Steep Gravity Gradient Research Articles

Tectonic Characteristics, Evolution, and Significance of the Zhouwang Fault, Lower Yangtze Area, Eastern China

AbstractFault geometry, kinematics, geophysics, the tectonic stress field and tectonic evolution of the Zhouwang fault in the southern Jiangnan tectonic transition zone of the Lower Yangtze region, eastern China are examined. Field observations show the fault is composed of a series of nearly E–W trending, N–S dipping faults, and four stages of tectonism (sinistral strike‐slip, thrust nappe, normal fault, and dextral strike‐slip) developed in turn. Geophysical data show that the fault trends almost linearly E–W along a flat, steep gravity gradient at shallow depth, with distinct gravity anomalies to the north and south and different in the north and south. Also, the deep part is characterized by northward dip and a gradual slowing down. Tectonic stress field analysis indicates that the fault experienced four tectonic movements: NNE–SSW compression, NNW–SSE compression, NEE–SWW extension, and E–W compression. Combined with regional tectonic background and previous research results, this indicates that: (1) the Zhouwang fault experienced sinistral strike‐slip movement during the Indosinian Period (260–200 Ma); (2) thrust nappes developed during the early Yanshanian Period (163–145 Ma); (3) a normal fault occurred in the late Yanshanian Period (125–65 Ma); and (4) dextral strike‐slip movement occurred in the Himalayan Period (ca. 50–37 Ma). The results reveal the tectonic evolution of the fault during Mesozoic deformation in the area, and also reveal the geological evolution and tectonic transformation of the Lower Yangtze region, which is key to our understanding of intracontinental deformation in eastern China.

A two million-year history of rifting and caldera volcanism imprinted in new gravity anomaly compilation of the Taupō Volcanic Zone, New Zealand

ABSTRACT The Taupō Volcanic Zone (TVZ) is characterised by a negative residual gravity anomaly that correlates with a zone of normal faulting, subsidence and voluminous silicic volcanism from c. 2 Ma. A steep gravity gradient is associated with downfaulting of the Mesozoic metasedimentary basement at its eastern and western margins, and all known calderas are associated with prominent local gravity lows. New Bouguer and residual gravity anomaly maps are presented that show improved detail over previous maps. The western margin of the TVZ shows as a series of parallel northeast-trending boundaries and northwest-trending step-over zones. The new map also reveals the complex boundaries of the Okataina caldera, and greater detail north of Taupō where gravity lows at Reporoa and Mihi form part of a larger and more extensive, elliptical low-gravity feature, suggesting that they may be separate collapse areas within a larger caldera complex. Around the margins of the TVZ, the Hauraki graben/rift and sedimentary basins at Hamilton, Waimiha, Galatea and Waiohau are also characterised by low residual gravity.

Mesoarchean convergent margin processes and crustal evolution: Petrologic, geochemical and zircon U–Pb and Lu–Hf data from the Mercara Suture Zone, southern India

