Neogene Collision Research Articles

Upper Triassic carbonate-platform facies, Timor-Leste: Foraminiferal indices and regional tectonostratigraphic association

Timor is part of the non-volcanic Outer Banda Arc with chaotic geology in the late Neogene collision zone between Sundaland (Southeast Eurasia) and the Australian continent. Confusion in the distinction between Triassic, Jurassic and Cenozoic shallow-marine carbonate units has led to a lack of appreciation of the widespread extent of Triassic carbonate-platform facies, at least in Timor-Leste. It also resulted in misunderstanding the different tectonostratigraphic/palaeogeographic affinities of the shallow-water limestones. Foraminiferal assemblages are critical in stratigraphic discrimination of these units. This study records for the first time the foraminiferal microfauna of Upper Triassic shallow-water carbonate deposits in Timor. Many of the foraminiferal species are known from North Africa and Europe and date these units to the Carnian–Rhaetian. The Upper Triassic Bandeira Group represents extensive sheet-like carbonate-platform deposits (possibly on horst plateaus as well as shelf areas adjacent land in the intracratonic sea). Contacts between the hard limestone, forming fatus (limestone peaks) and ridges, and friable mud-dominated surrounding units are often obscured under deep tropical soil cover. At all localities the Bandeira Group is closely associated in outcrop with coeval basinal mud-dominated deposits (Babulu and Aitutu groups). With these it forms part of the East Gondwana Interior Rift Association (EGIRA), similar to the classical Dachstein Platform–Hallstatt Basin facies associations found in the European Alps. The basinal units contain turbiditic and debris-slide deposits that include clasts derived from the carbonate platform. Shallow-water carbonate facies attributed to EGIRA, are present throughout the East Gondwana Interior Rift from the Papuan Basin in the north to Exmouth Plateau, part of the Northern Carnarvon Basin, in the south. These were deposited before Gondwanan breakup along this rift system. The Triassic carbonate-platform deposits have a different dominant biogenic mineralogy (aragonite rather than calcite) and a different architecture to the bryozoan-crinoidal carbonate mounds present in the Permian of EGIRA in Timor-Leste. No Lower and Middle Triassic shallow-water carbonate-platform deposits have been recognized in Timor-Leste. The Bandeira Group, confined to the Upper Triassic, belongs to a stratigraphic association (EGIRA; autochthonous) different both in constituent units and outcrop coverage from that of the allochthonous Overthrust Terrane Association to which the Lower Jurassic carbonate-platform deposits (viz. Perdido Group) belong. The Bandeira Group has been confused not only with the Perdido Group but also with other shallow-water limestone units of the Permian and Miocene in some former studies of Timor. The distinction between these units solves a major tectonostratigraphic problem in Timor, and may apply elsewhere along the Outer Banda Arc.

Petrogeochemical features of the Neogene collision volcanism of the Lesser Caucasus (Azerbaijan)

The article is devoted to the petrogeochemical features of Neogene collision volcanism in the central part of the Lesser Caucasus within Azerbaijan. The main goal of the study is to determine the thermodynamic conditions for the formation of Neogene volcanism in the central part of the Lesser Caucasus using the available petrogeochemical material. Using factor analysis, as well as the “IGPET”, “MINPET”, “Petrolog-3” programs, material balance calculations were performed that simulate the phenocryst fractionation process, the crystallization temperature, pressure, and figurative nature of the rock-forming minerals of the formation rocks were calculated. It was determined that at the early and middle stages of crystallization of the rocks of the andesite-dacite-rhyolite formation, the fractionation of amphibole played an important role in the formation of subsequent differentiates. Based on computer simulation, it was revealed that rocks of the andesite-dacite-rhyolite formation were formed by fractional crystallization of the initial high-alumina basaltic magma of high alkalinity in the intermediate magma foci. The calculations of the balance of the substance, simulating the process of fractionation of phenocrysts, as well as magnetite, confirmed the possibility of obtaining rock compositions from andesites to rhyolites as a result of this process. In this case, the process of crystallization differentiation was accompanied by processes of contamination, hybridism and mixing. Based on the geochemical features of rare and rare-earth elements, changes in their ratios, the nature of the mantle source and the type of fractionation process are determined. It was revealed that the enrichment of formation rocks by light rare earths, as well as by many incoherent elements, is associated with the evolution of enriched mantle material. Under high water pressure, as a result of the fractionation of olivine and pyroxene, high-alumina basalts are formed from primary high-magnesian magma, which can be considered parental magma. It was established that, in contrast to the elevated Transcaucasian zone in the more lowered East Caucasus, under conditions of increased fluid pressure and reduced temperature, the melt underwent fractional crystallization in the intermediate centers, being enriched with alkaline, large-ion lithophilic elements, light REEs, etc. This is evidenced by the presence of large crystals of feldspars, the contamination of these minerals by numerous crystals of biotite, magnetite, several generations of these minerals, zonality, as well as the presence of related “water” inclusions, such as hornblendites, hornblende gabbro, etc. The physicochemical conditions for the formation of Neogene volcanic rocks of the Lesser Caucasus are determined.

