Evolution Of Extensional Basins Research Articles

Magmatism during late Ordovician-early Silurian accretion of the Caledonides of Arctic Scandinavia: the Halti–Guolasjávri area revisited

In the Scandinavian Caledonides, evidence of syn-collisional magmatism related to extensional basin development immediately prior to late Silurian Baltica–Laurentia collision was considered restricted to exotic terranes until late Silurian ages were obtained from the Halti Igneous Complex (HIC), hosted by a thrust sheet (Corrovarre Nappe, CN) of continental affinity. Various orogenic models for the extension and magmatism, a. o. subduction flip, slab roll-back, and ridge subduction have been proposed. Crucial factors include the affinity (Baltican or exotic) of the CN, and the nature of the debated unconformity at the base of the overlying exotic Vaddas Nappe (Köli). This study reexamines a critical tectonostratigraphic section and reports U–Pb zircon ages (441–436 Ma) of palingenetic granitic dykes generated by the HIC. We reinterpret the CN as a slice of the continental margin and accreted with some other nappes of the Seve Nappe Complex (SNC) which decoupled from the continent-ocean transition (COT) at an early stage of subduction of the margin. The lower part of the bipartite Vaddas Nappe, composed of a very dense dolerite dyke swarm with screens of quartzite, marble and subordinate black schist, is reinterpreted as a continuation northwards of the uppermost nappe of the SNC in Indre Troms, derived from the Ediacaran outermost continental margin. The contact with the overlying telescoped succession of conglomerates, marbles, volcanics and turbidites of the Vaddas Nappe is tentatively interpreted as a tectonised unconformity. To a classical model of back-arc spreading outboard of Laurentia during closure of the Iapetus Ocean, we add the arrival of Baltica, with a prism of decoupled slices of the COT and exhumed/exhuming UHP nappes. The spreading centre of the basin provides a plausible setting for gabbros and pillow lavas of the Vaddas Nappe, and the shores of the basin for a Seve–Köli unconformity. Future PTt studies are required in order to be able to relate the proposed shallow, syn-exhumation HIC magmatism to the tectonometamorphic evolution of the HP–UHP nappes of the SNC.

Syn-rift to post-rift tectonic transition and drainage reorganization in continental rifting basins: Detrital zircon analysis from the Songliao Basin, NE China

Tectonic transition from a syn-rift stage to subsequent post-rift stage is an important mechanism in the evolution of extensional basins. The sedimentary infill records the crustal response to this process. We have obtained new detrital zircon U-Pb and Lu-Hf signatures from the Lower Cretaceous stratigraphic successions encompassing the commonly accepted syn- to post-rift transition boundary, the T4 unconformity, in the Songliao Basin, NE China. These constrain the Songliao Basin’s evolution from its center to distal margins, providing insights into the sediment provenance and dispersal pattern over the tectonic transition. Analysis of zircons from the syn-rift (the Shahezi and Yingcheng formations) and immediate post-rift (the Lower and Middle Denglouku Formation) stages reveals Phanerozoic age populations with positive ƐHf(t) values, which were derived from the proximal juvenile mantle-derived melt origin bedrocks of the Songliao Block. In contrast, the overlying samples from the Upper Denglouku Formation deposited in the subsequent post-rift stage contain exotic and ancient zircon populations with ages of 2.5 Ga & 1.8 Ga and complex hafnium signatures, characteristic of a mixed origin. These are interpreted to be transported from distant cratonic terranes via larger drainage networks. It is obvious that the sediment dispersal pattern switched from being a local and hydrologically closed “intraregional” pattern to a “transcontinental” pattern during the transition. The time lag between the development of the T4 unconformity and the drainage reorganization also ensures a distinguishable ∼3 Myr (106103 Ma, Late Albian) transition period of regional extent. During this transition stage, syn-rift faulting was replaced by post-rift thermal subsidence, exhibiting a uniform sag configuration. Our new findings are important for understanding other continental rift basins during syn- to post-rift transition, which often demonstrates a complex interaction between the linkage and integration of sub-basins, and the reorganization of fluvial drainages and catchment systems.