The Mercara Shear Zone is sandwiched between the Western Dharwar Craton and the Coorg Block in the Southern Granulite Terrain of India, and is marked by steep gravity gradients interpreted to suggest the presence of underplated high-density material in the lower crust. Here we present geological, petrological and geochemical data, together with zircon U–Pb ages and Lu–Hf isotopes from a suite of metaigneous (TTG-related gneisses, charnockite, metagabbro, mafic granulite) and metasedimentary (quartz mica schist, khondalite, garnet biotite gneiss, kyanite–sillimanite bearing metapelite) rocks from this zone. Geochemical data on the magmatic suite suggests formation through subduction-related arc magmatism, whereas the metasediments represent volcano-sedimentary trench sequences. Phase equilibrium modeling of mafic granulites from the Mercara Shear Zone suggests P–T range of 10–12kbar at 700°C to 900°C. The zircon data yield weighted mean 207Pb/206Pb ages of 3229±80Ma for metagabbro, 3168±25Ma for the charnockite, and 3181±20Ma for the mafic granulite. Ages ranging from 3248±28Ma to 3506±26Ma were obtained from zircons in the kyanite/sillimanite bearing metapelite, 3335±44Ma from khondalite, 3135±14Ma from garnet biotite gneiss, 3145±17Ma to 3292±57Ma from quartz mica schist and 3153±15Ma to 3252±36 from TTG gneiss. The tightly defined ages of 3.1 to 3.2Ga from igneous zircons in the magmatic suite suggest prominent Mesoarchean convergent margin magmatism. The timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca. 3.0Ga which might mark the collisional event between the Western Dharwar Craton and the Coorg Block. Hf isotope features suggest magma derivation mostly from juvenile sources and the Lu–Hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. Our study defines the Mercara Shear Zone as a terrane boundary, and possible Mesoarchean suture along which the Coorg Block was accreted to the Western Dharwar Craton. The accretion of these continental fragments might have coincided with the birth of the oldest supercontinent “Ur”.

Synchronous oceanic spreading and continental rifting in West Antarctica

AbstractMagnetic anomalies associated with new ocean crust formation in the Adare Basin off north‐western Ross Sea (43–26 Ma) can be traced directly into the Northern Basin that underlies the adjacent morphological continental shelf, implying a continuity in the emplacement of oceanic crust. Steep gravity gradients along the margins of the Northern Basin, particularly in the east, suggest that little extension and thinning of continental crust occurred before it ruptured and the new oceanic crust formed, unlike most other continental rifts and the Victoria Land Basin further south. A preexisting weak crust and localization of strain by strike‐slip faulting are proposed as the factors allowing the rapid rupture of continental crust.

Geometry of the inverted Cretaceous Chañarcillo Basin based on 2-D gravity and field data – an approach to the structure of the western Central Andes of northern Chile

Abstract. This paper discusses an integrated approach that provides new ideas about the structural geometry of the NNE-striking, Cretaceous Chañarcillo Basin located along the eastern Coastal Cordillera in the western Central Andes of northern Chile (27–28° S). The results obtained from the integration of two transverse (E–W) gravity profiles with previous geological information show that the architecture of this basin is defined by a large NNE–SSE-trending and east-vergent anticline ("Tierra Amarilla Anticlinorium"), which is related to the positive reactivation of a former Cretaceous normal fault (Elisa de Bordos Master Fault). Moreover, intercalations of high and low gravity anomalies and steep gravity gradients reveal a set of buried, west-tilted half-grabens associated with a synthetic normal fault pattern. These results, together with the uplift and folding style of the Cretaceous synextensional deposits recognized within the basin, suggest that its structure could be explained by an inverted fault system linked to the shortening of pre-existing Cretaceous normal fault systems. Ages of the synorogenic deposits exposed unconformably over the frontal limb of the Tierra Amarilla Anticlinorium confirm a Late Cretaceous age for the Andean deformation and tectonic inversion of the basin.

Gravity Anomaly Computed by GOCE Data in Northeastern Margin of Tibetan Plateau

In this paper, gravity anomaly in northeastern margin of Tibetan Plateau (90ºand 110º E, 28ºand 42º N) is computed using satellite gravity gradiometry data from GOCE satellite. The computed gravity anomaly is compared with the topographical data and location of some strong earthquakes in this region. The result shows that gravity anomaly has good conformity with the regional tectonic distribution and strong earthquake usually occurred in the steep gravity gradient zone.

