Lower crustal involvement in upper crustal thrusting

Summary. Basement structures mapped in the Devonian Adavale Basin, eastern Australia, indicate two styles of lower-crustal involvement in the formation of upper-crustal structures. The first style is typified by thrust features in the upper-crustal sedimentary section and basement, a response to lower-crustal shortening over a wide area. The second style includes lower-crustal thrusting and thickening in a limited region, with associated uplift of the upper crust. These two styles suggest that the upper and lower crust were mechanically decoupled during Palaeozoic compressive episodes.     

Evolution of a Neoproterozoic to Palaeozoic intracratonic setting, Officer Basin, South Australia

The Neoproterozoic basins of central Australia share many features of architecture and sedimentary fill, suggesting common large-scale extrinsic causal mechanisms. In an attempt to improve understanding of these mechanisms we have gathered and analysed new deep seismic reflection data and re-evaluated existing seismic and well-log data from the eastern Officer Basin, the largest and most poorly known of Australia's intracratonic basins. The Officer Basin is asymmetric and has a steep thrust-controlled northern margin paralleled by sub-basins as much as 10 km in depth. Further south the basin shallows gradually onto a broad platform. The basin rests on a thick crust (≈42 km) that is pervaded by a complex of northward-dipping surfaces most of which terminate erosionally against the sediments of the Officer Basin and are interpreted as prebasinal features, possibly faults. Some appear to have been zones of crustal weakness which were reactivated as thrust complexes and played a major role in basin evolution. The sedimentary succession can be subdivided into six megasequences separated by major tectonically and erosionally enhanced sequence boundaries. The megasequences have distinctive sequence stacking patterns suggesting that they were deposited in response to episodic subsidence induced by a major extrinsic tectonic event. The basin initially formed as part of a giant sag basin which incorporated all the present-day intracratonic basins (Amadeus, Georgina, Ngalia, Officer and Savory Basins) in a single large ‘superbasin’ perhaps as a response to mantle processes. Subsidence then ceased for ≈100 Myr producing a regional erosion surface. Beginning in the Torrensian or Sturtian five more major events of varying regional significance influenced the basin's evolution. Four were compressional events, the first of which activated major thrust complexes along the present basin margins, forming deep foreland sub-basins with elevated intervening basement blocks. Once activated, the thrust complexes and sub-basins persisted throughout the life of the intracratonic basins. From this epoch the intracratonic basins of central Australia were decoupled from the giant sag basin and became interrelated but independent features. Available information suggests that the Officer, Amadeus, Georgina, Ngalia and Savory Basins are related and are perhaps products of major tectonic events associated with the assembly and ultimate dispersal of the Proterozoic supercontinent. The closing phases of these basins were strongly influenced by events occurring along the newly created active eastern margin of the Australian continent in the Palaeozoic.     

Reflection mapping across the convergent margin of western Canada

Summary. Five marine multichannel seismic reflection profiles totalling 520 km were recorded across the western Canada convergent margin where the Juan de Fuca plate is subducting beneath North America. The data extend the results of LITHOPROBE on Vancouver Island. The primary objectives are definition of the offshore accretionary structures and clarification of the convergent interaction between the two plates. The main features of this preliminary interpretation are: (1) the subduction deformation front is complex with evidence of sediments being accreted and subducted; (2) the top of the oceanic crust and the Mono are imaged below the deep water sedimentary basin; (3) the top of the subducting plate is clearly imaged below the shelf; (4) beneath the inner shelf, one band of high reflectivity underlain by a zone of lesser reflectivity lies above the plate; (5) alternative interpretations place the present zone of decoupling at the base of the reflective band or the top of the plate; (6) the San Juan and Leech River faults that bound small accreted terranes are imaged as thrusts that merge at depth.     

The nature of deep crustal structures in the Mojave Desert, California

Summary. The character of multi-offset reflections from the deep crust in the Mojave Desert are examined to reveal the physical nature of the reflecting structures. We focus on distinguishing classical abrupt discontinuities, such as traditional models of the Conrad and Moho boundaries, from more unusual structures. Finite-difference modeling and simple interference relations show that pre-critical reflections exhibiting an increase in peak frequency with offset arise from thinly-layered horizontal structures, while reflections from step discontinuities show no change in frequency with offset. In the deep crust thin layers may result from sill intrusion or fault motion.
The sense of changes in Poisson's ratio and the relative strength of density changes determine whether reflection amplitudes will increase or decrease with offset. A simple linear regression on pre-critical reflection amplitudes against offset is adequate to separate reflections arising from increases in Poisson's ratio from those arising from decreases in Poisson's ratio and/or density changes. The latter condition may be the result of strong anisotropy or the presence of pore fluid. Comparisons of the properties of major deep reflectors across the Mojave Desert suggest that the effects of tectonic motion and fluid injection have penetrated all levels of the crust.     

