Imbricate Fan Research Articles

Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate

At several localities around the world, thrust belts have developed on both sides of oceanic island arcs (e.g., Java-Timor, Panama, Vanuatu, and the northeastern Caribbean). In these localities, the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt. For example, in the northeastern Caribbean, a north-verging accretionary prism lies to the north of the Eastern Greater Antilles arc (Hispaniola and Puerto Rico), whereas a south-verging thrust belt called the Muertos thrust belt lies to the south. Researchers have attributed such bivergent geometry to several processes, including: reversal of subduction polarity; subduction-driven mantle fl ow; stress transmission across the arc; gravitational spreading of the arc; and magmatic infl ation within the arc. New observations of deformational features in the Muertos thrust belt and of fault geometries produced in sandbox kinematic models, along with examination of published studies of island arcs, lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity, without subarc mantle fl ow, and without magmatic infl ation. We suggest that the Eastern Greater Antilles arc and comparable arcs are simply crustalscale bivergent (or “doubly vergent”) thrust wedges formed during unidirectional subduction. Sandbox kinematic modeling suggests, in addition, that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly. In such cases, the arc acts as a backstop that transmits compressive stress into the backarc region. Further, modeling shows that when arcs behave as rigid blocks, the strike-slip component of oblique convergence is accommodated entirely within the prowedge and the arc—the retrowedge hosts only dip-slip faulting (“frontal thrusting”). The existence of large retrowedges and the distribution of faulting in an island arc may, therefore, be evidence that the arc is relatively rigid. The rigidity of an island arc may arise from its mafi c composition and has implications for seismic-hazard analysis.

Transition from subduction to arc-continent collision: Geologic and neotectonic evolution of Savu Island, Indonesia

Geosphere, June 2009, v. 5, p. 152-171, doi:10.1130/GES00209.1, Animation 1 - Sand box experiment movie illustrating various phases of the arc-continent collision observed throughout the Banda Arc. You will need Windows Media Player or a multimedia player such as Quicktime Media Player to view this file. The model is designed to test the influence of inserting a flat-flexible backstop (forearc basement) into the accretionary wedge as observed in Savu and older parts of the Banda Orogen exposed in Timor. A stratigraphy of lime and sand is scaled to thicknesses observed for cover sequences of the Scott Plateau, while plasticine clay is used to represent rigid forearc basement. The rheological properties of these materials are well documented (i.e., Huiqi et al., 1992) and provide reasonable approximations of contrasts in mechanical strength found between Australian cover rocks and forearc basement. The model is scaled at a ratio of 1 cm = 1 km. A 1 cm layer of plasticine clay extends on top of a 3.7-cm thick section of alternating white lime and red lime/sand layers (Gondwana Sequence lower cover units of Australian continental margin). In front of the plasticine is a 1.2-cm thick section of alternating units (Australian Passive Margin Sequence of the upper cover of the Australian continental margin). Shear strengths are 161 and 176 (Pa) for the lime and sand/lime layers, respectively. The experiment and others like it produce an upper structural level of trench-ward verging imbricate thrust sheets of only the upper sections of the cover in front and above the backstop. The lower section of cover forms a duplex system of large isoclinal, recumbent folds, and out of syncline thrusts. Initially the upper thrust sheets and faults dip between 20°-30°, but with accretion of new thrust sheets to the front of the stack, older sheets are tilted progressively more arcward until they become near vertical and even slightly overturned. Maximum uplift occurs in the imbricate fan above the leading edge of the backstop. This part of the accretionary wedge would be the first to emerge above sea level and stripped by erosion, as observed in Savu. The backstop is also uplifted and folded into a concave down geometry. A highly attenuated mélange-like zone develops immediately beneath the plasticine forearc nappe due to very high shear strain. Insertion of the flat flexible backstop into the layered section produces two distinct structural domains: frontal accretion above and duplexing beneath the backstop. The same patterns of accretion and backstop deformation are found throughout the Banda arc-continent collision (Harris, 1991). Only the early stages of the model are like the Savu phase of collision. Later stages are more emblematic of the Timor region.

