que as estruturas dessa porção do feixe são de mesma idade, ou algo mais velhas que as transcorrências dextrais, que marcam o quadro tectônico do segmento NW da Faixa Ribeira.

A Neogene giant landslide in Tarapacá, northern Chile: A signal of instability of the westernmost Altiplano and palaeoseismicity effects

Giant landslides, which usually have volumes up to several tens of km 3, tend to be related to mountainous reliefs such as fault scarps or thrust fronts. The western flank of the Precordillera in southern Peru and northern Chile is characterized by the presence of such mega-landslides. A good example is the Latagualla Landslide (19°15′S), composed of ~ 5.4 km 3 of Miocene ignimbritic rock blocks located next to the Moquella Flexure, a structure resulting from the propagation of a west-vergent thrust blind fault that borders the Precordillera of the Central Depression. The landslide mass is very well preserved, allowing reconstitution of its movement and evolution in three main stages. The geomorphology of the landslide indicates that it preceded the incision of the present-day valleys during the late Miocene. Given the local geomorphological conditions 8–9 Ma ago (morphology, slopes and probably a high water table), large-magnitude earthquakes could have provided destabilization forces enough to cause the landslide. On the other hand, present seismic forces would not be sufficient to trigger such landslides; therefore the hazard related to them in the region is low.

3D seismic analysis of the Coast Shear Zone in SE Alaska and Western British Columbia: Broadside analysis of ACCRETE wide-angle data

The multidisciplinary ACCRETE project addresses the question of continental assemblage in southeast Alaska and western British Columbia by terrane accretion and magmatic addition. The previous studies of this project yielded important information for understanding the structures across the Coast Shear Zone (CSZ) and the formation of the CSZ and the Coast Mountains Batholith (CMB). The present study extends these interpretations into pseudo-3-D by using two additional wide-angle ACCRETE seismic lines. By analyzing the broadside wide-angle data using a series of laterally homogeneous 2-D models, we derive a lower-resolution 3-D velocity model of the outboard terranes and constrain variations in crustal thickness across and along the CSZ. Models of the broadside data confirms major structural and compositional trends extend along strike to the northwest. The key features are: a) a steep Moho ramp only ∼ 15-km wide is coincident with the CSZ and divides thin (∼25 ± 1 km) crust to the west below the west-vergent thrust belt (WTB) from thicker (∼ 31 ± 1 km) crust to the east below the CMB, (b) low-velocity mantle (7.7--7.9 km/s) extends beneath the entire study region indicating high crustal and upper-mantle temperatures below the WTB and CMB, and (c) the Alexander terrane is characterized by strong mid-crustal reflectivity and high lower crustal velocities that are consistent with gabbroic composition. This study extends the earlier interpretation and implies that the ramp is indeed likely associated with transpressional tectonics and magmatic crustal addition east of the CSZ.

Morphotectonics and sedimentation in convergent margin basins: An example from juxtaposed marginal sea basin and foreland basin, Northern South China Sea

Using reflection seismic profiles and bathymetric mapping this paper reveals the tectonic–sedimentary characteristics of the convergent margins in the northern South China Sea, where it is strongly related to flexure of Chinese rifted margin and overthrust of Taiwan orogen. Convergent margin tectonics of the South China Sea near southern Taiwan is characterized by a progressively northward transition from oceanic subduction along the Manila Trench to the incipient collision zone offshore southern Taiwan where the continental crust of the Eurasian plate subducts beneath the Philippine Sea plate. North of 21°N, dip angles of the Benioff zone increase up to 80° in the incipient collision zone where the Manila Trench becomes shallower, gradually loses its morphological identity and finally merges into the nearly N–S trending Penghu Canyon. Convergent margin tectonics in the initial collision zone in SW Taiwan is manifested by the beginning of flexure of the Chinese margin under the westward migrating overthrust belt of Taiwan, forming two distinct basins. On the passive Chinese margin, the marginal sea basin becomes smaller and is underlain by the South China Sea Slope, while on the active Taiwan margin, a wedge-top basin has formed above the frontal thrust sheets of the Taiwan orogenic wedge. Sediments derived from the Taiwan orogen progressively overlie the strata of the passive Chinese margin, resulting in sediment thickening and basin shallowing from south to north. Sedimentary facies shows that offshore deep-water mud is gradationally overlain by shallow marine sediments. Sediments of the wedge-top basin are being actively deformed into mud diapiric intrusions and a series of west-vergent thrusts and folds with their associated piggy-back basins, resulting in irregular topography of the sea floor with alternating sea ridges and troughs. Pliocene–Quaternary strata of the passive Chinese margin are a little deformed under the westward compression induced by the arc–continent collision in the southern Taiwan. The slope profile of the passive Chinese margin is characterized by a sigmoidal curvature, representing a typical primary depositional setting of a passive margin. Slope canyons occur mainly on the upper slope and cut the sea floor, resulting in irregular topography and representing effects of erosion. Being under the initial arc–continent condition, the offshore SW Taiwan has experienced the transition from a rifted margin to an overthrust belt and becomes a foreland basin, which is acting as a catch basin for orogenic debris derived from SW Taiwan. Arguments from tectonics and sedimentation suggest that the Koping shelf-slope region is considered to be an immature foreland basin rather than an accretionary wedge. More specifically, the Kaoping shelf-slope can be regarded as a depositional wedge top where exists an overlap area between rapid sedimentation and active deformation of the Kaoping shelf-slope sediments in the external orogenic wedge of southern Taiwan.