Seismic reflection studies using local earthquake sources

Summary. A method is presented for processing three-component digital recordings of micro-earthquakes to obtain near-vertical reflection profiles in regions of shallow seismicity. The processing includes magnitude and focal-depth normalization and event stacking, where stacking is by small localized groups, with ray theoretical time and distance corrections applied to compensate for varying focal depths. In areas with high seismicity, this procedure allows earthquakes to be treated as "controlled" sources to probe layered structures of the deep crust and upper mantle. The validity of our approach is demonstrated using S-waves from aftershocks of the Borah Peak, Idaho, earthquake (Ms = 7.3) of 1983.     

Deep seismic reflectors in the Campos basin, offshore Brazil

Summary. Some deep crustal features underlying the Campos basin are best recognized in a few reflection seismic sections that have been reprocessed recently to 10 s two-way traveltime. A prominent climbing-to-the-basin reflector is interpreted as the Moho, and a relatively steep fracture zone is, probably, the first example so far of an extensional fault crossing the whole crust and offsetting the Moho. Further constraints on the deep structure of the basin are provided by estimating the thinning of the crust from shallow seismic data and gravity modelling, and by cross-plotting backstripped subsidence curves against curves predicted by the lithospheric stretching model.     

南极半岛和东南极间南威德尔海陆架的地壳结构

重力资料结合磁性体埋深计算和地震折射资料的分析 ,表明在南极半岛和东南极克拉通之间的菲尔希内尔和罗纳冰架之下存在一减薄的地壳。推测该区地壳的减薄主要由南极半岛和东南极克拉通之间的近东西向拉张作用所造成 ,该作用很可能发生在陆架盆地发展的早期。     

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

We report results from the Seismic Wide-Angle and Broadband Survey carried out over the Mid North Sea High. This paper focuses on integrating the information from a conventional deep multichannel reflection profile and a coincident wide-angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new traveltime inversion scheme was developed (reported elsewhere) that was used to invert traveltime information from both the wide-angle OBH records and the reflection profile simultaneously. Results from the inversion were evaluated by producing synthetic seismograms from the final inversion model and comparing them with the observed wide-angle data, and an excellent match was obtained. It was possible to fine-tune velocities in less well-resolved parts of the model by considering the critical distance for the Moho reflection. The seismic velocity model was checked for compatibility with the gravity field, and used to migrate and depth-convert the reflection profile. The unreflective upper crust is characterized by a high velocity gradient, whilst the highly reflective lower crust is associated with a low velocity gradient. At the base of the crust there are several subhorizontal reflectors, a few kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike section through northward-dipping reflectors at the base of the crust, identified on orthogonal profiles by Freeman et al. (1988) as being slivers of subducted and imbricated oceanic crust, relics of the mid-Palaeozoic Iapetus Ocean.     

Deep seismic reflection profiles across the western Barents Sea

Summary. The continental crust beneath the western Barents Sea has been acoustically imaged down to Moho depths in a large scale deep seismic reflection experiment. A first-order pattern of crustal reflectivity has been established and the thickness of the crust determined. A number of features with important implications for the tectonics of the area have been discovered. The results are presented in the form of two transects.     

Deep seismic reflection studies in Israel - an update

Summary. The results of three deep crustal reflection lines are presently available from Israel. A 90 km line from near the Dead Sea rift to the Mediterranean coast was carried out for deep study. Two other lines in the Mediterranean coastal area were derived by recorrelation of oil exploration lines. The data shows a division between continental inner Israel and the coastal plain. In the first area a reflective lower crust is apparent with transparent upper crust and almost transparent upper mantle. Near the coast, in an area which was previously suggested as underlain by an ancient fossil oceanic crust, strong reflections characterize the uppermost mantle. Comparison between the reflection pattern and previous deep refraction and MT data indicates some agreement away from the coast and lack of correlation in the area of possible fossil oceanic crust near the coast.     