Geometria 3d de falhas de empurrão e dobras associadas como expressões da morfologia do descolamento basal: resultados de uma simulação computacional

D THRUST FAULT GEOMETRY AS AN EXPRESSION OF THE ASSOCIATED DETACHMENT MORPHOLOGY: RESULTS OF A COMPUTER SIMULATION Most two-dimensional fault propagation fold models are based upon the development and migration of kink bands and assume constant dips along each band, constant layer thickness and conservation of the section area. Modelling a fault-propagation fold requires the determination of the position and of the angle of each kink line that defines t he structure. Once modelling a series of dip sections of a fault propagation fold, it is possible to generate three dimensional (3 D) geometrical models by linking a large number of these sections. A computer program was created to generate 3D geometrical model s of single thrusts or imbricate fans. Using the program, a series of experiments were run to study the influence of the detachme nt morphology on geometry of the associated fault ramps. After modelling under variable conditions, following results were obtaine d: (a) the fault ramp surface is non-planar if the detachment surface is also non-planar and the ramp surface reflects all irregul arities of the detachment surface; (b) the trace of a thrust fault on a map is similar to the trace of the associated detachment surface a long a strike section; the fault trace is straight when the detachment surface is flat and is curved when the detachment surface is curved. Depressions in the detachment surface correspond to antitaxial curves (salients) in the fault surfaces. Detachment highs, in co ntrary, correspond to sintaxial curves (recesses) in the fault surfaces. The expressions of the deepest and the shallowest points of th e detachment are, respectively, the salient and the recess culminations. It is concluded that if indenters are not active during thrust nucleation and if no later deformation occurs, the 3D shape of single thrust fault and of thrust systems is determined by the morphology of the associated detachment. Simulation of the development of an imbricate fan linked to a trough-shaped detachment surface and its comparison with results of a natural example given by the Dom Bosco thrust system of the southern Quadrilatero Ferrifero, Brazil, resulted in a close correlation between the models and the Dom Bosco system. In both cases, faults exhibit pronounced curvature with increasing dips toward the hinterland. Fault traces converge in the areas of minimal depths of the detachment surface and diverge in the areas of maximal detachment depth.

A interação tectônica embasamento/cobertura em aulacógenos invertidos: um exemplo da Chapada Diamantina Ocidental

A basement-involved fold-trust belt is exposed along the western border of the Chapada Diamantina, a high plateau underlain by proterozoic units in the interior of the northern São Francisco craton. This NNW-trending and ENE-verging fold fold-thrust belt encompasses a substantial part of the eastern portion of the Paramirim aulacogen, which corresponds to a intracratonic rift basin that experienced a long history of development starting at ca. 1.75 Ga with sedimentation of Espinhaço Supergroup. After the deposition of its second major fill unit, the Neoproterozoic (Tonian-Cryogeninan) São Francisco Supergroup, the Paramirim rift experienced a vigorous positive inversion that generated the system of NNW-trending faults and folds. Aiming to contribute to better understanding of basement-involved inversion tectonics, we carried out a field based structural analysis along the western Chapada Diamantina. Our results indicated that the inversion process took place according to four coaxial phases of progressive deformation. Under an overall WNWESE oriented shortening, the first phase of deformation (D pdesc ) nucleated a thin-skinned system of ESE-verging structures, including layer controlled detachments, blind imbricate fans, strata-confined duplexes and a variety of small-scale structures. Reactivating pre-exiting structures, the subsequent deformation phases (D p1/p2 ) were responsible for the generation of basement-involved reverse faults, reverse to oblique-slip ductile shear zones and large-scale NNW-trending folds that dominate the structural picture of the western Chapada Diamantina. During the D p3 -phase trains of NNW-oriented and WSW-verging folds associated with a prominent crenulation cleavage overprint the previously mentioned structures in the high strain zones of the western border of the Chapada Diamantina.

A interação tectônica embasamento/cobertura em aulacógenos invertidos: um exemplo da Chapada Diamantina Ocidental

A basement-involved fold-trust belt is exposed along the western border of the Chapada Diamantina, a high plateau underlain by proterozoic units in the interior of the northern Sao Francisco craton. This NNW-trending and ENE-verging fold fold-thrust belt encompasses a substantial part of the eastern portion of the Paramirim aulacogen, which corresponds to a intracratonic rift basin that experienced a long history of development starting at ca. 1.75 Ga with sedimentation of Espinhaco Supergroup. After the deposition of its second major fill unit, the Neoproterozoic (Tonian-Cryogeninan) Sao Francisco Supergroup, the Paramirim rift experienced a vigorous positive inversion that generated the system of NNW-trending faults and folds. Aiming to contribute to better understanding of basement-involved inversion tectonics, we carried out a field based structural analysis along the western Chapada Diamantina. Our results indicated that the inversion process took place according to four coaxial phases of progressive deformation. Under an overall WNWESE oriented shortening, the first phase of deformation (D pdesc ) nucleated a thin-skinned system of ESE-verging structures, including layer controlled detachments, blind imbricate fans, strata-confined duplexes and a variety of small-scale structures. Reactivating pre-exiting structures, the subsequent deformation phases (D p1/p2 ) were responsible for the generation of basement-involved reverse faults, reverse to oblique-slip ductile shear zones and large-scale NNW-trending folds that dominate the structural picture of the western Chapada Diamantina. During the D p3 -phase trains of NNW-oriented and WSW-verging folds associated with a prominent crenulation cleavage overprint the previously mentioned structures in the high strain zones of the western border of the Chapada Diamantina.

