Passive Roof Thrust Research Articles

The India‐Eurasia convergence system: Late Oligocene to early Miocene passive roof thrusting driven by deep‐rooted duplex stacking

• The India-Eurasia convergence zone hosts a set of post-Eocene tectonic events. • Enhanced duplex stacking from continuous indian underthrusting. • Passive roof thrust system promoted the tectonic exchange. The Late Oligocene-Early Miocene epoch represents a critical stage in the collisional history of the India-Eurasia dominant collision zone. Within this timeframe, a set of tectonic events occurred simultaneously or progressively. However, the key responsible for triggering these nearly coeval events remains unclear. This study used an integrated analysis of both geological and geophysical data to document the tectonic interactions throughout the Himalayan Orogenic Belt from west to east. Deep seismic reflection profiles outline crustal geometry of a decoupled Indian subduction front, and the overlying sheets over the Main Himalayan Thrust (MHT) is evidenced with a sequence of detachment-associated ramp-anticlines. Meanwhile, regional magnetotelluric (MT) profiles document rheologic connections in the form of a high-conductivity anomaly running top-to-the south between the southernmost Tibet and the areas beyond the Yarlung-Zangbo Suture Zone to the south. The overall architecture provides a complete picture of the complex deformation pattern beneath the tectonic convergence system. Together with previous studies in surface geological investigations, we propose that the enhanced duplex stacking of the underthrusting Indian crust increased crustal shortening of the convergence system. The consequent sudden exposure of the northern Himalayan domes released the accumulated stress to trigger the onset of a south-dipping passive roof thrusting through the convergence system to the southernmost Tibetan Plateau. Recognition of this exchange pattern from crustal duplex stacking to passive roof thrusting replenished an understanding of the tectonic interactions of the ongoing India-Eurasia collision.

Structural Analysis of a Fault-related Anticline in the Southwestern Gyeonggi Massif, Korea Using an Unmanned Aerial Vehicle and Field Surveys: The Role of Rejoining Splays in a Duplex-like Structure

Kim, I.; Park, S.-I., and Lee, H.-J., 2020. Structural analysis of a fault-related anticline in the southwestern Gyeonggi Massif, Korea using an unmanned aerial vehicle and field surveys: The role of rejoining splays in a duplex-like structure. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 261-270. Coconut Creek (Florida), ISSN 0749-0208.In fold and thrust belts, various contractional structures accommodate horizontal shortening and vertical thickening in response to orogenic events. In this study, it was conducted a structural analysis of the Gomseom anticline, exposed along the coast in a part of a Phanerozoic orogenic belt in the southwestern Gyeonggi Massif, on the southern Korean Peninsula. To combat difficulties related to accessing the outcrop under tidal effects, high-resolution unmanned aerial vehicle (UAV) images alongside field data was used. The UAV photography technique allowed easy macro and mesoscale structural mapping and down-plunge projection, thereby enabling the efficient interpretation of the geometric and kinematic features of the Gomseom anticline. In the core of the Gomseom anticline, fault-bounded sheets are vertically stacked and folded, giving rise to a unique structural geometry analogous to duplexes. This duplex-like structure is bounded by a submerged thrust at the base and tightly folded thrust imbrication at the top and defined by intervening shortcut rejoining splays branching from the latter. Taking the kinematics of the sheet-bounding faults and intra-sheet minor faults into account, it was suggested an evolutionary model of the Gomseom anticline in which repeated fault-related folding and sequential propagation of rejoining splays give rise to the duplex-like structure. In the rejoining splay model, displacement on the basal thrust is transferred to a passive-roof thrust without hard links between them. Our result can be evaluated as a possible alternative duplex model that contrasts pre-existing models such as “Boyer-type”, “connecting splay”, and “fold” duplexes. However, further testing of the model is required.

