Secondary Normal Faults Research Articles

Surface-derived fluid percolation along detachment systems enhanced by syn-kinematic granites: uranium mineralization as an application

Detachment zones are privileged areas for the interaction between surface-derived fluids and rocks, potentially leading to ore deposition. However, the hydrodynamics of detachments and specifically the way by which surface-derived fluids reach crustal depths, remain enigmatic. This question is even more puzzling when the heating caused by the emplacement of a syn-kinematic granite increases the buoyancy of fluids, thus impeding their descent. Here, 2D hydrothermal numerical models are performed. The geometry comprises a detachment and secondary normal faults in the hanging wall. Sensitivity tests were carried out to assess the impact of topographic gradients, syn-tectonic magmatic activity and the depth-dependent permeability contrast between the detachment and the crust. Several flow indicators, integrated over time and combined with particle tracking, enable us to highlight the main controls of fluid circulations. Our study reveals that the infiltration of surface-derived fluids into detachment zones is enhanced by the presence of a heat source at depth, such as a syn-kinematic pluton. Secondary faults are the main percolation path for surface-derived fluids infiltrating the detachment. Plume-like thermal anomalies have been spotted between these faults. The dynamic permeabilities of magmatic intrusions, which depend on sub-solidus temperatures, spatially and temporally reproduce the conceptual model of uranium mineralization in the South Armorican Variscan Domain, which is used as an example.

Formation mechanism of fault accommodation zones under combined stress in graben basin: Implications from geomechanical modeling

A fault accommodation zone is a type of structure that is defined as regulating displacement and strain between faults structure. Increasing numbers of fault accommodation zones are being identified in graben basins, indicating the potential exploration target and petroleum accumulation areas. This study aims to analyze the formation mechanism and development of fault accommodation zones under combined stress by a numerical simulation method considering geomechanical modeling. Using three-dimensional (3-D) seismic interpretation and fractal dimension method, exampled with the Dongxin fault zone, the fault activity and fault combination pattern were conducted to quantitatively characterize the activity difference in fault accommodation zones. Combined with mechanical experiment test, a geomehcanical model was established for fault accommodation zones in a graben basin. Integrating the paleostress numerical simulations and structural physical simulation experiment, the developmental characteristics and genetic mechanism of fault accommodation zones were summarized. Influenced by multi movements and combined stresses, three significant tectonic evolution stages of the Dongxing Fault Zone (DXFZ) were distinguished: During the Es3 sedimentary period, the large difference in the stress, strain, and rupture distribution in various faults were significant, and this stage was the key generation period for the prototype of the DXFZ, including the FAZ between large-scale faults. During the Es2 sedimentary period, the EW-trending symmetric with opposite dipping normal faults and the NE-SW trending faults with large scale were furtherly developed. The junction area of two secondary normal faults were prone to be ruptured, performing significant period for inheriting and developing characteristics of fault accommodation zones. During the Es1 sedimentary period, the high-order faults in the DXFZ exhibited the obvious fault depressions and strike-slip activity, and the fault accommodation zones were furtherly inherited and developed. This stage was the molded and formative period of the FAZ, the low-order faults, and the depression in the DXFZ.

Basement structures exert a crucial influence on the structural configuration of later rift development: Insights from the Shanan and Chengbei sag, Bohai Bay Basin

The Early Cretaceous and Paleogene rift structures of the Shanan and Chengbei sags were inherited from basement tectonic deformation of Indosinian and Yanshanian tectonism. However, the architecture of the basement structures and its influence on the structural configuration of later rift development are poorly understood, which impeded hydrocarbon exploration efforts in the study area. Based on 2D high-resolution seismic reflection data in the study area and onshore field data in the Luxi area, we recognize the Indosinian NW-striking low-angle fold-thrust belts and Yanshanian NE-striking high-angle thrusts in basement of the study area. The pre-rift NW- and NE-striking basement structures exert a crucial influence on the two-phase rift architecture and structural evolution of the study area: (1) Low-angle NW-striking Indosinian thrusts are dominantly reactivated as dip-slip major normal faults during Early Cretaceous, such as the Shanan fault and Chengbei faults. The high-angle Yanshanian NE-striking thrusts are primarily reactivated as left-lateral strike-slip faults during this stage, such as the Shaxi fault. (2) During the Paleogene, oblique extension took place across the pre-existing NW-striking reactivated normal faults. A set of half-grabens formed along the major NW-striking oblique-slip faults and large amount of ENE-striking secondary normal faults developed in the NW-striking sags. The pre-existing left-lateral NNE-striking faults shifted to become right-lateral and extensional faults. The Early Cretaceous rift was primarily controlled by NE-SW-directed extension, which was derived from the large-scale left-lateral strike-slip movement of the Tan-Lu Fault Zone. The Paleogene oblique rift was induced by NNW-SSE-directed extension related to the upwelling water-rich mantle wedge of the Pacific subduction zone.