Seismic images of a collision zone offshore NW Sabah/Borneo

Multichannel reflection seismic data from the southern South China Sea, refraction and gravity modelling were used to investigate the compressional sedimentary structures of the collision-prone continental margin off NW Borneo. An elongated imbricate deepwater fan, the toe Thrust Zone bounds the Northwest Borneo Trough to the southeast. The faults separating the individual imbricates cut through post-Early Miocene sediments and curve down to a carbonate platform at the top of the subsiding continental Dangerous Grounds platform that forms the major detachment surface. The age of deformation migrates outward toward the front of the wedge. We propose crustal shortening mechanisms as the main reason for the formation of the imbricate fan. At the location of the in the past defined Lower Tertiary Thrust Sheet tectonostratigraphic province a high velocity body was found but with a much smaller extend than the previously defined structure. The high velocity structure may be interpreted either as carbonates that limit the transfer of seismic energy into the sedimentary layers beneath or as Paleogene Crocker sediments dissected by remnants of a proto-South China Sea oceanic crust that were overthrust onto a southward migrating attenuated continental block of the Dangerous Grounds during plate convergence.

Slip partitioning and inclined dextral transpression along the Zagros Thrust System, Iran

Flexural duplex structures and shear zones are well developed in the Heneshk area, in southwestern Iran. The Zagros Thrust System in this area consists of eight sheets of NW-striking, NE-dipping dextral strike-slip duplexes that are linked with imbricate fans and oblique-slip thrusts. The Zagros Thrust System is a portion of the internal zone of the Zagros orogenic belt. This internal zone is characterized by penetrative plastic deformation and metamorphism. Based on the kinematic vorticity number ( W k = 0.73 ± 0.02) calculated from quartz porphyroclasts of the Goshti shear zones along the Zagros Thrust System, the estimated θ angle between the maximum instantaneous strain axis (ISA) and the transpressional zone boundary is 33°. The estimated α angle of the plate convergence or the flow apophysis of the displacement field is 25°.The presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence. The mean estimated finite deformation ( W m ) value indicates relative contributions of 47% pure shear and 53% simple shear for the deformation which involved both strike-slip and oblique slip displacements. In this inclined transpression, about 40% strike-slip partitioning was required to accommodate the finite strain and re-orientation of instantaneous strain.

Dinosaur Tectonics: A Structural Analysis of Theropod Undertracks with a Reconstruction of Theropod Walking Dynamics

A dinosaur trackway in the Middle Jurassic eolian Entrada Sandstone of southern Utah, U.S.A., exposes three undertracks that we have modeled as isolated tectonic regimes showing the development of fold‐thrust ramp systems induced by the dinosaur’s feet. The faulted and folded sequence is comparable to crustal‐scale tectonics associated with plate tectonics and foreland fold‐thrust belts. A structural analysis of the dinosaur tracks shows the timing and direction of the forces exercised on the substrate by the animal’s foot during the stride. Based on the structural analysis, we establish a scenario for foot movements and weight distribution in the feet. During the end of the weight‐bearing phase of the stride, the weight of the animal was transferred to the front of the digits, creating a rotated disk below the foot that was bounded by an extensional fault at the front and a thrust ramp toward the back. As the body accelerated, the foot was forced backward. The rotated disk was forced backward along a detachment fault that was bounded by lateral ramps. The interramp segment matches the width of the dinosaur’s foot, which created an imbricate fan thrust system that extended to the far end of the undertrack. The total length of the tectonic disturbance created by the dinosaur is up to three times that of the original footprint. Early, near‐surface cementation gave the substrate the rheological properties necessary for development of the observed structures.

