Positive Gravity Anomalies Research Articles

Intrusive dike complexes, cumulate cores, and the extrusive growth of Hawaiian volcanoes

The Hawaiian Islands are the most geologically studied hot‐spot islands in the world yet surprisingly, the only large‐scale compilation of marine and land gravity data is more than 45 years old. Early surveys served as reconnaissance studies only, and detailed analyses of the crustal‐density structure have been limited. Here we present a new chain‐wide gravity compilation that incorporates historical island surveys, recently published work on the islands of Hawai‘i, Kaua‘i, and Ni‘ihau, and >122,000 km of newly compiled marine gravity data. Positive residual gravity anomalies reflect dense intrusive bodies, allowing us to locate current and former volcanic centers, major rift zones, and a previously suggested volcano on Ka‘ena Ridge. By inverting the residual gravity data, we generate a 3‐D view of the dense, intrusive complexes and olivine‐rich cumulate cores within individual volcanoes and rift zones. We find that the Hāna and Ka‘ena ridges are underlain by particularly high‐density intrusive material (>2.85 g/cm3) not observed beneath other Hawaiian rift zones. Contrary to previous estimates, volcanoes along the chain are shown to be composed of a small proportion of intrusive material (<30% by volume), implying that the islands are predominately built extrusively.

Uplift at lithospheric swells--II: is the Cape Verde mid-plate swell supported by a lithosphere of varying mechanical strength?

The Cape Verde mid-plate swell is the largest amplitude oceanic mid-plate swell on Earth at similar to 1800 km in diameter, with a crest similar to 2.2 km high, and long-wavelength positive geoid, gravity and heat flow anomalies of 8 m, 30 mGal and 10 15 mW m(-2), respectively. These characteristics and its location on the slow moving-to-stationary African Plate, which concentrates the volcanism and associated geophysical anomalies within a relatively small areal extent, makes it an ideal location to test various proposed mechanisms for swell support. Wide-angle seismic refraction data have been acquired along a similar to 474 km profile extending north-south from the swell crest. In this paper, the 2-D velocity-depth crustal model derived from forward modelling of phase traveltime picks is tested using two independent inversion approaches. The final crustal velocity-depth model derived from the combined modelling, shows no evidence for widespread thickened crust or for lower crustal velocities exceeding 7.3 km s(-1) that are indicative of undercrustal magmatic material. Using the final velocity-depth model to constrain the crust for 3-D 'whole plate' lithospheric flexure modelling of island loading alone, we show that the lithosphere of the Cape Verde region appears stronger than expected for its age. Regional-scale modelling suggests that the majority of the swell height is supported by dynamic upwelling within the asthenosphere coupled with, but to a lesser degree, the effect of a region of low density in the deeper lithosphere, originating most likely from conductive reheating of the overlying plate due to its slow-to-stationary motion. When this regional upward-acting buoyancy force is considered in the context of the shorter wavelength flexure associated with island loading, modelling suggests that the apparent high plate strength is a consequence of, in effect, a regional unbending of a lithosphere that has a long-term strength typical for its age

Preandean geological configuration of the eastern North Patagonian Massif, Argentina

The Preandean geological configuration of the eastern North Patagonian Massif is established through the use of geological and geophysical analysis. The positive gravity anomalies located near the Atlantic coast are due to 535 and 540 Ma old rocks belonging to the Pampean Orogeny (Precambrian–middle Cambrian), which are widely recognized in central and northern Argentina. The Famatinian Cycle (Ordovician–Devonian) is represented by a Silurian–Devonian marine basin equivalent to those of eastern-central Argentina and South Africa, and which was deformed at the end of the Devonian by an ∼E–W to WNW–ESE compressional event, part of the Famatinian Orogeny. Containing strong gravity gradients, the NW–SE belt is coincident with fault zones which were originated during the Gondwanide Orogeny. This event also produced NW–SE overthrusting of the Silurian–Devonian sequences and strike-slip faults that displaced blocks in the same direction. This deformation event belongs to the Gondwanide Orogeny that includes movements related to a counterclockwise rotation of blocks in northern Patagonia. The strong negative anomalies located in the western part of the area stem from the presence of rocks of the Jurassic Cañadón Asfalto basin interbedded in the Marifil Complex. These volcaniclastic sequences show mild deformation of accommodation zones in a pre-Jurassic paleorelief.

