Multiple Detachment Levels Research Articles

The Proterozoic–Cambrian salt detachment zone at Lakhar Kuh, Iran: a seismic-scale analogue for salt stringers, detachment folding and multiphase structural development

Abstract In the Middle East, significant evaporite units formed in the latest Precambrian–Cambrian, Triassic, Jurassic, Cretaceous and Cenozoic. The Precambrian–Cambrian period gave rise to the Ara Salt carbonate stringer plays, southern Oman and the giant Zagros anticline traps (Hormuz Formation). While outcrops of salt diapirs are common, basal detachment exposures are extremely rare. The Lakar Kuh area of Central Iran reveals the Precambrian–Cambrian basal salt detachment, on satellite images, in natural cross-section view through the entire Phanerozoic sedimentary section east of Ravar. This view illustrates how older diapiric structures (pillows and normal faults) were the focus of later contractional folds, particularly in the Jurassic and Cenozoic. The salt detachment zone contains many floating blocks (stringers) of clastic, carbonate and igneous rocks. Some blocks were stoped from the overlying beds, while most were layers originally interbedded with the evaporites. Block size, distribution and orientation is highly variable, and folding is infrequent. Lakar Kuh encompasses several key themes generally pertinent to structural geology and salt system research: the presence of sedimentary stringers within evaporites, multiple detachment levels within a thick (>5 km) stratigraphic section, detachment folding, multiphase salt activity, reactivation of older structures by newer ones and multi-stage development of salt bodies.

Interaction between salt and mobile shale in the East Breaks foldbelt, northwestern Gulf of Mexico

The East Breaks foldbelt comprises a series of dominantly slope-parallel folds and thrusts detached on a mostly evacuated salt canopy on the northern Gulf of Mexico slope. The canopy was emplaced near the end of the Eocene. The foldbelt formed mostly in the Oligocene, although minor amounts of shortening continued through the Miocene. We attribute the anomalous trend of structures in the foldbelt to convergent gravity gliding produced by the bend in topography in the northwest corner of the Gulf of Mexico. We suggest that convergent deformation was localized in the East Breaks foldbelt because the area comprised a shale-dominated slope between two sand-prone slope aprons during the Oligocene. These aprons were stronger than the shalier East Breaks area in between, allowing convergent deformation to be partitioned into weak shales.The fact that the Oligocene sequence in the area is shale prone means that shortening was accommodated on multiple detachment levels with fishtail thrusts, as well as in mobile shales, which occur alongside salt diapirs sourced from the canopy. The coincidence of salt structures and mobile shales allows a comparison of their behavior during shortening. Mobile-shale deformation is most intense near a minimally shortened salt massif, leading us to conclude that salt behaved more competently than mobile shale during shortening. This conclusion is counter to the general assumption that salt, where present, is the weakest part of any geologic system.

Mud volcanoes guided by thrusting in compressional settings

Mud volcanoes are fed largely by fracture systems, but fracture systems are common in fold-and-thrust belts (FTBs), and not all of them extrude mud volcanoes. One popular hypothesis is that mud volcanoes extrude up normal faults and fractures in the crests of thrust-related anticlines. Using a high-resolution depth-migrated seismic cube in the northwestern Gulf of Mexico, we demonstrate the occurrence of buried mud volcanoes (shale sheets) that appear to have been fed by fluids moving up the thrust faults themselves. Mobile shales in this deepwater FTB detach a mostly welded allochthonous salt sheet. The main phase of shortening was during the Miocene, accompanying the sedimentation. Shortening generated multiple detachment levels, stacked duplexes, thrusts, and folds affecting the Oligocene mobile shales. Above these detachments, syn-sedimentary shortening initially generated detachment folds that progressively evolved into upward-propagating listric thrusts and fault-propagation anticlines. Shale sheets formed near the central thrust domains, associated with higher fault dips and larger fault displacements. We argue that their feeder systems were probably thrust-related shear zones that widened upward and comprised networks of anastomosing fractures. Finally, we propose that the position of mud volcanoes with respect to the underlying thrust systems depends on the path of least resistance to the surface. Although overpressured muds may vertically reach the surface via extensional fractures in the crests of thrust-related anticlines, we have shown to the contrary that when thrusts are emergent in a deepwater FTB, fluids continue up the inclined thrusts and adjacent fractures, forming mud volcanoes at the thrust tips.

