Frontal Thrust Fault Research Articles

Latest Miocene to Quaternary deformation in the southern Chaiwopu Basin, northern Chinese Tian Shan foreland

AbstractBasinward propagation of fold and thrust belts is a crucial geological process accommodating Cenozoic crustal shortening within the India‐Eurasia collision zone. Anticlinal growth strata in the southern Chaiwopu Basin (a piggyback basin) of the northern Chinese Tian Shan foreland record basinward encroachment of the Tian Shan along the Junggar Frontal Thrust Fault. A new magnetostratigraphic section constrains the onset of syntectonic growth strata at circa 6.4 Ma and suggests synchronous basinward thrusting and propagation of the Tian Shan. The intense alluviation in the southern Chaiwopu Basin ceased at circa 0.55 Ma due to significant anticlinal growth and its resultant river incision. More recent anticlinal growth and deformation during the late Quaternary are revealed by folded river terraces developing across the anticline. The terrace height profile indicates that terrace T1Hhas been vertically offset about 0.6 m by thrust faulting since its formation at about 7 Ka. The stratigraphic and geomorphic data presented in this work are helpful to understand the initiation of thrust‐related folding, as well as aggradation and subsequent incision, in foreland basins of the Tian Shan in relation to the India‐Asia collision.

Slip model of the 2015 Mw 7.8 Gorkha (Nepal) earthquake from inversions of ALOS‐2 and GPS data

AbstractWe use surface deformation measurements including Interferometric Synthetic Aperture Radar data acquired by the ALOS‐2 mission of the Japanese Aerospace Exploration Agency and Global Positioning System (GPS) data to invert for the fault geometry and coseismic slip distribution of the 2015 Mw 7.8 Gorkha earthquake in Nepal. Assuming that the ruptured fault connects to the surface trace of the Main Frontal Thrust (MFT) fault between 84.34°E and 86.19°E, the best fitting model suggests a dip angle of 7°. The moment calculated from the slip model is 6.08 × 1020 Nm, corresponding to the moment magnitude of 7.79. The rupture of the 2015 Gorkha earthquake was dominated by thrust motion that was primarily concentrated in a 150 km long zone 50 to 100 km northward from the surface trace of the Main Frontal Thrust (MFT), with maximum slip of ∼ 5.8 m at a depth of ∼8 km. Data thus indicate that the 2015 Gorkha earthquake ruptured a deep part of the seismogenic zone, in contrast to the 1934 Bihar‐Nepal earthquake, which had ruptured a shallow part of the adjacent fault segment to the east.

Development of the Global Earthquake Model’s neotectonic fault database

The Global Earthquake Model (GEM) aims to develop uniform, openly available, standards, datasets and tools for worldwide seismic risk assessment through global collaboration, transparent communication and adapting state-of-the-art science. GEM Faulted Earth (GFE) is one of GEM’s global hazard module projects. This paper describes GFE’s development of a modern neotectonic fault database and a unique graphical interface for the compilation of new fault data. A key design principle is that of an electronic field notebook for capturing observations a geologist would make about a fault. The database is designed to accommodate abundant as well as sparse fault observations. It features two layers, one for capturing neotectonic faults and fold observations, and the other to calculate potential earthquake fault sources from the observations. In order to test the flexibility of the database structure and to start a global compilation, five preexisting databases have been uploaded to the first layer and two to the second. In addition, the GFE project has characterised the world’s approximately 55,000 km of subduction interfaces in a globally consistent manner as a basis for generating earthquake event sets for inclusion in earthquake hazard and risk modelling. Following the subduction interface fault schema and including the trace attributes of the GFE database schema, the 2500-km-long frontal thrust fault system of the Himalaya has also been characterised. We propose the database structure to be used widely, so that neotectonic fault data can make a more complete and beneficial contribution to seismic hazard and risk characterisation globally.

Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone

We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10−18 m2 at site C0004 and 9 × 10−18 m2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10−6 m2 s−1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.