The thrust-contact between the Eastern Ghats Belt and the adjoining Bastar craton (Eastern India): Evidence from mafic granulites and tectonic implications

Along its northwestern margin, the Meso-/Neo-proterozoic granulites of the Eastern Ghats Belt are juxtaposed against the Palaeoproterozoic/Archaean lithologies of the Bastar craton along a crustal ductile scale shear zone. In the shear zone hosted garnet-bearing mafic granulites of the Eastern Ghats Belt, garnet porphyroblasts pre-tectonic with respect to the shearing are mantled by coronal orthopyroxene + An-rich plagioclase vermicular intergrowths. The intergrowths typically overgrow the shear zone fabric defined by the shape preferred aggregates of hornblende, orthopyroxene ≫ clinopyroxene in a recrystallized mosaic of plagioclase. Schreinmakers analysis of reaction equilibria in the NCMASH system suggests that the post-shearing orthopyroxene + plagioclase (An-richer) symplectites formed at the expense of hornblende–garnet–An-poorer matrix plagioclase assemblage. The model reaction has high positive d P/d T slope with orthopyroxene + plagioclase stable in the high temperature side of the reaction. The textural relations are inferred to indicate prograde heating T max ∼ 750 °C, P max ∼ 7 kbar post-dating shearing. The gamut of available age data (K–Ar, Rb–Sr, Pb–Pb in zircon, and U–Pb–Th CHIME date in monazites) indicate that the youngest thermal event in and neighboring the shear zone cluster at ∼500–550 Ma, regardless of the vestigal Precambrian dates in rocks from across the shear zone. Since the closure temperature of thermally-driven Pb diffusion in minerals like monazite and zircon is ≥650–700 °C, the Early Palaeozoic age is likely to correspond with the ductile shearing and granulite facies prograde heating at T value higher than the blocking temperature of Pb. Since the shearing is correlated with top-to-the-west thrusting of the Eastern Ghats Belt over the cratonic foreland, the Early Palaeozoic prograde granulite facies metamorphism may relate to suturing of the two disparately evolved crustal blocks. The prograde heating may have been caused by burial of the Eastern Ghats granulite facies gneisses neighboring the shear zone due to emplacement of granulites from the hinterland along west-vergent thrust imbrications.

Exhumation history of the southern Altiplano plateau (southern Bolivia) constrained by apatite fission track thermochronology

Although the structural geometry of the Cenozoic Altiplano‐Puna plateau in the central Andes is well defined, the temporal evolution of this contractile deformation is poorly constrained. To address this shortcoming, we used apatite fission track thermochronology (AFT) to quantify the cooling and exhumation history along a transect at 21°S in southern Bolivia, through the deformed intermontane Altiplano basin, the doubly vergent thrust belt of the Eastern Cordillera and the inner foreland thrust belt east of the plateau (Interandean Zone). Thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories. Exhumation started during the late Eocene (40–36 Ma) in the central Eastern Cordillera, possibly due to bivergent thrusting and Cretaceous rift structure inversion. During the early Oligocene (33–27 Ma), exhumation spread across the study area as the current boundary thrusts of the Eastern Cordillera were activated. The inner west vergent thrust system became active in irregular order until circa 20 Ma, whereas the east vergent Interandean thrust belt formed by eastward propagating deformation since circa 30 Ma. Plateau exhumation continued at ∼0.2 mm/yr until shortening terminated by 11–7 Ma. Shortening within the plateau since circa 30 Ma did not evolve by lateral accretion of thrust wedges; the propagating deformation style is spatially confined to the foreland thrust belt, which initiated coeval to plateau deformation (Interandean Zone) but propagated mainly after circa 10 Ma (Subandean Zone). Early Oligocene plateau‐wide tectonically driven exhumation suggests that subduction‐related processes had already thermally weakened the continental lithosphere prior to the 27–25 Ma onset of volcanic activity.

