Thrust kinematics and transposition fabrics from a basal detachment zone, eastern Australia

Emplacement of an upper crustal, leading imbricate-fan thrust belt in the Lachlan Fold Belt of eastern Australia was accomplished along a 0.5–1 km thick zone of heterogeneously deformed, low grade phyllonite in pelitic rock. Continuous recrystallization and neocrystallization of mica in a zone of transposition layering has provided a weak zone at the base of a 100 km wide × 150 km exposed length × 10 km thick thrust system. The basal deformation zone is characterized by a low-moderately dipping, strong-intense transposition foliation enclosing elongate fault-bounded slices (up to 20 km long × 5 km wide in map view) of disrupted Cambrian metavolcanics and Upper Ordovician black shales and slates. These are derived from a structurally lower zone of duplexing or from the overturned limbs of anticlinorial structures. The detachment zone is a 10–15 km wide zone of intense deformation showing a transition from open, upright folds with weak cleavage to inclined, tightisoclinal folds with strong axial surface cleavage. The intensity of minor faults also increases into the zone. Leading imbricate fan thrust belts show maximum deformation effects along the basal detachment which forms the frontal or leading fault. The leading imbricate geometry is due to emplacement of the basal detachment zone up the lowest and last formed imbricate thrust. Movement is along a relatively ductile, low viscosity ‘layer’ at the base where strain softening occurs with development of transposition layering. This enables confined ‘flow’ along the basal zone with transport and emplacement of the fold system and duplex zone to higher structural levels. Reaction-enhanced ductility and grain boundary sliding may be important deformation mechanisms responsible for this flow. Localized polydeformation, marked by mesofolds and crenulation cleavage, reflects the interaction between thrust sheets and the movement on faults.     

东秋里塔格构造带构造特征及油气勘探区带

东秋里塔格构造带是塔里木盆地库车坳陷的前缘构造带,整体表现为受新近系吉迪克组膏盐岩滑脱层控制的分层变形构造特征。盐上构造层主要发育断层相关褶皱,盐岩层以塑性流动变形为主,盐下构造层发育断层相关褶皱、冲起构造、逆冲断块等多种构造样式。依据盐下构造层变形特征,自西向东将东秋里塔格构造带划分为过渡构造段、叠瓦扇构造段、前端冲起构造段以及后端冲起构造段四段。过渡构造段内膏盐岩层发育层位由库姆格列木群转变为吉迪克组,盐下发育叠瓦扇构造;叠瓦扇构造段盐下发育典型的逆冲叠瓦构造样式;前端冲起构造段盐下发育断弯褶皱背斜,并在反冲断层的复合作用下发生冲起,主逆冲断层之下发育次级逆冲夹片;后端冲起构造段盐下发育断弯褶皱,并在背斜北翼发育一个独立的冲起构造。东秋里塔格构造带具备有利生储条件,构造分析表明过渡构造段、叠瓦扇构造段以及前端冲起构造段的盐下次级逆冲夹片具备勘探潜力,是有利的油气勘探区带。     

A tertiary fold and thrust belt in the Valle del Cauca Basin, Colombian Andes

Surface geology and heophysical data, supplemented by regional structural interpretations, indicate that the Valle del Cauca basin and adjacent areas in west-central Colombia form a west-vergent, basement-involved fold and thrust belt. This belt is part of a Cenozoic orogen developed along the west side of the Romeral fault system. Structural analysis and geometrical constraints show that the Mesozoic ophiolitic basement and its Cenozoic sedimentary cover are involved in a “thick-skinned” west-vergent foreland style deformation. The rocks are transported and shortened by deeply rooted thrust faults and stacked in imbricate fashion. The faults have a NE---SW regional trend, are listric in shape, developed as splay faults which are interpreted as joining a common detachment at over 10 km depth. The faults carry Paleogene sedimentary strata and Cretaceous basement rocks westward over Miocene strata of the Valle del Cauca Basin. Fold axes trend parallel or sub parallel to the thrust faults. The folds are westwardly asymmetrical with parallel to kink geometry, and are interpreted to be fault-propagation folds stacked in an imbricate thrust system. Stratigraphic evidence suggests that the Valle del Cauca basin was deformed between Oligocene and upper Miocene time. The kinematic history outlined above is consistent with an oblique convergence between the Panama and South American plates during the Cenozoic.A negative residual Bouguer anomaly of 20–70 mgls in the central part of the Valle del Cauca basin indicates that a substantial volume of low density sedimentary rocks is concealed beneath the thrust sheets exposed at the land surface. The hydrocarbon potential of the Valle del Cauca should be reevaluated in light of the structural interpretations presented in this paper.     