Distribution of resistive and conductive structures in Nankai accretionary wedge reveals contrasting stress paths

In this article, we study the characteristics and spatial distribution of the deformation structures along the Kumano transect of the Nankai accretionary wedge, and use this information to interpret the stress path followed by the sediments. Deformation structures are identified from logging while drilling (LWD) resistivity images of the materials surrounding the drill hole and from 3-dimensional X-ray CT-images of cores acquired during the IODP NanTroSEIZE project. The relative resistivity of the structures identified on logs and the strike, dip, and density of structures identified on CT scan images are measured. The analysis of dip and strike of structures indicates that most of the resistive structures identified on logging data correspond to compactive shear bands. Results also indicate that conductive structures predominate at the toe of the prism and above the main out of sequence thrust, in locations where past and recent erosion occurred. We propose several mechanisms that could explain the relation between erosion and the absence of compactive shear bands. We conclude that sediments followed different stress paths depending on their location within the wedge, and that those differences explain the distribution of deformation structures within the wedge. We also show the coexistence of dilatant and compactant structures in fault zones including the frontal thrust and mega splay fault, and we interpret the coexistence of these structures as a possible consequence of a transient fluid pressure.

Seismic slip deficit in the Kashmir Himalaya from GPS observations

AbstractGPS measurements in Kashmir Himalaya reveal range‐normal convergence of 11 ± 1 mm/yr with dextral shear of 5 ± 1 mm/yr. The transition from a fully locked 170 km wide décollement to the unrestrained descending Indian plate occurs at ~25 km depth over an ~23 km wide transition zone. The convergence rate is consistent with the lower bounds of geological estimates for the Main Frontal Thrust, Riasi, and Balapora fault systems, on which no surface slip has been reported in the past millennium. Of the 14 damaging Kashmir earthquakes since 1123, none may have exceeded Mw = 7.6. Therefore, either a seismic moment deficit equivalent to a Mw ≈ 8.7 earthquake exists or the historical earthquake magnitudes have been underestimated. Alternatively, these earthquakes have occurred on reverse faults in the Kashmir Valley, and the décollement has been recently inactive. Although this can reconcile the inferred and theoretical moment release, it is quantitatively inconsistent with observed fault slip in Kashmir.

Stratigraphic uncertainty and errors in shortening from balanced sections in the North American Cordillera

Shortening from balanced cross sections is typically cited as a minimum estimate because of the uncertainty in the position of eroded hanging-wall cutoffs. We show here using area balancing that the single most significant source of error is that associated with the shape and thickness of the initial stratigraphic wedge involved in the deformation. New methods and software are introduced that, in combination with freely available scans of virtually all geologic maps published by the U.S. Geological Survey (USGS) and Geological Survey of Canada (GSC), allow one to make hundreds of map thickness measurements in just a few hours. These methods are applied to geologic maps and cross sections from the western North American Cordillera to determine the uncertainty of shortening in iconic cross sections by R.A. Price and P.R. Fermor, and by F. Royse. One-sigma stratigraphic thickness uncertainty, measured in homoclinal dip packages of prominent Paleozoic units within single thrust plates (and single quadrangles), ranges from 20% to 30% of average thickness in the Costigan and Sulfur Mountain plates in southern Canada and can be as high as 37% in the Darby and Absaroka plates of the Idaho-Wyoming (United States) thrust belt. Assuming that all individual Paleozoic units in the cross sections have similar errors, the entire Paleozoic section used in the area balancing would have 8%–11% uncertainty, assuming a normal or Gaussian distribution. Because the stratigraphic wedges in southern Canada and Idaho-Wyoming do not have a uniform original taper, our basic area balance underestimates shortening. By balancing the non-uniform taper back to a uniform taper, we can account for this discrepancy. Total shortening determined in this way is ∼104 ± 17 km, which is identical to that obtained by Price and Fermor. The Idaho-Wyoming section of Royse (with maps by the USGS) yields similarly large errors in shortening magnitude: 56 ± 10 km for the three frontal thrust faults. Of those errors, uncertainty in the shape and thickness of the initial stratigraphic wedge accounts for 56% and 70% of the total in southern Canada and western Wyoming, respectively. Eroded hanging-wall cutoffs account for just 15% in both areas, with the remainder coming from uncertainty in the subsurface geometry, including the position of the decollement.