Late Cenozoic deformation and uplift of the western flank of the Altiplano: Evidence from the depositional, tectonic, and geomorphologic evolution and shallow seismic activity (northern Chile at 19°30′S)

We analyze the west vergent thrust system (WTS) along the western flank of the Altiplano in northern Chile (18°S–21°S). In our study area (19°20′S–19°50′S), the WTS consists of three thrust propagation monocline folds (flexures) developing growth strata. The relative uplift accommodated by the flexures is rapid between 26 and 8 Ma (0.1 mm/yr), diminishing to 0.02 mm/yr after 8 Ma. Approximately 2000 m of relative surface uplift was accommodated by the flexures since the late Oligocene. Sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 Ma, coeval with the shifting of deformation from the Altiplano to the sub‐Andean zone, where the underthrusting of the Brazilian Craton would have resulted in crustal thickening, surface uplift in the orogen, and westward ductile subcrustal flow. Forearc tilting is accommodated by east vergent thrusts (ETS) issued from the Benioff zone beneath the Central Depression emerging into the Western Cordillera, contributing 500–1400 m of surface uplift. The WTS connects the ETS in the brittle‐ductile crustal transition (∼25 km depth), continuing farther east as the Altiplano low‐velocity zone, configuring the western Altiplano as a crustal‐scale fault bend fold. Forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ETS. Meanwhile, between 19°S and 21°S, the WTS accommodates dextral strike slip, and ∼3 km of N‐S shortening occurred in the Coastal Cordillera. Transcurrence and strain partitioning are probably the result of slight plate convergence obliquity, strong coupling within the interplate zone, westward continental concavity, and high elevation opposing horizontal contraction.

The structure of the south Urals foreland fold and thrust belt at the transition to the Precaspian Basin

The structural architecture of the south Urals foreland fold and thrust belt at the transition into the Precaspian Basin is that of a wedge-shaped, west-vergent thrust stack with a significant amount of basement involvement. The surface geology of this part of the thrust belt is dominated by the south Urals accretionary complex, whose internal structure was primarily developed during Late Devonian to Early Carboniferous arc–continent collision and which was subsequently incorporated into the thrust belt. A number of thrusts, identified in the field and in seismic profiles, breach the accretionary complex, suggesting that they post-date its emplacement and may be related to the development of the foreland fold and thrust belt. A basal detachment can be confidently defined only beneath the frontal part of the thrust belt, making it difficult to construct balanced and restored cross-sections and therefore to calculate the amount of shortening. Nevertheless, the amount of shortening appears to be of the order of only a few tens of kilometres. There is a major change in the structural style along the frontal part of the thrust belt that may be related to basement topography and/or basement response to the deformation across a basement fault oriented at a high angle to the structural grain of the thrust belt.

Geology and U–Pb geochronology of the Kipawa Syenite Complex — a thrust related alkaline pluton — and adjacent rocks in the Grenville Province of western Quebec

The Kipawa Syenite Complex, a thin, folded sheet of amphibole syenite, quartz syenite and minor nepheline syenite, lies along a west-vergent thrust separating a lower slice comprising the Kikwissi granodiorite and biotite tonalite dated at 2717 +15–11 Ma, and unconformably overlying metasedimentary rocks from an overlying slice containing the Red Pine Chute orthogneiss, an alkali granite gneiss, and the Mattawa Quartzite. The syenite complex, dated at 1033 ± 3 Ma, lies within the lower slice but has metasomatically altered the overlying slice. Texturally guided U–Pb spot analyses on partially metasomatised zircons from the alkali granite gneiss yield a cluster of 207Pb/206Pb ages at 1389 ± 8 Ma, interpreted as the time of igneous crystallization and four ages overlapping the time of syenite emplacement, interpreted as in situ, metasomatic growth. The highest structural slice comprises garnet amphibolite separated from lower slices by the Allochthon Boundary Thrust. Metamorphic grade increases upward from greenschist grade in the biotite tonalite to amphibolite grade (690 °C, 9 kbar (1 kbar = 100 MPa)) at the lower boundary of the alkali granite. Emplacement of the Kipawa Syenite Complex took place after assembly of the thrust stack had begun and after emplacement of the allochthon or hot slab responsible for the inverted metamorphic gradient. Origin of the syenite is tentatively ascribed to anatexis of material metasomatized by flow of alkaline solutions along a major shear surface. Crystallization of new zircon in the margins of the syenite shows that metasomatism continued from ca. 1035 to 990 Ma, redistributing alkalies, fluorine, rare-earth elements and zirconium.