Lithological control on thrust‐related deformation in the Sassa‐Guardistallo Basin (Northern Apennines hinterland, Italy)

ABSTRACTThe Sassa‐Guardistallo Basin (SGB) is located close to the Tyrrhenian Sea and represents one of the most internal Neogene–Quaternary hinterland basins of the Northern Apennines fold‐and‐thrust belt. Its sedimentary succession consists of ca. 400‐m‐thick Late Tortonian–Messinian continental – largely conglomeratic – units overstepping a mainly shaly substratum (Palombini Shales) and overlain by Late Messinian evaporites and marine to continental Pliocene–Pleistocene sediments. This stratigraphic succession can be approximated to a composite rheological multilayer that dictated the style of basin deformation. Detailed geological mapping and structural analysis revealed that basin deposits were affected by compressional deformations that can be found both at map and outcrop scales. Decametric splay thrusts emanating from the substratum–conglomerate interface locally double the continental succession and are bounded by a roof thrust along the Late Messinian evaporite décollement, defining a deformation pattern consistent with a duplex‐like structure. The time–space structural evolution of the basin inferred from the fieldwork was addressed and tested by analogue modelling that approximated the rheological stratification of the study area to a layered brittle–ductile system. The model results support the hypothesis that the evolution of the thrust system affecting the SGB started as an early floor imbricate fan thrust system that successively evolved to a duplex structure as the link thrusts propagated into the upper décollement layer that resulted from the deposition of the Late Messinian evaporites. Models display many structural features that may be compared with the natural prototype, and highlight the importance of syntectonic sedimentation in the development and evolution of tectonic structures. The results of this study retain relevant implications for the Neogene evolution of the Tyrrhenian Basin–Northern Apennines system. This study also supports that combining between field structural analyses and analogue modelling can give useful hints into the evolutionary history of tectonically complex areas.

Morphotectonics domains and structural styles in the Makran accretionary prism, offshore Iran

The Makran accretionary prism is regarded as one of the most extensive subduction complex on Earth. It provides an ideal example of accretionary prism to study processes related to subduction at plate boundaries such as frontal accretion and underplating. The rear portion of the wedge is uplifted and extended by normal faulting and ductile flow. Spectacular shale diapirs and mud volcanoes are present all along the external part of the prism and can be seen on the regional 2D-seismic section presented in this study. The Himalayan Turbidites sequence is the main detachment level for the imbricate fan and extensional faults of the Makran accretionary prism. It is also assumed to be the main source for rising shale diapirs and mud volcanoes along imbricated thrusts within the wedge. Evidence of active sediment remobilization is prevalent in the mid-slope morphotectonic province of the accretionary prism. It is proposed that the initiation of diapirism appears to be spatially coincident with the onset of underplating processes in the rear portion of the prism. The rapid uplift of the prism and the onset of extensional faults favour the extrusion of overpressured sediments and fluids/gas along thrust faults on the seaward side of the prism. The extensional faults above the deep zone of underplating have been mildly inverted implying episodic alternation of compression and extension. The geometry of the growth strata preserved in the limbs of the fault propagation folds in the outer part of the Makran accretionary prism reveals limb rotation as the main folding mechanism with uplift rate of the growing folds exceeding the sedimentation rate suggesting active thrusting at the wedge front. The growth strata patterns also indicate a synchronous activity of many of the thrusts of the imbricate fan system that exhibit a sequence of thrusting propagating toward the foreland.

Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels

Research Article| January 01, 2009 Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels David R. Montgomery; David R. Montgomery † 1Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA, Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA †E-mail: dave@ess.washington.edu Search for other works by this author on: GSW Google Scholar Sanjoy M. Som; Sanjoy M. Som 1Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA, Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar Martin P. A. Jackson; Martin P. A. Jackson 2Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78713-8924, USA Search for other works by this author on: GSW Google Scholar B. Charlotte Schreiber; B. Charlotte Schreiber 3Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar Alan R. Gillespie; Alan R. Gillespie 4Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar John B. Adams John B. Adams 5Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (1-2): 117–133. https://doi.org/10.1130/B26307.1 Article history received: 10 Aug 2007 rev-recd: 16 Feb 2008 accepted: 14 Mar 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation David R. Montgomery, Sanjoy M. Som, Martin P. A. Jackson, B. Charlotte Schreiber, Alan R. Gillespie, John B. Adams; Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels. GSA Bulletin 2009;; 121 (1-2): 117–133. doi: https://doi.org/10.1130/B26307.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A synthesis of deformation patterns within and around the Thaumasia Plateau, Mars, points to a new interpretation for regional deformation and the origin of Valles Marineris and associated outflow channels. The morphology of the Thaumasia Plateau is typical of thin-skinned deformation, akin to a “mega-slide,” in which extensional deformation in Syria Planum and Noctis Labyrinthus connects via lateral zones of transtension and strike-slip—Claritas Fossae and Valles Marineris—to a broad zone of compressional uplift and shortening defined by truncated craters and thrust faults along the Coprates Rise and Thaumasia Highlands. However, the low regional slope (~1°) results in gravitational body forces that are too small to deform the basaltic lava flows conventionally thought to compose the flanks of the Tharsis volcanic province. Instead, we conclude that geothermal heating and topographic loading of extensive buried deposits of salts and/or mixtures of salts, ice, and basaltic debris would allow for weak detachments and large-scale gravity spreading. We propose that the generally linear chasmata of Valles Marineris reflect extension, collapse, and excavation along fractures radial to Tharsis, either forming or reactivated as part of one lateral margin of the Thaumasia gravity-spreading system. The other, dextral, lateral margin is a massive splay of extensional faults forming the Claritas Fossae, which resembles a trailing extensional imbricate fan. The compressional mountain belt defined by the Coprates Rise and Thaumasia Highlands forms the toe of the “mega-slide.” Topographic observations and previous structural analyses reveal evidence for a failed volcanic plume below Syria Planum that could have provided further thermal energy and topographic potential for initiating regional deformation, either intrusively through inflation or extrusively through lava flow and/or ash fall emplacement. Higher heat flow during Noachian time, or geothermal heating due to burial by Tharsis-derived volcanic rocks, would have contributed to flow of salt deposits, as well as formation of groundwater from melting ice and dewatering of hydrous salts. We further propose that connection of overpressured groundwater from aquifers near the base of the detachment through the cryosphere to the martian surface created the outflow channels of Echus, Coprates, and Juventae chasmata at relatively uniform source elevations along the northern margin of the “mega-slide,” where regional groundwater flow would have been directed toward the surface. Our hypothesis provides a unifying framework to explain perplexing relationships between the rise of the Tharsis volcanic province, deformation of the Thaumasia Plateau, and the formation of Valles Marineris and associated outflow channels. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Neogene evolution of the Outer Latakia Basin and its extension into the Eastern Mesaoria Basin (Cyprus), Eastern Mediterranean

The Neogene stratigraphic and structural evolution of the eastern Mesaoria Basin is studied using multi-channel seismic reflection profiles from the eastern marine extension of the basin into the Outer Latakia Basin. The region includes three major tectonic elements: Troodos Larnaka culmination, Kyrenia fold/thrust belt and the intervening Mesaoria and Outer Latakia Basins. The Troodos Larnaka culmination is a large ophiolite-cored ramp anticline developed above a deep-seated S-verging thrust, complemented by a footwall imbricate thrust fan system extending into the Cyprus Basin. The Outer Latakia Basin is a large piggy-back basin, carried on the northern flank of the Troodos Larnaka culmination. The Kyrenia fold/thrust belt is developed as a S-verging trailing imbricate fan system. These three tectonic elements form part of a crustal-scale, S-directed, linked thrust system, which has a protracted tectonic history with major pulses of contraction in the Eo-Oligocene, the late Miocene and the middle Plio-Quaternary. This system extends southward well into the Cyprus Basin and probably links with the subduction zone between the African plate and the Anatolian microplate. Patterns of thrust activity in this linked thrust system exclude models of S-migration of the plate boundary from mid Tertiary to Recent. Instead, the upper plate is viewed as a broad zone of diffuse convergence, distributed across a number of active fold/thrust zones.

Salt tectonics in two convergent-margin basins of the Cyprus arc, Northeastern Mediterranean