Contractional salt-tectonic system in the south Dezful embayment, Zagros

The Dezful embayment in Zagros has traditionally been interpreted as a typical fold belt with multiple detachment levels, including a lower detachment at the basement-cover interface and an upper detachment along the Miocene Gachsaran halite-bearing evaporites. This paper constrains diapiric processes in the Gachsaran detachment coeval with regional shortening that resulted in complex and hitherto unstudied salt structures and flanking synclinal minibasins. In this paper, well-tied seismic interpretation is integrated with field and remote-sensing mapping, cross-section construction, and stepwise tectono-sedimentary restoration. The results indicate that after the deposition of Mishan–lower Aghajari pre-kinematic overburden sediments, detachment folding initiated above the Gachsaran evaporites during late Tortonian times as a result of regional shortening. The salt-cored anticlines later actively pierced their roof to subsequently rise as passive diapirs and evolve into salt walls. Synclinal minibasins developed due to salt evacuation into flanking walls during the deposition of the Lahbari Mb. in Messinian–Pliocene. Remarkable sedimentary thickening and lateral shifts in minibasin depocenter towards the salt walls, accompanied by overturned collars and hook halokinetic sequences, are diagnostics of near-surface, syndepositional passive growth of diapirs during this stage. From Pliocene to recent, the major episode of regional shortening has squeezed these precursor salt walls to form thrust-welds. In conclusion, the well-exposed structures of south Dezful show an early shortening stage, then a transition to a combination of shortening and passive diapirism, and final thrusting along diapirs. This evolution model may be more common in other salt-influenced fold-thrust belts than is currently recognized.

Structural inversion, imbricate wedging, and out-of-sequence thrusting in the southern Junggar fold-and-thrust belt, northern Tian Shan, China

Accurate definition of structural style in subsurface interpretation is critically important for understanding the deformation history of fold-and-thrust belts, as well as assessing the petroleum prospectivity of structural traps. Using two- and three-dimensional seismic reflection surveys, well data, field mapping, forward models, and balanced cross sections, we describe the structural styles across the actively deforming southern Junggar fold-and-thrust belt in northwestern China, a basin undergoing petroleum exploration and development operations. Subsurface interpretations indicate several folds in the basin overlie Jurassic normal faults that were tectonically inverted in the Late Jurassic to Early Cretaceous. Following inversion, multiple detachment levels propagated northward from the Tian Shan and formed a series of imbricated fault-related folds. The most prominent fold trend in southern Junggar consists of the Tugulu, Manas, and Huoerguosi anticlines, which trap hydrocarbons in clastic Eocene reservoirs. These structures exhibit complex internal geometries, with coeval forethrusts and backthrusts forming imbricated structural wedges. In the latest stages of deformation, and continuing at present, the uppermost thrust sheet, the Southern Junggar Thrust (SJT), truncated the backlimbs of these structural traps, implying the SJT is a tectonically active, out-of-sequence thrust. From these interpretations, we present a model for how the southern Junggar fold-and-thrust belt developed from Jurassic to present. Moreover, we detail how fold growth, fault activity, and structural style affected charge histories, trap formation, and reservoir compartmentalization. Our results have direct implications for assessment of the southern Junggar petroleum system as well as other complex fold-and-thrust belts.

Foreland basin stratigraphic control on thrust belt evolution

The link between orogenic activity and foreland basin stratigraphy is well established; however, potential controls by foreland basin stratigraphy on thrust belt architecture have not been fully evaluated. Mechanical properties of typical foreland basin stratigraphic successions influence the structural development of fold-thrust belts in predictable ways. Fundamental features of foreland basins include the onset of rapid subsidence and deposition of a coarsening-upward sedimentary succession. In the lower part of this succession are fine-grained, distal foreland basin deposits. Enlargement of the orogenic wedge through frontal accretion incorporates the foreland basin strata into the thrust belt, and distal foreland basin depositional units may be preferentially exploited as a thrust detachment zone, resulting in multiple detachment levels. We propose that foreland basin stratigraphic architecture has significant influence on the structural development of thrust belts and that, by extension, processes that influence foreland basin sedimentation may ultimately influence orogenic evolution far removed in time and space.

Detachment levels in the Marathon fold and thrust belt, west Texas

Surface and subsurface data are integrated to characterize the structural architecture of the Marathon fold and thrust belt in west Texas. Multiple detachment levels are present within the thrust belt and result in distinct structural domains. In addition to the basal décollement, whose stratigraphic position varies along strike, we recognize a regionally extensive detachment zone in the late Mississippian to early Pennsylvanian lower Tesnus Formation. The Lower Tesnus Detachment forms a structural domain boundary that can be observed along strike in the surface data and at depth in the subsurface. The stratigraphic intervals above and below this detachment exhibit characteristic patterns of deformation. The Lower Tesnus Detachment is folded by imbrication and the formation of duplexes in the early Mississippian to Ordovician section, suggesting that the detachment may have initially formed as a perched décollement in the foreland that was subsequently exploited as a roof thrust in a duplex system as deformation progressed in a break-forward sequence and older strata were incorporated into the toe of the allochthonous wedge. The structural model presented here for the Marathon region may be applicable across much of the Ouachita orogenic system.

