Abstract The San Juan–southern Gulf Islands Archipelago of Washington State, USA and western Canada is located on the upper plate of the Cascadia subduction zone, in the forearc between the trench and volcanic arc. Onland and island investigations show many faults within the region that primarily represent old, inactive faults associated with transport, subduction and accretion of tectonostratigraphic terranes. However, until recently little geologic investigation and mapping have been done in the offshore. From these narrow straits, channels and sounds we have collected and interpreted high-resolution multibeam echosounder bathymetric data, 3.5 kHz sub-bottom and Huntec seismic-reflection profiles, and piston-cores to identify and date recently active faults. Previous studies by us focused on the earlier recognized active Devils Mountain fault zone that bounds the southern part of the Archipelago and the recently reported newly mapped active Skipjack Island fault zone that bounds the northern part. These transcurrent fault zones appear to be deforming and rotating the Archipelago. We concentrate on the unique deformation occurring within the seaways to determine the relationship and styles of faulting associated with these active bounding fault zones and relate the fault geometry and kinematics to one other subduction complex, the New Hebrides island arc of Vanuatu.

ABSTRACT In a well-defined subrecess in the Appalachian thrust belt in northwestern Georgia, two distinct fold trains intersect at ~50° in the down-plunge depression of the Floyd synclinorium. A mushwad (ductile duplex) of tectonically thickened weak-layer rocks (primarily the shale-dominated Cambrian Conasauga Formation) filled the space beneath folds and faults of the overlying Cambrian–Ordovician regional stiff layer (mushwad roof). Measurements of the mushwad thickness from balanced cross sections provide the basis for three-dimensional (3-D) models. Tectonically thickened weak-layer shales in a model using a simple line-length balance of the stiff layer have a volume of ~64% of the volume in the deformed-state model, indicating that this balanced reconstruction is not appropriate. Previous work demonstrated deposition of a thick mud-dominated succession in a basement graben to balance the volume. A 3-D model incorporating a thick Conasauga Formation shale succession deposited in a basement graben yields good correspondence to the deformed-state mushwad volume. That model requires vertical separation on the graben boundary faults greater than the present small-magnitude separation; unconformable truncation of the upper part of the Cambrian–Ordovician carbonate succession documents Ordovician inversion of the graben boundary faults. In the 3-D models, the distribution of thickness in the deformed state suggests movement of weak-layer shale out of the planes of cross sections and up plunge away from the structural depression of the Floyd synclinorium. Out-of-plane tectonic translation is consistent with a relatively uniform depositional thickness of ~800 m, which allows calculation of the magnitude of vertical separation on basement faults during Conasauga Formation deposition.

Abstract The Diapir Fold Zone of the Carpathians is the most prolific onshore hydrocarbon area in Romania. Structural complexity, mainly due to the presence of salt, combined with poor seismic quality near and below the salt lead to contrasting structural models in the area. To gain insights into the mid-Miocene tectonic evolution, structural geometries and the effects of penetrative strain, we ran dual décollement scaled sandbox models with layered brittle and ductile materials. Results of two analogue models (20 and 33% shortening) revealed that the onset of the deformation sequence was mainly characterized by layer-parallel shortening. As shortening continued, a foreland-verging sequence of supra-salt detachment folds and sub-salt duplexes evolved. The sub-salt duplexes are located directly below the crests of the detachment folds, as the development of these large wavelength anticlines was related to sub-salt deformation. Salt flow was another controlling factor of the deformation style, as salt accumulated in the anticlinal cores and increased the coupling in the supra-salt synclinal axis. Our results offer insights into the effects of salt on the kinematic evolution of this area, help to predict geometries in areas of poor seismic quality, and highlight the important contribution of penetrative strain on deformation and reservoir quality.

Abstract The Bhatwari Gneiss of Bhagirathi Valley in the Garhwal Himalaya is a Paleoproterozoic crystalline rock from the Inner Lesser Himalayan Sequence. On the basis of field and petrographic analyses, we have classified the Bhatwari Gneiss into two parts: the Lower Bhatwari Gneiss (LBG) and the Upper Bhatwari Gneiss (UBG). The geochemical signatures of these rocks suggest a monzonitic protolith for the LBG and a granitic protolith for the UBG. The UBG has a calc-alkaline S-type granitoid protolith, whereas the LBG has an alkaline I-type granitoid protolith; the UBG is more fractionated. The trace element concentrations suggest a volcanic arc setting for the LBG and a within-plate setting for the UBG. The U–Pb geochronology of one sample from the LBG gives an upper intercept age of 1988 ± 12 Ma ( n = 10, MSWD = 2.5). One sample from the UBG gives an upper intercept age of 1895 ± 22 Ma ( n = 15, MSWD = 0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from c. 1940 to 1840 Ma. Based on these ages, we infer that the Bhatwari Gneiss has evolved due to arc magmatism and related back-arc rifting over a time period of c. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent.