Aptian to recent compressional deformation, foothills of the Neuquén Basin, Argentina

The Neuquén Basin lies in north-western Patagonia, on the eastern foreland of the Andes. It formed as a rift basin in the early Mesozoic and contains some 5000 m of Triassic to Aptian sediment, mainly marine. As a result of Andean orogeny, the western edge of the basin now forms part of the foothills. We describe some of the main compressional structures, drawing on surface and sub-surface data. We argue that regional eastward dips are due to west-verging thrust displacements on the Andacollo–Loncopué fault system. During a Late Cretaceous Peruvian phase (Aptian to Campanian), continental sediment accumulated in a retro-arc foreland basin, which was mainly of flexural origin. Nevertheless, more than 1500 m of Late Cretaceous sediment accumulated in the footwall of the east-verging Agua Amarga thrust. The Peruvian phase was responsible also for transpressional reactivation of a rift transfer zone, the Huincul Arch. The Incaic phase (Eocene) was responsible for north-westerly trending folds and thrusts, reactivation of Cretaceous thrusts, abundant andesitic lavas and high-level intrusions, and a swarm of north-easterly trending bitumen dykes. The Quechua phase (Miocene to Recent) produced northerly trending folds and thrusts and a Miocene foreland basin, containing some 1000 m of basalt lava flows. Structures that formed during the Incaic and Quechua phases are broadly compatible with the history of convergence at the Pacific margin of South America, as deduced from plate reconstructions.

Characteristics of repeated, detached, Miocene–Pliocene tectonic inversion events, in a large delta province on an active margin, Brunei Darussalam, Borneo

The Baram Delta province evolved during the Middle Miocene to present day from a foreland basin to a shelf margin. Episodic folding events affected the region, causing uplift of the hinterland, delta progradation, and inversion of gravity-related faults. Existing models place the N–S- and NE–SW-trending folds in a strike-slip transpressional setting. New geological mapping and re-interpretation of existing data suggest the region is better understood as the development of a west-verging thrust belt in a Middle Miocene foreland basin (filled by sandstones and shales of the Belait and Setap Formations), with key major folds (Jerudong and Belait anticlines, Belait syncline) forming during the Middle Miocene as fault bend and fault propagation folds. The deformation style is complicated by the predominantly shaley Middle Miocene–Pliocene Setap Formation becoming thicker and more overpressured from south to north. Onshore where the Setap Formation is thin or absent, the Belait formation is attached to the underlying Lower Miocene and older sequences. Offshore and in a narrow onshore strip the overpressured Setap Formation causes deformation in the Beltait Formation to be detached from the underlying Cretaceous–Tertiary accretionary prism ‘basement’. The detached style exhibits considerable structural complexity, including lift-off folds, growth faults, shale diapirs and pipes. Onshore thrust and inversion features are dominantly N–S-trending and began activity in the Middle Miocene; deformation is probably associated with an E–W maximum horizontal stress direction. In the Late Miocene (around 7.5 Ma; Watters et al., 1999) NE–SW striking inversion folds developed, located mostly over early counter-regional faults and associated reactive diapirs. Folds verge towards the NW when underlain by counter regional faults, and towards the S or SE when underlain by regional faults. Folds and diapirs along N–S trends were also reactivated. Continuation of this deformation into the Pliocene is largely confined to the offshore area. Onshore the N–S structures (no detachment) were not reactivated during the Pliocene.

Pan-African deformations in the basement of the Negele area, southern Ethiopia

Polydeformed and metamorphosed Neoproterozoic rocks of the East African Orogen in the Negele area constituted three lithostructurally distinct and thrust-bounded terranes. These are, from west to east, the Kenticha, Alghe and Bulbul terranes. The Kenticha and Bulbul terranes are metavolcano-sedimentary and ultramafic sequences, representing parts of the Arabian–Nubian Shield (ANS), which are welded to the central Alghe gneissic terrane of the Mozambique Belt affinity along N–S-trending sheared thrust contacts. Structural data suggest that the Negele basement had evolved through three phases of deformation. During D1 (folding) deformation, north–south upright and inclined folds with north-trending axes were developed. East and west-verging thrusts, right-lateral shearing along the north-oriented Kenticha and Bulbul thrust contacts and related structural elements were developed during D2 (thrusting) deformation. The pervasive D1 event is interpreted to have occurred at 620–610 Ma and the D2 event ended prior to 554 Ma. Right-lateral strike-slips along thrust contacts are interpreted to have been initiated during late D2. During D3, left-lateral strike-slip along the Wadera Shear Zone and respective strike-slip movements along conjugate set of shear zones were developed in the Alghe terrane, and are interpreted to have occurred later than 557 Ma. The structural data suggest that eastward thrusting of the Kenticha and westward tectonic transport of the Bulbul sequences over the Alghe gneissic terrane of the Mozambique Belt, during D2, were accompanied by right-lateral strike-slip displacements along thrust contacts. Right-lateral strike-slip movements along the Kenticha thrust contact, further suggest northward movement of the Kenticha sequence during the Pan-African orogeny in the Neoproterozoic. Left-lateral strike-slip along the orogen-parallel NNE–SSW Wadera Shear Zone and strike-slip movements along a conjugate set of shear zones completed final terrane amalgamation between the Arabian–Nubian Shield and the Mozambique Belt in Neoproterozoic southern Ethiopia.

