Passive-roof Duplex Research Articles

Topographic loading-dominated kinematics of the fold-and-thrust belts: Sandbox modeling of the Longmen Shan thrust belt in eastern Tibetan Plateau

The influence of topographic loading on the thrust kinematics of fold-and-thrust belts has long been poorly understood. The current Longmen Shan thrust belt (LSTB) in the eastern Tibetan Plateau has experienced progressive deformation, which might be influenced by preexisting topographic loading. Sandbox modeling was conducted herein to investigate the deformation of the LSTB. A reference model without topographic slope and five models with slopes of 6°, 14°, 17°, 19°, and 22°, each with two décollements, were evaluated using a sandbox instrument and particle image velocimetry. The critical slope angle for the experimental materials was determined to be 15°–17°. Subcritical wedges facilitated the formation of imbricate forethrusts and triangle zones with large-displacement backthrusts at the shallow level and active roof duplexes at the deep level. Furthermore, critical-supercritical wedges promoted the nucleation of imbricate forethrusts and rudimentary triangle zones with small-displacement backthrusts at the shallow level and passive roof duplexes at the deep level. The different kinematic assemblages were dominated by the localization of back-thrusting as topographic loading varied. The preferred deep backthrusts were likely associated with a brittle-ductile shear stress ratio exceeding 13.5 and with rotation of the stress σ1 axis towards hinterland. However, the shallow backthrusts tended to vanish due to increased ductile layer strength as initial backthrust tips showed increased horizontal velocity differences. Therefore, this study suggests that the Mesozoic topography probably created critical-supercritical to subcritical wedges from northern to southern segments, dominating the along-strike differential evolution of triangle zones and duplexes in the LSTB during the Cenozoic.

Seismic data, cross-section balancing, and hydrocarbon prospectivity of the western Sulaiman fold belt, northwest Pakistan

Over the past few decades, hydrocarbon exploration has considerably improved our understanding of foreland deformation. This study used the interpretation of seismic reflection profiles and cross-section balancing techniques to understand the passive-roof duplex geometry of foreland structures in the Western Sulaiman Fold Belt (SFB), Pakistan. Seismic reflection profiles were interpreted to show the presence of hinterland-vergent fault-propagation and fault-bend folds as prospects over a detachment located at a depth of ∼6 km in the pelitic Triassic succession. Cross-section balancing showed intraplate deformation in the hanging-wall flat of the frontal duplex. Clear seismic reflectors above 3 s two-way travel time (TWT), estimates of stratigraphic and tectonic thicknesses with field geology and isopach mapping were used to locate the décollement in the pelitic Permo-Triassic strata at a depth of ∼11 km and interpret stacked duplexes at the mountain front. The foreland deformation of the western SFB was compared with that of the central/eastern SFB. The comparison shows the presence of discovered gas fields, with intraplate deformation of the duplex in the western, and pop-ups in the roof-sequence in eastern SFB depicting the complexity of deformation for the assessment of hydrocarbon prospectivity in the fold belt. The 33 km deformed balanced cross-section was restored to ∼71 km, with 38 km (54%) shortening in the foreland. Approximately 12 km (17%) of shortening was passively accommodated by the hinterland-vergent out-of-sequence passive backthrust in the frontal horse. This description of foreland deformation using the example of the SFB adds to our understanding of the structures of the mountain front that are recognized as zones of prolific hydrocarbon prospects.

Several types of triangle zones from the Subandean ranges of Peru: Fish-tails, tectonic wedges and passive-roof duplexes