Oblique subduction and mantle flow control on upper plate deformation: 3D geodynamic modeling

Most subduction zones on Earth are oblique, i.e., the angle between the plate convergence vector and the trench notably differs from 90°. Therefore, modeling and understanding the strain partitioning in the forearc, the development of extensional basins in the back-arc region and the diachronous transition from subduction to collision require a 3D approach. Here, we assess how oblique oceanic subduction and subsequent collision and associated mantle flow around the subducted lithosphere control the thermo-mechanical evolution of active margins. We conducted a series of 3D thermo-mechanical subduction models and discuss the influence of different subduction obliquity angles, the role of mantle flow variations and their connection with sediment transport and back-arc deformation. Numerical models are complemented by scaled analogue models to visualize the mantle flow evolution. Oceanic subduction along an oblique trench results in asymmetric mantle return flow leading to the gradual decrease of the subduction obliquity angle driven by the gradual rotation of the lower plate and the along-trench variation of slab retreat. This creates laterally variable subduction velocities and slab geometries. Back-arc extension is governed by both the toroidal mantle flow along the slab edges and by the oblique subduction induced lateral mantle flow gradient. The diachronous transition from oceanic to continental subduction and collision facilitates the laterally variable trench advance and retreat and back-arc deformation. Tectonically induced lateral sediment transport in the trench and along the subduction interface decreases its strength and viscosity and can alter subduction velocities. Our model results provide critical insights into the evolution of oblique subduction and collisional systems, such as the Arabia-Eurasia convergence zone.

Coeval basin formation, plutonism and metamorphism in the Northern Tasmanides: extensional Cambro-Ordovician tectonism of the Charters Towers Province

The Charters Towers Province, of the northern Thomson Orogen, records conversion from a Neoproterozoic passive margin to a Cambrian active margin, as characteristic of the Tasmanides. The passive margin succession includes a thick metasedimentary unit derived from Mesoproterozoic rocks. The Cambrian active margin is represented by upper Cambrian–Lower Ordovician (500–460 Ma) basinal development (Seventy Mile Range Group), plutonism and metamorphism resulting from an enduring episode of arc–backarc crustal extension. Detrital zircon age spectra indicate that parts of the metamorphic basement of the Charters Towers Province (elements of the Argentine Metamorphics and Charters Towers Metamorphics) overlap in protolith age with the basal part of the Seventy Mile Range Group and thus were associated with extensional basin development. Detrital zircon age data from the extensional basin succession indicate it was derived from a far-field (Pacific-Gondwana) primary source. However, a young cluster (<510 Ma) is interpreted as reflecting a local igneous source related to active margin tectonism. Relict zircon in a tonalite phase of the Fat Hen Creek Complex suggests that active margin plutonism may have extended back to ca 530 Ma. Syntectonic plutonism in the western Charters Towers Province is dated at ca 485–480 Ma, close to timing of metamorphism (477–467 Ma) and plutonism more generally (508–455 Ma). The dominant structures in the metamorphic basement formed with gentle to subhorizontal dips and are inferred to have formed by extensional ductile deformation, while normal faulting developed at shallower depths, associated with heat advection by plutonism. Lower Silurian (Benambran) shortening, which affected metamorphic basement and extensional basin units, resulted in the dominant east–west-structural trends of the province. We consider that these trends reflect localised north–south shortening rather than rotation of the province as is consistent with the north–south paleogeographic alignment of extensional basin successions.KEY POINTSNorthern Tasmanide transition from passive to active margin tectonic mode had occurred by ca 510 Ma, perhaps as early as ca 530 Ma.Cambro-Ordovician active margin tectonism of the Charters Towers Province (northern Thomson Orogen) was characterised by crustal extension.Crustal extension resulted in the development of coeval (500–460 Ma) basin fill, granitic plutonism and metamorphism with rock assemblages as exposed across the Charters Towers Province developed at a wide range of crustal levels and expressing heterogeneous exhumation.Protoliths of metasedimentary assemblages of the Charters Towers Province include both Proterozoic passive margin successions and those emplaced as Cambrian extensional basin fill.

Constraints on Trenchward Arc Migration and Backarc Magmatism in the North Patagonian Andes in the Context of Nazca Plate Rollback

AbstractGeochemical and geochronological data reveal that late Oligocene‐early Miocene time is a break point in the evolution of Andean magmatism. The Patagonian Andes registered the onset of arc volcanism since the late Eocene forming part of the El Maitén Belt, whose development was driven by the subduction of the Farallon/Nazca plates beneath the Andean margin. During the Oligocene, the El Maitén Belt shows a change in the geochemical signature of its magmas from tholeiitic to calc‐alkaline compositions, reflecting a more mature stage in the magmatic arc evolution. Toward the early Miocene, a striking event is registered in Andean volcanic sequences as mafic tholeiitic lava flows of the El Maitén are interbedded with marine deposits, suggesting their development in the context of a fast subsiding regime. Geochemical analyses presented in this paper show that these rocks resemble enriched mid‐ocean ridge basalt‐like and ocean island basalt compositions, isotopically depleted, which strongly contrast with previous arc products. By this time, a global plate reorganization event had caused an increase in convergence rates, accelerated rollback, and a more orthogonal geometry of subduction, triggering widespread magmatism and the development of extensional basins in the overriding plate. Arc‐related volcanism during the early Miocene can be found only in the western slope of the Andes, suggesting the retreat of the volcanic front toward the trench. The proposed model highlights a strong linkage between the geochemical signature of magmatic products and changes in the subduction zone configuration and mantle dynamics during the evolution of the Patagonian Andes (41–44°S).