Gravity Interpretation of the Cameroon Mountain (West Central Africa) Based on the New and Existing Data

A new gravity survey of the Mount Cameroon area has enabled the definition of four major gravimetric do- mains, which coincide with the recognized structural units. In order to determine the nature of superficial and deep structures in this mountainous zone, new gravity data have been processed. These new gravity data was integrated to existing gravity data to propose the new complete Bouguer anomaly map of the region, and then to show major characteristics of the Bouguer gravity of this area. The interpretation of gravity patterns (bouguer maps) in terms of geological data, shows that the Mount Cameroon zone belongs to a wide positive anomaly; these anomalies display complex gravity domains, which seem to be similar to that due to major structural units in the region and volcanic activity of the mountain. In the mountain active zone in particular (between 2000 and 3800 m of altitude), the new anomalies map shows high gravity anomalies (from 11 to 60 mgal), coupled with low gravity at some stations (in the summit, 4060 m) where gravity anomaly is about –30 mgal. The steep WNW-ESE gravity gradients observed on the gravity maps mark the transition between positive in the south and negative anomalies.

Gravity Anomaly From Satellite Gravity Gradiometry Data by GOCE in Japan Ms9.0 Strong Earthquake Region

Abstract The great tsunami and secondary disasters caused by Ms9.0 strong earthquake occurred in northeastern Japan on March 11, 2011 has attracted widely public attention from all over the world. In this paper, gravity anomaly is calculated in the region between east longitude 132°and 145°, north latitude 35°and 50° by using satellite gravity gradiometry data observed by GOCE(Gravity field and steady-state Ocean Circulation Explorer). The computed gravity anomaly is compared with the topographical data and location of seismic activity in this region. The results show that gravity anomaly has great consistency with the regional tectonic distribution and Ms9.0 strong earthquake occurred in the steep gravity gradient zone. The spatial distribution of the gravity anomaly has obviously correspondence with extrusion action from the Pacific plate.

Tectonic significance and consequences of the Gondwanide orogeny in northern Patagonia, Argentina

The tectonic configuration of northern Patagonia is recognized through geological and geophysical studies, which define several areas characterized by positive and negative gravity anomalies with steep gravity gradients. Positive areas are interpreted as continental crust with Grenvillian and Pampean basement. The Pampean basement occurs to the north and south of the interpreted suture between Patagonia and Gondwana South America. Most negative anomalies are interpreted to represent Gondwana igneous and sedimentary rocks. Areas characterized by steep gravity gradients are coincident with shear zones and mylonitic belts that were active during Gondwana times, a period of great magmatic and tectonic activity in northern Patagonia. Dextral movement along the E–W-trending Huincul Fault Zone resulted in block collage, west–northwest crustal translations, and counterclockwise rotation of large crustal blocks belonging to the North Patagonia Massif. The indentation of blocks within the North Patagonia Massif resulted in tectonic escape of the surrounding blocks, each with a different trajectory. Escape tectonics may explain the diversity of stress directions during Gondwana Orogeny.

Gravity anomaly patterns in the south-central Zimbabwe Archaean craton and their geological interpretation

The granite-greenstone terrain of south-central Zimbabwe, encompassing the Belingwe (Mberengwa) greenstone belt and sections of the Great Dyke, provides important constraints on models for the evolution of the Zimbabwe craton and the Archaean crust in general. In this paper we enhance and model existing and recently acquired gravity data from the region and correlate the anomalies and their derivatives with the known basement geology to evaluate models for greenstone belt development. We also study the spatial gneiss-granite-greenstone association in general, and the geologic implications of models of the anomaly patterns in particular. Although the Belingwe greenstone belt has been mapped, its subsurface geometry is poorly known. Similarly, the Great Dyke is well studied, but no systematic study of the extent and cross-cutting relations of other mafic dykes in the Archaean crust has been undertaken. The regional gravity field shows no evidence for crustal thickness variations in the area and the gravity anomalies can be explained by lateral density variations of the supracrustal rocks. Prominent gravity highs are observed over the high density (⩽ 3000 kg/m 3) volcano-sedimentary piles (greenstone belts) and ultramafic complexes. Well-defined elongate, sub-oval/elliptical gravity lows are associated with intrusive granitic plutons. The granite-greenstone contacts are marked by steep gravity gradients of up to 5 mGal/km that imply steeply dipping or near-vertical contacts for the anomalous bodies. This is tested and confirmed by 2 1/ 2D modelling of gravity profiles across the Belingwe and Fort Rixon greenstone belts, constrained by measured densities and observed geological data. The modelling also indicates that these belts, and possibly all the belts in the study area (based on comparable densities and anomaly amplitudes), have limited depth extents in the range of 3–5 km. This is comparable to thicknesses obtained elsewhere from deep seismic reflection data and geoelectrical studies, but mapped stratigraphic thicknesses give a maximum depth extent of about 9.5 km. Present studies and previous work support the idea that the volcanics were extruded within rift zones and laid on older granitic crust, followed by subsidence and rapid deposition of sediments that were sourced from the adjacent basement terrains. The volcano-sedimentary sequences were subsequently deformed by intruding younger plutons and affected by late-stage strike-slip activity producing cross-cutting structures.