Surface geology, well data and 2D seismic interpretations reveal the presence of triangle zone structures at the contact of the Subandean Ranges and the Amazonian foreland in Peru. From north to south the Santiago, Marañon, Ucayali and Madre de Dios basins have been studied. Interpretations presented in this study show a great variety of structural styles at this contact area. Simple triangle zones with one back thrust, composite ones with several back thrusts, more complex double vergent ones with pop-up structures, best described as fish-tails, tectonic wedges and passive-roof duplexes are present in the study area. The role of pre-Andean tectonics and paleogeography is key to understanding the Andean evolution, due to the control on the location and geometry of the main detachment levels. The Santiago Basin is characterized by the presence of fish-tail structures with a Permian-Triassic evaporitic basal detachment located at the base of the Pucara Gr. At the Ucayali Basin, several detachment levels interact, the basal ones located within the Ordovician, Devonian, and Carboniferous defining triangle zones or tectonic wedges. At the Madre de Dios Basin, two detachment levels define the base of the triangle zone, Permian shales in the northwest at the Inambari Imbricates area, and Lower Devonian (Cabanillas Gr.) in the southeast at the Candamo area. A roof thrust is located within the Paleogene section at both the Madre de Dios and Ucayali basins, defining passive-roof duplex structures.Several detachment levels are rooted into the Lower Paleozoic; others within the Permian-Triassic section, being locally evaporitic, and finally others are located at the base of the Cretaceous. Paleozoic shales represent the main basal detachment level of the southern Subandean ranges. The role of these detachments and the role of the sedimentation rate are discussed as factors that control the development and geometry of the triangle zones described in the study area.Except for the San Martin-Sagari area of the Ucayali Basin, the triangle zones of the Santiago and Madre de Dios basins remain unexplored even though active petroleum systems are present. Sub-thrust structures at the Azulmayo area of the Madre de Dios Basin remain promising undrilled structures.

Structural Features and Evolution of the Northwestern Sichuan Basin: Insights From Discrete Numerical Simulations

The northwestern Sichuan Basin has experienced Meso-Cenozoic intracontinental compressional tectonic processes and formed multi-detachment stratigraphic distribution of foreland basins and fold-thrust belts, which have caused complicated structural deformations in the deep buried layers. Rapid uplift with accelerated erosion and two sets of detachments in the Lower Triassic and Lower Cambrian controlled the multilevel deformation structure. We conducted discrete numerical simulations with double weak detachments and erosion under extrusion conditions in order to examine the mechanics and kinematics of the frontalpiedmont zones of the NW Sichuan Basin. The following findings were made. (1) With continuous compression, the weak detachments promoted the decoupled and ladder-like deformation of the thrust belt, where the deformation above the slip layer extended further than it did below it. Rapid uplift and erosion at the thrust front contributed to the formation of a passive roof fault and a monocline in the upper layer, a series of forward and backward thin-skinned thrust-buried structures in the middle layer sandwiched between two weak detachments and stacking structures in the lower layer. (2) Erosion effectively prevents the deformation from propagating above the upper detachment, but can advance a horizontal transition in the deformation style generated within the middle brittle layer: from oblique and tight fault propagation folds to symmetrical, wide, and gentle detachment folds. (3) The model results consistent with tectonic deformation in the NW Sichuan Basin indicate a possible evolutionary mechanism under compression. There is hierarchical deformation of uncoordinated contraction controlled by the Lower Triassic and Early Cambrian weak layers, with the characteristics of the shallow monocline, the middle thin-skinned thrusts, and the deeper basement-involved folds. Continuous compression contributed a sequential pattern of steps as a whole, from the frontalpiedmont zones to the foreland basin, autochthonous stacking thrusts, and the huge buried structure in the NW Sichuan Basin. During the Himalayan period, syntectonic erosion along with the uplifted thrust front maintained the development of a passive-roof duplex and a huge forward buried structure.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of >300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Role of erosion in creating thrust recesses in a critical-taper wedge: An example from Eastern Tibet

