Regional Seismic Reflection Research Articles

4D architecture and tectonic evolution of the Laverton region, eastern Yilgarn Craton, Western Australia

The Laverton region, located in the eastern Yilgarn Craton (EYC) Western Australia, is second only to the Kalgoorlie region for gold endowment. The integration of high-density, potential-field data, regional- and camp-scale seismic reflection data, regional- and mine-scale structural analysis, and geochronologically-constrained stratigraphy, provided new insights into the 4D architecture and tectonic evolution of Laverton region. At a regional scale, the map patterns of Laverton are dominated by the Mt Margaret Dome in the northwest and the Kirgella Dome in the southeast. These domes are flanked to the east and west by north-northwest-striking shear zones, with the central zone between the two domes being dominated by north- to north-northeast-striking sigmoidal shear zones. These distinctly different strikes to the shear zones developed early in the tectonic evolution, and resulted in a favourable architecture for late-stage orogenic gold mineralisation. A series of regional seismic reflection lines have imaged the sub-surface architecture of the Laverton region, with a major crustal-penetrating shear extending to the Moho, mapped as the Laverton Tectonic Zone (LTZ). The LTZ consists of a basal fault, the north-northwest-striking Celia Fault, and a series of steeper faults, for example, the north- to north-northeast-striking Childe Harold Fault and Far Eastern Fault, which sole at depth into the Celia Fault beneath the Laverton region. Lower angle faults imaged in the seismic reflection data develop parallel to granite–gneiss domes that postdate the north-northwest-striking and north- to northeast-striking faults. The construction and integration of the new 3D map with the structural, stratigraphic and geochronological analysis, has defined six main deformation events and their effects on the architectural evolution of the region. This analysis suggests that early formed faults are genetically associated with late-stage orogenic gold mineralisation. Regional- and mine-scale structural analysis at the Wallaby and Sunrise Dam deposits, indicate that gold deposition occurred synchronously during the D4b event in both deposits. Both mines are separated by the LTZ, which records sinistral and later (D5) dextral movement. The kinematic model which best describes the observed architecture is a restraining step-over within a sinistral strike-slip system. A structural model, consistent with analogue modelling of strike-slip systems, is proposed, where two, early, extensional basement faults were reactivated during D4b northwest-southeast contraction as low-angle thrusts in a pop-up zone. This architecture has been mapped in 3D in the region between Wallaby and Sunrise Dam, where both deposits are located on the low-angle thrusts, on opposing sides of a sigmoidal structure developed during sinistral strike-slip movement. This strike-slip, step-over architecture provided significant opportunity for dilation and fluid flow. Late dextral movement of the basement faults during northeast-southwest contraction (D5) reactivated the low-angle thrusts at Sunrise Dam, producing extensional movement and further dilation within the overall restraining step-over. This event is not significant at Wallaby Mine. A detailed 3D stress analysis, at Sunrise Dam, revealed that the most favourable fault segments to reactivate during the D4b event were orientated at ∼30° towards 310°. A comparison of the spatial distribution of gold, defined from extensive production and resource estimation drilling, with the favourably orientated segments of faults was confirmed. The analysis also highlighted down-dip extensions, beyond the coverage of the drilling, of favourably oriented segments, which may provide additional targets during future near-mine exploration. The same approach was applied to the regional 3D map, and new targets generated beneath the major salt lake between Wallaby and Sunrise Dam.

Evolution of faulting and volcanism in a back-arc basin and its implications for subduction processes

[1] The formation of the Taranaki Basin, an active volcanic back-arc rift situated in the continental Australian Plate, is related to subduction of the Pacific Plate along the Hikurangi margin. The Taranaki Basin contains an almost complete Miocene-Recent sedimentary record of the evolution of faulting and submarine andesitic volcanoes in the back-arc. Detailed study of extensive regional seismic reflection and coastal outcrop data sets yields valuable information about the extent to which back-arc rifting and reverse faulting have been controlled by the evolution of the Hikurangi margin subduction. Normal faulting and andesitic volcanism commenced in the northern part of the basin at ∼12 and ∼16 Ma, respectively, and were synchronous with contraction in the southern part of the basin. The rift, contractional faults and folds, and volcanism migrated southward during the last 12 Ma. Southward migration of faulting was episodic and geologically instantaneous with 100–150 km increases in the length of the rift at ∼12–8 and ∼4 Ma. From ∼4 Ma, displacement rates in the northern basin slowed and ceased at ∼2 Ma. The death of normal faults in the northern Taranaki Basin together with sympathetic variations in the timing of faulting and the overlapping rift geometries between the Taranaki Basin and the Central Volcanic Region are attributed to displacement transfer between the two rift systems. Southward migration of andesitic volcanism, rifting, and contractional deformation are consistent with clockwise rotation of the subduction margin associated with slab rollback coupled with southward motion of the southern termination of subduction and mantle corner flow.