Delineation of a large ultramafic massif embedded within a major SW Pacific suture using gravity methods

Gravity studies have delineated the largest ultramafic massif in New Zealand, embedded within a buried major SW Pacific crustal suture zone. This suture records terrane collision onto the Gondwana margin during the Mesozoic and separates a forearc terrane from an outboard accretionary prism terrane. It can be traced throughout the length of New Zealand as the Junction Magnetic Anomaly and contains the Permian Dun Mountain Ophiolite Belt, which in the South Island of New Zealand is characterized by a string of isolated ultramafic massifs in a sheared matrix of serpentinite and sediment. Our analysis reveals a steep gravity gradient at the suture boundary which is attributed to a newly recognised density contrast (0.1 Mg m − 3 ) between terranes of the forearc and the accretionary prism. The massif itself is marked by the occurrence of a strong, elongate residual gravity anomaly (+ 120 g.u.) extending 50 km along the suture and coincident with the Junction Magnetic Anomaly. It is modelled, at its southern end, as a dense, 15 km wide source body, extending to at least 6 km in depth. In conjunction with detailed aeromagnetic data, this modeling indicates the presence of a spindle-shaped ultramafic massif, analogous to, but larger than similar bodies found within the Dun Mountain Ophiolite Belt elsewhere. This fabric of sheared serpentinites enclosing ultramafic massifs therefore extends at least the length of New Zealand and probably beyond. In part it may result from accretion of asperities in the subducting plate, but it is also due to disruption of larger ultramafic bodies during subsequent strike-slip motion, which caused the remarkable linearity of the Dun Mountain Belt. Given the common occurrence of the plate tectonic processes involved, it is likely that such structures can be found in other regions around the world using similar geophysical potential field methods.

Shallow crustal structure of Chicxulub impact crater imaged with seismic, gravity and magnetotelluric data: inferences about the central uplift

The structure of the onshore portion of the Chicxulub impact crater (Yucatan, Mexico) has been studied with seismic, gravity and magnetotelluric (MT) exploration methods. A dispersion analysis of Rayleigh waves along a 150-km long, east–west profile permitted the shallow Tertiary cover to be imaged to a depth of 400 m. Three layers were mapped. The thickness of the first two layers increase as we approach the sinkhole ring from the exterior of the crater (consistent with the existence of a central basin). Those thicknesses change from 100–150 m outside the crater rim to more than 200 m in the terrace zone. At the crater centre, the first layer is again approximately 100 m thick and has a low shear wave velocity (approximately 1 km s−1). Its velocity is slightly larger in the rest of the profile. Outside the central basin, we observed a third layer with a shear wave velocity larger than 1.8 km s−1, but its thickness could not be determined. The velocity distribution along our profile increases monotonously with depth, without low-velocity layers. The inferred inward slope of the two shallow layers immediately outside the central basin correlates well with a smooth gravity gradient. In addition, we present results of detailed gravity measurements along a profile that coincides (in part) with the line covered by the seismic experiment. These gravity measurements were inverted to obtain a density model. In this model (which is of course non-unique), the steep gravity gradients are explained by the faults along which the slumping of the terrace blocks took place. Two of these may correspond to the faults observed at radial distances of 85 and 96 km in offshore seismic data. Finally, we show results obtained using MT soundings. They provide additional, independent constraints on the structure of the Chicxulub crater. MT soundings at the centre of the crater show a sharper increase in resistivity than elsewhere in the impact structure. This sharper increase in resistivity may be related to the uplifted basement of the structural high at a depth of 4 km. Only the MT soundings located over the central gravity high display this feature. Our three different data sets are independently consistent with the existence of a central structural high in the crater and support a twin-peak shape for this high.