Carbonate complexes underlying Flysch belt and subsurface Neogene volcanic in the NE part of Slovakia – a potential for geothermal energy and raw materials

Abstract A positive gravity anomaly was observed in the Flysch belt realm. Based on this fact and available geological knowledge we suppose that the source of gravity anomaly might be carbonate rocks lying perhaps in the footwall of Flysch sediments. The carbonates belong perhaps to the Penninic crust cover (a part of Klippen belt), or to lower structural stage of the Flysch belt. Besides this it is interpreted more volume of Neogene subvolcanic bodies in the frame of the Flysch belt based on the results of the newest magnetic measurements in the NE part of Slovakia (Kucharič et. al., in press). These are accompanied by increasing heat flow and hydrothermal alteration within neighbouring rocks what may eventuate into creation of raw materials. These two factors - carbonates and subvolcanic bodies - are important items for appraisal of new perspective in this area not only from hydrocarbon occurrences point of view (a primary intend within this area) but also for enhancement of geothermal potential of Slovak Republic and opening possibilities for prognosis of raw material occurrences as well.

The origin of the non-mare mascon gravity anomalies in lunar basins

Many lunar basins are characterized by prominent positive gravity anomalies over the basin interiors, referred to as mass concentrations or mascons. While a significant fraction of some near-side mascon anomalies can be explained as a result of the flexural support of the mare basalts within the basins, a number of basins, including Orientale, exhibit mascons in excess of those that can be plausibly ascribed to the mare. Some basins exhibit mascons but lack mare altogether. Lunar gravity and topography data are used to map the isostatic anomaly, or the height of the surface above or below its isostatic level. Orientale is representative of the majority of lunar basins, in which the super-isostatic basin center is surrounded by a sub-isostatic annulus of comparable magnitude but greater area. The basin structure as a whole is found to be strongly sub-isostatic. High-resolution crustal thickness models of Orientale confirm that it is surrounded by an annulus of thickened but sub-isostatic crust. It is proposed that the flexural uplift of the annulus causes the uplift and positive gravity anomalies within the basin center. Finite element models are used to examine the flexural uplift of the sub-isostatic annulus and the basin center for a range of lithosphere thicknesses both outside the basin and in the basin interior. The uplift of the basin center can exceed 2km, increasing the central gravity anomaly by ∼200mGal. This annular uplift explains a significant fraction of the Orientale mascon, and is likely a dominant cause of non-mare mascons globally.

Geophysical investigations on the gravity and aeromagnetic anomalies of the region between Sapanca and Duzce, along the North Anatolian Fault, Turkey

In this paper, it is aimed to model subsurface structures to the east of the Gulf of Izmit through Duzce by using the gravity and aeromagnetic anomaly data. 1/500.000 scaled gravity anomaly map of the area was taken from the General Directorate of Mineral Research and Exploration (MTA) and it was digitized. The aeromagnetic anomaly data were obtained in the digital form. 3D and 2D models were constructed to reveal the subsurface structure in two different inset regions in the study area including most important negative and positive gravity anomalies. Seismic velocities obtained from the deep seismic recordings were converted to densities. In addition, density information from a previous research was also taken. These densities were used for construction of 3D and 2D gravity models where it was shown that there are narrow and long sedimentary basins and depressions with 0.5–3km depths. These sedimentary basins with the shape of negative flower structures indicating pull-apart basins are controlled by the active fault segments of the North Anatolian Fault (NAF). Earthquake epicenter data were also correlated with the constructed models from the gravity anomalies. Positive gravity anomalies are also caused by very shallow (about 2km) masses that are accepted as the crustal origin intrusions into the fractures of the NAF and, ophiolites and gabbro outcropping on the surface of the studied regions. These intrusives and remnants of the Tethys Ocean are located between the fault segments where the fault bifurcates and they also constitute barriers for straight extension of the NAF. Analytic signal method was applied to the aeromagnetic anomaly data to determine the locations and boundaries of the causative bodies. Those bodies are observed around Duzce, and to the E-SE of it, to the NW of Golyaka and a large mass between Adapazari and Sapanca. Shallow settlement of these magmatics was confirmed by the second vertical derivative of the aeromagnetic data. An anti-clockwise block rotation is evident in the northeast of the study area from the polarity changes of the magnetic anomalies.