The MARCONI reflection seismic data: A view into the eastern part of the Bay of Biscay

Seismic data acquisition for the MARCONI project took place in 2003 in the eastern half of the Bay of Biscay. The overall goal of the multichannel experiment was to collect seismic data to elucidate the structure and evolution of the southern margin of the Bay where an along-the-margin transition between oceanic subduction in the west and continental convergence in the east occurs, and its relationship to the Pyrenean realm. We describe the multichannel experiment and present an interpretation of the abyssal plain sector of the profiles. Altogether, eleven regional seismic reflection profiles were collected with a total length of 1800 km, between the longitudes 6° and 2°W and latitudes 43.5° and 46°N. The seismic profiles show first images of the thick sedimentary pile that covers the eastern abyssal plain of the Bay. The interpretation includes three sedimentary sequences separated by unconformities that correspond to the pre-, syn- and post-Alpine sequences. The post-orogenic deposits lay unconformably over the previous compressional structures in the eastern part of the Bay, floored by transitional crust. These compressional features are evident in the presence of a young accretionary prism sealed at its first stages of development. The overall structure and shape is similar to those of active subduction prisms worldwide, but evidencing differences in width and sediment wedge thickness from west to east along the margin. Buoyancy of the transitional crust and resistance to subduction led to stronger plate coupling towards the east, where oceanic crust is absent, and may be the main cause for these morphological changes along the east–west traverse of the wedge. The MARCONI multichannel deep seismic dataset provides the first estimation of the thickness of the sediment load on the 5000 m deep abyssal plain of Bay of Biscay, where no direct data were available.

Interactions between continental breakup dynamics and large‐scale delta system evolution: Insights from the Cretaceous Ceduna delta system, Bight Basin, Southern Australian margin

The interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern Australian margin (Bight Basin) are designed to analyze the influence of the Australia‐Antarctica continental breakup process on the kinematic evolution of the Cretaceous Ceduna delta system. The data show that the structural architecture of this delta system consists of two stacked delta systems. The lower White Pointer delta system (Late Albian‐Santonian) is an unstable tectonic wedge, regionally detached seaward above Late Albian ductile shales. Sequential restoration suggests that the overall gravitational sliding behavior of the White Pointer delta wedge (∼45 km of seaward extension, i.e., ∼27%) is partially balanced by the tectonic denudation of the subcontinental mantle. We are able to estimate the horizontal stretching rate of the mantle exhumation between ∼2 and 5 km Ma−1. The associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the White Pointer delta system. Lithospheric failure occurs at ∼84 Ma through the rapid exhumation of the mantle. The upper Hammerhead delta system (Late Santonian‐Maastrichtian) forms a stable tectonic wedge developed during initial, slow seafloor spreading and sag basin evolution of the Australian side margin. Lateral variation of basin slope (related to the geometry of the underlying White Pointer delta wedge) is associated with distal raft tectonic structures sustained by high sedimentation rates. Finally, we propose a conceptual low‐angle detachment fault model for the evolution of the Australian‐Antarctic conjugate margins, in which the Antarctic margin corresponds to the upper plate and the Australian margin to the lower plate.