Pull-down basin in the central part of Japan due to subduction-induced mantle flow

The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone. A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes.

Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal

SUMMARY We use two gravity profiles that we measured across Central Nepal, in conjunction with existing data, to constrain the mechanical behaviour and the petrological structure of the lithosphere in the Himalayan collision zone. The data show (1) overcompensation of the foreland and undercompensation of the Higher Himalaya, as expected from the flexural support of the range; (2) a steep gravity gradient of the order of 1.3 mgal km x1 beneath the Higher Himalaya, suggesting a locally steeper Moho; and (3) a 10 km wide hinge in southern Tibet. We compare these data with a 2-D mechanical model in which the Indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland. The model assumes brittle Coulomb failure and nonlinear ductile flow that depends on local temperature, which is computed from a steadystate thermal model. The computed Moho fits seismological constraints and is consistent with the main trends in the observed Bouguer anomaly. It predicts an equivalent elastic thickness of 40‐50 km in the foreland. The flexural rigidity decreases northwards due to thermal and flexural weakening, resulting in a steeper Moho dip beneath the high range. Residuals at short wavelengths (over distances of 20‐30 km) are interpreted in terms of (1) sediment compaction in the foreland (Dr=150 kg m x3 between the Lower and Middle Siwaliks); (2) the contact between the Tertiary molasse and the meta-sediments of the Lesser Himalaya at the MBT (Dr=220 kg m x3 ); and (3) the Palung granite intrusion in the Lesser Himalaya (Dr=80 kg m x3 ). Finally, if petrological transformations expected from the local (P, T) are assumed, a gravity signature of the order of 250 mgal is predicted north of the Lesser Himalaya, essentially due to eclogitization of the lower crust, which is inconsistent with the gravity data. We conclude that eclogitization of the Indian crust does not take place as expected from a steady-state local equilibrium assumption. We show, however, that eclogitization might actually occur beneath southern Tibet, where it could explain the hinge observed in the gravity data. We suspect that these eclogites are subducted with the Indian lithosphere.

Deep crustal structure of the Chicxulub impact crater

We present the results of a wide‐angle seismic survey conducted over the Chicxulub impact crater. Profile Chicx‐A/A1 is a chord across the offshore portion of the crater, and Chicx‐B/F is an onshore‐offshore profile through the center of the crater. We use travel times recorded by 32 ocean bottom seismograph and 19 land‐based receivers to model the crustal structure of the crater. The primary feature in the shallow velocity structure is a low‐velocity Tertiary basin, the edge of which correlates with a steep gravity gradient in some regions of our survey. Directly beneath the Tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks (either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet). This unit has a thickness of ∼1 km and a diameter of ∼100 km. Central uplift with a diameter of 40–60 km is observed on Chicx‐B/F. A lower limit on the vertical extent of uplift is 9 km. Shallow basement is also observed along the northwest portion of Chicx‐B/F, associated with a prominent northwest trending high in the gravity field. Superimposed on a regional trend is Moho uplift of ∼1 km near the center of the Chicx‐A/A1 profile, with adjacent Moho deepening of ∼1.25–1.5 km. The Moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity.