ABSTRACT The southern sub-Andean fold-thrust belt of Bolivia and northwestern Argentina is constructed from a ~10-km (~6-mi) thick stratigraphic pile of post-Ordovician to Neogene deposits that have been shortened above a detachment located in Silurian to Upper Ordovician horizons. Hydrocarbon accumulations in this fold-thrust belt include a variety of plays, with reservoirs ranging in age from Devonian to Neogene. Giant gas fields, however, are restricted to deep structures involving Devonian reservoirs. Exploration for this play relies on structural models, as seismic imaging is challenged by geological and topographical conditions. Duplex systems seem to be the dominant thrust system type, including passive, active, and composite roof-thrust geometries. Pure structural wedges are common either in structural plunges or early stage structures. The main controlling factor for the development of duplexes and structural wedges is the presence of two major detachments, the basal detachment, mainly located along the Silurian Kirusillas Formation and the Devonian Los Monos Formation. Once duplex horses, or wedges, start developing in the lower structural level, the overpressured Los Monos Formation is passively uplifted and lithostatic pressure decreases. As this happens, the overpressure increases significantly, triggering pseudoplastic deformation within the Los Monos Formation, which results in the classic complexities of the southern sub-Andean belt. Regional variations in the Silurian–Devonian stratigraphic package seem to be an important control on modes of deformation. Overall structural complexities in the lower structural level increase toward the most distal parts of the Silurian-Devonian basin, with additional detachments developed in the Icla Formation, and a marked decrease in the thickness and mechanical strength of the quartzite packages of the Devonian section. This results in complex and unpredictable trap geometries and a more challenging exploration for the deep plays.

ABSTRACT The sub-Andean system through Argentina and Bolivia is composed of a fold-and-thrust belt developed from 9 Ma until today, as a result of an east-northeast-verging compressive stress field. Depending on the area evaluated, thin- or thick-skinned deformation characterizes the structural style throughout this orogenic system. The differences in structural styles depend on variables such as the sedimentary column involved, internal facies and thickness changes, detachment level features, climatic influence, and the presence of inherited extensional and compressional structures. The existing balanced structural cross-sections sometimes present difficulties for solving the rate and chronology of the deformation. The aim of this chapter is to present suitable new deformation models integrating distinct kinematic characteristics and to analyze the variables involved in the southern sub-Andean thin-skinned fold-and-thrust belt. The structural framework proposed for the southern sub-Andean system in Bolivia and northwestern Argentina is based in the identification of four rheological levels. Levels 1 and 3, with a shale-dominated composition (Kirusillas and Los Monos Formations, respectively), are deformed as a weak isotropic material and can be simulated using Trishear kinematic modeling. On the other hand, rheological levels 2 and 4 (Tarabuco–Santa Rosa–Icla–Huamampampa Formations and Carboniferous–Cenozoic interval, respectively), with a sand/shale alternating composition, are structured as a strongly heterogeneous interval responding to the compressive stress field with parallel folding. A simple shear kinematic model could be used to simulate this deformation. This behavior has been tested as a feasible model for the deep structure in significant oil/gas fields in Argentina and Bolivia.

ABSTRACT This chapter focuses on the role of basement fabrics and inverted extensional faults that strongly affect the frontal zones of the fold-and-thrust faults of sub-Andean basins in Peru and Bolivia. This review examines the relationships of hinterland deformation in the basement with the Present Day topography from the Andean plateau to the sub-Andean foreland basin. Preexisting, steep basement–involved extensional faults that were inverted in the last phase of Andean deformation (~10 Ma to the Present Day) produced basement-cored uplifts that transferred thick-skinned shortening eastward onto the thin-skinned thrust fault and fold systems detached above the basement. Regional cross sections are reviewed and revised in the light of analysis of seismic data as well as mechanically feasible models of the hinterland to foreland transfer of displacement. Steep inverted faults with dominantly high vertical uplift in the hinterland exhume the older stratal packages together with crystalline basement, and these units provide the source for the largely Neogene to Holocene syn-tectonic foreland basins in front of the advancing thrust wedge of the sub-Andean system in Peru and Bolivia.

ABSTRACT Fold-and-thrust belts and their adjacent foreland basins provide a wealth of information about crustal shortening and mountain-building processes in convergent orogens. Erosion of the hanging walls of these structures is often thought to be synchronous with deformation and results in the exhumation and cooling of rocks exposed at the surface. Applications of low-temperature thermochronology and balanced cross sections in fold-and-thrust belts have linked the record of rock cooling with the timing of deformation and exhumation. The goal of these applications is to quantify the kinematic and thermal history of fold-and-thrust belts. In this review, we discuss different styles of deformation preserved in fold-and-thrust belts, and the ways in which these structural differences result in different rock cooling histories as rocks are exhumed to the surface. Our emphasis is on the way in which different numerical modeling approaches can be combined with low-temperature thermochronometry and balanced cross sections to resolve questions surrounding the age, rate, geometry, and kinematics of orogenesis.

Abstract: The internal deformation of the Appalachian Plateau décollement sheet has a distinctive style involving kink bands and thrusts. In areas where the décollement sheet is underlain by thin salt, the dominant structures are thrusts developed at shallow levels, underlain by a series of steep kink bands that terminate downwards at the Silurian salt décollement. Where the salt is thick, large asymmetrical anticlines developed with hinterland-verging kinks on their back-limbs that deformed the entire supra-salt sequence. In order to understand the constraints on deformation, we have used analytical mechanical modelling based on the maximum strength theorem. The simplified model consists of three layers: two are fluids and the third, intervening layer is a stratified competent material. The model is compressed horizontally and the predictions made are based on the kinematic approach of classical limit analysis. Two modes of deformation are investigated: the thrust and the kink band. The modelling shows that kink bands dominate deformation at large burial depth. At shallower depth and small regional bedding dip, the dominant mode is thrusting. In areas of open folding it is predicted that through-going hinterland-verging kink bands will form at a critical limb dip angle of about 10°. Supplementary material: Technical details of the mechanical theory behind this article are available at https://doi.org/10.6084/m9.figshare.c.3799492