Diachronous evolution of Late Jurassic–Cretaceous continental rifting in the northeast Atlantic (west Iberian margin)

Regional (2‐D) seismic reflection profiles, outcrop, and borehole data are used to characterize the evolution of deep offshore sedimentary basins in southwest Iberia (Alentejo Basin). The interpreted data indicate the bulk of Late Jurassic–earliest Cretaceous subsidence occurred in the present‐day continental slope area, as shown by (1) significant thickening of synrift strata basinward from a slope‐bounding fault system (SFS), west of which the total thickness of sediment can reach more than 9.0 km, and (2) relatively thin Mesozoic strata east of the SFS, where thickening of synrift units against principal faults is limited. Five principal regressive events and their basal unconformities reflect tectonic uplift and relative emersion in proximal basins, which were located on the rift shoulder to subsiding tilt blocks west of the SFS. These regressive events are correlated with major rift‐related events occurring on the deeper margin. Direct comparisons with the Peniche Basin of northwest Iberia reveal that significant portions of the Iberian lower plate margin were uplifted and eroded during the last stages of continental rifting. This process was repeated at different times (and in different areas) as the locus of rifting and continental breakup migrated northward. As a result, two distinct rift axes are recognized in west Iberia, a first axis extending from the Porto Basin to the Alentejo Basin and a second axis located on the outer proximal margin north of 38°30N. In addition, the SFS delimited (1) prograding deposits of Cretaceous‐Paleogene age and (2) late Cenozoic deposits draping the modern continental slope. These latter facts demonstrate that on lower plate passive margins, the relative position of the continental slope is established during the final rifting episode(s) preceding continental breakup.

Crustal-scale architecture and segmentation of the Argentine margin and its conjugate off South Africa

SUMMARY Integration of regional seismic reflection and refraction profiles and potential field data across the Argentine margin and its conjugate off South Africa, complemented by crustal-scale gravity modelling, is used to reveal and illustrate the whole-crust architecture, onshore–offshore crustal structure correlations, the character of the continent–ocean boundary/transition and the relationship of crustal structure to regional variation of potential field anomalies. The study reveals, within these two provinces, distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate transfer systems governing the margin segmentation and evolution, clearly implying structural inheritance. In particular, the Colorado transfer system on the Argentina margin, marks a distinct along-margin boundary in the distribution and volume of breakup-related magmatism. Similarly, the Hope transfer system on the conjugate South Africa margin also marks a distinct along-margin transition from a zone of relative magnetic quiescence to a zone of prominent magnetic anomalies. Furthermore, the study indicates that the ‘G-magnetic anomaly’ along the South Africa margin probably defines the eastern limit of the continent–ocean transition (COT) rather than a discrete continent– ocean boundary (COB). Potential field plate reconstructions of the South Atlantic suggest conjugate margin asymmetry, characterized by a rather broad Argentine margin conjugate to a narrow South Africa margin. In detail, the Argentine margin is characterized by a sharp and relatively constant COT, whereas the COT along the conjugate South Africa margin is considerably wider. An along-strike tectonomagmatic asymmetry variation is also observed and is expressed by the northward increase in width of the COT on the South African margin. The study clearly shows that integration of regional seismic reflection and refraction profiles, potential field data and gravity modelling provide a powerful resource for testing and validating alternative seismic profile structural interpretations and plate tectonic reconstructions, as well as geodynamic models for lithospheric breakup and early drift.

Imaging a potential geothermal target using Mt Isa regional seismic reflection and potential field geophysics, Queensland, Australia

A regional seismic survey in north Queensland, with acquisition parameters set for deep crustal imaging, shows a potential geothermal target beneath about 2 km of sediments. Beneath the sedimentary structure there appears to be an area of low seismic reflection signal from about 1 s to 4 s. Combined with the relatively low gravity signature over this location, this area of low seismic reflection signal could be interpreted as a large granite body, overlain by sediments. This body lies near an area of high crustal temperature and suggests a potential geothermal energy target.

Tectonic and Metallogenic Implications of Regional Seismic Profiles in the Timmins Mining Camp

Four regional seismic reflection profiles totalling 153 line km were acquired near Timmins, Ontario, within the southern Abitibi greenstone belt. Five additional high-resolution lines targeted specific metallogenically important features such as the Porcupine-Destor deformation zone. Interpretation of these profiles individually and in a composite north-south transect reveals a number of prominent bands of reflectors within the upper 15 km of the crust that define a series of folds or antiformal stacks of thrust nappes. Structures and stratigraphy mapped at the surface confirm structural culminations in these locations. At depths greater than 10 km the reflectors have generally shallower dips implying broad folding. Major ore deposits such as those at the Kidd Creek, Hollinger, McIntyre, and Dome mines are located on the northern, steeply dipping limbs of these antiformal stacks, implying that the fold structures focused post tectonic mineralizing fluids within the upper crust into the near surface. The Porcupine-Destor deformation zone is proximal to the large gold deposits near Timmins and is revealed by the new seismic data to be a composite of early fold structures and late transpressive fault arrays.