Mechanical models and analog experiments show that erosion plays an important role in controlling along-strike variations in thrust-belt deformation. Despite this fundamental insight, natural examples of focused erosion causing local variations of thrust-belt geometry, such as the formation of salients and recesses, remain underappreciated. In this study, we address this issue by integrating theoretical model predictions with structural, morphological, and erosion-pattern analyses along the thrust system defining the eastern margin of the Tibetan Plateau. Our work shows a nearly constant surface slope across and an overall cylindrical fault-related fold geometry along the thrust belt. These phenomena can be explained by the development of a critically-tapered thrust wedge along the plateau margin. A prominent recess exists at the intersection between the transverse Minjiang River and the range-bounding thrust. There, the surface trace displays a much higher curvature than the rest of the thrust belt and the local fold geometry is conical rather than cylindrical. We infer a warped passive-roof duplex under the recess. We build a classification diagram for recesses developing in critical-taper wedges, in which six independent variables lead to three morphotectonic scenarios regarding the effects of mass influx, mass efflux, and intrinsic properties of a thrust wedge. Formation of the Dujiangyan recess and its spatial correlation to the Minjiang River can be best attributed to localized fluvial erosion during forward propagation of the thrust wedge in eastern Tibet. This study sets a type example for map-view curves in thrust belts undergoing deformation-erosion interactions.

Structural interpretation of frontal folds and hydrocarbon exploration, western sulaiman fold belt, Pakistan

Abstract Seismic reflection profiles and borehole data combined with geological mapping was used to analyze foreland structures with 3-D structural variation, estimates of shortening, and kinematic evolution of western Sulaiman fold belt (SFB), Pakistan. The western SFB is interpreted with passive-roof duplex geometry. The floor thrust (decollement) is interpreted in Triassic strata rather than its inference in Precambrian-Cambrian evaporites. Whereas, the roof thrust (upper detachment) of hinterland vergence is located in Eocene Ghazij shale. Based on seismic data, the folds in the frontal duplex are interpreted as fault-propagation and fault-bend type. They are located over curved and staircase geometry of blind back thrusts. The folds-and-thrusts of EW trend show right-hand en-echelon stepping along NW-SE oriented mountain front due to drag along the western margin of the fold belt. The blind thrusts cut across the hanging-wall flat of the duplex horse with roots in base Triassic at depth of ~4–4.5 km 3D surfaces of reservoir strata show three areas of closed elliptical contours as potential prospects over blind thrusts, with maximum structural relief of ~2200 m. Shortening of 2100 (22%) and 9950 m (47%) is calculated along with thrusts in the frontal duplex with the interpretation of out-of-sequence structures. Passive roof duplex structures between foreland and hinterland vergent back-thrusts are reported from mountain belts around the world. We have identified out-of-sequence deformation in the frontal duplex with stacked structures. Identification of these features adds to our understanding of the development of mountain fronts in the context of Anderson's theory of faulting with the presence of a conjugate set of faults and foreland propagation of mountain fronts with secondary out-of-sequence deformation. Subsurface seismic data appears to be critical in their recognition for analysis of active foreland deformation and assessment of hydrocarbon plays in the fold belts.

Analogue modeling of the northern Longmen Shan thrust belt (eastern margin of the Tibetan plateau) and strain analysis based on Particle Image Velocimetry

In response to the multistage convergences across the western margin of the Sichuan Basin, a complex thrust belt developed along the Longmen Shan thrust belt (LTB). Compressional analogue modeling with double weak detachments is carried out to shed light on the development and kinematics of the triangle zone at the front of the northern LTB. (1) Two weak detachments create an essential configuration for the development of duplexes. Each detachment displays different mechanical behaviors during the evolution of the thrust belt and decouples deformation at different depths. (2) Regular erosion at the thrust front can effectively localize strain and promote the development of the back-thrust along the upper detachment to generate a sustained passive-roof duplex in the front. The similarity of the tectonic deformation between the analogue modeling and northern LTB implies a possible evolutionary mechanism of the Shuangyushi Structure. The Middle Triassic and early Cambrian weak layers act as two pivotal detachments, decoupling the tectonic styles at different depths. During the Indosinian and Yanshanian period, continuous compression contributed to the establishment of the main tectonic framework of a passive-roof duplex. In the following Himalayan period, the eastward compression complicated the middle structural layer and uplifted the thrust front, and was accompanied by regular erosion, contributing to the development of a passive-roof duplex.