Short-wavelength, high-amplitude gravity anomalies around the Banda Sea, and the collapse of the Sulawesi orogen

In eastern Indonesia, high-density ophiolitic rocks outcropping on islands surrounding the Banda Sea are in many cases associated with strong gravity anomalies and steep gravity gradients. However, the relationships are not always straightforward. Bouguer gravity levels and gradients over the extensive East Sulawesi Ophiolite are generally relatively low, although short-wavelength, high amplitude anomalies indicate rapid changes in thickness of high-density rocks in a few places. In the Banda Arc, most local positive anomalies due to ophiolites are superimposed on a steep regional gravity gradient but in one case, in western Seram, there is a distinct and important spatial separation between the two. On Buru, west of Seram, a gradient of more than 10mGal/km testifies to the presence of very dense rocks in the near subsurface, despite the absence of ophiolites in the outcrop.Gravity variations and ophiolite distribution around the Banda Sea are compatible with extension having occurred in the Sulawesi region following, and as a result of, Oligo–Miocene collision with an Australian-derived microcontinent. Similar histories have been proposed for many Mediterranean deep basins of similar size, shape and character, and emplacement of some of the high-density masses in the Banda Arc has probably resembled at least the later stages in the emplacement of peridotite massifs in the Rif-Betic belt. In both areas the present close association of the ultramafic rocks and their associated local anomalies with a strong regional gravity gradient is largely coincidental.

Geoid undulation modelling and interpretation at Ladak, NW Himalaya using GPS and levelling data

Fast and accurate relative positioning for baselines less than 20 km in length is possible using dual-frequency Global Positioning System (GPS) receivers. By measuring orthometric heights of a few GPS stations by differential levelling techniques, the geoid undulation can be modelled, which enables GPS to be used for orthometric height determination in a much faster and more economical way than terrestrial methods. The geoid undulation anomaly can be very useful for studying tectonic structure. GPS, levelling and gravity measurements were carried out along a 200-km-long highly undulating profile, at an average elevation of 4000 m, in the Ladak region of NW Himalaya, India. The geoid undulation and gravity anomaly were measured at 28 common GPS-levelling and 67 GPS-gravity stations. A regional geoid low of nearly −4 m coincident with a steep negative gravity gradient is compatible with very recent findings from other geophysical studies of a low-velocity layer 20–30 km thick to the north of the India–Tibet plate boundary, within the Tibetan plate. Topographic, gravity and geoid data possibly indicate that the actual plate boundary is situated further north of what is geologically known as the Indus Tsangpo Suture Zone, the traditionally supposed location of the plate boundary. Comparison of the measured geoid with that computed from OSU91 and EGM96 gravity models indicates that GPS alone can be used for orthometric height determination over the Higher Himalaya with 1–2 m accuracy.

西南日本の重力異常勾配と地震活動との関連

Relationship between distributions of shallow earthquakes and gradients of gravity anomaly field in Southwest Japan is investigated. Zones with steep horizontal gradient of gravity anomaly field are presumably caused by faults bounding different density structures in the crust. On the other hand, most shallow earthquakes are supposed to be caused by dislocations at faults in the crust. From both presumptions, it follows that the distribution of epicenters should be overlapped with that of the steep gravity gradient zones. This study verified the overlap in Southwest Japan. However, very low seismicity along the steep gradient zone of gravity anomaly was followed by the mainshock of the 1995 Hyogo-ken Nanbu earthquake (the Kobe earthquake). We also assessed such kind of seismic gaps beneath the steep gradient zones of gravity anomaly field. Many steep gradient zones of gravity anomalies with very low seismicities can be extracted. Some of them might be due to density contrasts at geological boundaries or inactive faults and some can be areas of future seismic hazards like the case of the Kobe earthquake.