Cenozoic tectonic evolution of the Qaidam basin and its surrounding regions (Part 3): Structural geology, sedimentation, and regional tectonic reconstruction

The Qaidam basin is the largest topographic depression inside the Tibetan plateau. Because of its central position, understanding the tectonic origin of the Qaidam basin has important implications for unraveling the formation mechanism and growth history of the Tibetan plateau. In order to achieve this goal, we analyzed regional seismic-reflection profiles across the basin and a series of thickness-distribution patterns of Cenozoic strata at different time slices. The first-order structure of the basin is a broad Cenozoic synclinorium, which has an amplitude ranging from >16 km in the west to 48% in the west to −15 s −1 to 1.3 × 10 −17 s −1 . The eastward decrease in upper-crustal shortening requires a progressive shift in crustal-thickening mechanisms across Qaidam basin, from dominantly upper-crustal shortening in the west to dominantly lower-crustal shortening in the east. Although sedimentation began synchronously at 65–50 Ma across the entire basin, the initiation ages of the southern and northern basin-bounding structures are significantly different; deformation started at 65–50 Ma in the north and at 29–24 Ma in the south. This information and the existing inference that the uplift of the Eastern Kunlun Range south of Qaidam basin began after 30–20 Ma imply that the Paleogene (65–24 Ma) Qaidam and Hoh Xil basins on both sides of the Eastern Kunlun Range may have been parts of a single topographic depression, >500 km wide in the north-south direction between the Qilian Shan and Fenghuo Shan thrust belts in the north and south. The development of this large Paleogene basin in central Tibet and its subsequent destruction and partitioning by the Neogene uplift of the Eastern Kunlun Range requires a highly irregular sequence of deformation, possibly controlled by preexist-ing weakness in the Tibetan lithosphere.

2D-3C high-resolution seismic data from the Abitibi Greenstone Belt, Canada

One high-resolution seismic profile acquired as part of a 3-component (three orthogonal seismometers) regional seismic reflection survey within the southern Abitibi greenstone belt was investigated in more detail using tomography and by processing all three components. Several high-resolution lines had targeted the Porcupine–Destor deformation zone, a zone proximal to the large gold deposits of the Timmins mining camp, and revealed by the new seismic data to be a composite of early fold structures and late transpressive fault arrays that host quartz-vein gold. Processing of the horizontal-component data assumed that seismic waves were converted from P-waves to S-waves between Vibrator source and receivers; S-wave static corrections proved especially important. Radial-component sections showed most features observed previously on vertical-component sections, but some amplitudes became enhanced, others reduced. The transverse-component stack demonstrated that a few structures are 3-D or contain significant seismic anisotropy. Two dimensional, P- and S-wave travel-time tomography analysis using data from the 10-km-long linear receiver array revealed low-resolution (> 200 m) physical property variations, possibly related to mineralization in near-surface structures of the Porcupine–Destor deformation zone; these variations were especially apparent using Poisson's ratio.

Northeastern Brazilian margin: Regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling

Integration of regional seismic reflection and potential field data along the northeastern Brazilian margin, complemented by crustal-scale gravity modelling, is used to reveal and illustrate onshore–offshore crustal structure correlation, the character of the continent–ocean boundary, and the relationship of crustal structure to regional variation of potential field anomalies. The study reveals distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate Brazil–West Africa transfer systems, governing the margin segmentation and evolution. Several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context. Furthermore, the constructed transects, the observed and modelled Moho relief, and the potential field anomalies indicate that the Recôncavo, Tucano and Jatobá rift system may reflect a polyphase deformation rifting-mode associated with a complex time-dependent thermal structure of the lithosphere. The constructed transects and available seismic reflection profiles, indicate that the northern part of the study area lacks major breakup-related magmatic activity, suggesting a rifted non-volcanic margin affinity. In contrast, the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity, suggesting that this region evolved, partially, with a rifted volcanic margin affinity and character.