Retroarc extension in the last 6 Ma in the South-Central Andes (36°S–40°S) evaluated through a 3-D gravity modelling

Abstract The Andean retroarc between 35° and 40°S is the locus of debate regarding its Pliocene to Quaternary tectonic setting. Retroarc volcanic eruptions since 6 Ma to the Present are, based on some hypotheses, associated with widespread extension. In these works, geological data point to the existence of normal faults affecting previous (Late Cretaceous to Miocene) contractional structures. In order to evaluate such interpretations we have collected data from various geological and geophysical studies and scales. Based on these data, an existing large-scale 3-D gravity model could be improved and used to investigate the lithospheric structure of this region. Moreover, using the gravity model, an attenuated crust could be localized and quantified throughout the retroarc area. Deep seismic data available from this region are limited to the forearc – arc area, while in general the retroarc zone lacks deep seismic constraints. The only deep seismic profile extending to the retroarc is a receiver function profile at 39°S, showing crustal attenuation. This observation correlates with the extensional activity recognized at the surface. When analysing the gravity field, positive residual anomalies are observed. They correlate with crustal attenuation at the areas of extension. Also, computed elastic thickness in the retroarc shows good correlation between the areas of crustal stretching and low flexural rigidity, explained by thermal processes. The present extensional deformation reflected in positive residual gravity anomalies points to the influence of reactivated Triassic rifting inherited from early phases of Pangea break-up. Finally, the present local uplift and consequent fluvial incision at the retroarc zone are explained by crustal stretching and not by crustal shortening, the common mechanism in Andean orogenesis.

Geophysical and structural pattern of the Knížecí Stolec pluton and its host rocks in the south-western part of the Moldanubian Zone, Bohemian Massif

*Corresponding author A detailed airborne magnetic and gamma-ray spectrometric as well as ground gravity survey in the south-western part of the Moldanubian Zone (Bohemian Massif) provided detailed geophysical characteristics of the principal geological units and large-scale tectonic features. The Knižeci Stolec durbachitic pluton with high contents of natural radioactive elements (Th, U and K) represents a body with no magnetically anomalous response but a pronounced positive gravity anomaly. The Křisťanov granulite massif, which is a host rock of the Knižeci Stolec pluton, exhibits low concentrations of Th and U, high abundances of K, slight magnetic and a negative gravity anomalies. The Plechý composite pluton is characterized by strikingly low gravity and an extremely monotonous magnetic field. One of its petrographic varieties (the Třistolicnik granite) is enriched in natural radioactive elements, especially in Th and U. Metamorphic complexes of the Monotonous and Varied groups contain multiple magnetic anomalies mostly related to the intercalated metabasic rocks; they also cause positive gravity anomalies. The Linsser density boundaries at a depth of 0.5 km clearly delimit the Plechý composite pluton and the Lhenice zone and indicate several structurally and/or lithologically different domains within the Křisťanov granulite massif and in the Knižeci Stolec pluton. The 2.5D gravity model indicates an asymmetric shape of the Knižeci Stolec pluton, which is deepest in its SE part (at least 4 km). The joint interpretation of the geophysical and structural data revealed that the maximum depth of the pluton is c. 4.5 km, with gently to moderately dipping intrusive contacts in the north and subvertical orientation in the south. The asymmetric shape of the pluton is consistent with its inferred syn-tectonic emplacement coeval with the regional subvertical contraction and development of regional flat-lying fabric. The gravity model combined with structural analyses also implies a considerable depth of the south Bohemian granulite massifs. This indicates their steep exhumation path and is at variance with the model of the Moldanubian Zone as a sequence of flat-lying nappes. The south-western part of the studied area is reworked by the NE-directed compression, referred to as the ‛Bavarian’ deformation phase.