SHRIMP U–Pb zircon ages of pyroclastic rocks in the Bansong Group, Taebaeksan Basin, South Korea and their implication for the Mesozoic tectonics

The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two well-known orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3 ± 1.5 and 187.2 ± 1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic. Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages.The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergence developed in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks, but an “intracontinental” thrust that developed after their juxtaposition.     

Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.     

The structure and timing of lateral escape during the Scandian Orogeny: A combined strain and geochronological investigation in Finnmark, Arctic Norwegian Caledonides

Determining the timing, duration and mechanism of tectonic events within an orogenic cycle, such as ocean subduction, continent–continent collision or gravitational collapse, is challenging, especially in ancient orogenic belts. Variations in the tectonic transport direction, however, can be used as a guide to these stages of orogeny. While thrust sheets within the Caledonian allochthon in north Norway were emplaced broadly eastwards perpendicular to the trend of the orogen, many features indicate material transport in other orientations. One dominant feature of the Magerøy Nappe, sitting above and infolded with the Kalak Nappe Complex, is the development of a strong N–S lineation orthogonal to the main transport direction. Strain measurements, in part determined by a new method, are used, in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen-parallel lateral escape. Quantitative 3-D strain estimation in the Magerøy Nappe indicates prolate deformation with c. 50% horizontal shortening parallel to the thrusting direction (E–W) and c. 200% extension along the orogenic strike (N–S) with c. 30% vertical shortening. Temporal constraint on this fabric is provided by Ar–Ar isotopic analysis of undeformed white mica in cross-cutting granitic pegmatites. These data show that prolate deformation occurred before the white mica cooling age of 416 ± 4 Ma, while the previously determined depositional age of the Hellefjord Schist indicates that it occurred after 438 ± 4 Ma. A granitic pegmatite that intruded the Hellefjord Schist after an initial deformation phase but during or prior to a later deformation, has been dated at 431 ± 2 Ma by U–Pb zircon. A previous lower age constraint on this deformation of 428 ± 5 Ma is given by metamorphic zircon overgrowths on fractured grains. These results constrain the continental collision between Baltica and Laurentia in Finnmark to the interval c. 431–428 Ma. Placed in a regional context, these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent–continent collision stage in the Scandian Orogeny before gravitationally driven collapse.     

A crustal thrust system in an intracratonic tectonic environment

An intracratonic thrust belt, developed during the early Carboniferous in central Australia, deformed the Amadeus Basin and its basement, the Arunta Block. This belt is characterized by a marked structural asymmetry (vergence) and by the deposition of a thick molasse basin on the foreland. A review of existing field data shows that décollement tectonics produced folding, thrusting, faulting and back-faulting of the sedimentary sequence. Thin-skinned tectonics extend into the basement to produce recumbent folds and têtes plongeantes of nappe structures rooted in steeply dipping mylonite zones of greenschist to amphibolite grade. Minimum horizontal shortening displacements are 50–100 km resulting in a 50–70% contraction of the upper part of the basement. The structures and shortening are best explained by a crustal duplex, characterized by a crustal-scale thrust system, i.e. a sole thrust and imbricate faults, responsible for an isostatic bending of the underthrust slab. The observed Bouguer anomaly profiles support this crustal model. The dynamic evolution of this thrust belt on the scale of the crust is of thin-skin type.     