The interplay between basement fabric, rifting, syn‐rift folding, and inversion in the Rio do Peixe Basin, NE Brazil

AbstractThe Rio do Peixe Basin is part of a series of aborted Cretaceous rifts formed within the Proterozoic Borborema Province, onshore NE Brazil in response to rifting between Africa and South America. The basin is remarkably well‐imaged and comprises fault‐bounded depocentres, the main ones being the NE‐oriented Brejo das Freiras and the E–W‐oriented Sousa half‐grabens. These grabens and their bounding faults are influenced by Neoproterozoic basement shear zones and present a complex framework of secondary normal faults and folds. Recent workers also interpret large reverse faults and regional post‐rift shortening driven by far‐field stresses from the Andes. For those reasons, the basin represents an ideal setting to investigate the multiphase history of rift basins. We thus combine borehole‐calibrated 2D and 3D seismic and magnetic data with section restoration and numerical modelling to investigate the architecture and evolution of this basin. We aim to understand: (i) the controls of the basement fabric in 3D fault architecture and kinematics and (ii) how syn‐rift faults controlled the geometry and development of fault‐related folds. By doing this, we also investigate the timing, kinematics, and magnitude of inversion in the basin to explore its multiphase history. We demonstrate that (i) the basement fabric controlled not only the strike of faults but also their geometry and polarity at depth, (ii) folds in the syn‐rift sequence are attributed simply to syn‐rift extension along stepped and/or curved faults, and (iii) inverted and/or reverse faults occur within the basin, but these are minor and appear to have formed during rifting. We explain this minor inversion by a change in plate kinematics related to the onset of the nearby transform margin to the north. These results have implications for understanding the 3D evolution of oblique grabens, the role of structural inheritance, and the recognition of inversion‐ versus extension‐related folds in rift basins worldwide.

3‐D Structure and Development of a Metamorphic Core Complex in the Northern South China Sea Rifted Margin

AbstractThe 3‐D structure of continental metamorphic core complexes (MCCs) and their coevolution along with the associated extensional detachments are still not well understood. In this study, analysis of a newly acquired high‐resolution 3‐D seismic reflection volume reveals for the first time a well‐imaged MCC in the proximal northern South China Sea (SCS) rifted margin, the Kaiping MCC (KP MCC). These data provide a 3‐D view of the KP MCC and the associated KP detachment fault. The KP MCC is characterized by ascend of ductile midcrustal materials, and it is partially exhumed in the KP9 High. The KP detachment fault displays a domed low‐angle geometry, and is characterized by pronounced NS‐plunging corrugations, among which two megacorrugations of tens of kilometers are revealed. Evidence show that the KP MCC developed according to the classical rolling‐hinge model. A group of secondary normal faults and fractures, which are parallel to the axis of the KP MCC and offset the KP detachment surface at the crest of the MCC, developed in response to inelastic bending during progressive warping of the footwall. The migration of the domal seismic reflection layers provides a visual evidence for the kinematic process of the rolling‐hinge activity, during which the brittle‐ductile transition and the rolling hinge gradually migrate as the detachment fault slips. The origin of the KP MCC in the northern SCS margin is suggested to have been favored by the existence of a pre‐existing midcrustal ductile layer and basement structures within the upper brittle crust.