Correlation Analysis Method and its Application to Interpretation of Regional Gravity and Magnetic Anomalies in Eastern Xinjiang, China

Based on detailed analysis of Bouguer gravity anomaly and magnetic anomaly by reduced to the pole, this paper uses correlation analysis method to analyze regional gravity and magnetic anomalies of different filter scales, including filter scales of 50km, 90km, and 130km, and then discusses the contributing factors of local gravity and magnetic anomalies in eastern Xinjiang. The results show that regional gravity and magnetic anomalies are cogenetic anomalies in some areas, such as Hangou, and northern Lamamiao, Jingrquan, and western Qijiaojing, gravity and magnetic anomalies are allogenic anomalies in other areas. The study reveals that regional negative gravity anomalies are chiefly caused by Cenozoic – Mesozoic with larger thickness and lower density, and regional positive magnetic anomalies are mainly caused by Precambrian metamorphic basement with stronger magnetic property uplifting in Turpan – Hami and Santanghu basin. Mantle substances uplifting mainly cause regional positive gravity and magnetic anomalies in Junggar basin. Regional positive gravity anomalies are mainly caused by Pre-Mesozoic with higher density uplifting, and regional negative magnetic anomalies are chiefly caused by sedimentary formation and intermediate acidity rock with larger thickness and weaker magnetism and Precambrian metamorphic basement.

Correlation Analysis Method and its Application to Interpretation of Local Gravity and Magnetic Anomalies in Eastern Tianshan, China

Based on detailed analysis of Bouguer gravity anomaly and magnetic anomaly by reduced to the pole, this paper uses correlation analysis method to analyze local gravity and magnetic anomalies of different filter scales, including filter scales of 50km, 90km, and 130km, and then discusses the contributing factors of local gravity and magnetic anomalies in eastern Tianshan. The results show that local gravity and magnetic anomalies are cogenetic anomalies in some areas, such as northeastern Santanghu, Shaqiuhe, southern Huangtuya, northern Dikan, and western Qijiaojing, and gravity and magnetic anomalies are allogenic anomalies in other areas. The study reveals that local gravity and magnetic anomalies are chiefly caused by Cenozoic – Mesozoic strata with lower density and weaker magnetism in northern Santanghu, and igneous rock with higher density and stronger magnetism in Shaqiuhe, and uplifting of older strata in northern Dikan, and Carboniferous strata with higher density and lower magnetism than its south and north areas in Qijiaojing. The study also reveals that local negative gravity and magnetic anomalies are chiefly caused by Cenozoic – Mesozoic strata with lower density and weaker magnetism, while local positive gravity and magnetic anomalies are mainly caused by Pre-Jurassic strata higher density and stronger magnetism in southern Huangtuya.

Locating a major Proterozoic crustal boundary beneath the Eastern Officer Basin, Australia

The Archaean to Mesoproterozoic basement of northern South Australia is almost completely overlain by thick Neoproterozoic and younger basins (≪1% outcrop), yet is likely to preserve an important record of the interactions between the Archean-Proterozoic Gawler Craton and the Proterozoic Musgrave Province during the amalgamation of Australia in the Proterozoic. However, constraints on the location and geometry of the boundary between these provinces are poor. We use potential field data to determine the 3D basement architecture and so constrain where this Palaeo-Mesoproterozoic boundary may be located beneath the Eastern Officer Basin. We establish the geometry and properties of the overlying basins and explicitly include them during forward and inverse modelling of potential field data to highlight the structure of the underlying basement. Our analysis identifies three crustal domains. (1) Southeast of the steep northeast–southwest Middle Bore Fault, positive gravity and magnetic anomalies are co-located and sourced from bodies in the upper crust, these bodies overlie middle to lower crust that is apparently uniform. (2) Between the Middle Bore Fault and the southern edge of the Munyarai Trough, the highest amplitude gravity and magnetic anomalies are not co-located and are sourced from large northwest dipping bodies. (3) To the northwest, the crust underlying the Munyarai Trough has similar properties to the Musgrave Province, suggesting that the Musgrave Province extends at least 50 km beneath the Eastern Officer Basin. Although details of the geology in the second (central) domain are poorly constrained, the domain preserves large crustal-scale Precambrian structures and is interpreted to mark the boundary between the Gawler Craton and the Musgrave Province. In particular, the multiply reactivated Middle Bore Fault forms a major crustal boundary and is interpreted to mark the northern limit of the Gawler Craton. The Middle Bore Fault may have formed as early as the Kimban Orogeny (∼1.7 Ga), although substantial reactivation and modification of the crustal architecture could have occurred between the Kimban Orogeny and intrusion of the cross-cutting Gairdner Dolerite Dykes (827 Ma).