Structural style and deformation mechanism of the southern Dabashan foreland fold-and-thrust belt, central China

The Daba Mountains define the southern margin of the East Qinling orogenic belt, and form the boundary of the Sichuan basin in the north and northeast. The Daba Mountains can be divided into two structural belts by the NW-striking Chengkou fault, namely the northern Dabashan thrust-nappe belt and the southern Dabashan foreland fold-and-thrust belt. The southern Dabashan fold-and-thrust belt is a southwestward extruding thin-skinned thrust wedge, showing obvious belted change in deformation style and deformation intensity along the dip direction, and can be divided further into three sub-belts, i.e. the imbricate thrust sub-belt characterized by imbricate stepped-thrust sheets, the thrust-fold sub-belt characterized by the combination of the equally-developed thrusts and related folds, and the detachment-fold sub-belt characterized by box folds and closed overturned-isoclinal folds on the outcrops. Several kinds of structures have been recognized or inferred, including imbricate thrust system, passive-roof duplex (triangle zone), fault-related folds, back-thrust system and pop-up structure. The NE-SW compressive stress from the Qinling orogenic belt and detachment layers in the covering strata are the two most important determinants of deformation style. After the collision between the North China block and Yangtze block at the end of the Middle Triassic, the northward intracontinental subduction along the southern edge of the Qinling orogenic belt was initiated, which led to the corresponding southward thrusting in the upper crust. The thrusting propagated towards the foreland through the Jurassic and extended to the southernmost part of the southern Daba Mountains around the end of the Early Cretaceous, with thrusting deformation to be preferentially developed along major detachment layers and progressing upwards from the Lower Sinian through the Lower Cambrian and Silurian to Middle-Lower Triassic. Translated from Geotectonica et Metallogenia, 2006, 30(3): 294–304 [译自: 大地构造与成矿学]     

滇西西盟地区前泥盆纪变质岩系的变形构造格架

滇西西盟一带是保山—掸邦地块在我国境内的一个基底岩系出露地区。该地区的前泥盆纪变质岩系可划分成两个构造层，下部为元古代构造层，由变质深度达角闪岩相的怕可杂岩系组成，发育３期南北向的变形构造；上部为早古生代构造层，由低绿片岩相变质的王雅组、允沟组组成，发育两期呈南北向的变形构造。变形构造表明，西盟变质岩系的主期构造格架以怕可—老街子背形叠瓦垛为主导构造要素，由背驮式扩展的向东逆冲的盲逆冲断裂系组成，王雅—允沟反冲叠瓦扇是盲逆冲断裂系的盖层响应变形系统，并以向西逆冲的推覆构造为特征     

Transpressional imbricate thrust zones controlling gold mineralization in the Central Eastern Desert of Egypt

Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D₁–D₃). D₁ produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D₂ formed a moderately to steeply dipping, NNW-trending S₂ foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F₂ folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D₂. D₃ (560–540 Ma) formed NNE-trending S₃ crenulation cleavage, tight F₃ folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.     

Tectonic setting, evolution and orogenic gold potential of the late Mesoarchaean Mosquito Creek Basin, North Pilbara Craton, Western Australia

The geology, evolution, and metallogenic potential of the Mesoarchaean Mosquito Creek Basin remains poorly understood, despite the presence of several orogenic gold deposits. The basin is dominated by medium- to coarse-grained, poorly sorted and chemically immature sandstone and conglomerates, characterised by very high Cr/Th, high Th/Sc, and low Zr/Sc relative to average continental crust. These features are consistent with the presence of significant mafic rocks in the source terrain(s), a limited role for sediment recycling, and deposition in an increasingly distal passive margin setting on the southeastern edge of the Palaeo- to Mesoarchaean East Pilbara Terrane.New U–Pb SHRIMP data on 358 detrital zircons indicate a conservative maximum depositional age of 2972 + 14/−37 Ma (robust median; 96.1% confidence). Zircon provenance spectra from conglomeratic rocks near the base of the unit are consistent with substantial derivation from the East Pilbara Terrane, but finer-grained sandstones higher in the stratigraphy appear to have been sourced elsewhere, as their zircon age spectra are not well matched by any of the exposed Pilbara terranes.The Mosquito Creek Basin was deformed before and during collision with the northern edge of the Mesoarchaean Kurrana Terrane, which resulted in the development of macroscopic north-verging folds, thrust faulting, and widespread sub-greenschist to greenschist facies metamorphism. This collisional event probably took place at ca. 2900 Ma, based on two identical Pb–Pb model ages of 2905 ± 9 Ma from epigenetic galena associated with vein-hosted gold–antimony mineralization. The metallogenic potential of the Mosquito Creek Basin remains largely unevaluated; however, the possibility of a passive margin setting and continental basement points to relatively limited potential for the formation of major orogenic gold deposits.     