Limitation of fluid flow at the Nankai Trough megasplay fault zone

Along the Nankai Trough megasplay fault off SE Japan, the effect of fluid migration on subduction-related seismogenesis and tsunamigenesis remains unresolved. To investigate the existence and role of fluid flow, a SmartPlug borehole observatory was installed at Site C0010 of the Integrated Ocean Drilling Program NanTroSEIZE Kumano transect, where a shallow branch of the fault was intersected and in situ fluid pressure monitored from August 2009 to November 2010. The tidal signal in the formation showed no phase shift relative to seafloor loading. The attenuation of 0.73 reflects the loading efficiency accurately, and enabled calculation of a formation compressibility of 1.0×10–9 Pa–1 and a hydraulic diffusivity (HD) of 1.5×10–5 m2 s–1. A similar HD is predicted by a tidal response model based on SmartPlug pressure data. By contrast, permeability measurements on intact samples from Site C0004 SE along-strike the splay fault and from Site C0006 in the frontal thrust zone were found to be similar and one magnitude smaller respectively, despite having a higher porosity. This is explained by the presence of fractures, which are covered by the larger scale of in situ measurements at Site C0010. Consequently, HD can be set to be at least 10–5 m2 s–1 for the splay fault and 10–6 m2 s–1 for the frontal thrust fault zone. Considering recent publications makes fluid flow at the splay fault unlikely, despite the presence of fractures. If the influence of fractures is limited, then processes leading to fault weakening may be enhanced.

Varying frontal thrust spacing in mono-vergent wedges: An insight from analogue models

Sandbox experiments are used to study frontal thrust fault spacing, which is a function of physical properties within the thrust wedge. We consider three styles of thrust progression in mono-vergent wedges: Style I, II and III. In Style I, frontal thrusts progress forelandward, maintaining a constant spacing, whereas Style II and Style III progression show increasing and decreasing spacing, respectively. The three styles are shown as a function of the following factors: basal friction (μ b ), initial surface slope (α) and basal slopes (β), and surface erosion. For high μ b (~0.46), thrust progression occurs in Style II when α 2° and β > 0.5°). Style II transforms to Style I when the wedge undergoes syn-thrusting surface erosion. In contrast, low-basal friction (μ b = 0.36) gives rise to either Style I or III, depending on the magnitudes of α and β. Conditions with α = β = 0 developed Style I, whereas Style III in conditions with any non-zero values of α and β. In this case, surface erosion caused the process of thrust progression unsteady, and prompted out-of-sequence thrusting in the wedge. This study finally presents an analysis of the three styles, taking into account the following two parameters: (1) instantaneous increase of hinterland thickness (ΔH e /H e ) and (2) forelandward gradient of wedge thickness (δH/δx). Experimental data suggest that thrust sequences develop in Style II for low δH/δx and large δH e /H e values and, in Style III as either δH/δx increases or ΔH e /H e drops.