Interpretation of the Eastern Sulaiman fold-and-thrust belt, Pakistan: A passive roof duplex

Surface geology, seismic reflection, and well data from the eastern Sulaiman fold-and-thrust belt (SFTB) were integrated to analyze its structural geometry, deformation style, foreland structure development, and tectonic shortening. The N-S oriented fold belt's deformation front has a monoclinal expression over ∼350 km along the eastern edge of the SFTB. The cross-section shows a ∼9 km thickness of platform sequences that are detached from the basement by an inferred decollement in Paleozoic strata. The base of Cretaceous strata is ∼6 km deep in the foredeep syncline, rising ∼7 km in the foreland in the absence of a major thrust, and is interpreted as a thin-skinned, passive-roof duplex style of deformation. The duplex sequence of Jurassic and older strata are separated from the Cretaceous and younger roof sequence, which is intact over foreland structures for ∼60 km. A balanced structural cross-section of this area restores to an original length of 172 km, implying ∼112 km (65%) of shortening since 6 ± 2 Ma, with rates of 18 mm/yr. This is greater than the ∼50% shortening along a 359 km long N-S balanced cross-section across the SFTB, but is likely to occur along stacked duplexes with high relief along the eastern SFTB.

Field documentation and genesis of back-structures in ductile and brittle regimes from the foreland part of a collisional orogen: examples from the Darjeeling–Sikkim Lesser Himalaya, India

In the foreland-side of an orogen, thrust-related structures develop that verge towards the foreland (fore-thrusts). Although much less abundant, structures with opposite vergence (i.e., towards hinterland), named, back-structures, also exist. We report back-structures (exposure scale back-thrusts and associated folds) from the Darjeeling Sikkim Lesser Himalaya, India. These structures occur mainly in three Back-Structure Zones (BSZ-A, -B, and C). Back-structures probably originated in the ductile regime and continued in the brittle domain. Tectonic setting of the study area, such as- critical taper, duplexing, etc. indicates a good potential for the presence of back-structures in this region. Along with the previous work on back-structures in collisional orogens, Himalaya in particular, these structures seem to be ubiquitous. The correlation of field observations with tectonics indicate that mechanisms, e.g., critical taper, sub-surface barriers, and passive roof duplex played roles in forming back-structures in this part of the Sikkim Lesser Himalaya.

Linked thick- to thin-skinned inversion in the central Kirthar Fold Belt of Pakistan

Abstract. The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.

Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

The mechanisms for crustal thickening and exhumation along the Yarlung (India-Asia) suture in southern Tibet are under debate, because the magnitudes, relative timing, and interaction between the two dominant structures—the Great Counter thrust and Gangdese thrust—are largely unconstrained. In this study, we present new geologic mapping results from the Yarlung suture zone in the Lazi region, located ∼350 km west of the city of Lhasa, along with new igneous (5 samples) and detrital (5 samples, 474 ages) U-Pb geochronology data to constrain the crystallization ages of Jurassic–Paleocene Gangdese arc rocks, the provenance of Tethyan Himalayan and Oligocene–Miocene Kailas Formation strata, and the minimum age (ca. 10 Ma) of the Great Counter thrust system. We supplement these data with a compilation of 124 previously published thermochronologic ages from Gangdese batholith, Kailas Formation, and Liuqu Formation rocks, revealing a dominance of 23–15 Ma cooling contemporaneous with slip across the Great Counter thrust system and other potentially linked structures. These data are systematically younger than 98 additional compiled thermochronologic ages from the northern Lhasa terrane, recording mainly Eocene cooling. Structural and thermochronologic data were combined with regional geological constraints, including International Deep Profiling of Tibet and the Himalaya (INDEPTH) seismic reflection data, to develop a new structural model for the Oligocene–Miocene evolution of the Tethyan Himalaya, Yarlung suture zone, and southern Lhasa terrane. We propose that a hinterland-dipping duplex beneath the Gangdese mountains, of which the Gangdese thrust is a component, is kinematically linked with a foreland-dipping passive roof duplex along the Yarlung suture zone, the Great Counter thrust system. The spatial and temporal convergence between the proposed duplex structures along the Yarlung suture zone and the South Tibetan detachment system indicate that they may be kinematically linked, though this relationship is not directly addressed in this study. Our interpretation, referred to as the Gangdese culmination model, explains why the Gangdese thrust system is only locally exposed (at relatively deeper structural levels) and provides a structural explanation for early Miocene crustal thickening along the Yarlung suture zone, relief generation along the modern Gangdese Mountains, early Miocene Yarlung River establishment, and creation of the modern internal drainage boundary along the southern Tibetan Plateau. The progression of deformation along the suture zone is consistent with tectonic models that implicate subduction dynamics as the dominant control on crustal deformation.