Highstand fans in the California borderland: The overlooked deep-water depositional systems

Contrary to widely used sequence-stratigraphic models, lowstand fans are only part of the turbidite depositional record; our analysis reveals that a comparable volume of coarse-grained sediment has been deposited in California borderland deep-water basins regardless of sea level. Sedimentation rates and periods of active sediment transport have been determined for deep-water canyon-channel systems contributing to the southeastern Gulf of Santa Catalina and San Diego Trough since 40 ka using an extensive grid of high-resolution and deep-penetration seismic-reflection data. A regional seismic-reflection horizon (40 ka) has been correlated across the study area using radiocarbon age dates from the Mohole borehole and U.S. Geological Survey piston cores. This study focused on the submarine fans fed by the Oceanside, Carlsbad, and La Jolla Canyons, all of which head within the length of the Ocean-side littoral cell. The Oceanside Canyon–channel system was active from 45 to 13 ka, and the Carlsbad system was active from 50 (or earlier) to 10 ka. The La Jolla system was active over two periods, from 50 (or earlier) to 40 ka, and from 13 ka to the present. One or more of these canyon-channel systems have been active regardless of sea level. During sea-level fluctuation, shelf width between the canyon head and the littoral zone is the primary control on canyon-channel system activity. Highstand fan deposition occurs when a majority of the sediment within the Oceanside littoral cell is intercepted by one of the canyon heads, currently La Jolla Canyon. Since 40 ka, the sedimentation rate on the La Jolla highstand fan has been >2 times the combined rates on the Oceanside and Carlsbad lowstand fans.

Insights into the structure of the Australian Crust, results from ANSIR?s Seismic Reflection Profiling Program

ANSIR, the National Facility for Earth Sounding, has been responsible for the acquisition of 30 regional deep seismic reflection surveys across many different Australian geological regions and structural environments. The seismic data has provided exciting new images of the Australian Crust at depth. Each survey has produced the unexpected and has usually dispelled some `sacred-cow' in geological understanding of the depth-dimension of that region. ANSIR's deep seismic reflection equipment provides both high quality whole-of-crust images of the Australia continent as well as detailed district and mine scale seismic images. These seismic surveys are providing a clear picture of the architecture of a region and providing information to develop tectonic models a region. In particular, the seismic data is successfully imaging the major basement geological units and structures and therefore providing the necessarydepth constraints to construct 3D geological models ofa region. The seismic data is suggesting possible mineral systems that have operated and allowing explorers to assess the potential of an area for mineral deposits.

Defining regolith and beyond from regional seismic reflection data: A case study from the Tanami.

Regional seismic reflection data in hard rock areas contains more shallow information than might first be supposed. Here I use a subset of the 2005 Tanami Seismic Survey data to show that near surface features can be defined, including paleochannels, Palaeozoic basins and structures within the Proterozoic basement. Successful imaging depends on correct determination of refraction statics, including identification of refractor branches, and use of a floating or intermediate datum during seismic reflection processing. Recognition of steep stacking velocity gradients associated with surface referenced processing aids velocity analysis and can further delineate areas of thicker regolith in palaeochannels. The first arrival refraction analysis can also be applied in more detail to estimating thickness of regolith and depth to economic basement in areas of sedimentary cover.

From Proterozoic strata to a synthesized seismic reflection trace: implications for regional seismic reflection patterns in northwestern Canada

Calculation of a synthetic seismic reflection trace from detailed descriptions of exposed Proterozoic strata in northwestern Canada permits correlation of reflections on regional seismic profiles to surface outcrop. Approximately 5.4 km (composite thickness) of Paleo- and Mesoproterozoic strata are exposed in the Muskwa anticlinorium that is located within the foreland of the Cordillera in northeastern British Columbia. The Tuchodi anticline is the easternmost structure of the Muskwa anticlinorium and has the deepest levels of Proterozoic strata exposed. At this location, prominent seismic reflection layering rises toward the surface and is easily correlated to the deeper formations of the Muskwa assemblage stratigraphy. These layers are followed westward into the middle crust, where they are overlain by dramatically thickened (by about five times) strata, primarily of the Tuchodi Formation. Along the same line of section, the Muskwa assemblage reflections overlie additional subparallel layered reflections at depth whose lithology and origin are unknown. However, coupled with other observations, including regional refraction results that indicate the crustal layers have both low seismic p-wave velocities and low ratios of p- and s-velocities, regional gravity observations that indicate the layers are low density, and correlation to similar layers on other seismic profiles that exhibit characteristic seismic stratigraphic features, the subparallel layers that are present beneath the known Muskwa assemblage are most easily interpreted as layered Proterozoic (meta-) sedimentary rocks. These results provide the basis for interpreting the Muskwa anticlinorium as a crustal-scale structure that formed when a deep basin of Proterozoic strata was inverted and thrust over an ~20 km high footwall ramp during Cordilleran orogenesis.