Tectonic wedging, back-thrusting and basin development in the frontal parts of the Mesoproterozoic Karagwe-Ankole belt in NW Tanzania

Structural complexities in the Mesoproterozoic Karagwe-Ankole fold belt in northwest Tanzania have led to conflicting interpretations of regional kinematics and the geodynamic significance of the belt. Structural mapping of an eastern portion of the belt indicates that the regional-scale (>100km) Mugera-Nyakahura basement inlier may be considered a forethrusted tectonic wedge. Tectonic wedging in the frontal parts of the belt occurred during top-to-the southeast thick-skinned thrusting of the gneissic Archaean basement. The diagnostic feature of tectonic wedging is the reversal of vergence directions of kinematic fabrics on either side of the basement wedge, resulting in hinterland-directed, top-to-the northwest kinematics in front and on top of the wedge. Strain is localised into the often graphitic metapelitic rocks of the Upper Muyaga Group. The mainly coarse-grained clastic Mesoproterozoic sediments of the Bukoba Group represent the foreland, molasse-type deposits of the Karagwe-Ankole fold belt. The only gently folded Bukoba Group is separated from the underthrusted, highly deformed Muyaga Group by a passive roof thrust. This corresponds to the regional-scale asymmetry of the synclinal structure of the Bukoba basin in the frontal parts of the belt. The gentle folding is the result of the underthrusting and lifting of the Bukoba sediments above the basement wedge creating a triangle zone. The kinematics and geometry of the frontal parts of the Karagwe-Ankole belt described here confirm the belt to represent a top-to-the-east and -southeast verging foreland fold-and-thrust belt. The actual timing of deformation is, at present, unknown, but regional-scale kinematics and the metamorphic zonation are compatible with an origin of the belt during convergence between the Congo and Tanzania Cratons in the west.

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.

Structural Evolution of the Eastern Qiulitagh Fold and Thrust Belt, Northern Tarim Basin, China

Abstract:The eastern Qiulitagh fold and thrust belt (EQFTB) is part of the active Kuqa fold and thrust belts of the northern Tarim Basin. Seismic reflection profiles have been integrated with surface geologic and drill data to examine the deformation and structure style of the EQFTB, particularly the deformational history of the Dina 2 gas field. Seismic interpretations suggest that Dongqiu 8 is overall a duplex structure developed beneath a passive roof thrust, which generated from a tipline in the Miocene Jidike Formation, and the sole thrust was initiated from the same Jidike Formation evaporite zone that extends westward beneath the Kuqatawu anticline. Dongqiu 5 is a pop‐up structure at the western part of the EQFTB, also developed beneath the Jidike Formation evaporite. Very gentle basement dip and steep dipping topographic slope in the EQFTB suggest that the Jidike Formation salt provides effective decoupling. The strong deformation in the EQFTB appears to have developed further south, in an area where evaporite may be lacking. Since the Pliocene, the EQFTB has moved farther south over the evaporite and reached the Yaken area. Restoring a balanced cross‐section suggests that the minimum shortening across the EQFTB is more than 7800 m. Assuming that this shortening occurred during the 5.3 Ma timespan, the shortening rate is approximately 1.47 mm/year.

Cretaceous‐Tertiary structural evolution of the north central Lhasa terrane, Tibet

In the north central Lhasa terrane of Tibet, two distinct structural levels of an east–west striking thrust system are exposed along the north trending late Cenozoic Xiagangjiang rift. Upper Paleozoic strata deformed by the south directed Langgadong La thrust, and Cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels, respectively. These two structural levels are separated by the Tagua Ri passive roof thrust. Balanced cross section restoration suggests that the thrust system accommodated ∼103 km (∼53%) shortening. The40Ar/39Ar results, together with an interpretation of synthrust deposition of Upper Cretaceous strata, suggest that the majority of shortening occurred during the Late Cretaceous–Paleocene. Cretaceous strata lie unconformable on Permian rocks; volcanic tuffs directly above the unconformity yield U‐Pb zircon ages of ∼131 Ma. Upper Cretaceous strata record a change from shallow marine to nonmarine deposition, indicating uplift above sea level during this time. The overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central Tibet by large‐scale northward underthrusting of Lhasa terrane basement beneath the Qiantang terrane prior to the Indo‐Asian collision. The documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of Tibet from at least the Bangong suture in the north to the Tethyan Himalaya to the south. This style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening.