Late Cretaceous to Oligocene burial and collision in western Papua New Guinea: Indications from low-temperature thermochronology and thermal modelling

Recent field and analytical work have considerably improved our understanding of the structure of the Muller Range along the frontal trend of the Papuan Fold and Thrust Belt (PFTB) in western Papua New Guinea (PNG). However numerous questions remain unresolved concerning the geological evolution of the region. In particular, the Late Cretaceous to Oligocene history of the region is largely unknown due to the absence of a continuous stratigraphic record. Here, we use both new and existing low-temperature thermochronology data to investigate the geological history of the Muller Range. Thermal history models based on these data suggest two major Cenozoic cooling episodes. The youngest, and best constrained, is clearly recorded in the stratigraphic record and relates to Neogene collision at the northern margin of the Australian continent. An older episode of comparable or greater magnitude occurred in the Eocene to Oligocene and may relate to the removal of 1500–3000 m of Late Cretaceous to Eocene section across the Muller Range prior to the widespread deposition of the shelfal Darai Limestone. We suggest that extension along major faults beneath the Muller Range accommodated sedimentation from the Late Cretaceous to the Eocene, consistent with long-lived extensional structures observed in neighbouring regions. In contrast to the Muller Range area, an almost continuous Late Cretaceous to Eocene stratigraphic sequence is preserved in the hinterland <50 km to the northeast. The selective removal of this sequence across the Muller Range suggests it was uplifted in the Eocene to Oligocene, possibly in part facilitated by the inversion of extensional faults in the Muller Range area. We suggest that this inversion was related to the Eocene to Oligocene collision of the expansive Sepik Terrane to the northwest of the PNG margin. The new data and interpretations presented here have significant implications for the evolution of the PFTB and for tectonic reconstructions of PNG.

Basin Evolution Palispatic Model of Bonaparte Basin, Australia Northwest Shelf

This research area is located on the Australian NW Shelf close to the westernedge of the Sahul Platform. This research is aimed to generate the palispatic basin model of Bonaparte Basin, Australian Northwest Shelf. It is to predict the impact of Neogene collision on the petroleum system distribution on Australian Northwest Shelf. The main data used in this research are seismic data using qualitative method analysis. The well data is used to well-seismic tied. After data acquisition, the seismic data are interpreted based on the horizon and structure interpretation. These interpretation are to reconstruct the basin evolution thorough geologic time. According to data analysis, the basin evolution palispatic model are divided into Paleo-proterozoic, Paleozoic, Triassic, Early Jurassic, Middle Jurassic, Late Jurassic, Early Cretaceous, Late Cretaceous, Early Eocene, Late Miocene and Recent condition. Regional tectonically there are at least three important events in NW Shelf: Middle Triassic-Jurassic NNE–SSW extension phase, Late Jurassic NE–SW extension phase and the Neogen collision phase; the Neogen collision effects on Northwest Shelf Australia. These three events contributed in forming and disturbing the Paleozoic and Mesozoic petroleum system in Bonaparte basin especially.