Duplex thrusting in the South Dabashan arcuate belt, central China

Due to later tectonic superpositioning and reworking, the South Dabashan arcuate belt extending NW to SE has experienced several episodes of deformation. The earlier deformational style and formation mechanism of this belt remain controversial. Seismic interpretations and fieldwork show that the curved orogen can be divided into three sub-belts perpendicular to the strike of the orogen, the imbricate thrust fault belt, the detachment fold belt and the frontal belt from NE to SW. The imbricate thrust fault belt is characterized by a series of SW-directed thrust faults and nappes. Two regional detachment layers at different depths have been recognized in the detachment fold and frontal belts, and these detachment layers divide the sub-belts into three structural layers: the lower, middle, and upper structural layers. The middle structural layer is characterized by a passive roof duplex structure, which is composed of a roof thrust at the top of the Sinian units, a floor thrust in the upper Lower Triassic units, and horses in between. Apatite fission track dating results and regional structural analyses indicate that the imbricate thrust fault belt may have formed during the latest Early Cretaceous to earliest Paleogene and that the detachment fold belt may have formed during the latest Late Cretaceous to earliest Neogene. Our findings provide important reference values for researching intra-continental orogenic and deformation mechanisms in foreland fold-thrust belts.

Fault‐related fold styles and progressions in fold‐thrust belts: Insights from sandbox modeling

AbstractFault‐related folds of variable structural styles and assemblages commonly coexist in orogenic belts with competent‐incompetent interlayered sequences. Despite their commonality, the kinematic evolution of these structural styles and assemblages are often loosely constrained because multiple solutions exist in their structural progression during tectonic restoration. We use a sandbox modeling instrument with a particle image velocimetry monitor to test four designed sandbox models with multilayer competent‐incompetent materials. Test results reveal that decollement folds initiate along selected incompetent layers with decreasing velocity difference and constant vorticity difference between the hanging wall and footwall of the initial fault tips. The decollement folds are progressively converted to fault‐propagation folds and fault‐bend folds through development of fault ramps breaking across competent layers and are followed by propagation into fault flats within an upper incompetent layer. Thick‐skinned thrust is produced by initiating a decollement fault within the metamorphic basement. Progressive thrusting and uplifting of the thick‐skinned thrust trigger initiation of the uppermost incompetent decollement with formation of a decollement fold and subsequent converting to fault‐propagation and fault‐bend folds, which combine together to form imbricate thrust. Breakouts at the base of the early formed fault ramps along the lowest incompetent layers, which may correspond to basement‐cover contacts, domes the upmost decollement and imbricate thrusts to form passive roof duplexes and constitute the thin‐skinned thrust belt. Structural styles and assemblages in each of tectonic stages are similar to that in the representative orogenic belts in the South China, Southern Appalachians, and Alpine orogenic belts.

Formation of passive-roof duplexes in the Colombian Subandes and Perú

Passive-roof duplexes are important features for accommodating shortening in active orogens, but their occurrences have been previously demonstrated only with significant subsurface data or after their exhumation. In this study, we describe a series of thin-skinned passive-roof duplexes along the Subandean front in Colombia and compare them with potential analogues in southern Peru. We suggest type localities for this structural style, which display conditions favorable for formation of these structures. It appears that passive-roof duplexes in the Subandean zones are mostly late Miocene features. Our main data for placing temporal and thermokinematic constraints on their development are 25 apatite fission-track (AFT) analyses, including track-length distributions. We show that these areas require high shortening, surface erosion, and downstream sedimentation rates at the time of their formation, as well as two distinct low-friction detachments. These features, which have been previously described by analog models, appear to have been conditioned by a phase of late Miocene topographic growth and denudation in the hinterland and subsequent increase in accommodation as well as sedimentation rates in the foreland.