Kinematic evolution of the Patagonian retroarc fold-and-thrust belt and Magallanes foreland basin, Chile and Argentina, 51 30'S

The kinematic evolution of the Patagonian fold-and-thrust belt and cogenetic Magallanes retroarc foreland basin is reconstructed using new geologic mapping, two-dimensional (2-D) seismic-reflection data, and zircon U/Pb geochronology. Results span an ~160-km-wide transect of the thrust belt and Magallanes Basin near 51°30′S and highlight the influence of inherited extensional structures on basin paleogeography, syntectonic sedimentation, and Late Cretaceous–Neogene foreland shortening. South of 50°S, the Patagonian fold-and-thrust belt developed on oceanic and attenuated crust of the predecessor Late Jurassic Rocas Verdes rift basin, resulting in a collisional nature to early fold-and-thrust belt development and foreland sedimentation atop rifted South American crust. We identify six principal stages of development between Late Cretaceous and Neogene time. A palinspastic restoration indicates ~32–40 km (~19%–23%) of retroarc shortening following closure of the Rocas Verdes Basin and incipient growth of the thrust belt. More than half of the estimated crustal shortening occurred synchronously with the deep-water phase of Late Cretaceous foreland basin sedimentation. Subsequent deformation migrated into the foreland, accounting for ~12 km of shortening across the Cretaceous–early Miocene basin fill. Thick-skinned thrust faulting along multiple detachment levels in Paleozoic metamorphic basement resulted in –5 km of foreland uplift and exposure of preforeland basin deposits. The final phase of early Miocene deformation ca. 21–18 Ma may reflect enhanced coupling between the continental and oceanic lithospheres, causing foreland basement uplifts as the Chile Ridge spreading ridge approached the trench. We speculate that Neogene foreland shortening was accommodated by reactivation of Mesozoic normal faults zones and accounts for broad uplift of the Patagonian fold-and-thrust belt.

Normal fault development in a sedimentary succession with multiple detachment levels: the Lower Cretaceous Oliete sub‐basin, Eastern Spain

ABSTRACTField exposures of Lower Cretaceous strata in the Oliete sub‐basin (eastern Spain) allow identification of syn‐rift features such as listric and planar normal faults, rotated fault blocks, fault‐related folds, sharp thickness variations and wedge‐shaped sedimentary geometries, as well as intra‐rift angular unconformities defined by the erosive truncation of rotated fault blocks and the onlap of upper units. The combined use of both stratigraphic and extensional tectonic features at the outcrop scale has allowed us to characterise different syn‐sedimentary tectonic events and their correlation between the footwall and the hangingwall block of the major extensional Gargallo fault. Such events have been interpreted as induced by the major Gargallo fault activity, and they are the basis for proposing a polyphase evolutionary model for this master fault. Data indicate that the deformation tends not to be concentrated on the major fault; instead, it is distributed over a wide area. We interpret that both the interlayered detachment levels in the pre‐rift (especially the Late Triassic Keuper Facies) and syn‐rift series, together with the rheology of the sedimentary pile, play an important role in transmitting deformation from master faults to hangingwall and footwall blocks.

Multiple detachment levels and their control on fold styles in the compressional domain of the deepwater west Niger Delta

ABSTRACTWe interpret recently acquired two‐dimensional (2D) and 3D seismic data from the contractional domain of the Tertiary deepwater west Niger Delta, which is an area of current hydrocarbon exploration and development to show that during its gravitational collapse, multiple detachments were active. Detachments are located within (1) what we herein refer to as the ‘Dahomey unit’, (2) the transition between the Agbada and Akata formations (Top Akata) and (3) the Akata formation. Seismic interpretation and quantitative measurements of fault displacements show that the utilisation of different detachments results in contrasting styles of thrust propagation and fold growth. Two geographical zones are defined. In zone A (NW sector of the study area), the stratigraphically shallowest Dahomey detachment is dominant and is associated with thrust truncated folds. In zone B (SE sector of the study area), a stratigraphically lower detachment approximately at the Agbada–Akata formation boundary is associated with thrust propagation folds. A third detachment, within the Akata formation, is locally developed and is also associated with thrust propagation folds. The different deformational histories are probably related to the mechanical stratigraphy and the pore‐pressure characteristics of the succession.