Gravity anomalies and flexure of the lithosphere at Ascension Island

SUMMARY Ascension Island, in the northern South Atlantic, forms the summit of a volcanic edifice 60 km in diameter which places a substantial load on the underlying young oceanic lithosphere. An analysis of a combined data set of recent and historical surface-gravity and bathymetry measurements on and around the island suggests that the lithosphere responds to this load by flexure equivalent to that of an elastic plate only z3 km thick, and that the mean density of the volcanic edifice is ~2500 kg m-3. A steep gravity gradient across the island cannot be explained by a simple flexural model and must be attributed to lateral density variations within the volcano itself. The effective elastic thickness is considerably less than the expected z 12 km mechanical thickness of the ~6 Ma lithosphere loaded by the volcano, and less even than zero-age elastic thicknesses commonly observed at slow-spreading ridges with axial rift valleys. The unusually small elastic thickness may be attributed to the combined effects of the high curvature beneath the island, which produces bending stresses that are limited by the yield stress envelope, localized heating of the lithosphere during emplacement of the island, and crustal thickening. When these factors are taken into account, the observed flexure is consistent with rheological models based on experimental rock mechanics.

Hydrocarbon Prospect Mapping Using Balanced Cross Sections and Gravity Modeling, Onin and Kumawa Peninsulas, Irian Jaya, Indonesia: ABSTRACT

Exploration field geology mapping and acquisition of gravity data has been conducted on approximately 650 line kilometres of26 surveyed traverse lines across the Onin and Kumawa Peninsulas of western Irian Jaya. Karstified New Guinea Limestone with a maximum thickness of 2,150 m is the predominant surface outcrop, and precludes use of seismic. However, integrated use offield geology data, balanced cross-sections, and gravity modelling has enabled us to identify two giant hydrocarbon prospects. The Onin and Kumawa Peninsulas lie at the margin of Jurassic age faulting associated with the Australian Northwest Shelf. Jurassic rift sands of the Lower Kembelangan Formation are the primary reservoir target. During the Plio-Pleistocene, collision of the Australian plate. and the Banda Arc inverted sections of the rift system, including the Onin and Kumawa Peninsulas. A better understanding of the regional structure was gained by integrating the Mobil Oil gravity data (of 1992) and that collected by Shell Oil (in the 1950's) in the structurally less deformed Bomberai region east ofOnin and Kumawa. Bouguer reduction was carried out using 2.4 glcm3 density, GRS 1967 and IGSN 1971. A sequence of gravity maps were generated, including Bouguer, regional, residual, downward continued, and second derivative. Spectral analyses indicate that basement is about 3 km depth at Onin and about 6 km in the Bomberai area. The Bomberai and Onin-Kumawa regions are separated from one another by a steep gravity gradient which has a SE-NW strike direction. This gravity gradient may represent a change of lithology. Prospect definition was obtained by evaluating traverse profile data. Balanced cross-sections were constructed, using detailed biostratigraphy to determine formation thicknesses and amount of fault offsets. Where possible, onshore balanced section profiles were tied to offshore seismic profiles and wells. Two dimensional forward gravity models were calculated, using formation densities from well data. The calculated profiles were then compared to profile observed values. Differences between calculated and observed profiles were resolved by adhering to the exploration model, which required that the main thickness changes would be in the Lower Kembelangan Formation rift section. Density values for the seven formations were held constant. Constraining the variables to one (Lower Kembelangan thickness) in the gravity profile modelling maintains credibility of the technique. Errors inherent in the structural maps derived from the cross­ sections are likely to be vertical shift which would be approximately constant across the prospects. The interactive work between geologists and geophysicists was effective in producing a logical representation of subsurface geology, which in turn allowed selection of drill sites with some degree of confidence. Subsequent to completing the modelling, offshore seismic data across the western plunge of the Onin Peninsula was obtained. The structural style demonstrated in the modelling and the seismic data are comparable. This enhances our confidence in the modelling results.