Three-dimensional structural evolution and kinematics of the Piedemonte Llanero, Central Llanos foothills, Eastern Cordillera, Colombia

The Piedemonte Llanero is a wedge duplex zone in the Llanos foothills on the eastern flank of the Eastern Cordillera of Colombia. It is located northeast of the Cusiana and Cupiagua hydrocarbon fields. The area is characterized by a series of moderate to high-angle duplexes with east-southeast verging thin-skinned and thick-skinned tectonics. We present a structural model constrained by 2-D and 3-D seismic reflection data, surface geology, and well data. The structural analysis is based on backward modeling (kinematic restoration) and forward modeling using transfer-flexural slip and fault slip fold algorithms. The structures are significantly tighter in the northern segment compared to the southern segment of the overthrust trend. We estimate approximately 17 km of shortening in the northern duplex zone, and about 26 km total shortening for the southern duplex zone. We propose that thin-skinned in-sequence imbricate thrust stack deformation produced most of the shortening. The main Andean deformation (80% of the total shortening) commenced in the Piedemonte area about 6 Ma with rapid shortening and uplift in the area resulting in the development of an active-roof duplex structure as the cover was bulldozed forward by a horse block (Monterralo anticline) ramping up to a detachment at the base of C2, then ramping to the surface as the Yopal thrust. Later the horse blocks in the wedge rose, underthrusting the cover in a passive-roof duplex triangle zone. This was followed by an out-of-sequence Laramide-style thick-skinned basement-uplift of the range which produced much of the structural relief of the Eastern Cordillera. Cenozoic deformation in the Eastern Cordillera has been primarily range-normal, but has involved an increasing component of mountain-parallel right-lateral shear in the last 2 Ma.

Unraveling the central Appalachian fold-thrust belt, Pennsylvania: The power of sequentially restored balanced cross sections for a blind fold-thrust belt

We present a kinematic model for the sequential development of the Appalachian fold-thrust belt (eastern U.S.) across a classic transect through the Pennsylvania salient. New map and strain data are used to create a balanced geologic cross section from the southern edge of the Valley and Ridge Province to the northern Appalachian Plateau. This region of the central Appalachian fold-thrust belt is an ideal location to illustrate the incorporation of strain data in balanced cross sections, because it cannot be balanced without quantifying grain-scale strain. We use a sequentially restored, balanced cross section to show how layer-parallel shortening (LPS) is distributed above and ahead of thrust and fold shortening and constrain the geometric and kinematic evolution of a passive roof duplex. By combining line length and area balancing of a kinematically viable cross section with LPS estimates in both the Valley and Ridge Province (20%) and Appalachian Plateau (13%), we document the total magnitude of shortening in both the folded cover sequence and the duplexed lower layer of the fold-thrust belt. Restoration of the cross section indicates a total of 77 km (22%) of shortening between the southern margin of the Valley and Ridge Province in central Pennsylvania and a pin line immediately north of the northern limit of documented LPS in the foreland. The 24 km (13%) of LPS on the Appalachian Plateau is interpreted as being above the Salina (salt) decollement. This magnitude of shortening is 14 km greater than the amount of displacement on the Nittany Anticlinorium, the northernmost structure of the fold-thrust belt that cuts upsection from the Cambrian Waynesboro Formation to the Silurian Salina decollement. Because the fault that cores the Nittany Anticlinorium can only facilitate 10 km of shortening on the plateau, an early history of Appalachian Plateau LPS in Silurian and younger rocks is required to balance the section. We propose that the additional 14 km of LPS on the plateau occurred early in the deformation history and was kinematically linked to two fault-bend folds that have a lower decollement in the Cambrian Waynesboro Formation and an upper, subhorizontal detachment in the Silurian Wills Creek Formation (in the Valley and Ridge) and the Salina Group on the Appalachian Plateau. This upper detachment feeds displacement from these early horses in the duplex system onto the Appalachian Plateau and is expressed there as LPS shortening. This early shortening is followed by the development of in-sequence horses that repeat the mainly thrust-faulted Cambrian–Ordovician sequence using both the main decollement in the Cambrian Waynesboro and the Ordovician Reedsville Formations as an upper detachment horizon. In the south, shortening in the Late Ordovician through Devonian layers is accommodated by both LPS and forced folding of the overlying folded cover sequence. We propose that the Reedsville Formation becomes weaker to the north, facilitating shorter wavelength detachment folds. The development of gentle open folds on the Appalachian Plateau, as well as the last 10 km of LPS on the plateau, is linked to the most forelandward horse in the duplex. This horse forms the broad Nittany Anticlinorium, the northern boundary of the Valley and Ridge.