Primary surface ruptures of the great Himalayan earthquakes in 1934 and 1255

It is unclear where plate boundary thrusts generate giant rather than great earthquakes. Along the Himalayas, the source sizes and recurrence times of large seismic events are particularly uncertain, since no surface signatures were found for those that shook the range in the twentieth century. Here we challenge the consensus that these events remained blind and did not rupture the surface. We use geomorphological mapping of fluvial deposits, palaeo-seismological logging of river-cut cliffs and trench walls, and modelling of calibrated 14C ages, to show that the Mw 8.2 Bihar–Nepal earthquake on 15 January 1934 did break the surface: traces of the rupture are clear along at least 150 km of the Main Frontal Thrust fault in Nepal, between 85° 50′ and 87° 20′ E. Furthermore, we date collapse wedges in the Sir Valley and find that the 7 June AD 1255 earthquake, an event that devastated Kathmandu and mortally wounded the Nepalese King Abhaya Malla, also ruptured the surface along this stretch of the mega-thrust. Thus, in the past 1,000 years, two great earthquakes, 679 years apart, rather than one giant eleventh-century AD event, contributed to the frontal uplift of young river terraces in eastern Nepal. The rare surface expression of these earthquakes implies that surface ruptures of other reputedly blind great Himalayan events might exist. The recurrence times of great Himalayan earthquakes are difficult to assess because they rarely rupture the surface. Field mapping and 14C dating of offset fluvial deposits are used to identify two great Himalayan quakes that ruptured the surface along the main plate boundary fault in AD 1255 and 1934.

Postseismic deformation following the 1999 Chi‐Chi earthquake, Taiwan: Implication for lower‐crust rheology

On 1999 September 21, the Mw 7.6 Chi‐Chi earthquake ruptured a segment of the Chelungpu Fault, a frontal thrust fault of the Western Foothills of Taiwan. The stress perturbation induced by the rupture triggered a transient deformation across the island, which was well recorded by a wide network of continuously operating GPS stations. The analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements, with relaxation times varying by a factor of more than ten, and large cumulative displacements at great distances, in particular along the Longitudinal Valley in eastern Taiwan, where relaxation times are also longer. We show that while afterslip is the dominant relaxation process in the epicentral area, viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale. We model the spatiotemporal behavior of the transient deformation as the result of afterslip on the décollement that extends downdip of the Chelungpu thrust, and viscoelastic flow in the lower crust and in the mid‐crust below the Central Range. We construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled. The model is compatible with the shorter relaxation times observed in the near field, which are due to continued fault slip, and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the Central Range. Our preferred model shows a viscosity of 0.5–1 × 1019Pa s at lower‐crustal depths and 5 × 1017Pa s in the mid‐crust below the Central Range, between 10 and 30 km depth. The low‐viscosity zone at mid‐crustal depth below the Central Range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures, estimated to be between 300°C and 600°C from the modeling of thermo‐chronology and surface heat flow data.

Interseismic crustal deformation of frontal thrust fault system in the Chiayi–Tainan area, Taiwan

We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi–Tainan area located in the deformation front of the Taiwan mountain belt. The crustal motion with respect to Penghu shows large velocities of about 33–44mm/yr in the west-northwest to west directions in the Western Foothills and the velocities decrease westward to 0–5mm/yr in the coastal area. Significant uplift rates of 5–20mm/yr are observed at sites to the east of the Jiuchiunken–Muchiliao–Liuchia Fault (JMLF) system. We use a block model, a buried dislocation model, and a two-dimensional fault model to invert for fault geometries and slip rates on major frontal thrust faults. Modeling results from a block model show the inferred long-term slip rate of 42mm/yr in the direction of 280° and the maximum back-slip rate of 38mm/yr on a 23° east dipping fault extending to 13km at depth. On the other hand, the buried dislocation model results in a horizontal décollement at a depth of 8km with a uniform slip rate of 41.6mm/yr. If we connect the top edge of décollement to the surface trace of JMLF as a potential future rupture, a 22° east-dipping fault is required. Results from both block model and buried dislocation model suggest the JMLF is nearly fully locked. The results of two-dimensional fault models show the frontal thrust faults have slip rates of less than 2mm/yr at shallow depths and the inferred décollement is sub-horizontal (5°–7°) at a depth of 10km with slip rates of 44–46mm/yr. Results of various approaches show general agreement on fault geometries and slip rates and reveal that the frontal thrust fault system has a high potential for large earthquakes.