MesozoicndashCenozoic evolution of North Atlantic continental-slope basins: The Peniche basin, western Iberian margin

New regional (two-dimensional) seismic reflection data, published Deep-Sea Drilling Project–Ocean Drilling Program reports, and unpublished shallow-offshore well information characterize the Mesozoic–Cenozoic evolution of the western Iberian continental slope north of 3845N. Two distinct sectors bounded by first-order transfer faults exist between the Galicia Bank and the Nazar fault. The northernmost sector 1 is filled by Triassic–Aptian (prebreakup) sequences, reaching more than 3.5 s two-way traveltime (TWTT) in thickness in distinct half grabens. Salt pillows, salt ridges, minibasins, and salt-detached overburden faults were generated during the Mesozoic and reactivated in the Cenozoic. Sector 2 shows Triassic–Jurassic units more than 2.0 s TWTT thick, underlying east-tilting half grabens of Early Cretaceous age. Salt structures in this sector evolved into mature salt diapirs. Postbreakup units are up to 2.0 s TWTT thick in both sectors. The evolution of the study area replicates evolutionary settings that have previously been proposed for nonvolcanic passive margins. However, some distinct features are noted: (1) widespread Triassic–Berriasian units deposited over rotated tilt blocks represent the early rifting stage; (2) Early Cretaceous subbasins showing rift-climax units, most likely formed during the advanced rifting stage, are spatially constrained to an approximately 100-km (62-mi)-wide region stretched along the continental slope; and (3) listric blocks and their associated low-angle (deep) detachment faults, formed on the distal margin during the advanced rifting and transition to sea-floor spreading stages, show no developed rift-climax units above them. From the early rifting stage onward, Mesozoic faults and halokinetic structures induced local differences in the thickness and character of seismic facies. Cenozoic (Alpine) tectonism promoted the reactivation of older Mesozoic structures.

Upper Triassic–Lower Cretaceous seismic sequence stratigraphy and basin tectonics at Bornholm, Denmark, Tornquist Zone, NW Europe

An integrated analysis of seismic sequence stratigraphy and structural evolution of a dynamic intraplate fault basin was conducted along a segment of the Tornquist Zone at the boundary between the Northwest European Craton and the Baltic Shield-East European Platform. High-quality regional seismic-reflection lines document the Late Triassic through early Late Cretaceous tectonic history and depositional sequence architecture. The stratigraphic package was divided into three major transgressive/regressive cycles (1st-order sub-cycles). The upper Upper Triassic through Lower Cretaceous siliciclastic cycles I and II are divided into 2nd-order transgressive/regressive facies cycles composed of higher frequency 3rd-order sequences. The lowstand basin fill comprises stacked successions of lowstand systems tracts produced by relative sea-level rises and falls. During the 3rd-order regressive intervals highstand-dominated depositional conditions prevailed to the northwest while lowstand-dominated conditions occurred to the southeast. Cycle III records an Upper Cretaceous major siliciclastic–chalk cycle. The magnitude of the relative sea-level highs and lows increased up through the studied interval. A two-dimensional structural-evolution analysis is based on the variation of the sediment thickness across the fault blocks up through the sedimentary column. Basin subsidence can be divided into successive structural/tectonic regimes. Fault block rotation and structural inversion characterized the early subsidence of the basin floor. During the second interval, inversion ceased while fault-block rotation continued. In the third interval, the rotation also stopped and no deformation of the basin is recognized. The three major transgressive/regressive facies cycles and associated relative sea-level cycles are linked with the structural regimes. Contemporaneous deposition of 3rd-order highstand- and lowstand-dominated complexes related to differential subsidence and uplift is also evidence of basin tectonics as the controlling factor on relative sea-level changes. Comparison of the relative sea-level curve developed for the study area with eustatic sea-level curves shows that they follow different trends. The deviations between these curves, and the correlation of sequence boundaries with changes of the tectonic regime, support the conclusion that tectonics was a major control on relative sea-level fluctuations in the study area. The 2nd-order cycles linked with deformation episodes along the Tornqist Zone correlate with Lower-Middle Jurassic 2nd-order cycles in England. Deformation along the Tornquist Zone may thus have influenced the sequence stratigraphy across the entire Northwest European Craton.