Seismic reflection profiles from the Adana–Cilicia and Iskenderun–Latakia Basin complexes in the NE Mediterranean Sea show the presence of linked extensional–contractional fault systems, detached at the base of the Messinian evaporites. The extensional domain is developed within the thick proximal part of the Plio-Quaternary delta succession in the inner part of each basin. It is characterized by imbricate fans of listric normal faults, which delineate turtle anticlines and their intervening syncline. The faults sole into the gently southwest-dipping detachment surface. In the inner basins, salt is largely evacuated toward the outer basins, but remnants occur as rollers in the footwalls of the listric extensional faults and small subdued pillows beneath turtle anticlines. The narrow synclines between the two turtle back growth anticlines record a more complicated history of rise and fall of the salt related to switch in vergence of the extensional fault fan. A large salt wall is developed at the toe of the delta complex in both basins. Two wide peripheral sinks are present within the Plio-Quaternary units on both flanks of the salt wall. Locally small segments of salt welds are present near the base of the salt wall. The contractional domain is characterized by a relatively thin Plio-Quaternary cover overlying a relatively uniform, 500–800 ms-thick salt substrate. A series of salt-cored growth folds are commonly associated with thrusts of variable vergence. The halokinetic structures in the Cilicia and Latakia Basins display a remarkable resemblance to detached and linked systems of extensional and contractional fault fans documented in classical salt provinces on passive margins. The study area also resembles scaled experiments which document synchronous development of extensional and contractional fault systems detached in a ductile substrate. However, a number of differences also exist, which may arise from several factors, including the nature of ductility contrast between the ductile portion of the evaporite layer and the supra-salt succession, the thickness ratio of overburden to evaporite unit, and the rate of progradation and the nature of sediment distribution during the evolution of the delta complex. The special aspect of these associations of linked gravity-driven extensional and contractional salt structures is that they are developed as a self-contained system wholly situated within a Plio-Quaternary transtensional basin.

Fault zone development and strain partitioning in an extensional strike-slip duplex: A case study from the Mesozoic Atacama fault system, Northern Chile

Upper crustal strike-slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system. Detailed field mapping of the Mesozoic Atacama fault system in the Coastal Cordillera of Northern Chile documents the progressive development of second- and third-order faults forming a duplex at a dilational jog between two overstepping master faults: the sinistral strike-slip, NNW-striking, Jorgillo and Bolfin faults. These are constituted by a meter-wide core of foliated S-C ultracataclasite and cataclasite, flanked by a damage zone of protocataclasite, splay faults and veins. Lateral separation of markers along master faults is on the order of a few kilometers. Second-order, NW-striking, oblique-slip subsidiary fault zones do not show foliated ultracataclasite; lateral sinistral separations are in the range of ∼ 10 to 200 m with a relatively minor normal dip-slip component. In turn, third-order, east–west striking normal faults exhibit centimetric displacement. Oblique-slip (sinistral–normal) fault zones located at the southern termination of the Bolfin fault form a well-developed imbricate fan structure. They exhibit a relatively simple architecture of extensional and extensional-shear fractures bound by low displacement shear fractures. Kinematic analysis of fault slip data from mesoscopic faults within the duplex area, document that the NW-striking and the EW-striking faults accommodate transtension and extension, respectively. Examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure. Faults started from arrays of en echelon extensional/extensional-shear fractures that then coalesced into throughgoing strike-slip faults. Further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement. We interpret that the duplex formed by progressive linkage of horsetail-like structures at the southern tip of the Bolfin fault that joined splay faults coming from the Jorgillo and Coloso faults. The geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike-slip fault zones in the upper crust.

Early Mesozoic thrust tectonics of the northwest Zhejiang region (Southeast China)

The NW Zhejiang region of South China occupies a key tectonic position near the suture zone of the Yangtze and Cathaysian blocks and is of critical importance for the assembly of East Asia. Sedimentological and tectonic analyses indicate that the region had a SE-dipping paleoslope in the late Paleozoic to Early Triassic. A transitional sedimentary environment from deep sea to continental molasse is documented in the early Triassic– late Triassic interval. Associated structures are NW-vergent folds and thrusts that root southeastward beneath the high-grade Chencai metamorphic complex. The structural styles of this foreland fold-and-thrust belt are characterized by multifold duplexes and individual folds that are together zoned from SE to NW as follows: (1) core zone characterized by shear folds and ductile thrusts; (2) SE belt with out-of-sequence thrusting of multifold duplexes and an average shortening of 50%; (3) central belt with duplexes, imbricate fans, and an average shortening of 40%; and (4) NW belt with Jura-type folds and a shortening of ~10%. A tectonic model for the foreland fold-and-thrust belt is discussed in relation to the early Mesozoic archipelago paleogeography of South China.