New fission-track age constraints on the exhumation of the central Santander Massif: Implications for the tectonic evolution of the Northern Andes, Colombia

The Late Cretaceous to late Neogene exhumation history of the central Santander Massif in the Northern Andes of Colombia is controlled by the geodynamic interactions between the Caribbean, South American and Nazca plates, as well as the Neogene collision and accretion of the Panama arc. Slab-breakoff of the Caribbean plate, with the tip of the slab tear presently being located beneath Bucaramanga, and the east-west oriented Caldas tear are the main structures relating seismic activity and Late Miocene to Pleistocene magmatic/hydrothermal activity and associated gold mineralization in the central Santander Massif.Here we present new apatite (AFT) and zircon fission-track (ZFT) data from 18 samples collected along two profiles in the California–Vetas block (including the Rio Charta), to the south of the Rio Charta fault, and from Bucaramanga to Picacho on the western flank of the central Santander Massif. The fission-track data are used for time–temperature history modelling and for estimating long-term average exhumation rates. The California–Vetas block in the central Santander Massif to the north of the Rio Charta fault cooled rapidly at a rate of about 24°C/Myr between 10 and 5Ma. Fast cooling was not related to post-magmatic cooling or hydrothermal activity, but rather to exhumation, with rates based on apatite fission-track cooling ages on the order of 0.3–0.4km/Myr. However, long-term average exhumation rates since the Late Cretaceous, based on zircon fission-track data, were only on the order of 0.1–0.2km/Myr. Our data indicate that next to the Rio Charta fault also the Surata fault contributed to the exhumation of the California–Vetas block.The western flank of the central Santander Massif, shows a more complete thermal history along the Bucaramanga–Picacho profile, with the exposure of an exhumed zircon fission-track partial annealing zone. Thermal history modelling of zircon fission-track data of this profile shows that after burial and heating from about 150Ma on cooling at rates of 7–10°C/Myr started at about 25Ma. For the lower part of the profile, the early Miocene ZFT ages indicate exhumation at rates of 0.3–0.5km/Myr along the Bucaramanga fault, but were only about 0.1km/Myr on the high plateau of the central Santander Massif.

U-Pb zircon geochronology of rocks from west Central Sulawesi, Indonesia: Extension-related metamorphism and magmatism during the early stages of mountain building

Sulawesi has generally been interpreted as the product of convergence in the Cretaceous and Cenozoic, and high mountains in west Central Sulawesi have been considered the product of magmatism and metamorphism related to Neogene collision. New SHRIMP and LA-ICP-MS U-Pb zircon dating of metamorphic and granitoid rocks has identified protoliths and sources of melts, and indicates an important role for extension. Schists, gneisses and granitoids have inherited Proterozoic, Paleozoic, Mesozoic and Paleogene zircons. Mesoproterozoic and Triassic age populations are similar to those from the Bird’s Head region. Their protoliths included sediments and granitoids interpreted as part of an Australian-origin block. We suggest this rifted from the Australian margin of Gondwana in the Jurassic and accreted to Sundaland to form NW Sulawesi in the Late Cretaceous. Some metamorphic rocks have Cretaceous and/or Late Eocene magmatic zircons indicating metamorphism cannot be older than Late Eocene, and were not Australian-origin basement. Instead, they were metamorphosed in the Neogene after Sula Spur collision and subsequent major extension. Associated magmatism in west Central Sulawesi produced a K-rich shoshonitic (HK) suite in the Middle Miocene to Early Pliocene. A later episode of magmatism in the Late Miocene to Pliocene formed mainly shoshonitic to high-K calc-alkaline (CAK) rocks. I-type and silica-rich I-type granitoids and diorites of the CAK suite record a widespread short interval of magmatism between 8.5 and 4Ma. Inherited zircon ages indicate the I-type CAK rocks were the product of partial melting of the HK suite. S-type CAK magmatism between c. 5 and 2.5Ma and zircon rim ages from gneisses record contemporaneous metamorphism that accompanied extension. Despite its position in a convergent setting in Indonesia, NW Sulawesi illustrates the importance of melting and metamorphism in an extensional setting during the early stages of mountain building.

Hf and O isotopic evidence for metamorphic crystallization of zircon during contact metamorphism of Fenniaolin metabasalts, Tananao complex, Taiwan