Passive-roof thrust model for the emplacement of the Pelona-Orocopia Schist in southern California, United States

The early Tertiary Orocopia-Chocolate fault in southern California was originally interpreted as a southwest-directed thrust assisting tectonic underplating of the Orocopia Schist beneath North America during subduction. However, the predicted thrusting direction is opposite to that observed for the fault. Although this problem appeared to be resolved by interpreting the Orocopia-Chocolate fault as a normal fault, this alternative is inconsistent with the fact that neither an early Tertiary breakaway nor early Tertiary upper plate extension has been observed. I suggest that the fault was a northeast-directed passive-roof thrust that linked with a southwest-directed basal decollement (the Vincent thrust) during early Tertiary flat subduction of the Farallon plate beneath North America. This model may also explain a similar structural relationship between the Main Central thrust and the South Tibet detachment fault in the Himalaya.

Tectonic inheritance, crustal architecture, and contrasting structural styles in the Venezuela Andes

Surface and subsurface data image contrasting structural styles for the two flanks of the Venezuela Andes. In the north, a flexural basin developed in Neogene times between the Andes and the Lake Maracaibo. North verging thrusts are mainly detached in the pre‐Cretaceous substratum and form a deeply buried antiformal stack. Secondary décollement levels occur both in the Upper Cretaceous and Tertiary strata, accounting for the passive roof thrust of a conventional frontal triangle zone. In the south, the Barinas basin hardly compares with a flexural basin. It is largely dominated by both north and south verging basement‐involving structures. Paleogene normal faults are locally inverted, and Caribbean nappes are frequently reactivated or refolded by younger oblique Neogene Andean structures. Seismic profiles in this area also attest to the strong Neogene structural inversion of Upper Jurassic‐Lower Cretaceous grabens. Hercynian, or more likely Early Paleozoic structures account for local preexisting crustal heterogeneities, reactivated during both the Tethyan rifting and the Andean deformations. Two trans‐Andean regional profiles have been balanced and constrained by an inversion of the gravimetric data. They imply a progressive deepening of the northern Moho and a south dipping subduction of the infracontinental lithospheric mantle of the Maracaibo block. The shortening for both sections averages 60 km. Palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the Andes and minimize the possible effects of a progressive right‐lateral escape of the Maracaibo block with respect to stable South America along the Bocono Fault. Strain partitioning during Neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary, strike‐slip motion in the allochthon along the Bocono Fault, and an asymmetric subduction (wedging) at depth.

Structure and Evolution of the Northern Potwar Deformed Zone, Pakistan

The northern Potwar deformed zone (NPDZ) is part of the active foreland fold and thrust belt of the Salt Range and Potwar Plateau in northern Pakistan. About 500 km of seismic reflection profiles are integrated with surface geologic and drilling data to examine the deformation style and structure of the NPDZ with particular emphasis on history of deformation of the Dhurnal oil field. The seismic lines suggest that the overall structure of the eastern NPDZ is a duplex structure developed beneath a passive roof thrust. The roof thrust is generated from a tipline in the Miocene Murree Formation, and the sole thrust is initiated from the same Eocambrian evaporite zone that extends 80 km southward beneath the Soan syncline and Salt Range. The Dhurnal oil field structure is a pop-up at the southern margin of the NPDZ, and developed beneath the passive roof thrust. The passive roof thrust crops out just north of Dhurnal on the steep, northern limb of the Soan syncline. An overstep passive roof thrust (Sakhwal fault) is interpreted west of Dhurnal; this fault developed due to