川东-鄂西郁江逆冲断层轨迹结构及扩展过程

野外地质调查和剖面测量揭示:郁江断层带的轨迹结构整体上表现为南北分段、东西分带的后展式叠瓦状逆冲断层结构;其中,底板缓倾角逆冲断层多层次(区域尺度、露头尺度和微观尺度)的断坪、断坡轨迹,特别是沿断坡方向的破裂分解,是分支断层呈上叠后展式扩展增殖的基础。根据运动学标志,郁江逆冲断层从北西向南东逆冲,最大断距小于1km,一般断距为10～100m级;后缘高倾角正断层的累计断距与前缘逆冲断距基本对应。结合区域构造分析,郁江逆冲断层轨迹结构定型于燕山期,轨迹扩展过程表现为:上叠分支断层沿底板逆冲断层的断坡方向逐次从前缘向后缘扩展增殖,属于典型的上叠后展式逆冲扩展结构;其中,后缘正断层的形成是断层轨迹结构反馈、运动调整和断层自组织行为的必然结果;而喜马拉雅主期区域应力场的激发,只是强化和加速了后缘正断层的扩展规模和进程。     

A thin-skinned thrust model for Variscan Pembrokeshire, Wales

The east and west coasts of Pembrokeshire (SW Wales) provide two sections through the Variscan fold and thrust belt. The evolution of these structures is interpreted in terms of a thin-skinned tectonic model. Balanced cross-sections are constructed for the high-level imbricate sequences, and these allow reasonably accurate estimates of shortening to be made. Basement control on structures developed in the Upper Carboniferous cover rocks is minimal, though some thrust ramp positions may be determined by the location of earlier normal faults.The thrust belt may be divided into two parts, according to the depth to the décollement horizon. In the north, imbricate fans developed from a shallow-level detachment (<1 km) which dips gently south. In the southern part, a deeper level of décollement and thicker sedimentary pile gave rise to large-amplitude folds.Shortening is heterogeneous, and both thrust periodicity and fold style are partly determined by rheology. Cumulative tectonic displacement increases to the west across Pembrokeshire, resulting in a net clockwise rotation of about 40°.     

Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc–Ontong Java Plateau convergent zone)

Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an accretionary prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide accretionary prism. We name this prism the “Malaita accretionary prism” or “MAP” after Malaita, the largest and best-studied island exposure of the accretionary prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.We divide the MAP into four distinct, folded and thrust fault-bounded structural domains interpreted to have formed by diachronous, southeast-to-northwest, and highly oblique entry of the Ontong Java Plateau into a former trench now marked by the Kia–Kaipito–Korigole (KKK) left-lateral strike-slip fault zone along the suture between the Solomon arc and the MAP. The structural style within each of the four structural domains consists of a parallel series of three to four fault propagation folds formed by the seaward propagation of thrust faults roughly parallel to sub-horizontal layering in the upper crystalline part of the OJP. Thrust fault offsets, spacing between thrusts, and the amplitude of related fault propagation folds progressively decrease to the west in the youngest zone of active MAP accretion (Choiseul structural domain). Surficial faulting and folding in the most recently deformed, northwestern domain show active accretion of greater than 1 km of sedimentary rock and 6 km, or about 20%, of the upper crystalline part of the OJP. The eastern MAP (Malaita and Ulawa domains) underwent an earlier, similar style of partial plateau accretion. A pre-late Pliocene age of accretion (3.4 Ma) is constrained by an onshore and offshore major angular unconformity separating Pliocene reefal limestone and conglomerate from folded and faulted pelagic limestone of Cretaceous to Miocene age. The lower 80% of the Ontong Java Plateau crust beneath the MAP thrust decollement appears unfaulted and unfolded and is continuous with a southwestward-dipping subducted slab of presumably denser plateau material beneath most of the MAP, and is traceable to depths >200 km in the mantle beneath the Solomon Islands.     