Oil origin, charging history and crucial controls in the carboniferous of western Junggar Basin, China: Formation mechanisms for igneous rock reservoirs

The Carboniferous igneous rocks in the western margin of Junggar Basin, western China developed large-scale oil reservoirs. Hongshan-Zhongguai Bulge, the location of Jinlong Oilfield where oil and gas exploration in igneous rocks has achieved efficient breakthroughs in recent years, was studied as an example, because a large number of igneous rocks and crude oil samples are available in this area. The Carboniferous igneous rock reservoirs are typical fault-controlled block reservoirs, and are distributed in a stepped-like manner. Based on analysis of biomarkers, inclusions and reservoir cores under microscope, the oil sources, filling history and major controls for the igneous rock reservoirs that associated with accumulation mechanisms were studied. The biomarker geochemistry reveals that the oil in the igneous rocks was derived from the Permian source rocks including formations of Jiamuhe, Fengcheng and Lower Wuerhe (P2w), and the P2w is the primary source rock. Three important periods of oil charging in the igneous rock reservoirs are suggested: Late Triassic to Early Jurassic, Early Cretaceous, and Late Cretaceous to Early Tertiary. Volcanic breccia of eruptive facies is most favorable for igneous rock reservoir, followed by andesite of effusive facies. Strike-slip faults, overthrust faults and secondary normal faults formed into a fault network, interwoven with the unconformities in the top of the Carboniferous system and within the Permian strata, forming a vertical and horizontal three-dimensional petroleum migration pathway. Hydrocarbons migrated to igneous rock reservoirs from the Permian source rocks along strike-slip faults. The faults communicate the hydrocarbon source rocks with the upper weathering crust reservoirs and control the petroleum distribution. Due to weathering and leaching below the unconformity at the top of the igneous rock reservoirs, oil and gas are most abundant in the upper 150 m. In the fault belt the rocks are cemented by minerals of carbonate, clay, zeolite, quartz, and serpentine, and these types of cementation along faults were recognized as important factors for laterally sealing oil and gas in igneous rock reservoirs.

Characterizing damage zones of normal faults using seismic variance in the Wangxuzhuang oilfield, China

We have investigated the distribution and thickness of damage zones for a system of secondary normal faults in the subsurface of the Wangxuzhuang oilfield, China. Based on seismic variance analysis, we find (1) four isolated faults with approximately 2 km length and approximately 200 m damage-zone thickness. The damage zones of these isolated faults reveal a decaying intensity of deformation from the fault core to the protolith, which fits a power-law form [Formula: see text] similar to that observed in the field. (2) A merged fault with approximately 400 m thickness. (3) A bifurcated fault with approximately 400 m thickness and three linked segments. Damage zones that consist of several subsidiary faults are thicker than those of isolated faults. The displacement-length analyses of the four isolated faults suggest the constant-length growth of the limestone in this case. We determine the potential to apply seismic variance to systematically characterize damage zones as potential fluid migration conduits on the basin scale.

TECTONIC EVOLUTION OF THE NORTHERN MARGIN OF THE CENOZOIC ARARAT BASIN, LESSER CAUCASUS, ARMENIA

This paper investigates the structure of the northern margin of the Ararat depression in a study area in SE Armenia. The depression is a Cenozoic intermontane basin located to the south of the Lesser Caucasus. The purpose is to improve understanding of the basin's structure and origin within a regional tectonic framework which has been dominated since the Late Cretaceous by the closure of Neotethys and the Arabia‐Eurasia collision. We suggest that the depression is not a graben controlled by normal faults; rather, based on detailed observations, structures in the study area are interpreted as oblique‐slip reverse and thrust faults activated in post Oligocene‐Miocene times. These compressional faults resulted in the formation of asymmetric fold structures including the Lanjanist and Urts anticlines which are well expressed in the surface relief to the north of the Ararat depression. In general the structural pattern is complicated by secondary normal faults which resulted in superimposed gravitational slope processes and erosion.Major structures in the study area originated in a compressional setting associated with the closure of Neotethys since the latest Cretaceous. Post‐collisional strike‐slip faulting was linked to convergence of the Arabian and Eurasian plate margins. Pliocene and Quaternary structures, some still active, show evidence of structural inheritance.The Armenian portion of the Ararat depression contains obduction‐related nappes, anticlines and thrust faults which have potential as structural traps for hydrocarbons. These should be investigated in detail using advanced geophysical methods including 2D and 3D seismic analyses.