Crustal thickening under the palaeo-meso-Proterozoic Delhi Fold Belt in northwestern India: evidence from deep reflection profiling

The results of deep reflection profiling studies carried out across the palaeo-meso-Proterozoic Delhi Fold Belt (DFB) and the Archaean Bhilwara Gneissic Complex (BGC) in the northwest Indian platform are discussed in this paper. This region is a zone of Proterozoic collision. The collision appears to be responsible for listric faults in the upper crust, which represent the boundaries of the Delhi exposures. In these blocks the lower crust appears to lie NW of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks. A fairly reflective lower crust northwest of the DFB exposures appears to be the downward continuation of the DFB upper crust. The poorly reflective lower crust under the exposed DFB may be the westward extension of the BGC upper crust at depth. Thus, the lower crust in this region can be divided into the fairly reflective Marwar Basin (MB)-DFB crust and a poorly reflective BGC crust. Vertically oriented igneous intrusions may have disturbed the lamellar lower-crustal structure of the BGC, resulting in a dome-shaped poorly reflective lower crust whose base, not traceable in the reflection data, may have a maximum depth of about 50 km, as indicated by the gravity modelling.
The DFB appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. This has resulted in three Moho reflection bands, two of which are dipping SE from 12.5 to 15.0 s two-way time (TWT) and from 14.5 to 16.0 s TWT. Another band of subhorizontal Moho reflections, at ≈ 12.5 s TWT, may have developed during the crustal perturbations related to a post-Delhi tectonic orogeny. The signatures of the Proterozoic collision, in the form of strong SE-dipping reflections in the lower crust and Moho, have been preserved in the DFB, indicating that the crust here has not undergone any significant ductile deformation since at least after the Delhi rifting event.     

Crust‐scale 3D model of the Western Bredasdorp Basin (Southern South Africa): data‐based insights from combined isostatic and 3D gravity modelling

The southern South African continental margin documents a complex margin system that has undergone both continental rifting and transform processes in a manner that its present‐day architecture and geodynamic evolution can only be better understood through the application of a multidisciplinary and multi‐scale geo‐modelling procedure. In this study, we focus on the proximal section of the larger Bredasdorp sub‐basin (the westernmost of the five southern South African offshore Mesozoic sub‐basins), which is hereto referred as the Western Bredasdorp Basin. Integration of 1200 km of 2D seismic‐reflection profiles, well‐logs and cores yields a consistent 3D structural model of the Upper Jurassic‐Cenozoic sedimentary megasequence comprising six stratigraphic layers that represent the syn‐rift to post‐rift successions with geometric information and lithology‐depth‐dependent properties (porosities and densities). We subsequently applied a combined approach based on Airy's isostatic concept and 3D gravity modelling to predict the depth to the crust‐mantle boundary (Moho) as well as the density structure of the deep crust. The best‐fit 3D model with the measured gravity field is only achievable by considering a heterogeneous deep crustal domain, consisting of an uppermost less dense prerift meta‐sedimentary layer [ρ = 2600 kg m^?3] with a series of structural domains. To reproduce the observed density variations for the Upper Cenomanian–Cenozoic sequence, our model predicts a cumulative eroded thickness of ca. 800–1200 m of Tertiary sediments, which may be related to the Late Miocene margin uplift. Analyses of the key features of the first crust‐scale 3D model of the basin, ranging from thickness distribution pattern, Moho shallowing trend, sub‐crustal thinning to shallow and deep crustal extensional regimes, suggest that basin initiation is typical of a mantle involvement deep‐seated pull‐apart setting that is associated with the development of the Agulhas‐Falkland dextral shear zone, and that the system is not in isostatic equilibrium at present day due to a mass excess in the eastern domain of the basin that may be linked to a compensating rise of the asthenospheric mantle during crustal extension. Further corroborating the strike‐slip setting is the variations of sedimentation rates through time. The estimated syn‐rift sedimentation rates are three to four times higher than the post‐rift sedimentation, thereby indicating that a rather fast and short‐lived subsidence during the syn‐rift phase is succeeded by a significantly poor passive margin development in the post‐rift phase. Moreover, the derived lithospheric stretching factors [β = 1.5–1.75] for the main basin axis do not conform to the weak post‐rift subsidence. This therefore suggests that a differential thinning of the crust and the mantle‐lithosphere typical for strike‐slip basins, rather than the classical uniform stretching model, may be applicable to the Western Bredasdorp Basin.     

Anomalous passive subsidence of deep‐water sedimentary basins: a prearc basin example,southern New Caledonia Trough and Taranaki Basin,New Zealand

Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.     

A deep seismic profile of 800 km length recorded in southern Queensland, Australia

Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.     

The Acre Basin basement (NW Brazil) and the transition from the intracratonic to retroarc foreland basin system

The understanding of the crustal transition between orogenic zones and cratonic portions in distal regions of foreland basins has received increasing attention, but the analysis is often hampered by the sedimentary cover. Despite the peculiar location of the Acre Basin, specifically between the Amazonian Craton and the sub-Andean zone, local basement studies are still scarce due to lacking seismic data and exploratory wells. Therefore, this work aims to map basement depths, estimate crustal compositions and identify the main depocenters, structures and limits of Acre Basin using an integrated analysis to understand better the region lithospheric evolution, its relationship with the Amazonian Craton and its positioning within the Andean orogeny. For that, we used well, 2D seismic reflection, airborne and ground gravity and magnetic data as well as the EMG2008. Tilt Depth estimates indicate basement depths between 500 and 7800 m and larger sedimentary thicknesses in the northern portion. Additionally, we modelled groups of potential sources between 0.1 and 22 km and Moho depths between 26 and 37 km. Compositionally, the upper crust consists dominantly of meta-sedimentary and low-grade metamorphic rocks and granites, indicating that the sub-Andean and Acre Basins share a similar basement. Thus, there are indications that the basement of the Acre Basin is essentially formed by the Sunsás province in the Amazonian Craton. However, local differences in basement depth, magnetic susceptibility and exploratory potential led to the subdivision into Divisor and Xapuri sub-basins, north and south of the Fitzcarrald Arch, respectively. Finally, it was possible to establish the limits of the Andean orogeny influence in the Acre Basin and delimit the area of the Western Amazon Foredeep installed during the Neogene.     

Coincident seismic reflection/refraction studies of the continental lithosphere: a global review

Summary. Vertical-incidence reflection profiling has identified several characteristic features of the continental Iithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling.     

Seismic reflection and refraction studies of the deep structure of the Agulhas Bank

Summary. A 12 s two-way time seismic reflection profile, 46 km in length and straddling the Cape Seal Arch, was surveyed on the Agulhas Bank during 1985. The contact between the marine sediments and the pre-Mesozoic basement produces a strong reflection at 2 s. The folded Cape and Kaaimans sediments give rise to occasional strong reflections from 2-6 s. Strongly reflecting segments occur between 9 and 10 s, and with a time-to-depth conversion made using refraction velocities, this zone of occasional strong reflections is identified as the Moho. The section from 6–9 s does not give rise to significant coherent reflections, and is considered to represent the Archaean crust. An analysis of the faults active during Gondwana break-up, revealed by reflection seismology, show the Agulhas Fracture Zone to be a divergent wrench fault system.     

Statistical analysis of spatial point patterns on deep seismic reflection data: a preliminary test

Spatial point patterns generated from bitmaps of images of processed reflection seismic profiles are analysed to quantify the reflectivity patterns. The point process characteristics for two different regions of a deep seismic reflection profile in northwestern Canada demonstrate that in both cases the points are not randomly distributed and that the point pattern distribution is different between the regions. The cluster effects for small point pair distances are stronger for the region of data where there is strong sedimentary layering than for the region where the layering is less distinct. As a result, it appears that future developments in point pattern analysis may provide a new tool for analysing spatial variations in reflection data.     

Pre-drilling reflection survey of the Black Forest, SW Germany

Summary. The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection meaasurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25–27 km above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is' also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.     

Fault‐controlled fluid flow inferred from hydrothermal vents imaged in 3D seismic reflection data,offshore NW Australia

Fluid migration pathways in the subsurface are heavily influenced by pre‐existing faults. Although studies of active fluid‐escape structures can provide insights into the relationships between faults and fluid flow, they cannot fully constrain the geometry of and controls on the contemporaneous subsurface fluid flow pathways. We use 3D seismic reflection data from offshore NW Australia to map 121 ancient hydrothermal vents, likely related to magmatic activity, and a normal fault array considered to form fluid pathways. The buried vents consist of craters up to 264 m deep, which host a mound of disaggregated sedimentary material up to 518 m thick. There is a correlation between vent alignment and underlying fault traces. Seismic‐stratigraphic observations and fault kinematic analyses reveal that the vents were emplaced on an intra‐Tithonian seabed in response to the explosive release of fluids hosted within the fault array. We speculate that during the Late Jurassic the convex‐upwards morphology of the upper tip‐lines of individual faults acted to channelize ascending fluids and control where fluid expulsion and vent formation occurred. This contribution highlights the usefulness of 3D seismic reflection data to constraining normal fault‐controlled subsurface fluid flow.