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex,Central Eastern Desert,Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.     

Regional structural controls of gold mineralisation,Bendigo and Castlemaine goldfields,Central Victoria,Australia

The Bendigo and Castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the Bendigo Zone of central Victoria, SE Australia. Detailed mapping and biostratigraphic interpretation has led to a better understanding of the regional structural controls of this type of gold-quartz mineralisation. Mineralised quartz veins are hosted by the Castlemaine Group, an Early-to-Middle Ordovician turbidite succession at least 3,000 m thick. Gold deposits are controlled by low-displacement faults that are clustered into several belts (the goldfields) indicating a regional structural control. The timing of mineralisation overlapped with that of the major period of deformation including folding, cleavage development and regional faulting. The Bendigo and Castlemaine goldfields are located in an area termed the Whitelaw thrust sheet bounded by two unmineralised, high-displacement, regional-scale faults. Mapping has revealed an interrelationship between the regional-scale faults, regional structural style and goldfield location. The goldfields lie immediately west of the boundary between the upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub-horizontal to synclinal enveloping surfaces, relatively low co-axial strains and moderate cleavage development. The non-gold-bearing areas immediately east of each goldfield correspond with the lower part of the Whitelaw thrust sheet and are characterised by higher non-coaxial strains, stronger cleavage and folds with wide west-dipping limbs giving rise to easterly vergent sections and steeply west-dipping enveloping surfaces. That mineralisation was an integral part of the thin-skinned style of deformation in the central Bendigo Zone is indicated by timing relationships and the interrelationship between local-scale mineralised structures and regional-scale features such as large-displacement unmineralised faults, regional variations in fold style and overall thrust sheet geometry. The work supports previous models that suggest mineralised fluids were focussed along a linked system of deep-seated faults. The primary conduits may have been major regional-scale ‘intrazone’ faults, which are inferred to sole into detachments near the base of the Castlemaine Group. It is proposed that these structures linked with minor intrazone faults and then with networks of low-displacement mineralised faults that were strongly controlled by folds. The location of minor intrazone faults was probably controlled by internal thrust sheet geometry. The distribution of gold deposits and of gold production suggests that maximum fluid flow was concentrated along the eastern margins of networks of low-displacement faults.     

Mountain building across a lithospheric boundary during arc construction: The Cretaceous Peninsular Ranges batholith in the Sierra San Pedro Martir of Baja California, Mexico