Structure and Holocene Rupture of the Morelia Fault, Trans‐Mexican Volcanic Belt, and Their Significance for Seismic‐Hazard Assessment

Abstract The Trans‐Mexican volcanic belt is an active volcanic arc being deformed by an intra‐arc extensional fault network. The Morelia fault, with a length of 14 km and a maximum throw of 260 m, is a major normal fault in the central part of this volcanic arc and forms part of a 36‐km‐long fault array. Slickenlines measured on the primary fault surface indicate north–south extensional dip slip with a minor left‐lateral component. In a well‐exposed cross section in the hanging wall of the Morelia fault, some few meters from the master fault trace, two buried soil layers, containing ceramic shards and stone tools, yielded radiocarbon ages of A.D. 420–660 (prehispanic classic period, Chupicuaro culture) and A.D. 1290–1435 (postclassic period), respectively. The underlying bedrock (Atecuaro ignimbrite) is cut in a staircase pattern by three synthetic secondary normal faults that delimit two colluvial wedges. Three slip events, with displacements ranging between 46 and 185 cm (average 109 cm), can be inferred from this section. The latest of these events must have occurred after A.D. 1290–1435, during the postclassic period or possibly during early novohispanic times. The Morelia fault, which is capable of generating M w 6–7 earthquakes, poses a major seismic ground‐shaking hazard and locally a surface fault‐rupture hazard within the metropolitan Morelia region with a population of more than 900,000 habitants. The foci of a sequence of seven small 2007 earthquakes on the Morelia fault were located below the center of town, at a depth of 11 km, and the fault trace crosses urban residential areas. Moreover, the town repeatedly experienced severe ground shaking during historical earthquakes. The best documented is the 19 June 1858 earthquake, which caused Morelia extensive damage corresponding to a macroseismic intensity of VIII–IX on the modified Mercalli scale. Online Material: Radiocarbon analysis reports, local 1894–1904 and 1909–1911 earthquake catalogs.

Paleoseismological evidence of Holocene activity on Los Tollos Fault (Murcia, SE Spain): A lately formed Quaternary tectonic feature of the Eastern Betic Shear Zone.

The Los Tollos Fault is a recent and important feature of the Eastern Betic Shear Zone, one of the major tectonic structures in South Iberia accommodating the convergence between Nubian and Eurasian plates in the western Mediterranean. The Los Tollos Fault became active by the end of Middle Pleistocene introducing some paleogeographical modifications. Previously mapped as a secondary normal fault related to the Carrascoy Fault, recent research evidences that the Los Tollos Fault is an independent Holocene active left-lateral reverse fault extending for at least 15 km. Data analysis from 4 trenches dug across the fault has revealed the occurrence of at least two paleo-earthquake events during the Holocene. The most recent event is dated between 2,740 and 2,140 yr BP (8th to 2nd centuries BC). The size of the paleoevents is calculated to be Mw 6.3 – 6.6 following empirical regressions on surface rupture length. The recurrence interval is estimated to be between 2,200-6,860 years, fitting a slip rate for the fault between 0.12 and 0.17 mm/yr. Such parameters highlight the Los Tollos Fault as a tectonic structure with a considerable activity located relatively close to densely populated areas. These seismogenic parameters should be considered in future reassessments of the seismic hazard of the region.

DEM-aided study of shear band formation in dip-slip faulting through granular soils

A GPU-based two-dimensional discrete element methodology allowing for grain rolling resistance is adopted to study the engineering and fundamental aspects of shear band formation in normal and reverse faulting through an initially dense sand; of course, the paper emphasizes on fundamental aspects. The employed DEM modeling methodology is verified with the experimental results and FEM prediction of the pattern of shear bands developed through a trap-door centrifuge model test. Regarding the engineering aspects, a parametric study is carried out and the obtained results from the DEM simulations are compared with the experimental results. Concerning the fundamental aspects, various micro and macro aspects of shear banding, such as void ratio and coordination number, the evolution of particle-scale energies and second order work within the localized areas along the shear bands are studied. Moreover, a link is established between the micro and macro events occurring inside the shear bands. Regarding the engineering results, the analyses show that the reverse fault ruptures may deviate toward the hanging wall or footing wall, depending on the faulting angle; however, the primary and secondary normal fault ruptures refract at the soil–bedrock interface, deviating toward the hanging wall. Regarding the fundamental results, the analyses show that the pre-failure disintegration of strong contacts whose directions are aligned between the minimum principal strain and zero-extension line directions within the localized areas plays a fundamental role in the development of shear bands at micro scale. This leads to the local rotation (buckling) of columnar chains within shear bands at failure, being associated with a softening behavior, development of large voids, generation of high particle rotation gradients, rapid decrease of internal energy and rapid increase of dissipated energy within the shear bands at macro scale. In addition, the rolling mechanism plays a significant role in energy dissipation within the shear bands even though the sliding mechanism has a dominant role. Furthermore, the second order work becomes negative within the localized areas by the occurrence of shear banding.