Abstract The eHf and δ18O compositions of zircons separated from an amphibolite in the Tananao complex of northeast Taiwan have been analyzed in order to test the hypothesis that these zircons are metamorphic in origin. A previous study found the U–Pb age of 18 zircons to be 86 ± 1 Ma. The 176Hf/177Hf ratio of 13 of these zircons is rather constant with an average value of 0.283263 ± 0.000037 (2σ SD) and a weighted mean of initial eHf(t) value of + 19.2. The δ18O compositions of these same 13 grains and of 7 other grains vary from + 1.2 to + 2.0‰ with a weighted mean of + 1.5 ± 0.43‰ (2σ SD). The very positive eHf(t) values show that the protolith of this amphibolite had a primitive composition, consistent with N-MORB. The low δ18O values are not consistent with typical mantle values of ~ 5.3; rather we interpreted them to reflect metamorphic growth of zircon at greenschist facies conditions in a basalt already hydrothermally altered by seawater. In keeping with other studies, we conclude [1] that the host amphibolite was basalt of MORB composition extensively altered by seawater shortly after extrusion, [2] that the crystallization of the metamorphic zircons was caused by contact metamorphism of the host amphibolite by the 87 Ma intrusion of the adjacent granodiorite, and [3] that this metamorphism was not caused by the Neogene collision of the Philippine Sea plate with the south China margin.

Formation of a metamorphic complex along an obliquely convergent margin: Structural and thermochronological evolution of the Chugach Metamorphic Complex, southern Alaska

This study documents the structural and metamorphic evolution of middle to lower crust along an oblique convergent curved continental margin during a time span of ∼60 Myr. Our study documents the importance of variable obliquity during convergence which led to the development of overprinting fabrics and distinct exhumation histories along strike of the margin. We present structural analyses,40Ar/39Ar, Rb/Sr, and zircon fission track ages from middle to lower crust exposed along the southern Alaskan margin in the Chugach Metamorphic Complex. Together with the metamorphic history and additional geochronology from the literature we derive the following tectonic evolution for this area: accretion of sediments during dextrally oblique convergence led to strain‐partitioned D1structures and greenschist‐facies metamorphism prior to circa 55 Ma. At ∼55–51 Ma, a margin‐parallel stretching phase with vertical thinning (D2) affected the margin and led to andalusite‐sillimanite grade metamorphism and the onset of partial melting. A switch back to dextral transpression (D3) shortly after D2led to rapid cooling of the western and central parts of the complex associated with exhumation of parts of the core of the complex until circa 46 Ma. The southeastern part of the complex cooled and exhumed regularly and slowly until circa 5 Ma due to its highly oblique orientation relative to the convergence direction. An increase in cooling and exhumation occurred after circa 5 Ma in the entire southeastern part of the complex, associated with the Neogene collision of the Yakutat terrane.

The Euphrates volcanic field, northeastern Syria: petrogenesis of Cenozoic basanites and alkali basalts

AbstractThe Plio-Quaternary Euphrates volcanic field of NE Syria includes large discontinuous exposures of basanitic and basaltic lava flows (1200 km2in area). It represents the northern segment of the Cenozoic volcanic province of the Middle East and is located near the Bitlis collision suture. The rocks consist of olivine (15–20%), clinopyroxene (30–35%), plagioclase (45–55%) and opaque phases. Chemically, the rocks are largely ultrabasic (SiO238.2–45.5 wt%, MgO 8.7–13.0 wt% and average Mg number of 0.65). They are enriched in incompatible trace elements such as Zr (133–276 ppm), Nb (25–71 ppm) and Y (17–28 ppm). The REE patterns are strongly fractionated ((La/Yb)N= 19.6), indicative of a garnet-bearing source. The143Nd/144Nd isotopic compositions range from 0.512868 to 0.512940 (εNd= 4.5 to 5.9), and87Sr/86Sr from 0.70309 to 0.70352. These chemical and isotopic compositions reflect strong affinities to OIB. Elemental ratios such as K/P (3.4), La/Ta (13) and La/Nb (0.77), and the low SiO2values, suggest that the Euphrates magma was subjected to minimal crustal contamination. Petrogenetic modelling has been carried out using a variety of mantle source materials, different degrees of partial melting (0.1 to 10%), and a number of scenarios including metasomatized sources. Modelling suggests that the magma could have been produced as a result of a small degree of partial melting of either (1) a garnet-bearing depleted source enriched with a small addition of metasomatizing fluids, or (2) a garnet-bearing fertile source. The overall chemical and petrological characteristics are more consistent with the generation of the Euphrates magma by a small degree of partial melting (F = 1%) of a primitive, garnet-lherzolite mantle source, possibly containing a minor spinel component. The Neogene collision of the Arabian plate with Eurasia along the Bitlis suture resulted in reactivation (beneath the Euphrates basin) of deep-seated fractures, along which lavas may have penetrated the crust.