Tectonic controls on sediment provenance evolution in rift basins: Detrital zircon U–Pb and Hf isotope analysis from the Perth Basin, Western Australia

The role of tectonics in controlling temporal and spatial variations in sediment provenance during the evolution of extensional basins from initial rifting to continental breakup and passive margin development are not well established. We test the influence of tectonics in a rift basin that has experienced minimal uplift but significant extension throughout its history: the Perth Basin, Western Australia. We use published zircon U–Pb and Hf isotope data from basin inception through to continental drift and complement this with new data from samples deposited synchronously with the continental breakup of eastern Gondwana. Three primary source regions are inferred, namely the Archean Yilgarn Craton to the east, the Paleo- and Mesoproterozoic Albany–Fraser–Wilkes Orogen to the south and east, and the Mesoproterozoic and Ediacaran–Cambrian Pinjarra Orogen underlying the rift basin and comprising the dominant crustal components to the west and southwest. From mid-Paleozoic basin inception to Early Cretaceous breakup of eastern Gondwana, drainage in the Perth Basin was primarily north- to northwest-directed as evidenced by the dominant Mesoproterozoic detrital zircon cargo, paleodrainage patterns and paleocurrent directions. Thus, provenance was primarily parallel to the rift axis and perpendicular to the extension direction, particularly during periods of thermal subsidence. During episodes of mechanical extension, detrital zircon ages are polymodal and consistently dominated by Paleo- and Mesoproterozoic grains derived from the Albany–Fraser–Wilkes Orogen, but with significant Archean and Neoproterozoic inputs from the rift margins. It is inferred that during mechanical extension the rate of subsidence exceeded sediment supply, which generated basin-margin scarps and enhanced direct input from the rift shoulders. Detrital zircon spectra from temporally-equivalent samples at the rift margin and in the rift axis reveal that distinct sedimentary routing operated on the flanks. In summary, sediment provenance in the Perth Basin (and probably other rift basins) is tectonically controlled by: (1) extension direction, (2) episodes of mechanical extension (rift) or thermal subsidence (post-rift), and (3) proximity to rift axis or rift margin.

Pliocene–Pleistocene sedimentary development of the syntectonic Polis graben, NW Cyprus: evidence from facies analysis, nannofossil biochronology and strontium isotope dating

AbstractThe recently uplifted and exposed Pliocene and Pleistocene sedimentary infill of the neotectonic Polis graben provides an excellent opportunity to understand extensional basin development in a marine setting. Fieldwork, facies analysis and dating using nannofossils and strontium isotopes reveal how the sedimentary conditions evolved during infill of the Polis graben during Pliocene and Pleistocene time, and allow a composite succession for the depocentre to be determined for the first time. Six lithofacies are recognized in the northern Polis graben, allowing evolving palaeoenvironments to be inferred. By the end of Miocene time (Messinian) a major c. N–S-trending graben was established; extensional faulting continued during the Pliocene–Pleistocene until recent time. Post-Messinian salinity crisis deposition began with deposition of hemipelagic muds (c. 5.08–2.76 Ma), equivalent to the Nicosia Formation. This was followed by upwards incoming of repeated normal-graded bioclastic carbonates (couplets) (c. 2.76–1.6 Ma), which are interpreted as age-equivalents of the Athalassa Formation elsewhere in Cyprus. The upwards sudden facies change is explained by tectonically controlled shallowing which enabled neritic carbonate production on the basin margins. The appearance of basement-derived material (e.g. ophiolitic extrusive detritus) in the highest stratigraphic levels of the basin fill in the north (c. 1.7–1.6 Ma) reflects onset of rapid surface uplift focused on the Troodos ophiolitic massif. Overall, the syntectonic basin infill appears to document a two-stage, pulsed uplift related to early-stage collision of the African and Eurasian plates in the easternmost Mediterranean region.