Active faulting and folding along the Jumilla Fault Zone, northeastern Betics, Spain

The Jumilla Fault Zone (JFZ) is an ENE-WSW topographic lineament in the external part of the eastern Betic Cordillera. It represents an active fault. Three small basins are aligned along the JFZ, the La Celia-, Alqueria- and Jumilla basins. The tectonic geomorphology of the La Celia- and Alqueria basins consists of folds, a set of normal fault scarps, strike-slip lineaments, fault-springs, tectonically-modified drainage lines and elevated gypsum/anhydrite diapirs. Two of the scarps are normal faults generated by the extensional collapse of one of the folds. The other scarps are secondary normal faults generated by transtensional left-lateral motions along the JFZ. Normal fault scarps that developed on conglomerates are considerably steeper (~30°) than those affecting softer marly materials (<10°) as a consequence of their different erodibilities. Two natural springs reveal the sealing nature of the normal faults, which has resulted in a segmented groundwater system. These springs are the sources of the two largest ramblas (gullies) in the area.The onset of fault activity is constrained by the late Tortonian–Messinian age of the sedimentary rocks in the basins. Interbedded lavaflows and lamproitic dykes in the lacustrine sediments have been dated at 7.6–7.2Ma. However, the morphology of fault scarps and relief of the diapirs suggest a younger age of the deformation. Results from modelling of fault scarp evolution indicate Middle Pleistocene and younger ages of faulting and scarp formation. Furthermore, in at least one of the fault scarps, Quaternary alluvial-pediment deposits are involved in the deformation. The drainage pattern anomalies and moderate earthquake activity (Mb 4.2) in the JFZ also indicate slow ongoing tectonic activity. In addition, the ENE prolongation of the JFZ (outside the study area) shows geodetic displacement rates of approximately 2mm/year. We thus conclude that the JFZ is still active, despite the general lack of post-Messinian deposits in the studied basins. The discrepancy between the age of the tectonic landforms and the late Neogene age of the basin infill can be explained by an endo- to exhorheic change in the drainage system, due to the capture of the ancient basin lake system by tributaries of the nearby Segura river. The cessation of sedimentation in the basins resulted in the preservation of tectonic landforms.

Geophysical characterisation of two segments of the Møre-Trøndelag Fault Complex, Mid Norway

Abstract. The Møre-Trøndelag Fault Complex (MTFC) has controlled the tectonic evolution of Mid Norway and its shelf for the past 400 Myr through repeated reactivations during Palaeozoic, Mesozoic and perhaps Cenozoic times, the very last phase of reactivation involving normal to oblique-slip faulting. Despite its pronounced signature in the landscape, its deep structure has largely remained unresolved until now. We focused on two specific segments of the MTFC (i.e. the Tjellefonna and Bæverdalen faults) and acquired multiple geophysical datasets (i.e. gravity, magnetic, resistivity and shallow refraction profiles). A 100–200 m-wide zone of gouge and/or brecciated bedrock steeply dipping to the south is interpreted as being the Tjellefonna fault sensu stricto. The fault appears to be flanked by two additional but minor damage zones. A secondary normal fault also steeply dipping to the south but involving indurated breccias was detected ~1 km farther north. The Bæverdalen fault, ~12 km farther north, is interpreted as a ~700 m-wide and highly deformed zone involving fault gouge, breccias and lenses of intact bedrock. As such, it is probably the most important fault segment in the studied area and accommodated most of the strain during presumably Late Jurassic normal faulting. Our geophysical data are indicative of a Bæverdalen fault dipping steeply towards the south, in agreement with the average orientation of the local tectonic grain. Our findings suggest that the influence of Mesozoic normal faulting along the MTFC on landscape development is more complex than previously thought.