Backthrusts and passive roof duplexes in fold-and-thrust belts: The case of Central-Western Sicily based on seismic reflection data

Seismic reflection profiles crossing the area bounded by the M. Kumeta Ridge and the M. Maranfusa-M. Galiello-Rocca Busambra morphostructural feature (central-western Sicily) were used to constrain the deep structural setting of the Sicilian–Maghrebian fold-and-thrust belt (FTB). By integrating seismic, field and well data, we attempted to correlate outcropping and deep-seated contractional structures, shedding light on the internal geometries of the thrust system. Results of the new seismostratigraphic analysis presented in this paper reveal structural variations along the tectonic edifice. This suggests dividing the study area into two sectors: the western sector is characterized by foreland-verging thrusts comparable to the already known structural model of the chain, conversely in the easternmost sector duplexes and hinterland-verging structures, here highlighted for the first time, prevail. In fact, the main carbonate relief of this area (Rocca Busambra Ridge) appears as the outcropping part of a thick tectonic pile bounded on its northern side by a high-angle backthrust, while at depth a blind southern verging (i.e. forelandward) thrust can been recognized; this structural setting suggests that underthrusting of the buried S-verging carbonate body induced the N-verging (i.e. hinterlandward) dislocation of the overlying units. On the whole, this structure can be interpreted as a triangle zone, where the outcropping relief represents a passive-roof duplex bounded, on its northern side, by the high-angle backthrust deeply connected with a low-angle décollement layer. The recognized backthrust appears as the product of a deep-seated tectonic activity, that played a key role for the upper thrust-sheet emplacement.

Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

A scenario for late Neogene Andean shortening transfer in the Camisea Subandean zone (Peru, 12 S): Implications for growth of the northern Andean Plateau

Precise knowledge of the timing of deformation in the Subandean zone of the Andean Plateau is a prerequisite for deciphering the late Neogene growth of the Andean Plateau. In this paper, we report new apatite fission-track (AFT) and vitrinite reflectance (Ro) data for a regional balanced cross section of the Camisea Basin in the central Peruvian Subandean zone, adjacent to the northern Andean Plateau. The balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust- related anticlines. The balanced cross-section restoration shows 53 km (39%) of total horizontal shortening. We sampled Paleozoic to Cenozoic sedimentary strata for AFT and Ro analyses along the ∼4-km-thick vertical profile of the Mainique back thrust (passive roof thrust), the innermost preserved Subandean structure. Young components of AFT ages are spread between ca. 6 Ma and ca. 24 Ma. A break in the slope in the AFT ages determines the geometry of the Miocene partial annealing zone and the exhumation of the Mainique back thrust at ca. 6 Ma. Sequential restoration calibrated by AFT and Ro data indicates that the last ∼23 km horizontal shortening were accommodated by the Camisea thrust system over the past ∼6 m.y., giving a mean shortening rate of 3.8 mm/yr. Using this shortening rate for the first ∼30 km horizontal shortening, we calculate that the Andean shortening transfer into the Peruvian Subandean zone initially started at ca. 14 Ma. This result suggests that the transfer of shortening from the northern Andean Plateau to the Subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca. 10 Ma and ca. 7 Ma. We rather propose that the late Neogene growth of the northern Andean Plateau mostly resulted from a continuous crustal shortening combined with lower-crustal flow.