Timing of sediment-hosted Cu-Ag mineralization in the Trans-Hudson orogen at Janice Lake, Wollaston Domain, Saskatchewan, Canada

The Janice Lake Cu-Ag mineralization in the Wollaston Domain of northern Saskatchewan is hosted by a metasedimentary sequence in the upper part of the Wollaston Supergroup of the Trans-Hudson orogen. The Wollaston Supergroup was deposited between ~ 2070 and 1865 Ma in a foreland basin setting constructed over Archean basement of the Hearne craton. The Trans-Hudson orogen underwent final collision and peak metamorphism at ~ 1810 Ma, during consolidation of Laurentia and its amalgamation with the Columbia supercontinent. Titanite is a common constituent of the post-peak metamorphic assemblages of Trans-Hudson lithotectonic units and accompanied disseminated sediment-hosted Cu sulfide mineralization at Janice Lake. Titanite crystals, intergrown with chalcocite over a strike-length of ~ 2 km of Cu-bearing stratigraphy, were dated by the ID-TIMS and LA-ICP-MS U-Pb methods, returning an age range from 1780 to 1760 Ma and a weighted average age of 1775 ± 10 Ma. The titanite ages effectively date the associated chalcocite-dominated sediment-hosted Cu-Ag mineralization and its formation during initial post-orogenic uplift and cooling, ~ 30 myr after peak metamorphism. The age-range and tectonic setting of the Janice Lake mineralization confirms that sediment-hosted Cu mineralization was an integral part of the metallogenic endowment of Columbia and that its emplacement coincided with the continental-scale Trans-Hudson orogeny rather than with diagenesis and extensional basin development ~ 100 myr earlier.

Geology of the Eastern Prebetic Zone at the Jumilla region (SE Iberia)

ABSTRACTThis article presents a geological map and cross-sections at 1:50,000 scale covering an area of 609 km2 of the Eastern Prebetic Zone (SE Iberia). The structure of the studied area is characterized by an NW-directed fold-and-thrust belt and inactive salt diapirs that are parallel to the ENE- to NE-regional trend of the eastern Betic Cordillera. This regional trend is locally disrupted by the NW-trending Matamoros Basin, which is flanked by the active Jumilla and La Rosa diapirs. The geological map, the cross-sections and the outcrop observations support the hypothesis that the major Mesozoic rifting phase affecting the Eastern Prebetic Zone occurred during the Upper Jurassic to Santonian times coeval to the development of extensional basins in the Western Tethyan area. The proximal part of this passive margin was subsequently incorporated into the external part of the Betic thin-skinned fold-and-thrust belt. The Upper Cretaceous to Cenozoic tectonic evolution of the study area encompassed the following stages: a Campanian to Aquitanian NW-directed contraction; a Burdigalian to upper Miocene extensional reactivation of the main subsalt faults; and a Serravallian NW-directed contractional reactivation. In this scenario, the combined effect of the previous contractional reactivation of pre-existing salt structures together with the Miocene subsalt extension triggered passive salt extrusion of the La Rosa and Jumilla diapirs.

Diagenetic controls on the evolution of fault-zone architecture and permeability structure: Implications for episodicity of fault-zone fluid transport in extensional basins

Diagenesis demonstrably changes both the mechanical and hydrologic properties of sediments. However, the effect of these changes on the distribution and types of structures that develop in fault zones over time, and their impact on fault-zone fluid flow has not previously been systematically investigated. We explored the impact of diagenesis on the mechanical and hydrologic properties of initially unlithified sand cut by a syndepositional normal fault in an extensional basin. Field, microstructural, and geochemical data document a diagenetic record of initially continuous fluid flow through a hanging-wall damage zone up to 10 m wide. In this zone, initial deformation via particulate flow is recorded by a foliation defined by a grain shape preferred orientation, preserved by subsequently precipitated, pore-filling calcite cement. A transition to episodic fluid flow is demonstrated by calcite veins that crosscut the previously cemented, foliated sandstone within relatively narrow (≤5 m from the fault core) segments of the older damage zone. Unlike the widespread record of particulate flow, veins are restricted to the vicinity of a mapped relay zone between overlapping fault segments, and an inferred, partially covered relay zone. Vein microstructures record repeated fracture opening and sealing. Veins in breccia zones have δ 13 C values as high as +6.0‰, suggesting degassing of CO 2 - and/or CH 4 -charged fluids. These data collectively suggest that relatively early damage zone cementation strengthened and stiffened the foliated damage zone, affecting the localization of brittle fractures and fluid flow in relay zones. Our results highlight systematic changes in the character and locus of deformation and fluid flow during the development of normal faults and provide a basis for predicting how these structures may act to trap or transmit fluids during the development of extensional basins.

Age and tectonic setting of the Udokan sediment-hosted copper-silver deposit, Transbaikalia, Russia