Listric versus planar normal fault geometry: an example from the Eisenstadt-Sopron Basin (E Austria)

In a gravel pit at the eastern margin of the Eisenstadt-Sopron Basin, a satellite of Vienna Basin (Austria), Neogene sediments are exposed in the hanging wall of a major normal fault. The anticlinal structure and associated conjugated secondary normal faults were previously interpreted as a rollover anticline above a listric normal fault. The spatial orientation and distribution of sedimentary horizons and crosscutting faults were mapped in detail on a laser scan of the outcrop wall. Subsequently, in order to assess the 3D distribution and geometry of this fault system, a series of parallel ground penetrating radar (GPR) profiles were recorded behind the outcrop wall. Both outcrop and GPR data were compiled in a 3D structural model, providing the basis for a kinematic reconstruction of the fault plane using balanced cross-section techniques. However, the kinematic reconstruction results in a geologically meaningless normal fault cutting down- and up-section. Additionally, no evidence for a weak layer serving as ductile detachment horizon (i.e. salt or clay horizon) can be identified in stratigraphic profiles. Instead, the observed deflection of stratigraphic horizons may be caused by a displacement gradient along a planar master fault, with a maximum displacement in the fault centre, decreasing towards the fault tips. Accordingly, the observed deflection of markers in the hanging wall—and in a nearby location in the footwall of the normal fault—is interpreted as large-scale fault drag along a planar fault that records a displacement gradient, instead of a rollover anticline related to a listric fault.

Temporal evolution of surface rupture deduced from coseismic multi-mode secondary fractures: Insights from the October 8, 2005 (Mw 7.6) Kashmir earthquake, NW Himalaya

Detailed rupture–fracture analyses of some of the well-studied earthquakes have revealed that the geometrical arrangement of secondary faults and fractures can be used as a geological tool to understand the temporal evolution of slip produced during the mainshock. The October 8, 2005 Mw 7.6 Kashmir earthquake, NW Himalaya, surface rupture provides an opportunity to study a complex network of secondary fractures developed on the hanging wall of the fault scarp. The main fault scarp is clearly thrust-type, rupture length is ~ 75 ± 5 km and the overall trend of the rupture is NW–SE. We present the results of our detailed structural mapping of secondary faults and fractures at 1:100 scale, on the hanging wall of the southern end of the rupture in the vicinity of the Sar Pain. Secondary ruptures can be broadly classified as two main types, 1) normal faults and, (2) right-lateral strike-slip ‘Riedel’ fractures. The secondary normal faults are NW–SE striking, with a maximum 3.3 meter vertical displacement and 2.5 meter horizontal displacement. Estimated total horizontal extension across the secondary normal faults is 3.1–3.5%. We propose that the bending-moment and coseismic stress relaxation can explain the formation of secondary normal faults on the hanging wall of the thrust fault. The strike-slip ‘Riedel’ fractures form distinct sets of tension (T) and shear fractures (R′, R, Y) with right-lateral displacement. Field observations revealed that the ‘Riedel’ fractures (T) cut the secondary normal faults. In addition, there is kinematic incompatibility and magnitude mismatch between the secondary normal faults and strike-slip ‘Riedel’ fractures. The cross-cutting relationship, geometric and magnitude incoherence implies a temporal evolution of slip from dip- to strike-slip during the mainshock faulting. The interpretation is consistent with the thrust fault plane solution with minor right-lateral strike-slip component.

Structural deformation and fault activity of the Tan-Lu Fault zone in the Bohai Sea since the late Pleistocene

The Tan-Lu Fault zone (TLFZ) is a significant fractural zone in eastern China and also a seismicity belt in North China. Based on total 4000-km-long shallow penetrated single seismic data with high-resolution, structural deformation and fault activity of the TLFZ in the Bohai Sea since the late Pleistocene are discussed in detail. The results show that the TLFZ with a discontinuous distribution and a general NNE-trending consists of 14 active subfaults with an NNE or NE strike in the Bohai Sea. Seismic data reveal that deformation zones along the subfaults in the central Bohai Sea and the Laizhou Bay are wider and more complex than those in the Liaodong Gulf. Related folds and lots of secondary normal faults which are characterized by nearly vertical fault planes and a same or reverse dip construct the fractural zone in the Laizhou Bay and the central Bohai Sea. Usually, micro symmetrical grabens develop on the top of anticlines. In the Liaodong Gulf, subfault fractural zones usually consist of secondary normal faults with the same inclination or opposite inclination. Ages of seismic sequences and cutting relation between subfaults and seismic sequences suggest that the latest faulting age of the TLFZ is the end of the late Pleistocene in the Liaodong Gulf and the early Holocene in the Laizhou Bay and the central Bohai Sea. There is a good match between distribution of earthquakes and that of the subfaults in the Laizhou Bay and the central Bohai. Statistical result shows that total vertical offset of the TLFZ since the late Pleistocene ranges from 6 to 11 m. On the basis of offsets of the subfaults, the vertical slip rate is calculated and results show that average vertical slip rates in the central Bohai Sea are larger than those in the Liaodong Gulf and the Laizhou Bay. Slip rates more than 0.06 mm/a during 23-10 ka B.P. and 85-65 ka B.P. are larger than those in other stages. The TLFZ was mainly dominated by tensional normal component since the late Pleistocene. Synthesizing shallow deformation, activity and distribution of earthquakes, the TLFZ in the Bohai Sea can be divided into three segments: the Laizhou-Bay segment, the Bozhong segment, and the Liaodong-Gulf segment.