New insights into the sources of magnetic anomalies in the Levant

The magmatic characteristics of the Levant were investigated by combining recompiled regional scale magnetic maps, revised petrophysical and gravity databases and previous interpretations. In the study area, the inclination of the Earth's magnetic field vector is low and the total magnetic intensity map is, therefore, difficult to interpret for location and strike of magnetic bodies. Hence a pole-reduced magnetic map was compiled. The integrated interpretation of magnetic and local gravity anomalies allowed for the estimation of depth and size of magmatic bodies and, in some cases, permitted identification of their composition. Reliability and accuracy of the results were approximated by forward modeling and by comparison with seismic reflection and drill-hole data. The study area was divided into five regions of consistent regional-scale magnetic patterns with more or less distinct boundaries. This was based not only on recognition of formal magnetic patterns but also on correlation with known (or expected) geology. The strong coincident magnetic and positive gravity anomalies probably correspond to ophiolite massifs in the northern part of the study area and basic magmatic intrusions in the southern part. Strong magnetic anomalies, which are not associated with positive gravity anomalies, most likely correspond to Mesozoic and Cenozoic volcanics located mainly in the central part of the region. The absence of magnetic anomalies in some areas suggests the lack of basic magmatic bodies (volcanic or plutonic) but provides no information on the presence of acid magmatics (granite, etc.).

Gravity anomalies of the Antarctic lithosphere

Anomalous free-air gravity signals in and around the Antarctic continent have been reported for some decades. Recent definition of the Antarctic gravity field from field-based oversnow traverses and supporting data from Earth-orbiting satellites reveal discrete regions of both negative and positive free-air gravity anomalies. The data from these observations have enabled us to construct a free-air gravity anomaly map of Antarctica. Negative free-air gravity anomalies are found to occur mainly on the Antarctic continent, in particular, in the Wilkes Land, Ross Sea, central continental, and Weddell Sea sectors. Positive free-air gravity anomalies are found to occur mainly in the offshore circum-continental sectors. While each of these regions of anomalies provides excellent opportunities for further investigation, including identification of the causes of the negative and positive free-air gravity anomalies, special attention is given to the negative free-air gravity anomaly sites of the continent proper. Three potential sources of the negative free-air gravity anomalies are identified: the mantle, lithosphere, and crust. Examination of thermally induced density variations in the mantle based upon seismic tomography, and analysis of mantle-related gravity anomaly wavelengths favor a gravity anomaly source other than the mantle. Examination of the subcrustal lithosphere based on upper-mantle thermal structure, the origin of the lithosphere, and crustal influences on the underlying lithosphere, including radiogenic heat, implies that the source of the negative free-air gravity anomalies is less likely to be the subcrustal lithosphere and more likely to be located in the Antarctic crust. Examination of possible crustal features that might account for these anomalies leads to a consideration of subglacial topography and specific locales of anomalously low rock density.

Gravity anomalies of the Northern Hawaiian Islands: Implications on the shield evolutions of Kauai and Niihau

New land and marine gravity data reveal two positive residual gravity anomalies in the Northern Hawaiian Islands: one over Kaua'i, the other between the islands of Kaua'i and Ni'ihau. These gravitational highs are similar in size and magnitude to those of other Hawaiian volcanoes, indicating local zones of high‐density crust, attributed to olivine cumulates in solidified magma reservoirs. The residual gravity high over Kaua'i is located in the Līhu'e Basin, offset 8‐12 km east of Kaua'i's geologically mapped caldera. This offset suggests that the mapped caldera is a collapsed feature later filled in with lava and not the long‐term center of Kaua'i shield volcanism. A second residual gravity high, in the submarine channel between Kaua'i and Ni'ihau, marks the volcanic center of the Ni'ihau shield volcano. This second residual gravity anomaly implies that Ni'ihau's eastern boundary extended ∼20 km east of its present location. Through inversion, the residual gravity anomalies were modeled as being produced by two solidified magma reservoirs with average densities of 3100 kg/m3 and volumes between 2470 and 2540 km3. Considering the locations and sizes of the residual gravity anomalies/magma reservoirs, the extent of the two islands' paleoshorelines and potassium‐argon dating of shield‐stage lavas, we conclude that the two islands were not connected subaerially during their respective shield stages and that Ni'ihau's topographic summit was removed by an eastern flank collapse between 4.3 and 5.6 Ma. Continued constructional volcanism on western Kaua'i likely covered much of the submerged remains of eastern Ni'ihau.