The Udokan Cu–Ag deposit in Transbaikalian Russia is one of the largest individual examples of the sediment-hosted stratiform type. Mineralization is hosted by the Udokan Complex, a ~12,000-m-thick metasedimentary sequence deposited between ~2.2 and 2.06Ga in a large intra- to peri-cratonic basin constructed over Archean basement of the Siberian craton. The basin was inverted and metamorphosed to dominantly greenschist facies at ~1.9Ga, as a consequence of the collision between the Aldan and Stanovoi domains.Titanite is a common constituent of the greenschist metamorphic assemblages of the Udokan Complex as well as being an integral part of the stratiform disseminated and associated veinlet Cu sulfide mineralization at Udokan. Titanite crystals were separated from a sample collected in the Medny area of the deposit where typical, high-grade, disseminated and veinlet chalcocite-bornite mineralization is well exposed. Titanite crystals from the disseminated and veinlet fractions were separately dated by the ID-TIMS U–Pb method, with the disseminated fraction returning a concordia age of 1895.3±9.7Ma and the veinlet fraction a concordia age of 1896.7±7.8Ma. Combination of both titanite fractions produced a concordia age of 1896.2±6.2Ma.The titanite age effectively dates the associated Cu sulfide mineralization. Since there is no evidence that titanite growth is appreciably younger than either the mineralization in the Medny area or the Udokan deposit as a whole, a ~1.9-Ga age for the stratiform Cu mineralization is considered likely. This age is undistinguishable from that of regional metamorphism during the collisional orogeny that led to final amalgamation of the Siberian craton and its incorporation into Columbia. The age also confirms Udokan as the world's oldest sediment-hosted Cu deposit and that its formation coincided with a continental-scale orogeny rather than with diagenesis and extensional basin development ~150m.y. earlier.

Eurydesma–Lyonia fauna (Early Permian) from the Itararé group, Paraná Basin (Brazil): A paleobiogeographic W–E trans-Gondwanan marine connection

Here, the biocorrelation of the marine invertebrate assemblages of the post-glacial succession in the uppermost portion of the Late Paleozoic Itararé Group (Paraná Basin, Brazil) is for the first time firmly constrained with other well-dated Gondwanan faunas. The correlation and ages of these marine assemblages are among the main controversial issues related to Brazilian Gondwana geology. In total, 118 brachiopod specimens were analyzed, and at least seven species were identified: Lyonia rochacamposi sp. nov., Langella imbituvensis (Oliveira),? Streptorhynchus sp.,? Cyrtella sp., Tomiopsis sp. cf. T. harringtoni Archbold and Thomas, Quinquenella rionegrensis (Oliveira) and Biconvexiella roxoi Oliveira. The presence of Tomiopsis sp. cf. T. harringtoni and the bivalve Atomodesma (Aphanaia) orbirugata (Harrington) in the Teixeira Soares beds plus Myonia argentinensis (Harrington) and Heteropecten paranaensis Neves et al., both of which recorded in deposits in the Teixeira Soares and Mafra beds (Butiá), suggests a biocorrelation with the Eurydesma fauna from the Bonete Formation, Sauce Grande-Colorado Basin, Argentina. Furthermore, the presence of Lyonia Archbold and Praeundulomya cf. subelongata Dickins in the Taciba Formation indicates affinities with deposits in the Lyons Group, Carnarvon Basin, and the Fossil Cliff Member, Perth Basin (Western Australia), suggesting a late Asselian–early Sakmarian age. Even more importantly, the collected data suggest the existence of an W–E trans-Gondwanan marine seaway between the Paraná (Brazil), Sauce Grande-Colorado (Argentina), Huab (Hardap shale of the Dwyka Group, Aranos area, Namibia, southwest Africa), the Carnavon (Western Australia) basins, and beyond eastward to the Cimmerian region. A V proto-rift system through two major axes of extensional basin development facilitated the W–E marine connection. Main axes formed by a north-northwesterly trending axis paralleling the future South Atlantic and a broadly north-northeasterly trending line of separation related to the future Indian Ocean. This proto-rift system coupled with a sea-level rise of at least 100m, favored the establishment of a long narrow shallow seaway, allowing the exchange of Eurydesma fauna between eastern and western Gondwana.

The multiply deformed foreland fold-thrust belt of the Balkan orogen, northern Bulgaria