Mari-Bugti pop-up zone in the central Sulaiman fold belt, Pakistan

The Sulaiman fold-and-thrust belt is an active tectonic feature of the Himalayan mountain system in Pakistan. Seismic reflection profiles, borehole, surface geology data and Bouguer gravity modeling suggested a ‘passive-roof duplex’ geometry over a transitional crust related to the former passive margin of the Indian subcontinent. In the frontal part of the Sulaiman fold belt, a passive-roof sequence of Cretaceous and younger rocks is structurally uplifted. At the surface, the roof sequence displays a coherent stratigraphy over the underlying duplex sequence of Jurassic and older strata. The folds in the roof sequence reflect blind faults in the duplex sequence. The duplex style of deformation persists throughout the central Sulaiman fold belt. However, unlike the frontal Sulaiman fold belt, stratigraphy at the surface in the central Sulaiman is disrupted by E-W- and NE-trending faults, with apparent map lengths of tens of kilometers. These foreland- and hinterland-verging high-angle faults juxtapose Cretaceous rocks in the cores of tight, symmetrical anticlines against Eocene Ghazij Shale and Kirthar Limestone. According to seismic reflection data, they have only minor vertical offsets of 1–2 km and are mostly restricted to the roof sequence. As a result Cretaceous rocks bounded between reverse faults are exposed at the surface in the cores of tight anticlines as pop-up structures. This implies that: (1) the exposed faults in the central Sulaiman fold belt are not primary structures with major shortening; and (2) recognition of these faults in the roof sequence may reflect an early stage of development of overstep back thrusts from the upper décollement (passive-roof thrust).

Seismic Data, Geometry, Evolution, and Shortening in the Active Sulaiman Fold-and-Thrust Belt of Pakistan, Southwest of the Himalayas

Surface and subsurface data from the Sulaiman fold-and-thrust belt are integrated to analyze the deep structure, tectonic shortening, and kinematics of the Sulaiman fold-and-thrust belt at the western margin of the Indian subcontinent. Seismic reflection data show that nearly all the 10-km-thick sequence of dominantly platform (>7 km) and molasse strata is detached at the deformation front. These strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland. A balanced structural cross-section suggests that structural uplift in the Sulaiman fold-and-thrust belt is a result of a thin-skinned, passive-roof duplex style of deformation. The duplex sequence consists of Jurassic and older strata. It is separated from the roof equence by a passive-roof thrust in thick Cretaceous shales that has a backthrust sense of displacement over foreland-propagating duplexes. The roof sequence with incipient out-of- sequence thrusting is intact for about 150 km northwards, where it emerges along a passive backthrust. Sequential restoration of the balanced section reveals a series of structural and geometrical features including: (1) development of low-amplitude, broad concentric (detachment) folds (Sui and Loti) at the tip of the decollement; (2) increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures; and (3) out-of-sequence thrusting to create required critical taper for an outward translation of the foreland fold-and-thrust belt. A balanced structural cross-section 349 km long from the Sulaiman fold-and-thrust belt restores to an original length of 727 km, suggesting a maximum of 378 km of shortening since 21 Ma in the cover strata of the Indian subcontinent. Calculation of displacement rates over the Sulaiman fold-and-thrust belt (18 mm/yr) added to the resolved rate of the Chaman fault vector for the component parallel to the plate convergence direction (15 mm/yr) are close to the current India-Asia plate convergence rate (37 mm/yr).

‘Passive-roof’ duplex geometry in the frontal structures of the Kirthar and Sulaiman mountain belts, Pakistan

Exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures. In this study we introduce and discuss the concept of the ‘Passive-roof duplex’, using as the main example the Kirthar and Sulaiman Ranges in the Baluchistan Province of Pakistan. Structures similar to those described here have been recognized previously in other mountain belts, and they appear to exist as a common feature in many more frontal regions of mountain belts. Our example of a Passive-roof duplex which we describe from Pakistan is compared briefly with similar structures reported by others. The Passive-roof duplex is here defined as a duplex whose roof thrust has backthrust sense ( Passive-roof thrust) and whose roof sequence (those rocks lying above the roof thrust) remains relatively ‘stationary’ during foreland directed piggy-back style propagation of horses within the duplex.