The Jura-Cretaceous Peninsular Ranges batholith (PRB) of Southern and Baja California contains a remarkable example of variation in crustal composition and structure across a batholith-parallel lithospheric-scale discontinuity. This lithospheric boundary between western oceanic-floored and eastern continental-floored crust influenced contractional deformation, arc magmatism, and differential exhumation of western and eastern zones in the batholith during its evolution.In the Sierra San Pedro Martir of Baja California, Mexico, a ca. 20 km wide, doubly vergent fan structure occurs across the PRB basement transition that consists of inward-dipping mylonite thrust sheets on the sides of the fan that gradually transition to a steeply-dipping tectonized zone in the center. A dramatic inverted metamorphic gradient occurs on the western side of this structure where mid-crustal amphibolite metamorphic grade rocks with peak pressures of 5–6 kbar in the center of the fan were thrust over upper-crustal sub-greenschist grade rocks (peak pressures < 2 kbar) in the western zone footwall. An inverted but smaller gradient occurs on the eastern side of the structure where rocks of the fan interior have been thrust eastwards over amphibolite to upper greenschist grade rocks (peak pressures 4–5 kbar).Gradients in cooling ages determined by ⁴⁰Ar/³⁹Ar biotite and K-feldspar and apatite fission track methods coupled with U–Pb zircon ages and Al in hornblende thermobarometry studies on plutons across this zone indicate that structures focused along the transition zone between contrasting lithosphere in the PRB accommodated nearly 15 km of the differential exhumation of western and eastern basement in the orogen. The western zone of the batholith was a major forearc depo-center for thick clastic sequences derived from the uplifting eastern PRB and remained at low average elevation during the Late Cretaceous and Paleogene. In contrast the eastern zone experienced dramatic uplift subsequent to achieving a crustal thickness in excess of 55 km by mid-Cretaceous time. This region had the isostatic potential for 4–5 km surface elevations, and likely formed a topographically high orogenic plateau. Exhumation of the fan structure initiated after 100 Ma and was largely complete by 85 Ma. Eastward-migrating unroofing of the rest of the eastern PRB continued into the Paleogene.A variety of factors were responsible for exhumation in this region. Structural thickening of the eastern zone of the orogen resulted from more than 30 million years of episodic contractional deformation in the fan structure, much of which followed island arc accretion of the western zone along this segment of the batholith. An episode of voluminous magmatism involving the intrusion of the 99–92 Ma La Posta-type magmatic suite across the eastern zone of the PRB triggered exhumation in the fan structure. Denudation in this region appears to have been solely by erosion; no evidence has been found for extensional tectonics during this time. This arc orogen demonstrates the important influence of inherited tectonic boundaries in controlling the spatial distribution of structural thickening and magmatism. It also displays the complex interrelationships among structural thickening, exhumation, and the role of magmatism in triggering exhumation episodes within orogens.     

Basement control on oblique thrust sheet evolution: seismic imaging of the active deformation front of the Central Andes in Bolivia

In the area of the Bolivian Orocline, we examine the deformation pattern associated with the active development of a new thrust sheet. A dense grid of reprocessed 2-D seismic lines from hydrocarbon exploration industry is interpreted and a 3-D simplified structural and kinematic model is deduced. In the Boomerang Hills, onlapping Paleozoic and foredeep sediments are detached from the underlying S-dipping basement. They are thrust northeastwards by less than 2 km. Two zones can be differentiated along the Andean deformation front: (1) a W–E to NW–SE striking frontal segment of predominantly orthogonal shortening, comprising a thrust and anticline system; (2) a WSW–ENE striking lateral zone of oblique shortening within a complex system of thin-skinned strike–slip faults and minor folds. The deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the Paleozoic basin. The observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography, which provides a near oblique ramp for the onlapping wedge of sediments. The SW–NE thrusting direction is divided into orthogonal and tangential components. These are accommodated by convergent and strike–slip structures, respectively, which sole into a common detachment horizon. The structural evolution of the new thrust sheet in the Bolivian Orocline is primarily controlled by the paleorelief of the Brazilian Shield because: (1) the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and (2) small asperities in/close to the top of the basement promote fault localization. The coincidence of a relatively high basement position and a structural high of the Eastern Cordillera leads to the conclusion that the shape of the Brazilian Shield also controls the structural evolution of the pronounced eastern border of the Bolivian Orocline.     

南大巴山前陆冲断带构造样式及变形机制分析

大巴山构造带位于秦岭造山带和四川盆地的过渡部位,形成于印支-燕山期,定型于喜山期。按照构造变形样式及其组合特征,从北东向南西可依次划分为北大巴山逆冲推覆构造带、南大巴山前陆褶皱-冲断带(又包括叠瓦断层带、断层-褶皱带和滑脱褶皱带等3个亚带)和四川盆地东北部低缓构造区等3个构造带(区)。南大巴山冲断带地表构造以类侏罗山式褶皱为显著特征,主要发育叠瓦断层系、断层相关褶皱、被动顶板双重构造、反冲断层系和冲起构造等变形样式。北东-南西向挤压应力和滑脱层是控制南大巴山及其前缘构造变形的主要因素,结合区域地质研究成果,建立了南大巴山及其前缘地区依次从震旦系-下寒武统-志留系-中下三叠统逐渐抬高的多层次滑脱前展模式。