Eastward extent of the Late Eocene–Early Oligocene onset of deformation across the northern Andes: constraints from the northern portion of the Eastern Cordillera fold belt, Colombia

New geologic information supports an interpretation of the initiation of tectonic uplift in the eastern margin of the Eastern Cordillera of Colombia and the onset of deformation across the northern Andes during the Late Eocene–Early Oligocene. The eastern edge of Late Eocene–Early Oligocene deformation was located at the present site of the eastern margin of the Eastern Cordillera. This uplift can be linked to tectonic events already described for the western margin of the Eastern Cordillera. On the basis of an interpretation of geologic maps, a new regional balanced cross-section across the region, seismic reflection profiles, remote sensing images, and biostratigraphic data, three major deformation events are invoked to describe the evolution of this fold belt. The first event occurred early during the Late Eocene–Early Oligocene and formed a NE-vergent imbricate fan. This imbricate fan was eroded and covered by Late Oligocene deposits, which formed a major angular unconformity. A subsequent compressional event in Miocene–Pliocene times (Andean orogeny) reactivated preexisting thrust faults and created new ones that folded those formed during the earlier event. These younger thrust faults also offset and folded the angular unconformity. Finally, a more recent tectonic event refolded the structures from the previous events. The thin-skinned thrust system of the northeastern margin of the Eastern Cordillera of Colombia can be linked to a foot wall shortcut of the inverted Servitá fault formed during the Late Eocene–Recent inversion of a Jurassic–Cretaceous extensional basin.

Geophysical interpretation and modelling of the Olary Domain under cover: A hidden inverted rift system

Interpretation and 2‐D forward modelling of aeromagnetic datasets from the Olary Domain to the north of the outcropping Kalabity Inlier, South Australia, is consistent with a buried structural architecture characterised by isolated anticlines (also referred to as growth anticlines) bounded by steeply dipping reverse faults. The isolated anticlines are interpreted to have formed by half‐graben inversion during crustal shortening associated with the ca 1600–1580 Ma Olarian Orogeny. We interpret the bounding reverse faults as reactivated high‐angle normal faults, originating from a listric extensional fault architecture. As shortening increased, ‘break‐back bypass’ and ‘short‐cut‘ thrusts developed because of buttressing of the hangingwall successions against the footwall. The resulting architecture resembles a combination of a thrust‐related imbricate fan and an accumulation of inverted basins. Using this structural architecture, synrift sediments proximal to interpreted normal faults were identified as prospective for sediment‐hosted massive sulfide Pb–Zn–Ag mineralisation.

Role of the Wan-Na fault system in the western Nansha Islands (Southern South China Sea)

Since 1987, the South China Sea Institute of Oceanology, Chinese Academy of Sciences, has acquired about 20,000 km of seismic data in Nansha (Spratly) Islands waters, southern South China Sea (SCS). Interpretation and correlation of the profiles crossing the western part of the waters, indicate that the Wan-Na Fault Zone is a dextral strike-slip system composed of the following: (1) Ranai-Sarawak strike-slip-contractional imbricate fan at its southern segment, (2) a strike-slip-extensional imbricate fan at its northern segment in the southwestern end of the Southwest subbasin (SWSB) of the South China Sea, and (3) a strike-slip-pull-apart duplex of the Wan'an basin in the central segment. The northern segment is characterized by the faults that tend to converge toward the side of the Wan-Na fault zone to the southwest and splay toward the Southwest subbasin to the northeast, down-faulted step by step. Lithospheric delamination beneath the South China-Indochina continental margins and seafloor spreading of the South China Sea led to the development of the Wan-Na fault zone as a plate-bounding tectonic element in the region. As a whole, its major activity occurred during the Eocene to Early Miocene. The strike-slip system and pull-apart duplex directly resulted in the generation of the Wan'an basin.

Numerical models for the control of inherited basin geometries on structures and emplacement of the Klippen nappe (Swiss Prealps)

Cover nappes commonly deform above a shallow basal detachment surface located above rigid crystalline basement rocks. Inherited basin geometries are presumed to control the kinematic evolution and detachment of cover nappes in an accretionary wedge. We compare results from two-dimensional finite element modelling with the structural style of a natural cover nappe, the Klippen nappe, which was detached from its basement along an evaporitic detachment horizon and thrust over a distance of roughly 100 km. First-order characteristics of the Klippen nappe paleo-basin include a complex distribution of weak detachment rocks and various changes in the sediment composition and layer thicknesses; two types of structural style can be distinguished: an imbricate fan at the rear end of the nappe (Préalpes Médianes Rigides) and a fold-dominated region at the front (Préalpes Médianes Plastiques). The model results resemble the first-order characteristics of the Klippen nappe. The model experiments suggest that the formation of imbricates and fault-related folds are controlled by the discontinuity of detachment horizons. Discontinuities are locations where thrust ramps are triggered and layer heterogeneities control the locations for the initiation of detachment folds. Basement horsts may lead to the formation of recumbent folds, which develop a mélange zone on the highly sheared inverted limb. The experiments also suggest that the thickness of the basal detachment horizon of the Médianes Plastiques decreases gradually from the foreland to the hinterland.