On the Geologic Structure at the Epicenter of the 1886 Charleston, South Carolina, Earthquake

The study focuses on evidence of Cenozoic faulting in the epicentral area of the 1886 Charleston, South Carolina, earthquake and its connection with Mesozoic structure. The seismic data consist of several reflection profiles collected near Summerville, South Carolina, in the period 1975–1983. Reprocessing of the data reveals an extensive early Mesozoic extensional basin, approximately 20 km in width, between Summerville and Charleston. The basin is delineated by the geometry of reflections that image early Mesozoic volcanic and sedimentary rocks and by positive magnetic and gravity anomalies. Cenozoic compressional reactivation of Mesozoic extensional faults is imaged in the interior of the basin. The northwestern boundary of the basin is marked by a sharp gradient in the magnetic field. Folded Cretaceous and Tertiary Atlantic Coastal Plain sediments in association with diffractions and truncated reflections from the early Mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted. Instrumentally located earthquakes are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern basin margin. Modeling of magnetic and gravity data indicates that the upper crust beneath the seismically imaged structural basin is composed largely of mafic rocks to a depth of at least 4 km. We propose that the Charleston earthquake occurred due to compressional reactivation of a Mesozoic fault in a localized zone of intense early Mesozoic continental rifting.

Bidimensional empirical mode decomposition (BEMD) for extraction of gravity anomalies associated with gold mineralization in the Tongshi gold field, Western Shandong Uplifted Block, Eastern China

A bidimensional empirical mode decomposition (BEMD) program on a MATLAB platform was effectively used to handle gravity signals for the Tongshi gold field. This yielded a two-dimensional intrinsic mode function (IMF 3) image that meticulously depicts the spatial distribution relationship between various gold deposits and the different geological units of the gold field. By combining the IMF 3 image with geological features, the Tongshi gold field can be divided into three geological units. Unit I is made up of both Archean greenstone and Cambrian–Ordovician carbonate rocks and is defined as a concealed basement that exhibits weak negative and positive gravity anomalies. Unit II is defined as a Mesozoic volcanic sediment basin consisting of Jurassic and Cretaceous volcanic-sedimentary rocks, which developed on the concealed basement and exhibits an obvious negative gravity anomaly. Unit III, located on the concealed basement on the SW side of the Mesozoic volcanic-sedimentary basin, can be subdivided into subunits IIIa and IIIb. The former, with a negative circular gravity anomaly, indicates that the Tongshi complex consists of both Yanshan diorite-porphyrite and syenite-porphyry. Unit IIIb, with a positive ring gravity anomaly distributed around a negative circular gravity anomaly, indicates a possible contact metasomatic mineralization zone around the Tongshi complex. Almost all types of gold deposits are located within this contact metasomatic mineralization zone and thus the zone is a prospective area for gold deposits.

Characterization of a proposed Palaeoproterozoic suture in the crust beneath the Curnamona Province, Australia

We use constrained forward and inverse modeling of Bouguer gravity data to investigate a regional positive gravity anomaly that defines, but is not confined to, the eastern margin of the Curnamona Province. Best-fit results imply that the positive gravity values are sourced from a relatively dense terrane (+ 0.04 Mg m − 3 with respect to surrounding crust) located at middle crustal levels (16–27 km depth), which is likely dismembered and offset into discrete slivers by later tectonism. Consideration of pre-1500 Ma plate reconstructions of the Australian continent correlates this terrane with an interpreted buried suture in the Mount Isa Inlier of northern Australia, where allochthonous crust was amalgamated to the Australian margin at ca. 2.20–1.85 Ga. Bouguer gravity forward and inverse modeling also delineates a shallow, west-dipping crustal-scale structure that possibly represents a primary boundary between Precambrian basement of Australia and the eastern fold belts accreted to the Precambrian margin during the Palaeozoic.