Structural styles in the deep-water fold and thrust belts of the Niger Delta

The deep-water Niger Delta includes two large fold and thrust belts, products of contraction caused by gravity-driven extension on the shelf that exhibit complex styles of thrusting. These contractional structures formed above multiple detachment levels in the overpressured shales of the Akata Formation. Using the patterns of growth sedimentation, fold shapes, fault-plane seismic reflections, and combined conventional and shear fault-bend folding theories, we describe and model the structural styles and kinematics of the fault-related folds and imbricate thrust systems that compose these belts. Individual fault-related folds, involving both forethrusts and backthrusts, are characterized by long planar backlimbs that dip less than the associated fault ramps, with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation. Forelimbs are short compared to backlimbs, but growth strata show more consistent dips that suggest a component of folding by kink-band migration. Thus, we employ a combination of classic and shear fault-bend fold theories to describe these structures, including the influence of a weak basal detachment zone in the overpressured shales. We expand upon these theories to model the kinematics of imbricate thrust systems, which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta. Regional patterns of folded growth strata are used to define break-forward, break-backward, and coeval thrusting involving single and multiple detachment levels. We define two main types of imbricate thrust systems: type I system with a single basal detachment level and type II imbricate system with multiple basal detachment levels, which cause massive structural thickening of the Akata Formation and refolding of shallow thrust sheets. Through the sequential restoration of two regional cross sections across these systems, we resolve the structural styles, the timing and sequences of thrusting, as well as the regional amounts of shortening, all of which have important implications for hydrocarbon maturation and charge in the deep-water Niger Delta.

Multiple detachment levels in thrust tectonics: Sandbox experiments and palinspastic reconstruction

Abstract Sandbox models of thrust tectonics with multiple detachment levels allow alternative interpretations of fault patterns and evolution, even though they are well controlled experiments. Palinspastic reconstruction resolves these ambiguities and leads to refined interpretations. It reveals that if there are thick viscous interlayers, all non-fault related contraction can be accommodated by folding, without tectonic compaction. It also confirms the complete decoupling of faulting above and below a thick weak viscous layer, and the initial straightness and regular spacing and dips of thrusts above a decollement of low basal friction. A weak elasto-plastic interlayer or decollement merely has a lubricating effect, without complete decoupling. Many natural thrust systems are underlain by a ductile substratum and have more than one decollement level. These examples almost always exhibit a complex thrust belt geometry and are apparently formed by different thrusting events that can be difficult to interpret and to time relative to one another. Weak decollement materials and interlayers have a strong effect on the structural style of thrust belts at the moment of detachment. However, other material properties, in particular the viscosity, play a dominant role during the structural development following detachment (the overthrust phase). The objective of this study was to improve structural interpretation of data from areas of thin-skinned thrust tectonics with weak ductile detachment horizons (i.e. excluding large scale basement involvement). The approach involved: • the use of sandbox experiments to provide controlled structural models with different decollement rheologies, and • the refinement and validation of selected cross-section interpretations with palinspastic reconstruction. The result is a set of better constraints on structural interpretation of cross-sections in thrust tectonics. In natural cases, the most common weak decollement lithologies are over-pressured shales and evaporites, in particular rock salt. In our experiments we have simulated rock salt by using silicone putty (‘SP’), a viscous material that can flow slowly as a Newtonian fluid. We modelled overpressured and/or very soft clay with an oil-water emulsion (‘OWE’), which is a ductile, weak elasto-plastic, non-viscous material in which localized shear zones can form, as in any other elasto-plastic solid.

Thrust sequences

The sequential evolution of arrays of thrusts can occur by a number of geometric models: these models form the subject of this contribution. The most common of these is a simple foreland-directed (piggy-back) series which develops into either an emergent imbricate fan or, if the faults converge upwards, a duplex structure. Depending on the geometry and distribution of thrust ramps and the relative magnitude of displacements on the faults, a range of duplex geometries can result where the imbricate slices have either hinterland-dipping, antiformal stack or foreland-dipping attitudes. A frequently recognized complexity occurs when a later fault climbs across from the footwall into the hanging wall of an earlier one leading to a breached geometry. A less common generation of imbricate fans occurs by the hinterland-directed progression of thrusts in a break-back sequence. This can lead to an overstep geometry where earlier structures are truncated in the footwall of a later fault. The evolution of hinterland-directed, back-thrusts is also considered. Both internally piggy-back (regionally hinterland-directed) and break-back (regionally foreland-directed) sequences can develop, depending on the ability of the active displacement front to propagate forwards. Simple imbricate geometries can develop with an internally break-back sequence but piggy-back propagation will generally require substantial strains within the back-thrust slices or the generation of multiple detachment levels. The range of structures which can result from these fore-thrust and back-thrust sequences is illustrated using hypothetical cross-sections and natural examples from the Scottish Moine thrust belt and the western Alps. The implications of each geometry for estimates of orogenic contraction and the construction of balanced cross-sections are also emphasized.