Two Stages of Deformation and Fluid Migration in the West-Central Brooks Range Fold and Thrust Belt, Northern Alaska

The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (60 and 45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (250–300C) event that was buffered by limestone and a younger, lower temperature (150C) event that had distinctly lower 13C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160–120 Ma, whereas the lower temperature event probably occurred at about 60–45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by far-traveled allochthons with relatively low structural relief, because it involved a thin (1–4-km [0.6–2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5–10 km (3–6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100–90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Glaciation across the Oligocene–Miocene boundary in southern McMurdo Sound, Antarctica: new chronology from the CIROS-1 drill hole

Few Palaeogene and Neogene sediment cores from the Antarctic continental margin have been dated with sufficient precision to enable establishment of direct linkages between glacial events on the Antarctic continent and marked events in deep-sea δ 18O records. As a result, much of our knowledge of the gradual, but stepwise, shift from ‘greenhouse’ climates of the Cretaceous to the ‘ice-house’ climates of the Quaternary is inferred from well-dated and more continuously deposited deep-sea sediments. In this study, we present new magnetostratigraphic results from the CIROS-1 drill core from McMurdo Sound, Antarctica, along with a reinterpretation of a published diatom biostratigraphic zonation that is constrained by correlation to a high-precision age model from the nearby CRP-2/2A drill core. Our results suggest that most of the upper 350 m of the CIROS-1 drill core represents rapid sediment accumulation during a short time interval spanning the Oligocene–Miocene boundary. Chronostratigraphic control is precise enough to enable correlation of this interval of glacimarine sedimentation with the Mi-1 deep-sea δ 18O event, which confirms that the Mi-1 event was related to a major expansion of Antarctic ice. A major unconformity at 366 m in the CIROS-1 drill core, which is widely observed in regional seismic reflection studies, represents 9 Myr of missing time. This unconformity can be traced offshore into the Ross Sea using seismic stratigraphy and is interpreted to indicate significant East Antarctic ice sheet development during the Mi-1 glaciation. The stratigraphic expression of this ∼400-kyr glacial event is evidently multiphase and complex in the Victoria Land Basin, probably because it was punctuated by higher-frequency orbitally induced glacial oscillations. The presence of Nothofagidites pollen throughout the CIROS-1 drill core and the presence of a Nothofagus (Southern beech) leaf within the early Miocene portion of the core indicate that Antarctic mean summer temperatures did not decrease below 5°C throughout the Mi-1 glaciation. These temperatures are significantly warmer than present-day mean summer temperatures at sea level in McMurdo Sound. The persistence of a Nothofagus forest in coastal southern Victoria Land throughout this time interval suggests that the present state of deep refrigeration was not reached until some time after the Mi-1 glaciation.

Tau-p velocity imaging of regolith structure

The tau-p velocity imaging method, first developed for obtaining the velocity field from marine multichannel seismic data, has been applied to refracted waves from the regolith in regional seismic reflection surveys on land. The technique converts travel time picks from the refracted wavefield into two-dimensional velocity models, by transforming from time-offset into the tau-p domain. Each arrival is mapped individually, and the ?true? velocity and position of the ray turning point is obtained by considering reversed raypaths. Thus the data are transformed directly into a depth or two-way time image of the subsurface displayed in seismic velocity. The method is extremely fast and involves no interpretive steps or iteration. Ideal datasets contain the refracted wavefield sampled densely and equally in the shot and receiver domains. It was therefore decided to test the application of the method for mapping the velocity structure of a portion of regolith and to compare the results with those obtained using more conventional methods. The area chosen for study was part of a regional seismic reflection line across the Lachlan River palaeo-valley in central NSW. The data set consisted of the first break picks for 240 channels with receivers spaced every 40 m and vibration points every 40 m. The velocity images were produced as both time and depth sections and compared with one and two layer refractor models from a refraction tomographic approach. A low velocity region on the image corresponds to the deepest part of the refractor model, interpreted as the thickest part of the palaeo-valley. Bedrock velocity variations are also mapped and agree with the changes along the (lowest) refractor velocity profile. While further tuning may be required for land work, the technique has the advantage that velocities can be directly imaged and potentially related to regolith physical properties.