The generally east-west–trending Balkan orogen (eastern Europe) consists of a northern belt of folded and thrusted Mesozoic and Cenozoic strata that forms the external fold-thrust belt of late Mesozoic and early Cenozoic age, and a southern belt that consists of deformed igneous and metamorphic rocks overprinted by Cenozoic extensional basins. Unlike most foreland fold-thrust belts, wherein deformation commonly migrates toward the foreland, the fold-thrust belt within the Balkan orogen is marginal to the Moesian Platform to the north, but was deformed in at least three events related to three different dynamic systems caused by changes in plate interactions. The earliest event of late-Early to early-Late Cretaceous deformed strata deposited within the Moesian continental margin and within a continental rifted belt containing deep-water flysch of Late Jurassic–Early Cretaceous age, a probable eastward extension of oceanic troughs from the Southern Carpathians. The shortening was a consequence of south or southwest synthetic subduction within the Vardar zone along the southern margin of the Balkan orogen. In Late Cretaceous time a backarc and/or intraarc rift zone developed along the southern margin of the fold belt, terminating shortening. The backarc and/or intraarc basin closed in Late Cretaceous–early Paleocene time, deforming the fold-thrust belt for a second time, but antithetically to north or northeast subduction in the Vardar zone. North- and northwest-vergent subduction within the Vardar zone caused magmatism, metamorphism, and deformation within the Rhodope area of southern Bulgaria south of the foreland thrust belt. In Paleogene time the southern part of the Balkan orogen became extensional with development of extensional basins and abundant magmatism due to trench rollback. The time of the final foreland fold-thrust belt deformation was late Eocene extending into Oligocene or early Miocene, contemporaneous with the extension to the south. The deformation within the fold-thrust belt was caused by a transfer of transpressional right shear within north Bulgaria and the Southern Carpathians as crustal units were translated northward west of the Moesian foreland crust and moved northeast and eastward into the eastern Carpathian west-dipping subduction zone. During the third event of deformation crustal units were molded around the Moesian foreland crust. The shortening ceased by early Miocene time and the right shear west of Moesian foreland crust was manifested by discrete right-slip faults to the present. During this third event southern Bulgaria was in an extensional regime that dominated the south- to southwest-vergent Hellenide orogen throughout the Cenozoic, thus dividing the Balkan orogen into two different deformational regions.

Tectono-sedimentary evolution of the Permian–Triassic extension event in the Zagros basin (Iran): results from analogue modelling

Since the 1970s, the largest oil and gas reservoirs have been discovered in the Permian–Early Triassic formations in Saudi Arabia. Thus, this time period is important for the discovery of new oil reserves in Iran. The Arabian passive continental margin has undergone lithospheric extension during the Permian–Triassic, which led to the formation of the Neo-Tethys. The aim of this paper is to describe the development of the continental rift basin in the Zagros region based on the tectono-sedimentological evolution. We have studied well-log data to specify the distribution of synrift deposits in the Zagros and have related this information to the modelling. Environmental changes indicated by various sedimentary sequences, from a siliciclastic basin to a carbonate platform setting, are described. The Cambrian Hormuz salt, which overlies the metamorphosed Precambrian basement, becomes effective as a basal detachment layer influencing the style of overburden deformation during the Permian–Triassic extension event. We have investigated the formation of various structures linked to the presence or absence of the Hormuz layer by analogue modelling and relating these structures to the Late Palaeozoic sedimentation. Based on results of the analogue modelling, we argue that the basal detachment layer (Hormuz series) has contributed to the various structural styles of the extensional basin development in the Fars domain and the Lorestan domain.