Elastic flexure and distributed deformation along Australia's North West Shelf: Neogene tectonics of the Bonaparte and Browse basins

Abstract Neogene collision between Australia and the Banda Arc modified the adjacent Bonaparte and Browse basins of the North West Shelf of Australia. Modification comprised both continuous long-wavelength amplification of Permo-Carboniferous basement topography and flexure and normal faulting of Triassic–Recent sedimentary cover. Deformation was continuous across the Browse and Bonaparte basins, despite the basins beings separated by a rupture-barrier style accommodation zone, the Browse–Bonaparte Transition. The degree of basement control and mechanisms of fault linkage vary significantly across this transition, and reflect differences in the structural relief, amplitude and depth of rifted basement either side of the transition. Neogene collision amplified the architectural divide. Amplification of basement topography over wavelengths of several hundred kilometres was associated with negligible horizontal length change. The transcurrent component of Neogene deformation was partitioned outboard of any continuous flexural amplification.

Tectonic modification of the Australian North‐West Shelf: episodic rejuvenation of long‐lived basin divisions

AbstractNeogene collision between Australia and the Banda Arc modified two adjacent depocentres within Australia's North‐West Shelf, the Browse and Bonaparte Basins. We identify two components of this modification: (1) continuous long‐wavelength amplification of Permo‐Carboniferous basement topography, and (2) flexure and normal faulting of Triassic–Recent sedimentary cover. Although this deformation was continuous across the Browse and Bonaparte Basins, the degree of basement architectural control, mechanisms of fault linkage and distribution of syntectonic accommodation space varied significantly between the two basins. These variations reflect fundamental differences in the structural relief, amplitude and depth of rifted basement on either side of a rupture‐barrier‐style accommodation zone, the Browse/Bonaparte Transition. This long‐lived architectural divide, of which there is no discrete structural expression, was amplified by Neogene collision. We examine tectonic rejuvenation of the Browse/Bonaparte Transition and describe a mechanism for actively sustaining long‐lived segmentation of the continental shelf.

POST–EARLY CARBONIFEROUS THERMAL HISTORY RECONSTRUCTION FROM WELL DATA IN THE AMADEUS BASIN, CENTRAL AUSTRALIA

Apatite Fission Track Analysis (AFTA) combined with maturity data has revealed that four (possibly five) cooling events affected the northern margin of the Amadeus Basin since the Early Carboniferous. A consistent regional thermal history framework is developed, with recognition of cooling events beginning in the Carboniferous/Early Permian (between ~360 and 290 Ma), the Early Jurassic (~200 Ma), Late Cretaceous (between ~80 and 70 Ma) and Tertiary (between ~ 25 and 20 Ma). We suggest the first of these reflects uplift and erosion associated with the Alice Springs Orogeny, while Early Jurassic cooling reflects uplift and erosion associated with the Fitzroy Movement. Uplift and erosion in the Late Cretaceous probably relates to the breakup of Australia and Antarctica (opening of the Tasman Sea) at about this time. Later uplift and erosion in the Miocene may reflect Neogene collision of the Australian and SE Asian Plates in the region of the Banda Arc.In Tyler–1 (northern Amadeus Basin), maturation modelling using paleotemperature constraints from AFTA and VR equivalent data suggests the main source rock horizon (Ordovician Horn Valley Siltstone), went through the dry gas window during burial associated with the latter stages of the Alice Springs Orogeny. Basinward (south) of this foreland wedge, the influence of Devonian-Carboniferous loading decreases enabling oil expulsion from the Horn Valley Siltstone to have charged the Mereenie Structure. This was later partially displaced by gas.To date the Neoproterozoic sequences have yielded only dry gas (at Dingo field, Ooraminna–1 and Magee–1) which could be due to original source rock characteristics, but more likely to high maturity levels in the main depocentres. Previous notions that the Ordovician petroleum system was probably only active in the northern portion of the basin appear correct, but gas charged traps at the level of the Neoproterozoic should be ubiquitous.