Fold and thrust partitioning in a contracting fold belt: Insights from the 1931 Mach earthquake in Baluchistan

Surface deformation associated with the 27 August 1931 earthquake near Mach in Baluchistan is quantified from spirit‐leveling data and from detailed structural sections of the region interpreted from seismic reflection data constrained by numerous well logs. Mean slip on the west dipping Dezghat/Bannh fault system amounted to 1.2 m on a 42 km × 72 km thrust plane with slip locally attaining 3.2 m up dip of an inferred locking line at ∼9 km depth. Slip also occurred at depths below the interseismic locking line. In contrast, negligible slip occurred in the 4 km near the interseismic locking line. The absence of slip here in the 4 years following the earthquake suggests that elastic energy there must either dissipate slowly in the interseismic cycle, or that a slip deficit remains, pending its release in a large future earthquake. Elastic models of the earthquake cycle in this fold and thrust belt suggest that slip on the frontal thrust fault is reduced by a factor of 2 to 8 compared to that anticipated from convergence of the hinterland, a partitioning process that is presumably responsible for thickening of the fold and thrust belt at the expense of slip on the frontal thrust. Near the latitude of Quetta, GPS measurements indicate that convergence is ∼5 mm/yr. Hence the minimum renewal time between earthquakes with 1.2‐m mean displacement should be as little as 240 years. However, when the partitioning of fold belt convergence to frontal thrust slip is taken into account the minimum renewal time may exceed 2000 years.

南海トラフにおける間隙水挙動に関する研究の現状と間隙水圧観測の意義

Experimental monitoring of fluid pressures was initiated in June 2001 at 2 underwaterholes drilled during the Ocean Drilling Program (ODP) Leg 196 to investigate the relationship between deformation and fluid flow processes in the Nankai accretionary prism. ODP Leg 196 visited Sites 1173 (drilled on Leg 190) and 808 (drilled through the frontal thrust on Leg 131), installing 2 Advanced Circulation Obviation Retrofit Kits (ACORKs) to monitor fluid pressures along the walls of the drilled holes. Site 808 penetrates the frontal thrust fault and the deconement, while Site 1173 is located about 11 km seaward from the Nankai Trough deformation front. Formation water freshening around the decollement was first observed on ODP Leg 131, and geochemists have been investigating whether or not the freshening was due to the production of deep-sourced dehydration processes. We now know that the role of smectite dehydration and dehydrant quartz-cristobalite phase transition should be estimated quantitatively. One of the important findings of fluid pressure monitoring is that the formation fluid pressures seem to reflect the change in the stress state in and around the accretionary prism. We believe that such fluid pressure monitoring in the accretionary prism and in the sediments on top of the subducting oceanic plate in terms of stress field and dehydrant process of minerals is a key to deepening our knowledge in future investigations of seismogenic processes.

Evidence for a Great Medieval Earthquake (~1100 A.D.) in the Central Himalayas, Nepal

The Himalayan orogen has produced three thrust earthquakes with moment magnitude (Mw) 7.8 to 8.5 during the past century, yet no surface ruptures associated with these great earthquakes have been documented. Here, we present paleoseismic evidence from east central Nepal that, since approximately 700 A.D., a single earthquake ruptured the Frontal Thrust fault at approximately 1100 A.D., with a surface displacement of approximately 17 (+5/-3) meters and a lateral extent and size that could have exceeded 240 kilometers and approximately Mw 8.8, respectively. Ruptures associated with Mw <8.2 events would contribute to the frontal Himalayas folding but would stop before reaching the surface. These findings could require substantial modifications to current regional seismic hazard models.