Ediacaran Araba Complex of Jordan

ABSTRACTThe Ediacaran Araba Complex in Jordan is defined and described for the first time in lexicon style, with an emphasis on the sedimentary, volcanic and volcaniclastic units outcropping adjacent to Wadi Araba, and from seismic and deep exploration well data. The Araba Complex ranges in age from ca. 605 to 550 Ma and comprises a major cycle of sedimentary, volcanic and volcaniclastic, and igneous rocks emplaced in an overall extensional tectonic regime that followed intrusion and amalgamation of the granitoid and metamorphic Aqaba Complex, a part of the Gondwanan Arabian-Nubian Shield (ANS; ca. 900 to 610 Ma).The Araba Complex is bounded by two major erosional unconformities, the newly defined Ediacaran Araba Unconformity (ca. 605 Ma) at its base, underlain by the Aqaba Complex, and the post-extensional, regional lower Cambrian Ram Unconformity (ca. 530 Ma) that is marked by the widespread deposition of thick alluvial and marginal-marine siliciclastics (Ram Group).Two sub-cycles can be recognised in the Araba Complex mega-cycle. The earliest (Safi Group) followed suturing and extensional rifting of the Aqaba Complex that resulted in rapid basinal subsidence and the deposition of coarse-grained, polymict conglomerates (Saramuj Formation) in predominantly proximal, but evolving to more distal, alluvial fan settings. The early extensional basin appears to have been orientated approximately north-south (depocentre to the west) and can be traced from north Sinai to Lebanon, approximately parallel to the present-day Dead Sea Transform. Rounded clasts, up to boulder-size, include a variety of local to regionally-derived basement lithologies, including granites, diorites, metamorphic rocks; doleritic and rhyolitic dyke rocks. Rapid isostatic uplift and weathering of the granitoid basement resulted in high sediment flux that kept pace with rapid basin subsidence; this, in turn, led to erosion and partial peneplanation of the hinterland ANS. Regional detrital zircon ages from the conglomerate clasts and matrix indicate age ranges from ca. 650 to 600 Ma with a minor cluster between 750 to 700 Ma, indicating mostly a local or, at least, near-field provenance. Subsequent to this early, rapid basin-fill, continued crustal extension resulted in tapping of rhyolitic and basaltic effusive volcanics and volcaniclastics (Haiyala Volcaniclastics and Museimir Effusives, ca. 598–595 Ma), including flow-banded rhyolitic lavas and air-fall tuffs, the latter deposited in a lacustrine or shallow-water environments.The second Araba sub-cycle (595–586 Ma) is characterised by renewed basinal subsidence, very low burial metamorphism to about 6 km depth, and associated stock-like intrusion of the Qunaia Monzogabbro (595 ± 2 Ma) that resulted in thermal contact metamorphism of the Saramuj conglomerate, as well as granite plutons (e.g. Feinan-Humrat intrusions) and dolerite dykes. The second cycle is characterised by renewed extension, rifting and the deposition of volcanic rocks, agglomerates (Aheimir Volcanics) and monomict conglomerates (Umm Ghaddah Formation) that were sourced, locally, from volcanic rocks on the rift margins.To the east, in the sub-surface of south-central Jordan, the early Safi sub-cycle is absent. Deep exploration wells and seismic data in the Jafr area demonstrate that the Araba Complex comprises terrestrial lavas (Ma’an Formation) with weathered soil horizons, unconformably overlying weathered Aqaba Complex granitic basement (Araba Unconformity). Seismic data in the Jafr region records the eruption of lavas in north-south trending graben and half-graben settings, and possible northwest-trending bounding faults similar to the Najd basins in Saudi Arabia. Again, in contrast to the outcrop areas to the west, the upper part of the Araba Complex, hereabouts, consists of fine-grained, in part carbonate-cemented sandstone and claystone, together with anhydrite (Jafr Formation) suggesting a shallow-marine or coastal sabkha setting, and a possible link to similar shallow-marine extensional basin-fills that developed widely within NW-trending Najd basins across the ANS in Saudi Arabia (e.g. Jibalah and Antaq basins). To date, no Ediacaran biotas have been described from the Araba Complex, but the Jafr Formation, which post-dates the appearance of soft-bodied faunas around 579 Ma, and which was probably deposited in marginal-marine environments, is a potential candidate for these enigmatic fossils.Subsequent to the final Araba extensional rifting phase, renewed regional uplift, far to the south, of the ANS hinterland during the early Cambrian, led to widespread deposition of alluvial and shallow-marine siliciclastics as a progradational ‘sand-sea’ (Ram Group) that blanketed the now peneplained Aqaba Complex in south Jordan and surrounding countries (Ram Unconformity). However, the younger Ediacaran Araba Complex outcrops adjacent to Wadi Araba remained, in places, as a relatively immature palaeotopography. It was not until early mid-Cambrian times (ca. 509 Ma), during the Burj marine transgression that this late Ediacaran palaeotopography was finally buried.The Araba Complex in Jordan with its multi-cycle development provides an insight to the regional development of Ediacaran extensional basins in the Arabian-Nubian Shield, an important phase in the evolution and transition from Neoproterozoic to Phanerozoic crustal tectonics and associated basin-fill.

Extensional basin development and subsequent inversion in Greymouth Coalfield

The validity of the history of basin development in the east of Greymouth Coalfield, as broadly accepted for the past 60 years, is examined through reinterpretation of the structure and stratigraphy. Rather than a Late Cretaceous–Paleocene fault-angle basin (extensional half-graben) west from the Mount Davy Fault Zone, a downwarping basin extended across the location of that future fault zone. This basin developed within regionally extensive deposition of Late Cretaceous sediments. Uplift and erosion in the east was followed by the development of an Eocene fault-angle basin extending from the Montgomerie Fault west across the developing Mount Davy Fault Zone. A thick Miocene sequence accumulated in the Grey Valley Trough to the east, and a thinner sequence is inferred to have covered the whole coalfield. Inversion in the coalfield, rather than being intermittent throughout the whole Palaeogene, began c. 4.5 Ma ago with development of the Brunner Anticline and reversal of movement on both the Mount Davy Fault Zone and the Montgomerie Fault.

Orogenesis without collision: Stabilizing the Terra Australis accretionary orogen, eastern Australia

The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis. At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting. The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis. Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.