Early-stage orogenesis in the Timor Sea region, NW Australia

Neogene collision between the Australian, Eurasia and Pacific plates was coeval with the growth of major depocentres in the Timor Sea. Seismic cross-sections across these troughs identify a characteristic architecture: all three lie inboard of diffuse horst-like structural highs and all mimic older depressions in the top-Permian and upper-Jurassic surfaces. Distortion of pre-tectonic (Aptian–Oligo-Miocene) sequences indicates apparent trough subsidence was coupled to uplift of outboard highs. Rudimentary cross-section restoration identifies negligible strike-normal length change. We interpret apparent Neogene subsidence to reflect amplification of basement topography. This was not accommodated by discrete structural inversion. Rather, landward propagation of contractional strain caused continuous vertical amplification of basement topography. At shallow levels, normal faulting accommodated flexure and thin-skinned collapse of detached sedimentary cover, permitting it to shed away from structural highs and pond in adjacent troughs. We infer that shortening of the North West Shelf accommodated oblique convergence between Australia and the Eurasia–Pacific arc and speculate that the transcurrent component of this deformation was partitioned well outboard. Neogene modification is interpreted to reflect the earliest stages of collisional orogenesis.

HYDROCARBON CHARGE HISTORY OF THE NORTHERN LONDONDERRY HIGH: IMPLICATIONS FOR TRAP INTEGRITY AND FUTURE PROSPECTIVITY

Re-appraisal of the oil charge history of the northern Londonderry High has identified numerous palaeo-oil columns of up to 80 m in height. An integration of the oil charge history, stress field analysis and contemporary seepage data allows a subdivision of the well results into three distinct provinces. These each have distinct charge histories that reflect differences in potential source kitchens and all have been adversely affected by the Neogene collision of the Australian and Southeast Asian plates. Traps located on the northern and northeastern Londonderry High have experienced high oil charge rates at the Mesozoic level, with nearly all valid traps showing evidence of prior oil accumulation. Breaching of these oil columns in the Neogene appears to be related to the orientation of the contemporary stress field, which promotes shear failure on the faults reliant for seal. Present day hydrocarbon migration indicators, such as Synthetic Aperture Radar (SAR) data show differences in seepage response between the northern and northeastern Londonderry High, with prolific current day seepage restricted to the northern province. Rapid subsidence associated with plate collision has accelerated maturation in the northern province to create these strong seepage anomalies over this region. The absence of seepage over the breached oil columns of the northeastern province indicates that either, oil charge has ceased to this area or that hydrocarbon leakage is episodic in nature.In contrast, results from the northwestern province show no evidence of prior oil accumulation, despite many wells having tested valid traps. These data point to either a lack of connected oil migration pathways or an impoverished source kitchen for liquid hydrocarbons. Low levels of seepage in the northwestern Londonderry High detected by the SAR data are minor compared with other parts of the Timor Sea and consistent with migration continuing at the current day. The overall prospectivity for fault bound traps in the study area appears to be low, due to extensive fault reactivation producing low fault seal integrity. Stratigraphic plays that do not rely on faults for seal, particularly in the northern and northeastern provinces, represent an alternative play concept at the Jurassic level. At shallower levels in the Cretaceous, subtle four-way dip closed structures are often enhanced by the reactivation process and could be ideally positioned to receive remigrated oil from breached Jurassic oil accumulations.