Late Cenozoic evolution and earthquake potential of an active listric thrust complex above the Hikurangi subduction zone, New Zealand

In the center of the frontal wedge of the Hikurangi subduction zone, New Zealand, Mahia Peninsula and its submarine continuation, Lachlan Ridge, are being uplifted and folded above an active landward- dipping thrust-fault complex that is 80 km long. High-quality marine seismic reflection profiles reveal complex deformation of a Cretaceous to Holocene sedimentary section and enable a detailed analysis of the stratigraphy, structural evolution, deformation rates, and future earthquake potential. The structural analysis is facilitated by uplifted marine terraces on Mahia Peninsula and by 14 submarine unconformities in the hanging-wall sequence, five of which are correlated across the eroded crest of Lachlan Ridge and into the footwall basin. The ages of the unconformities are determined by seismic ties to an offshore exploration well, onshore outcrops on the peninsula, and seabed samples dated by pollen, coccolith nannoflora, and foraminifera biostratigraphy. Nine regional Quaternary unconformities, which developed in response to eustatic fluctuations in sea level and are not older than ca. 1 Ma, are correlated with oxygen isotope stages in equatorial Pacific Ocean Drilling Project core 677. The ages of fault-growth strata and progressive restorations of deformed stratigraphy indicate that Lachlan Ridge developed during three phases of deformation since subduction of the Pacific plate commenced beneath the Australian plate in the early Miocene. These include an initial phase of thrust faulting, a subsequent phase dominated by extensional faulting, and the current, mainly Pleistocene to Holocene phase of structural inversion, reactivated listric thrust faulting, and folding. Early to middle Miocene thrusts in the deeper core of the complex developed out of sequence, by sequentially stepping up into the hanging-wall section, creating an imbricate fan with emerging thrust tips now buried within the forelimb basin. In the middle Miocene–early Pliocene, listric extensional faults developed in the active thrust wedge—possibly as a result of substantial relief on the subducted Pacific plate—and controlled the development of Lachlan Basin to the west of the ridge. The principal active thrust is the Lachlan fault, a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right-stepping segments. Depth-converted seismic profiles indicate that the fault dips westward at 15°–20°, 6–8 km beneath the western flank of Lachlan Ridge, and steepens to 55°–70° or even steeper in the upper 1–2 km of section beneath the eastern flank. Syninversion Pleistocene fault-growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold-limb rotation resulting from the listric-fault geometry. A geometric analysis of the fault-growth strata and deformed terraces was used to derive a maximum dip-slip displacement rate of as fast as 3.0–6.5 mm/yr. The implied shortening rate of 2.6–6.3 mm/yr represents ∼8%–20% of the total 31 mm/yr of orthogonal plate convergence across this part of the upper plate of the Hikurangi subduction zone. The top of the pre–fault-growth Paleogene section reveals as much as 5.8 ± 1.5 km of vertical separation in the north, decreasing to ∼30%–50% of this value in the south. Temporal (10 3 –10 6 yr) and spatial (10 3 –10 4 m of strike length) variations in vertical-deformation rate have occurred during the past 1 m.y.; maximum rates occurred in the Holocene and middle Quaternary. A long-term increase in vertical- separation rate on all segments during the Pleistocene largely reflects a change in thrust kinematics associated with structural inversion. The relatively greater increase in uplift rate on the northern part of the fault during the past 1 m.y. could be related to the possibility that a subducted seamount lies >10 km beneath the peninsula. Estimates of earthquake source parameters, incorporating paleoseismic uplift data from Mahia Peninsula, indicate a potential moment magnitude of up to M w 7.6–8.0 for an earthquake that ruptures all three segments of the Lachlan fault. The average recurrence interval for such events is estimated to be 615–2333 yr, which is consistent with a mean recurrence interval of 1062 yr for four late Holocene earthquakes. Thus, the uplift and folding of Mahia Peninsula and Lachlan Ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large-magnitude earthquakes.