Role of lateral thickness variations on the development of oblique structures at the Western end of the South Pyrenean Central Unit

The Montsec unit is one of the most important detached South-verging nappes within the South Pyrenean Central Unit (SPCU, Southern Pyrenees). A N–S cross-section of its Western sector, based on seismic reflection profiles, shows a hangingwall ramp geometry in Mesozoic strata, overlain by a syntectonic series of Lower Eocene sediments with growth geometry. The geometry of growth strata constrains the age of its movement between the Paleocene and the Middle Eocene. The geometry of the Western, oblique ramp of the South Pyrenean Central Unit is defined by a series of N–S folds, in some cases associated with underlying West-verging thrusts, as indicated by seismic reflection profiles and field data. In this paper, we propose that the geometry of the thrust wedge of Mesozoic units, progressively thinning from East to West, strongly contributed to constrain the location and geometry of the Western termination of the Montsec thrust. The hypothesis proposed is checked by a series of experimental wedges developed in a sandpack with lateral and three-dimensional thickness variations. Oblique structures form as thrusting progresses at the tip of the sand wedge.

Source Parameters of Regional Earthquakes in Taiwan: 1999-2000 Including the Ch-Chi Earthquake Sequence

We report source parameters of 277 earthquakes that occurred during 99.2000 in the Taiwan region, a time period dominated by the Chi-Chi earthquake sequence. We used an improved CMT inversion algorithm to handle high background noise and structural heterogeneity from complex tectonic settings near Taiwan. We also searched the best fitting solutions at an increment of 1 km of focal depths for all events that occurred after the :hi-Chi main shock on September 20, 1999. In general, central and northern Taiwan was seismically quiet before the Chi-Chi main shock. Aftershocks of this sequence dominated the regional seismicity for approximately 3 months afterward. The Chi-Chi sequence consists of a wide range of focal mechanisms, but the overall pattern is a west-verging thrust block bound strike-slip tear faults near the northern and southern termini of the surface rupture – the Chelungpu fault. Seismicity outside of the source zone of the Chi-Chi sequence generally agrees with known active tectonics from previous studies. Nonetheless, under high elevations of the eastern Central Ranges, there are several intriguing cases of changing focal mechanisms from normal faulting near the surface to thrust at depth. To facilitate further research of the Chi-Chi sequence, results of our inversion and original waveforms from the Broadband Array in Taiwan for Seismology (BATS) of 170 events within this sequence are ready for distribution on CD-ROM. The dataset is also online at the BATS web site (http://bats.earth.sinica.edu.tw/Chi-Chi_CMT).

Structural evidence for the allochthonous nature of the Bulbul terrane in southern Ethiopia: A west-verging thrust nappe

The Neoproterozoic basement of southern Ethiopia links the low-grade Pan-African province of the Arabian–Nubian Shield to the high-grade Mozambique Belt to the south. In this intervening area, a northward terminating low-grade metavolcano–sedimentary and mafic–ultramafic sequence of the Bulbul terrane gently overlies moderately to steeply dipping granitic migmatites of the Alghe gneissic terrane. The contact between the two terranes is a right lateral thrust. In the Bulbul terrane, rocks are part of an overturned sequence with a gently east-dipping composite D1/D2 foliation containing downdip and NE-plunging stretching lineations and westerly verging intrafolial folds. These structures are interpreted to have been developed during westward thrusting. The Bulbul sequence was therefore detached and tectonically transported to the west as a thrust nappe of which the lower inverted limb is still preserved. Structural evidence suggest yet another smaller nappe sequence (terrane) is present in southern Ethiopia and further suggest that Neoproterozoic obducted crust of the Arabian–Nubian Shield in eastern Ethiopia may be located beneath Phanerozoic cover.