Extension during late Neogene collision in east Indonesia and New Guinea

Extension during late Neogene collision in east Indonesia and New Guinea

Ontong Java Plateau and late Neogene changes in Pacific plate motion

Aided by a new geometrical technique called hot spotting, we recently proposed a new model for the absolute motion of the Pacific plate. This model, called WK97, assumed the Hawaiian hotspot is under Loihi and that there was a change in plate motion at 3–6 Ma. The model located the Louisville hotspot close to the Hollister Ridge, a shallow volcanic ridge associated with a geoid high, seismicity, and recent volcanism. Most of the tectonic implications of WK97, however, were not addressed. Furthermore, although several recent studies suggest that the geochemistry of Hollister lavas may indicate a mixture between Louisville melt material and Pacific‐Antarctic Ridge mid‐ocean ridge basalt, these studies strongly disagree with WK97 and consider it seriously flawed. Here we discuss numerous tectonic predictions of WK97 and present a large amount of observational evidence in support of it. We find that a late Neogene collision between the Ontong Java Plateau and the northern margin of the Australia plate appears to have altered the motion of the Pacific plate, as inferred from hotspot volcanism, by forcing it to rotate counter‐clockwise. This rotation, about a low‐latitude pole, appears to have induced right‐lateral shear stress along the entire Pacific plate divergent boundary, which resulted in the formation of extensional transform faults, microplates, and propagating ridges. This change in motion also appears to have triggered concomitant circum‐Pacific tectonism, including trench migration and back arc rifting. The WK97 suggests that long‐lived, plume‐fed volcanism on the Pacific plate appears very limited, perhaps only to five to six hotspots: Hawaii, Louisville, Cobb, Caroline, and perhaps Marquesas and Bowie; the geometry of these chains are consistent with our model. Despite some evidence for volcanic age progressions, most other Pacific Neogene hotspots are most likely “crack spots”, i.e., sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses. Indeed, several volcanic ridges, such as Hollister Ridge near the Louisville hotspot, appear to have formed in response to stresses following the change in absolute plate motion. WK97 also obviates the need for simultaneous, different directions of mantle flow in the south central Pacific. Transpression at the San Andreas and Alpine strike‐slip faults and Aleutian Arc explosivity since 6 Ma also appear to be consequences of the changes in Pacific plate motion. Furthermore, both the WK97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion. Our findings support slab pull as the dominant plate tectonic driving force, highlight the rheological difference between strong oceanic lithosphere and weak continental crust, and suggest that WK97 presently provides a unifying, albeit preliminary, framework for relating both intraplate and circum‐Pacific tectonism and volcanism in the late Neogene.

NEOGENE TECTONICS, GREATER TIMOR SEA, OFFSHORE AUSTRALIA: IMPLICATIONS FOR TRAP RISK

The present day tectonic configuration of the Timor Sea area is the product of multiple phases of deformation occurring from the Proterozoic to the Holocene. Each major phase of deformation has produced structural hydrocarbon traps, which, in many cases, have subsequently undergone considerable reactivation, and in some cases, breaching. The last phase of tectonism affecting the region is the Neogene collision of the Australian and SE Asian Plates in the Banda Arc region. Although its importance in trap modification and integrity has been acknowledged, a cohesive tectonic model integrating observations that cross the international border has been lacking. The observed Neogene structural deformation and subsidence/uplift trends of the Australian shelf in the Greater Timor Sea can be explained by a combination of strike-slip faulting and flexure as the result of oblique convergence and partial subduction of the Indo-Australian Plate under the Southeast Asian plate. Neogene subsidence and deformation of the Cartier Trough and Malita Graben are interpreted to be the result of rhomb graben (i.e. pull-apart) basin development associated with sinistral shear and reactivation of relict rift structures. These basins developed along 'releasing bends' on the west-northwestward dipping flanks of rigid basement highs. Conversely, in areas where existing basement structure trends define 'confining bends' (e.g. southeast-eastward dipping flanks of basement high features), older rift structures have been reactivated transpressionally, in some cases resulting in large-scale inversion. The present day bathymetry suggests that these rhomb-grabens are currently active. In addition to the wrench-related deformation associated with oblique plate convergence and partial subduction, the Australian shelf has undergone flexure resulting in extension. It is proposed that the Neogene palaeo-and the present day stress fields of the Australian shelf reflect the combination of shear stresses related to wrenching and the tensional stresses related to flexure as the Australian Plate is obliquely subducted. Understanding of this modus operandi has direct bearing to hydrocarbon exploration as areas of structural risk (i.e. structures prone to trap breaching) can be delineated. In general trap breaching risk is lower in areas lying away from the shelf margin and away from 'releasing bends' along shear zones.

The Maturation History of the Paleozoic Hydrocarbon System of the Arabian Platform: ABSTRACT