Geomorphic evidence of active deformation and uplift in a modern continental wedge-top–foredeep transition: Example of the eastern Ecuadorian Andes

The eastern Ecuadorian Andes appear as a fold-and-thrust belt adjacent to a continental foredeep represented by one of the world's largest tropical alluvial megafans, the Pastaza megafan, debouching into the Amazonian lowland. The apex of the Pliocene–Pleistocene megafan situated in the present-day wedge top (Subandean Zone) has been cut by an erosion surface, the western part of which has been uplifted of ∼500 m along the frontal thrust, forming a poorly dissected plateau, the Mera plateau. This erosion surface erased most of the previous fluvial landscape but preserved a large thrust-related anticlinal hinge deforming less erodible underlying strata, the Mirador fold and smaller-sized anticlines. This surface has been then incised by two antecedent major rivers, the Pastaza and the Napo, and few tributaries. The plateau edge is marked by a series of large scale gently sloping landslides clustered along a 70 km long concave eastward line associated with the frontal thrust fault. The newly formed immature rivers issued from the landslides or sourced within east-dipping remnants of the erosion surface downstream of the landslide line constitute the greatest part of the streams feeding the Ecuadorian Amazonian basin. At 70 to 100 km from the landslide line, the drainage abruptly changes from highly immature to mature with a well-defined hinge line representing the outer limit of landslide and tectonic control. The diversions of the Pastaza River indicate ongoing fold growth since at least the late Pleistocene in the Eastern Cordillera, and the early Holocene in the Mera plateau. The preserved terraces of the Pastaza valley are all degradational and are ascribed to periods of tectonic (seismic) activity alternating with periods of tectonic quiescence or decreased seismic activity rather than to climatic events. 14C dating of the plateau erosion surface and of the upper Pastaza terraces indicates that the minimum average incision rate since ∼18,000 years BP varies locally in the upper Pastaza valley from 0.5 to 0.67 cm year −1 , increasing from ∼18,000 years BP to now. A comparison of these incision rates with fold-and-thrust fault uplift rates indicates that incision in the upper Pastaza valley was a result of rapid uplift (up to 1 cm year −1) along the Mirador fold-and-thrust which caused a restoration of the local equilibrium profile of the upper reach, combined with smaller local fault uplift along the westernmost thrust faults. The uplift of the whole Mera plateau with respect to the upper Amazonian basin gives a minimum average uplift rate of 2.8 cm year −1 since 18,000 years BP. The overall uplift of the Mera plateau and the Eastern Cordillera is likely to have been caused by a regional-scale low angle thrust ramp emerging as the frontal thrust fault.

Tectonic evolution of a continental collision zone: A thermomechanical numerical model

We model evolution of a continent‐continent collision and draw some parallels with the tectonic evolution of the Himalaya. We use a large‐scale visco‐plasto‐elastic thermomechanical model that has a free upper surface, accounts for erosion and deposition and allows for all modes of lithospheric deformation. For quartz/olivine rheology and 60 mm/yr convergence rate, the continental subduction is stable, and the model predicts three distinct phases. During the phase 1 (120 km or 6% of shortening), deformation is characterized by back thrusting around the suture zone. Some amount of delaminated lower crust accumulates at depth. During phase 2 (120 km–420 km or 6%–22% of shortening), this crustal root is exhumed (medium‐ to high‐grade rocks) along a newly formed major thrust fault. This stage bears similarities with the period of coeval activity of the Main Central thrust and of the South Tibetan Detachment between 20–16 Myr ago. During phase 3 (&gt;420 km or 22% of shortening), the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge. Deformation is localized around frontal thrust faults. This kinematics should produce only low‐ to medium‐grade exhumation. This stage might be compared with the tectonics that has prevailed in the Himalaya over the last 15 Myr allowing for the formation of the Lesser Himalaya. The experiment is conducted at constant convergence rate, which implies increasing compressive force. Considering that this force is constant in nature, this result may be equivalent to a slowing down of the convergence rate as was observed during the India‐Asia collision.