Role of sedimentation during basin inversion in analogue modelling

Sand-box experiments were designed to reproduce the effect of sedimentation during extensional basin development and tectonic inversion. These experiments were motivated by growing evidence for tectonic inversion in early Miocene times of an extensional basin (Abanico Basin) in the central Chilean Andes with contemporaneous intense volcanic activity. The experiments perform an extension followed by compression. The first series of experiments were performed without sedimentation in the developing basin; the second series were performed with partial filling of the basin, and the third series with total filling of the extensional basin. The effect of sedimentation on the sides of the basin was also controlled. Compression experiments reproducing tectonic inversion of the previously developed extensional basin were also performed with and without coeval sedimentation (series with null, partial or total fill, and also external sedimentation). The analogue experiments show that high amounts of sediments accumulated during the evolution of an extensional basin enhance subsidence and lead to the development of shortcuts in the footwall of the basin bounding faults during tectonic inversion. During extension the earliest appearance of normal faults occurs in the compartment with the greater sedimentary fill, and this causes asymmetric basin development. The post-extension compressional models show that the greater burden caused by a thicker accumulation of sediments within the basin inhibits tectonic inversion, causing the development of thrust structures rooted in the basin bounding faults. The accumulated sedimentary load affects both the extension and compression phases of the experiments, but has greater influence in the case of sedimentation during extension. A higher burden on the outer edges of the basin compared to the interior enhances the reactivation of normal faults and hampers the development of new thrust structures. The thrust fault(s) originating from the most loaded basin compartment develops as a footwall shortcut from the reactivated fault. With increasing sedimentation the shortcut is developed more superficially. The shortcut absorbs less deformation than the associated main fault.

Late Palaeozoic-Early Cenozoic tectonic development of the Eastern Pontides (Artvin area), Turkey: stages of closure of Tethys along the southern margin of Eurasia

Abstract Early Carboniferous–Eocene units exposed in the Arvin area document the development of the southerly, active continental margin of Eurasia. The oldest rocks exposed in the area are Early Carboniferous granites that regionally intrude schists and gneisses. The continental terrane rifted along the entire length of the Pontides (&gt;1000 km east–west) during the Early–Middle Jurassic. Subsidence of the rift basin in the Artvin area was accompanied by terrigenous debris flows, turbidites and deep-sea radiolarian muds, and was associated with local extrusion of chemically ‘enriched’ basalts. Swarms of subduction-influenced basic, intermediate, to locally silicic dykes, intruded high-grade metamorphic basement within the rift. A basement horst within the rift was covered by condensed pink ammonite-bearing pelagic facies. Large volumes of subduction-influenced basalts erupted during the later stages of extensional basin development (Mid-Jurassic), associated with volcaniclastic sedimentation. The Artvin Basin is interpreted as a supra-subduction rift associated with incipient arc magmatism. The basin was stratigraphically inverted in response to Late Middle Jurassic ‘Neo-Cimmerian’ deformation. It was then partially eroded and covered by Upper Jurassic continental, to shallow-marine sediments, together with localized eruption of ‘enriched’ (non-subduction-influenced) basalts. The margin collapsed during the Late Jurassic–Early Cretaceous, initiating deposition of pelagic carbonates and mixed terrigenous, biogenic and volcaniclastic gravity flows. Subduction during the Late Cretaceous then constructed the east Pontide magmatic arc and a thick volcaniclastic fore-arc apron to the south. Supra-subduction-type ophiolites and accretionary melange formed within Neotethys to the south during the Late Cretaceous and were emplaced regionally northwards onto the leading edge of the Pontide active continental margin during the latest Cretaceous. Continental collision during the Mid-Eocene telescoped the distal part of the active margin which was emplaced northwards onto the east Pontide continental basement. The geological evolution of Artvin area correlates with the Pontides further west and with the southern and northern Transcaucasus to the east. Our favoured tectonic model involves long-lived, episodic, northward subduction of Tethys. Finally, there is no evidence of ‘Palaeotethyan’ ophiolites in the eastern Pontides region.

CRETACEOUS RIDGE SUBDUCTION ALONG THE LOWER YANGTZE RIVER BELT, EASTERN CHINA

The Lower Yangtze river belt is one of the most important metallogenic belts in China. The mechanisms responsible for ore genesis and the formation of related Cretaceous igneous rocks, such as adakite, A-type granitoid, and Nb-enriched basalt, remain controversial. Mesozoic granitoids in the Lower Yangtze river belt were mostly formed in the Early Cretaceous (140–125 Ma), and three granitoid belts—the inner, the south, and the north—have been defined according to petrological and geochemical characteristics. Previously, based mainly on negative eNd and high initial Sr isotope values, the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust, both of which require crustal thickening. Mesozoic crustal thickening, however, is not supported by the development of extensional basins in the region. From the Late Jurassic to Cretaceous, eastern China was closely associated with subduction of the Pacific plate in the south and the Izanagi plate in the north. The midocean ridge (MOR) between these two plates was drifting toward and likely subducting under the Lower Yangtze river belt. A ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt. Partial melting of subducting young, hot oceanic slabs close to the ridge formed adakitic rocks. The negative eNd values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle, and/or crustal materials through AFC process. A slab window opened during ridge subduction as indicated by A-type granitoids in the center of the inner belt. Nb-enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths.