Basement Density Research Articles

Extension modes and breakup processes of the southeast China-Northwest Palawan conjugate rifted margins

Abstract Our understanding of continent-ocean transition structures and magmatism in the absence of excessive magmatic additions has been guided by the observations and models developed at the magma-poor Iberia-Newfoundland conjugate margins. Recently these models have been challenged in the South China Sea in light of new IODP Expeditions 367-368-368X. We have used an integrated analysis of high quality seismic reflection and gravity anomaly data, calibrated against recent deep sea drilling results, to investigate margin structure and tectono-magmatic interplay during continental breakup and early seafloor spreading between the SE China-NW Palawan conjugate margins. The Eocene-Oligocene South China Sea rifting initiates in a heterogeneous and likely thermally un-equilibrated lithosphere formed by the Mesozoic Yenshanian orogeny. Gravity and joint inversion methods confirm lateral variation of basement densities across the conjugate margins. Lithospheric and basement heterogeneities induced a rifting style characterized by a series of highly thinned rift basins associated with extensional faulting soling out at various crustal levels. Final rifting in late Eocene triggered decompression melting forming mid-ocean ridge type magmatism, which emplaced within thinned continental crust as deep intrusions and shallow extrusive rocks. This initial magmatic activity was concomitant with continued deformation of continental crust by extensional faulting. Integrated analysis of seismic reflection profiles and gravity anomaly data combined with deep-sea boreholes accurately locate the continent-ocean boundary. We show that the initial igneous crust, continentward of oceanic magnetic anomaly C10n, is asymmetric in width and in morphology for the conjugate margins. The wider and faulted newly accreted domain on the SE China side indicates that magmatic accretion is associated with tectonic faulting during the formation of initial oceanic lithosphere. We suggest that deformation was not symmetrically distributed between the conjugate margins during the initiation of seafloor spreading but evolved asymmetrically until the stabilisation of the spreading ocean ridge around C10n. The analysis of the South China Sea breakup reveals a transient interplay between faulting, magmatic budget and extension rates during the formation of the continent-ocean transition and initial emplacement of igneous crust.

Simple 2D gravity–density inversion for the modeling of the basin basement: example from the Banat area, Serbia

We have developed a technique to calculate lateral density distribution of the sedimentary basin basement by combining linear gravity–density inversion and 2D forward modeling. The procedure requires gravity anomaly data, depth-to-basement data and density data for the sediments (density–depth distribution). Gravity effect of density variations in the basement was extracted from the total gravity anomaly by removing the joint effect of the sediments with vertical density variations and homogeneous basement of average density contrast (calculated by 2D modeling). Gravity effect of the sediments was calculated using depth-to-basement data and density–depth function, based on borehole logging data. Bouguer slab formula was used in order to simplify basement model and calculate the operator for the linear inversion of gravity effect of the basement into lateral density distribution. The results were tested by forward modeling, and differences between observed and modeled gravity data were used for the next iteration of the inversion and correction of density values in each point along the profile. The procedure was tested using synthetic models and field example, and both results were satisfying, especially taking into account the simplicity of the inversion technique. The main problem was the effect of abrupt changes in the basement topography on density distribution, but it was downsized by filtering. Basement density maps were compiled based upon the density distribution along the profiles.

3D mapping and gravity modelling of Capel and Faust basins

The Capel and Faust basins lie at water depths of 1500- 3000 m, 800 km east of Brisbane. Geoscience Australia began a petroleum prospectivity study of these remote frontier basins with acquisition of reflection and refraction seismic, gravity, magnetic and multi-beam bathymetry data during 2006/07. The approach mapped a complex distribution of sub-basins via traditional 2D reflection seismic interpretation techniques and 3D mapping and gravity modelling. Gravity models were used to inform the seismic interpretation of basement. Gravity models had three sediment layers with average densities inferred from refraction modelling of 1.85, 2.13, 2.31 t/m3 overlying a pre-rift basement of density 2.54 t/m3, itself consisting in part of intruded older basin material. Gravity modelling of the simple density model arising from the initial interpretation of reflection seismic data indicated a first order agreement between observed and calculated data. Second order misfits were accounted for by adjustments to the sediment density values, localised adjustments to basement depths, and heterogeneity in the basement densities. The study concluded that sediment of average velocity 3500 m/s exceeds 6000 m thickness in the northwest of the area, which is sufficient for potential petroleum generation.

Sequential Inversion for Geoacoustic Parameters in the South China Sea Using Modal Dispersion Curves

This paper presents an inversion scheme that uses two wideband explosive sources based on modal dispersion characteristics to synchronously obtain sound speed profile and bottom parameters. The signal received on a single hydrophone in a vertical line array is decomposed into a series of propagation modes within the framework of normal mode theory, and the dispersive characteristics of the modes are analyzed using time–frequency analysis. Time-warping transform is applied to resolve the propagation modes of explosive sources from the South China Sea in 2007. The relative travel time differences of the propagation modes are used to invert the environmental parameters, including empirical orthogonal function coefficients, sediment thickness, sound speed, density, basement sound speed and density. Sediment density and basement density are constrained by Hamilton’s empirical relationship, and relative mode energy ratio is used to estimate the bottom attenuation coefficient. An adaptive simplex simulated annealing algorithm is used for the inversion. The reliability of the inversion results is verified via uncertainty analysis. The transmission loss calculated using the deduced parameters matches the experimental data well.

Rift processes in the Westralian Superbasin, North West Shelf, Australia: insights from 2D deep reflection seismic interpretation and potential fields modelling

Acquisition of long-offset (8–10 km), long-record length (12–18 sec), 2D reflection seismic and ship-borne potential fields data (WestraliaSpan by Ion/GXT and New Dawn by PGS) on the North West Shelf of Australia provide the opportunity to study rift processes in the context of modern models for rifted margins (Manatschal, 2004). Basement and Moho surfaces were interpreted on seismic reflection data. Refraction models from Geoscience Australia constrain Moho depth and initial densities for gravity modelling through standard velocity-density transformation. 2D joint inversion of seismic reflection and gravity data for Moho depth and basement density constrain depth to basement on seismic. 2D gravity and magnetic intensity forward modelling of key seismic lines constrain basement thickness, type and density. Late Permian and Jurassic-Early Cretaceous rift zones were mapped on seismic reflection data and constrained further by inversion and forward modelling of potential fields data. The Westralian Superbasin formed as a marginal basin in Eastern Gondwana during the Late Permian rifting of the Sibumasu terrane. Crustal necking was localised along mechanically-weak Proterozoic suture belts or Early Paleozoic sedimentary basins (such as Paterson and Canning). Mechanically-strong cratons (such as Pilbara and Kimberley) remained intact, resulting in necking and hyper-extension at their edges. Late Permian hyper-extended areas (such as Exmouth Plateau) behaved as mechanically-strong blocks during the Jurassic to Early Cretaceous continental break-up. Late Permian necking zones were reactivated as failed-rift basins and localised the deposition of the Jurassic oil-prone source rocks that have generated much of the oil discovered on the North West Shelf.

Gravity anomalies of sedimentary basins and their mechanical implications: Application to the Ross Sea basins, West Antarctica

In general, sedimentary basins are characterized by negative free-air and Bouguer gravity anomalies. However, the extensional basins of the Ross Sea are paradoxical in that positive gravity anomalies overlay the Victoria Land Basin, Northern Basin, Central Trough and Northern Central Trough while basement highs are associated with negative gravity anomalies. Measured basement densities from DSDP basement cores give values between 2600–2800 kg/m 3 while bulk sediment densities range from 1210–2200 kg/m 3, indicating a normal density relationship between basement and sediment infill. In contrast, the relatively young and narrow Terror Rift is associated with negative free-air and Bouguer gravity anomalies, but has a different geological history as compared to the larger Ross Sea basins. Process-oriented gravity modeling indicates that magmatic underplating and crustal intrusions are inconsistent with the observed gravity and basement geometry of the Ross Sea basins. The magma volume necessary to account for the distribution and amplitude of the positive gravity anomaly of the Central Basin and be isostatically balanced would need to be comparable to the tholeiitic flood basalt volume of the Columbia River province—it is thus unlikely that the volume of Neogene volcanics of the Ross Sea region is sufficient to explain the observed gravity relationship by modifying the bulk density of the crust. We demonstrate that positive free-air and Bouguer gravity anomalies over extensional basins are the consequence of a relatively low flexural strength of the lithosphere during rifting being contrasted by higher flexural strengths later during sedimentation. As the difference between the rigidity of the lithosphere during sedimentation increases relative to the rigidity of the rifted lithosphere, the gravity over the basin becomes progressively more positive but only for a limited range of wavelengths. The narrow width of the Terror Rift precludes it from having a positive gravity anomaly while the opposite is true for the large Ross Sea basins. For the Ross Sea region, such a loading scenario requires a significant delay between extension and the timing of sediment infilling of the basins, consistent with the late Cretaceous extension of the Ross Sea region and the sedimentary succession being dominated by large-scale late Eocene–Neogene glaciogenic progradational sequences. Sediment source was presumably from the denudation of the Transantarctic Mountains, which commenced in the late Paleogene. The time delay between the late Cretaceous formation of the Transantarctic Mountains, late Paleogene exhumation, and the generation of significant Paleogene paleobathymetry requires either the Ross Sea region to be sub-aerial and sediment starved for most of the Paleogene and/or the Paleogene climate was ineffective in producing clastics until the onset of glaciation in the late Eocene–early Oligocene.

A high resolution gravity survey near McArthur River uranium deposit, Athabasca Basin, Canada

A high resolution gravity traverse (27.2 km-long, 50 m or 100 m station-spacing) across the unconformity-associated McArthur River uranium deposit, Athabasca Basin, Canada was conducted to investigate basement geology and locate alteration zones related to mineralization. In this geological environment, amplitudes of gravity anomalies relevant to uranium exploration (related to offsets of the basement unconformity, basement relief and alteration zones) are typically <1 mGal, but range up to 2 mGal. Alteration near the McArthur River deposit is commonly present as silicification of the basin sandstones, located above basement quartzite units. Many short to intermediate wavelength (250-1800 m) anomalies, amplitudes <0.5 mGal, are identified along the traverse profile, superposed on longer wavelength and larger amplitude (up to 6000 m, and 1.8 mGal, respectively) anomalies. Preliminary modeling of anomalies, constrained by magnetic data, drill-hole logs and density information, and reflection seismic data, indicates a variety of geological contributions. Short wavelength anomalies are attributed to changes in thickness of the glacial overburden, thickening in some places being linked to preferential erosion along an underlying fault. Long wavelength anomalies are explained by variations in basement density and offsets of the basement unconformity. Importantly, from an exploration perspective, most intermediate wavelength positive anomalies are explained by a combination of silicification within the Athabasca Sandstone, and associated basement relief produced by underlying basement quartzite units. Desilicification is also modeled, but limited. The gravity method, apparently, has been successful in identifying alteration zones that may be of significant interest for uranium exploration, because of their association with mineralizing fluids, and because patterns of silicification/desilicification are important for studies of fluid flow.

3D Gravity Modelling and Interpretation for the 1:250,000 Boulia Map Sheet, Queensland

Sedimentary cover thickness strongly affects area selection for exploration in Proterozoic and Archean basement. Depth-to-basement is often inferred from gravity data. Interpretation of gravity is, however, fraught with ambiguity. The uncertainty can be minimised if all available information is exploited. In this paper drill logs, aeromagnetic depth-to-basement and basement geology interpretations, and a compilation of density values, as well as gravity data, are used to constrain a 3D density model of the Boulia 1:250,000 map area, Queensland. Four recent events riggered this work: acquisition of 4km-spaced gravity data by the Queensland Department of Mines and Energy; publication of an aeromagnetic interpretation by AGSO; development of a computer program for constrained 3D inversion of gravity; and availability of sophisticated 3D geological modelling and visualisation software. A 3D starting model was constructed, with basement topography, lithology, and density conforming to drill hole data and aeromagnetic interpretations. Paleozoic cover was represented as a layer of uniform density. The calculated gravity response of the starting model resembled the observed gravity in gross terms, but did not provide a satisfactory fit. Shallow magnetic basement correlated with low gravity. Denser basement units tended to be deeply buried. The basement densities and elevations have been adjusted via inversion, to achieve a close fit to the data. In the areas of shallow basement (less than 1 km), basement depths have changed by up to 300m. In the NW corner and the central south of the map sheet, the interpreted depth of undivided granite basement has in places been reduced to 100m and 200m respectively. The Boulia 3D density model represents a starting point for future refinement as additional holes are drilled and as new gravity and density data are collected. In particular, the gravity contribution from limestones in the cover sequence is not well understood. Additionally, combined magnetic and gravity inversion could further advance geological understanding of the area.

Exploring Through Cover - The Integrated Interpretation of High Resolution Aeromagnetic, Airborne Electromagnetic and Ground Gravity Data from the Grant’s Patch Area, Eastern Goldfields Province, Archaean Yilgarn Craton Part B: Gravity Inversion as a Bedrock and Regolith Mapping Tool

The cost and risk associated with mineral exploration in Australia increases significantly as companies move into deeper regolith-covered terrain. The ability to map the bedrock and the depth of weathering within an area has the potential to decrease this risk and increase the effectiveness of exploration programs.This paper is the second in a trilogy concerning the Grant’s Patch area of the Eastern Goldfields. The recent development of the VPmg potential field inversion program in conjunction with the acquisition of high-resolution gravity data over an area with extensive drilling provided an opportunity to evaluate threedimensional gravity inversion as a bedrock and regolith mapping tool.An apparent density model of the study area was constructed, with the ground represented as adjoining 200 m by 200 m vertical rectangular prisms. During inversion VPmg incrementally adjusted the density of each prism until the free-air gravity response of the model replicated the observed data. For the Grant’s Patch study area, this image of the apparent density values proved easier to interpret than the Bouguer gravity image.A regolith layer was introduced into the model and realistic fresh-rock densities assigned to each basement prism according to its interpreted lithology. With the basement and regolith densities fixed, the VPmg inversion algorithm adjusted the depth to fresh basement until the misfit between the calculated and observed gravity response was minimised. The resulting geometry of the bedrock/regolith contact largely replicated the base of weathering indicated by drilling with predicted depth of weathering values from gravity inversion typically within 15% of those logged during RAB and RC drilling.

Magnetic and gravity anomaly patterns related to hydrocarbon fields in northern West Siberia

A study of the features of gravity and magnetic fields in the vicinity of oil and gas reservoirs in West Siberia demonstrated a spatial relationship with the hydrocarbon deposits. The relevant magnetic and gravity anomalies cover approximately [Formula: see text] in northern West Siberia. Amplitude and frequency were investigated initially using double Fourier spectrum (DFS) analysis. This was followed by (1) application of transformations, filtering, and “moving windows” analysis; (2) compilation of maps of regional and local anomalies, and potential field derivatives; and (3) investigation of the distribution of parameters in areas of known deposits. Hydrocarbon deposits are located mostly at the slopes of positive regional gravity and magnetic anomalies which are interpreted as relating to deep riftogenic structures. At the same time, it is established that the location of hydrocarbon depositions coincides commonly with local gravity and magnetic minima generated by lows in basement density and magnetization. All known hydrocarbon deposits in northern West Siberia are in areas characterized by comparatively high gradients of constituent of gravity anomalies with a wavelength of about 90–100 km. These newly revealed links between reservoirs and potential field parameters may be a means to predict new discoveries in poorly explored territories and seas, primarily in Russia's Arctic shelf.

Parameter estimation in flexural plate motion; elastic, plastic-elastic and thermo-elastic models applied to the Chukchi Basin, Alaska

Models are presented of flexural plate basement motion (elastic, plastic-elastic and thermo-elastic) due to: (1) compressive or extensional stress; (2) variation of lithospheric basement density along the flexural plate; (3) spatial variations of plate rigidity; and (4) variations of initial plate angle, hinge point position, plate length, initial end load and plate curvature. A non-linear inverse method determines parameters. Synthetic test results and a case history from the Central Chukchi Basin illustrate the accuracy and resolution of the inversion procedure. The Central Chukchi Basin has an unloaded geometry mainly controlled by a Palaeozoic fault system. Plastic or thermal effects produce a narrow unloaded basin geometry not conforming to the Paleozoic fault distribution or to the isopach map of the Upper Ellesmerian sediments. Inverse elastic model results agree with both the known fault system development (from seismic data) and the direction of the Brooks Range motion.

The Kenya rift axial gravity high: a re-interpretation

Since KRISP 85 did not provide overwhelming evidence for the massive intrusion that was originally suggested to explain the axial gravity high yet did provide a velocity section for the upper crust along the axis of the Kenya Rift, it is appropriate to use this section to control a re-interpretation of the gravity anomalies. A 2 1 2 D inversion procedure has been used to model a number of isostatic anomaly profiles between Lake Baringo and Suswa. There are too many unknowns and gravity station coverage is too sparse for the results to be unique. Nevertheless, certain conclusions can be drawn. One of those is that some relatively dense material exists within the basement all along the Rift axis, since the axial isostatic anomalies are positive even though they occur where there are several thousand metres of Cenozoic volcanics of relatively low density (inferred from their seismic velocity of 3.7–5.1 km/s). The dense material is envisaged as a zone of dyke injection and assigned a density of 2.75–2.76 g/cm 3 (corresponding to its 6.05 km/s velocity) compared to a normal basement density of 2.70 g/cm 3. It is assumed to extend down to 22 km—the top of the 7.1 km/s layer. The KRISP 85 line passed just east of Menengai, where the basement velocity increases to about 6.6 km/s over a distance of about 20 km. On an east-west gravity profile through Menengai there is a gravity high corresponding to this velocity increase which has been modelled as a basic intrusion (density 2.93 g/cm 3) underlying the caldera.

Three‐Dimensional Gravity Modeling of the Geologic Structure of Long Valley Caldera

A 48‐mGal gravity low coincides with Long Valley caldera and is mainly attributed to low‐density caldera fill. Gravity measurements by Unocal Geothermal have been integrated with U.S. Geological Survey data, vastly improving gravity station coverage throughout the caldera. A strong regional gravity trend is mainly attributed to isostasy. A “best fitting” (based on regional control of basement densities) Airy‐Heiskanen isostatic model was used for the regional correction. A three‐dimensional, multiple‐unit gravity modeling program with iterative capabilities was developed to model the residual gravity. The density structure of Long Valley caldera and vicinity was modeled with 22 discrete density units, most of which were based on geologic units. Information from drill hole lithologies, surface geology, and structural geology interpretations constrain the model. Some important points revealed by the three‐dimensional gravity modeling are that (1) the volume of ejected magma associated with the Bishop Tuff eruption is greater than previously thought, (2) the caldera structure is strongly influenced by precaldera topography and the extensions of major, active faults, (3) the main west ring fracture is coincident with the Inyo Domes‐Mono Craters fracture system, (4) a relatively low‐density region probably underlies the caldera, and (5) a silicic magma chamber may underlie Devils Postpile.

Isostatic response of the large-offset Atlantic Equatorial fracture zones

Spectral analysis techniques have been applied to data sets of gravity and topography selected across fracture zones with large offset in the Equatorial and Central Atlantic Ocean and processed independently for each fracture zone. Three simple compensation models, two in local isostatic equilibrium (Airy I and II) and one in regional equilibrium (Plate model) have been tested. It is found that the free-air anomalies are primarily controlled by the topography and its isostatic response. For short wavelengths, admittance can be explained either by the effect of uncompensated sea floor topography with high density basement or by the effect of uncompensated sea floor topography with normal oceanic basement density but accompanied by a crust of constant thickness. For intermediate wavelengths, admittance for the Romanche fracture zone agrees best with a local isostatic model in which compensation is achieved by a less dense material in the upper mantle. No such evidence exists for the Vema and Chain fracture zones for which topography could just as well be regionally supported by an elastic plate 3 to 10 km thick. For longer wavelengths, the admittances computed for the three fracture zones are compatible with a thermal compensation of the topography.

Powder River Basin, Wyoming: Structural Development, Hydrocarbon Migration, and Accumulation: ABSTRACT

The geographical location of oil accumulations in the Powder River basin, Wyoming, is closely related to present basement structure. About 70% of the basin's cumulative oil production has been obtained from only 12 fields or 23% of the total fields. Each major oil field lies in an area of a pronounced positive Bouguer gravity anomaly and in the path of preferred regional hydrocarbon migration. Powder River basin Bouguer gravity anomalies most likely are caused by a combination of present basement structure and density changes in post-Paleozoic sediments; the latter are the result of synsedimentary basement structure and/or related topographic features influencing post-Paleozoic sedimentation. Stratigraphic and structural traps occur in close interrelationships across the asin. Published geochemical data in connection with available regional subsurface data permit mapping the preferred migration paths for oil and gas across the basin. Future discoveries of major hydrocarbon fields will be made in these hydrocarbon migration paths and areas in and around regional positive Bouguer gravity anomalies. Powder River oil field distribution follows general rules known from practically all producing basins but rarely used for lack of sufficient integration of geological and geophysical data. Gas field distribution is expected to be similar to oil field distribution. End_of_Article - Last_Page 298------------

Oil Basins of Peru: ABSTRACT

The principal oil basin of Peru, confined between the Andes and the Pacific Ocean, is a part of the long narrow belt (750×50 miles) of chiefly marine Tertiary sediments extending from northern Peru to western Colombia. Oil is produced on the Talara-Negritos, Lobitos, and El Alto uplifts from innumerable normal fault traps, which appear to have developed due to differences in basement density and/or rigidity rather than tectonic folding. The faults reflect the persistent tensional stress which prevailed from early Tertiary to early Quaternary. The area produces about 50,000 b/d of 37° gravity oil; cumulative production to date is about 600 million barrels. The other oil-producing area of Peru is the heavily jungle-covered, scarcely explored Maranon Basin, which occupies the upper Amazon tributary drainage east of the Andes. This basin is a part of the sub-Andean trough, an elongate downwarp which persists from eastern Venezuela, through eastern Colombia, Ecuador, Peru, Bolivia, into western Argentina, lying between the overriding Andean uplift and the Guiana and Brazilian shields. The basin contains up to 12,000 feet of Tertiary and Quaternary fluviatile and lacustrine sediments which mask the Mesozoic and upper Paleozoic objectives. From outcrops and seepages in the Andean foothills, these objectives should constitute substantial reservoir and source rocks. The Ganso Azul and recently discovered Maquia fields, lying on the southwestern margin of the Maranon Basin in the belt of surface folds, are the only producing structures in the basin to date, with a daily production of less than 3,000 barrels, and an ultimate yield of probably less than 15 million barrels. Attention is directed to the many geological similarities exhibited by North and South America in structural framework, tectonics, paleogeography, and stratigraphy. End_of_Article - Last_Page 255------------