Gravity and tectonic patterns of Mercury: Effect of tidal deformation, spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation

We consider the effect of spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation on Mercury's gravity and tectonic pattern. Large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values,J2/C22= 10/3 and (b−c)/(a−c) = 1/4, arise because of nonsynchronous rotation and nonzero eccentricity even in the absence of reorientation or despinning. Reorientation or despinning induces additional variations. The large gravity coefficientsJ2= (6 ± 2) × 10−5andC22= (1 ± 0.5) × 10−5estimated from the Mariner 10 flybys cannot be attributed to Caloris alone since the required mass excess in this case would have caused Caloris to migrate to one of Mercury's hot poles. Similarly, a large remnant bulge due to a smaller semimajor axis and spin‐orbit resonance can be dismissed since the required semimajor axis is unphysically small (<0.1 AU). Reorientation of a large remnant bulge recording an epoch of faster rotation (without significant semimajor axis variations) can explain the large gravity coefficients. This requires initial rotation rates ≳20 times the present value and a positive gravity anomaly associated with Caloris capable of driving ∼10°–45° equatorward reorientation. The required gravity anomaly can be explained by infilling of the basin with material of thicknesses ≳7 km or an annulus of volcanic plains emplaced around the basin with an annulus width ∼1200 km and fill thicknesses ≳2 km. The predicted tectonic pattern due to these despinning and reorientation scenarios, including some radial contraction, is in good agreement with the lobate scarp pattern observed by Mariner 10. We also predict that lobate scarps will follow a NE–SW orientation in the eastern hemisphere and a positive gravity anomaly (of a few hundred mGal) associated with Caloris.

A Rapid Graphical Solution for Determining the Presence of Remnant Magnetization in the Basement Rocks of Western Iraq

A Rapid Graphical Solution for Determining the Presence of Remnant Magnetization in the Basement Rocks of Western Iraq

INTEGRATED GRAVITY AND SEISMIC INVESTIGATION OVER THE JABAL HAFIT STRUCTURE: IMPLICATIONS FOR BASEMENT CONFIGURATION OF THE FRONTAL FOLD‐AND‐THRUST BELT OF THE NORTHERN OMAN MOUNTAINS

A gravity survey was conducted over and around Jabal Hafit, located on the border between the United Arab Emirates (UAE) and Oman on the western edge of the northern Oman Mountains, as part of a study to investigate the subsurface structures and sedimentary sequences of the area. This new data, together with outcrop geology, well data and measurements of physical properties of rock samples, was integrated with a new interpretation of reprocessed commercial seismic reflection profiles recorded across the Jabal Hafit anticline.We recognize five major tectonostratigraphic sequences in the seismic profiles: Mesozoic shelf carbonates; Upper Cretaceous foreland (primarily Fiqa Formation); the Hawasina allochthon; and Upper Cretaceous to Lower Tertiary, and Upper Tertiary sequences. The seismic sections suggest that Jabal Hafit represents a backfolded anticline associated with a steep east‐vergent thrust that probably lies above a blind thrust fault along the base of the foreland basin sequence at depth. The lack of onlapping and thinning in the Lower Tertiary sequence suggests that the initiation of folding may have started after the Oligocene‐Miocene and corresponds to the beginning of the collision of Arabia and central Iran along the Zagros suture in Iran. In addition, the seismic sections suggest a pop‐up structure beneath the Jabal Hafit structure at the base of the foreland basin sequence. The structure is bounded by high‐angle listric reverse faults that cut downward through the Mesozoic shelf carbonates and are interpreted to be related to the inversion of deep structures.NNW to SSE trending positive residual gravity anomalies of up to +8 mGal occur on the Jabal Hafit and Al‐Ain anticlines. These positive gravity highs are attributed to uplifted basement structures which probably resulted from the reactivation of deep‐seated fault blocks. The residual Bouguer anomaly values decline gradually to the east and west of Jabal Hafit probably due to the deepening of the basement. A large negative gravity anomaly of <−5 mGal is observed east of Jabal Hafit and coincides with a syncline which caused thickening of the Upper Cretaceous and Tertiary sequences.