This poster defines the Paleozoic hydrocarbon system of the Arabian Platform, presents an interpretation of its thermal and burial history and uses those results to describe the maturation history of the Lower Silurian source rock. Geohistory analysts were used to delineate source rock kitchens and to guide searches for previously unrecognized sources. The Paleozoic hydrocarbon system is one of the areally largest hydrocarbon systems in the world and one of the least drilled. It contains one of the world`s largest gas fields, is one of the largest potential gas resources on earth, and contains one of the largest emerging oil plays. It is present in both the Zagros Foldbelt and the Arabian Platform. The Lower Silurian Qusaiba source and the Triassic Sudair regional seal define the boundaries of the system. The Paleozoic hc system evolved in basically five stages: (1) simultaneous sediment accumulation and structural formation during the Paleozoic; (2) major tectonism and erosion in Late Paleozoic (Hercynian); (3) Triassic deposition of a regional seal; (4) Mesozoic charging primarily from the Lower Silurian sources; and (5) re-initiation of maturation of Paleozoic sources and charging of the Zagros compressional traps during Neogene collision with Eurasia. We have also interpreted the fillingmore » history of North Dome to explain why it contains gas rather than oil. A combination of late trap growth and late gas generation apparently displaced the previously resident oils and left a trap with a gas rich in condensate. The Silurian may have been exhausted by this time, thus indicating that other sources may have contributed significantly to North Dome.« less

Tectonics and Terranes of the Southeastern Caribbean

The southeastern Caribbean plate (Ca) is comprised of the following terranes: Tobago, Grenada Basin, St. Vincent, Araya-Margarita, and Paria-Trinidad-Barbardos (PTB). All are alient relative to South America (SA) east of Caracas except for PTB, which is of continental provenance and parautochthonous and lies within the principal movement zone of the Ca-Sa plate boundary. The Tobago terrane extends between the eastern Venezuelan coastline and the Grenada Basin. On its south, the Tobago terrane overrode PTB and the South American passive margin during Neogene oblique collision. The Mesozoic tectonostratigraphy of the Tobago terrane is not unlike that of the Colombian Basin, suggesting the Tobago may belong to the Pacific-derived Caribbean plate. The Grenada Basin terrane consists of Eocene and older oceanic crust that now occupies the southern Lesser Antilles arc platform and the southern Grenada Basin. Such crust abducted southward below the Tobago terrane in mid-Cenozoic time, probably taking up boundary-normal shortening during oblique collision of the Ca and Sa plates. The oceanic crust of the GB terrane arose by backarc spreading of unknown orientation. The St. Vincent terrane extends north in the Antilles from St. Vincent; it is defined by thick crust, perhaps an old arc basement. The Araya-Margarita terrane ismore » a probable subduction complex of Mesozoic age of metamorphism that has been transported far eastward from an unknown site with the Ca plate.« less

Geology of a Miocene collision complex, Buton, eastern Indonesia

The island of Buton in eastern Indonesia is part of a Neogene collision zone that encompasses much of the eastern margin of Sulawesi. Miocene collision of microcontinents with a west-dipping subduction zone emplaced the Tukang Besi platform (TBP) against Buton, and the Sula platform (SP) against the East Arm of Sulawesi, and thrust large ophiolite sheets eastward over the imbricated margins of the platforms. Reconnaissance observations of the resulting collision complex in Buton (Wolio Complex) indicate that imbricate west-dipping thrust sheets and overturned folds are the predominant structural element, although later steep faults offset the imbricate stack and largely control present map patterns. The Wolio Complex consists of the sedimentary Turumbia sequence, peridotite, and two distinct groups of metamorphic rocks. The Turumbia sequence makes up most of the eastern part of the Wolio Complex and consists mostly of deep-water limestone ranging in age from Late Triassic through late Eocene or Oligocene. It is interpreted as a deep-water facies of the western TBP margin. Ophiolitic rocks are represented in present outcrop only by massive peridotite in the western part of the Wolio Complex, but the compositions of clasts in conglomerates overlying the Wolio Complex indicate that a full ophiolite succession was exposed to erosion during the collision. Metabasite and metachert ranging in grade from greenschist to amphibolite facies are found locally in fault contact with peridotite and are interpreted as the remnants of a metamorphic sole developed at the base of the ophiolite. Pelitic phyllite and quartzite in northeastern Buton probably represent a slice of the continental basement of the TBP, for similar rocks have been dredged from the northeast margin of the TBP and also form the pre-Mesozoic basement of the SP. The middle to upper Miocene Tondo Formation in Buton consists of clastic strata derived from uplift and erosion of the Wolio Complex, placing an upper limit on the age of TBP collision. We infer a middle Miocene time of collision, with oblique convergence continuing into late Miocene time. In contrast, sparse biostratigraphic data place the age of the SP-East Arm collision in the late Miocene. Separate microcontinents may have collided successively with different parts of the Buton-Sulawesi convergent margin, or a single large microcontinent may have been fragmented during oblique collision.