Acoustic stratigraphy of Bear Lake, Utah–Idaho—Late Quaternary sedimentation patterns in a simple half-graben

A 277-km network of high-resolution seismic-reflection profiles, supplemented with a sidescan-sonar mosaic of the lake floor, was collected in Bear Lake, Utah–Idaho, in order to explore the sedimentary framework of the lake's paleoclimate record. The acoustic stratigraphy is tied to a 120 m deep, continuously cored drill hole in the lake. Based on the age model for the drill core, the oldest continuously mapped acoustic reflector in the data set has an age of about 100 ka, although older sediments were locally imaged. The acoustic stratigraphy of the sediments below the lake indicates that the basin developed primarily as a simple half-graben, with a steep normal-fault margin on the east and a flexural margin on the west. As expected for a basin controlled by a listric master fault, seismic reflections steepen and diverge toward the fault, bounding eastward-thickening sediment wedges. Secondary normal faults west of the master fault were imaged beneath the lake and many of these faults show progressively increasing offset with depth and age. Several faults cut the youngest sediments in the lake as well as the modern lake floor. The relative simplicity of the sedimentary sequence is interrupted in the northwestern part of the basin by a unit that is interpreted as a large (4 × 10 km) paleodelta of the Bear River. The delta overlies a horizon with an age of about 97 ka, outcrops at the lake floor and is onlapped by much of the uppermost sequence of lake sediments. A feature interpreted as a wave-cut bench occurs in many places on the western side of the lake. The base of this bench occurs at a depth (22–24 m) similar to that (20–25 m) of the distal surface of the paleodelta. Pinch-outs of sedimentary units are common in relatively shallow water on the gentle western margin of the basin and little Holocene sediment has accumulated in water depths of less than 30 m. On the steep eastern margin of the basin, sediments commonly onlap the hanging wall of the East Bear Lake Fault. However, no major erosional or depositional features suggestive of shoreline processes were observed on acoustic profiles in water deeper than about 20–25 m.

Geometric and experimental models of extensional fault-bend folds

Abstract We use geometric and experimental models to study the development of extensional fault-bend folds. The geometric models show that fault shape, fault displacement, and patterns of aggradation/erosion profoundly affect the distribution of growth beds, the magnitude and direction of dip of pregrowth and growth beds, and the location and dip of the outer limit of folding in pregrowth and growth beds. Complex structural and stratigraphic patterns develop if the rate of aggradation/erosion relative to the rate of fault displacement changes through time. The experimental models (with dry sand and wet clay) show that several deformational styles can accommodate extensional fault-bend folding. In sand models, a few, relatively major, secondary antithetic normal faults accommodate most hanging wall deformation. Pregrowth layers, although faulted, remain flat. The effective shear direction parallels the antithetic normal faults, and the shear angle is about 60°–65°. In clay models, numerous, relatively minor, secondary normal faults (antithetic and synthetic) and cataclastic flow accommodate most hanging wall deformation. The deformed pregrowth and growth layers dip gently toward the main fault. The effective shear angle (35°–50°) is considerably less than the dip of the antithetic normal faults. In the sand models and geometric models with a large shear angle (60°), more displacement occurs on the main normal fault and the hanging wall collapses in a relatively narrow zone. In the clay models and geometric models with a small shear angle (35°), less displacement occurs on the main normal fault. Instead, the hanging wall stretches substantially and collapses in a relatively wide zone.

Joint exploration of crustal structure in Fuzhou basin and its vicinities by deep seismic reflection and high-resolution refraction as well as wide-angle reflection/refraction

The Fuzhou basin and its vicinities are located at the northern end of the southeastern coastal seismic zone of the mainland of China. By the joint explorations of high-resolution seismic refraction and wide-angle reflection/refraction as well as deep seismic reflection in the region, which is the first synthetic profile in China, its fine velocity structure and geometric structure from the ground to Moho discontinuity is obtained. The result shows that the crust is obviously layered with a thickness of about 32 km. Basically, it consists of two parts: upper crust and lower crust. The velocity of the upper crust is 5.9―6.2 km/s in which there is a 3―4-km-thick weak low-velocity layer between the depths of 10―15 km, while the velocity of the lower crust in the range of 6.3―7.2 km/s. There exists a strong velocity gradient layer about 3 km thick above the Moho discontinuity whose velocity increases from 6.5 to 7.27 km/s. There exist high-angle normal faults that are small in size and extend only in the shallow crust. These faults are the secondary developed on the hanging wall of westward dipping low-angle normal faults. Thus, their seismogenic ability is limited, however, there exists a high-angle deep fault in the crust from Changle-Zaoan fault zone to seashore fault zone. This deep fault has cut the interface between the upper and lower crusts and Moho discontinuity, and may be the deep structure to trigger destructive earthquake source in future to affect Fuzhou City. These results have advanced the detecting precision of the deep structure in the southeastern coastal seismic zone of the mainland of China. In the aspect of the combined feature of the deep and shallow extensional structures that consist of the westward dipping low-angle normal faults and secondary normal faults on their hanging walls in the upper crust, it is firstly obtained that a united structural interpretation has deepened the knowledge about the deep dynamic process in the southeastern coastal seismic zone. At the same time, in synthetic application of deep seismic detecting methods for deep tectonic background, it possesses a wide referenced meaning to the other regions.

Magallanes‐Fagnano continental transform fault (Tierra del Fuego, southernmost South America)

Multichannel seismic reflection profiles, gravity measurements, and bathymetric soundings, in conjunction with field geological reconnaissance and remote sensing images, reveal with unprecedented detail the morphostructure of a major segment of the South America–Scotia plate boundary in the Tierra del Fuego region. This segment, known as the Magallanes‐Fagnano fault system, is a continental transform margin arranged in an en echelon geometry, along which prominent asymmetric basins were developed. Data acquired off the Atlantic coast of Isla Grande (the main island of Tierra del Fuego), in its central and eastern part, and in the central and western Magallanes Strait image the surface and subsurface structure of the transform fault and its associated basins. The Magallanes‐Fagnano fault system is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near‐vertical faults. In the Atlantic sector, the fault system trends broadly N70°E and seems to be composed by a single master fault, along which a highly asymmetric basin has formed. At around 63°W, the fault terminates by splaying into secondary normal faults that dissipate the horizontal displacement along the system. In the central eastern part of Isla Grande, the fault segments have been principally identified from analyses of remote sensing images on the basis of their morphological expression. These segments are located within river valleys and are generally associated with localized gravity minima. Lago Fagnano, a 105‐km‐long, E‐W trending depression, is a large, mostly asymmetric pull‐apart basin developed within the principal displacement zone of the Magallanes‐Fagnano fault system. Restraining bends and overlapping step‐over geometry characterize the central part of the Magallanes Strait. Along the western part of the fault system, in the vicinity of the Pacific entrance of the Magallanes Strait, asymmetric sedimentary basins have also developed. The sedimentary architecture of the basins formed within the principal displacement zone of the fault, in which the thick end of the depositional wedge abuts the transform segment, suggest simultaneous strike‐slip motion and transform‐normal extension, a common feature found in other continental transtensional environments. Strike‐slip faulting in the Tierra del Fuego region is also documented along other prominent lineaments which parallel the Magallanes‐Fagnano fault system. Along at least two of these lineaments, characterized by a remarkable morphological expression, widespread Quaternary activity occurs. The present‐day motion between the South America and Scotia plates is slow (&lt;5 mm/yr). Also the modern seismicity monitored in the Tierra del Fuego region is low (individual events &lt;3.5 in magnitude). The low seismicity may be explained by the slow relative motion between plates and may be further affected by slip partitioning along the different segments which make up the Magallanes‐Fagnano fault array, and along the subsidiary wrench lineaments that traverse the region.