Genetic relationship of rift-stage crustal structure, terrane accretion, and foreland tectonics along the southern Appalachian-Ouachita orogen

Abrupt along-strike variations in tectonostratigraphic composition, internal structural style, and detachment level in the southern Appalachian and Ouachita foreland thrust belts are defined at a large-scale bend in strike and a truncation of Ouachita structures by the frontal Appalachian thrust fault. The along-strike variations correspond to differences in the pre-orogenic rifted Laurentian margin, in the history and nature of terrane accretion, and in the response of the foreland to these differences. Within the Ouachita embayment of the Laurentian margin, diachronous arc-continent collision migrated northwestward along a rift-stage transform margin from the Black Warrior foreland basin on the southeast in Late Mississippian time to a short-wavelength, high-amplitude foreland basin (Arkoma basin) on the northwest in front of the Ouachita thrust-belt salient in Early-Middle Pennsylvanian time. Off-shelf, deep-water strata of both passive-margin and synorogenic facies comprise an accretionary prism and subduction complex, and the Ouachita allochthon consists of mud-dominated thrust sheets that are internally disharmonic and folded. The allochthon of off-shelf strata was thrust over the passive-margin carbonate shelf, which remains in the Ouachita footwall. Along the southeast side of the Alabama promontory of the Laurentian margin, passive-margin shelf carbonates are imbricated in the Appalachian thrust belt, which is characterized by internally coherent thrust sheets and high-amplitude frontal ramps. The palinspastic extent of shelf-carbonate rocks corresponds to the extent of structurally shallow basement rocks on the upper-plate rift-stage margin of the Alabama promontory of Laurentian crust. Terranes accreted to the Laurentian margin during the Taconic and Acadian orogenies were driven over the shallow basement by continent-continent collision of Laurentia with Africa (Gondwana). Emplacement of the thrust-translated terranes tectonically stripped and replaced the shelf carbonate. The frontal thrust fault of the Appalachian thrust belt truncates the southeastern end of the slightly older frontal Ouachita thrust belt, as well as the southeastern part of the greater Black Warrior basin in the Ouachita foreland. Shallow basement beneath the Appalachian thrust belt extends cratonward beneath the low-amplitude Appalachian foreland basin.

Dynamics and inversion of the Mesozoic Basin of the Weald–Boulonnais area: role of basement reactivation

The geometry and dynamics of the Mesozoic basins of the Weald–Boulonnais area have been controlled by the distribution of preexisting Variscan structures. The emergent Variscan frontal thrust faults are predominantly E–W oriented in southern England while in northern France they have a largely NW–SE orientation. Extension related to Tethyan and Atlantic opening has reactivated these faults and generated new faults that, together, have conditioned the resultant Mesozoic basin geometries. Jurassic to Cretaceous N–S extension gave the Weald–Boulonnais basin an asymmetric geometry with the greatest subsidence located along its NW margin. Late Cretaceous–Palaeogene N–S oriented Alpine (s.l.) compression inverted the basin and produced an E–W symmetrical anticline associated with many subsidiary anticlines or monoclines and reverse faults. In the Boulonnais extensional and contractional faults that controlled sedimentation and inversion of the Mesozoic basin are examined in the light of new field and reprocessed gravity data to establish possible controls exerted by preexisting Variscan structures.

Estimating basal friction in accretionary wedges from the geometry and spacing of frontal faults

Elastic theory applied to the deformation in accretionary wedges is used to calculate the condition for slip along an active frontal fault and the basal décollement. The equations for calculating the stresses can be solved for the coefficient of basal friction in the situation of the formation of a new frontal thrust fault. This allows us to calculate the efficient coefficient of basal friction, which includes the weakening effect of pore-fluid pressure, from geometric parameters and material properties only. The geometric parameters, like fault dip and layer thickness, can be derived from high-resolution seismic cross-sections. Application of our analysis to the Makran and the Nankai accretionary wedge allows us to estimate the upper limit of the effective coefficient of basal friction, μb≈0.16 and μb≈0.2, in these two areas respectively.