Isostatic Residual Gravity Anomalies Research Articles

Crustal structure on west-central Baja California peninsula, México, inferred by 3D linear inverse modeling of gravity data

Gravity data from the west-central portion of the Baja California Peninsula, in NW Mexico, were processed on the ellipsoidal reference frame to obtain isostatic residual gravity anomalies. This is a nearly flat region close to sea level and blanked by sediments, where a gravity high bounded by two prominent minima occurs. The gravity high is oriented in a NE-SW direction, and it is possibly the most prominent gravity anomaly transverse to the Baja California Peninsula. The gravity minima, located towards the NW and SE from the gravity high, are respectively related with the Sebastián Vizcaíno and Purísima sedimentary basins. These basins were developed since Early Cretaceous times during the subduction of the Farallon Plate under the North American plate. To infer the crustal structure and the basement geometry in this region, inverse linear modeling is carried on a multilevel data set of upward continued isostatic residual gravity data. Assuming that compaction is the primary diagenetic process on the sediments filling the basins, the initial model is constrained by the surface geology expressed as a density contrast that decreases with depth. The outcome of the inversion is density contrast in a cuboid model, constituted by 67,379 prisms, and its three-dimensional representation suggests that these two basins accommodate as much as five km-thick sedimentary sequences –formed by shallow siliciclastic and deep volcaniclastic rocks penetrated by wells drilled in the region– lying above the inferred basement, which shallows beneath the Lagunitas gravity high, consistently with depth to crystalline rocks penetrated by wells.

Stress field estimation by the geoid undulations of the Samos-Kuşadası Bay and implications for seismogenic behavior

The strong earthquake with magnitude 6.9 occurred offshore at the northernmost edge of the Samos Island and was strongly felt in the north Aegean islands and Izmir metropolitan city. In this study, the effective elastic thicknesses of the lithosphere and seismogenic layer thickness were correlated with each other in order to understand the nature of the earthquakes. We determined that the upper and lower depth limits of seismogenic layer are in a range of 5–15 km, meaning that only the upper crust is mostly involved in earthquakes in the study area. The fact that seismogenic layer and effective elastic thicknesses are close to each other indicates that the earthquake potential may be within the seismogenic layer. Following that, we estimate the stress field from the geoid undulations as a proxy of gravity potential energy in order to analyze the amplitude and orientation of the stress vectors and seismogenic behavior implications. The discrete wavelet transform has been carried out to decompose the isostatic residual gravity anomalies into horizontal, vertical and diagonal detail coefficients. The results delineated edges of gravity anomalies that reveal some previously unknown features.

Preliminary interpretation of isostatic residual gravity anomalies within the central portion of the Equatorial Atlantic African region

Airy’s isostatic residual gravity anomaly map and profiles were interpreted for the central portion of the Equatorial Atlantic African region with a view to understanding the lithospheric dynamics, hence imminent vertical crustal movements and thus predict the future outlook of evolving landscape within the study area. In order to compute the isostatic residual gravity anomaly, the AIRYROOT algorithm of the United States Geological Survey was used and isostatically overcompensated, undercompensated and completely compensated portions were identified on the isostatic residual gravity anomaly map and profiles. The Airy’s isostatic residual gravity anomaly map showed three distinct parts which were the oceanic south-western part characterised by negative isostatic residual gravity anomaly values (− 11 to − 200 mGal), the volcanic eastern and extreme northcentral parts characterised by positive isostatic residual gravity anomaly values (+ 10 to + 90 mGal) and the central part having a network of sedimentary basins, characterised by isostatic residual gravity anomaly values that approach zero (− 10 to + 9 mGal). It was discovered that the oceanic south-western part was isostatically overcompensated, the uplifted eastern part and the Younger Granite Province of the extreme northcentral part were undercompensated and the central part consisting of the Niger Delta, Benue Trough and the Mid-Niger (Nupe) Trough was completely compensated. It was consequently predicted that vertical uplift is imminent in the isostatically overcompensated oceanic region while subsidence is expected over the region of the eastern upland and the Younger Granite Province which were associated with isostatic undercompensation. The sedimentary basins within the study area are expected to remain stable.

Isostatic Residual Gravity Anomaly Grid of Onshore Australia

SUMMARY Gravity anomalies from geological features in the upper crust are masked by large amplitude long wavelength gravity variations from isostatic roots. An isostatic residual (IR) gravity anomaly grid of onshore Australia has been produced by Geoscience Australia which has these long wavelength features removed. This gravity map reveals more clearly the density distributions of geological interest within the upper crust. The depth to mantle model and subsequent isostatic corrections were produced using a modified version of the USGS program AIRYROOT provided by Intrepid Geophysics. Geoscience Australia’s 2009 Bathymetry and Topography Grid was used to calculate the depth to crustal bottom following the Airy-Heiskanen crustal-root model. The isostatic corrections were then applied to the complete Bouguer anomalies to produce the Isostatic Residual Gravity Anomaly Grid of Australia.

Major tectonic structures in northeastern Greece deduced from geophysical and seismological data

We apply lineaments analysis on the gravity and magnetic data of NE Greece, and combine seismological and geophysical data in order to delineate the major structural features. These methods are frequently used for extracting the dimensional and physical parameters of the buried structures that stimulate gravity and magnetic fields. These estimates concern the location, local depth, strike, dip and physical quantity contrast, of potential field contacts. We used results from previous studies in order to correct the Bouguer data for the gravity effect of the crust. The isostatic residual gravity anomalies, produced from the subtraction of the effect of the crust, are related to near-surface features. Noise suppression was achieved by slightly upward continuing the data by one cell size. Geologic significance of detected lineaments is confirmed by comparisons with the known geology, active tectonics and seismicity as well as with topographic lineaments

Isostatic Residual Gravity Anomaly Grid of Onshore Australia

Isostatic Residual Gravity Anomaly Grid of Onshore Australia

A gravity model of the North Eurasia crust and upper mantle: 1. Mantle and isostatic residual gravity anomalies

A gravity model of the North Eurasia crust and upper mantle: 1. Mantle and isostatic residual gravity anomalies

Crustal structure beneath the Tell and Atlas Mountains (Algeria and Tunisia) through the analysis of gravity data

Bouguer and isostatic residual gravity anomaly maps were analyzed for the general crustal structure under the Tell and Atlas Mountains of Algeria and Tunisia. Two-dimensional gravity models and profiles of topography, free-air and Bouguer gravity anomalies were constructed to provide estimates of the crustal thickness and the degree of isostatic compensation. The thickest crust (38–40 km) was found beneath the Saharian Atlas in northern Algeria, whereas under the Tunisian Atlas it is approximately 34–36 km. There is little evidence of crustal thickening due to the formation of the Tell Mountains. Large amplitude negative Bouguer, small amplitude, positive free-air gravity and small amplitude, positive isostatic residual gravity anomalies suggest that Saharian and Tunisian Atlas are partially isostatically compensated with regions of possible overcompensation in Algeria. The boundary between the Saharian and Tunisian Atlas is characterized by negative isostatic residual anomalies, suggesting overcompensation and a thick crustal root; however, the cause may be low density material in the upper crust. The amplitude and width of Bouguer gravity anomalies, and two-dimensional gravity models across the Saharian and Tunisian Atlas indicate that there was a greater amount of crustal shortening in Algeria than in Tunisia.

The Isostatic Residual Gravity Anomaly of the Central Andes, 12° to 29° S: A Guide to Interpreting Crustal Structure and Deeper Lithospheric Processes

The isostatic residual gravity field of the Central Andean Plateau region contains laterally continuous, elongated anomalies that reflect the tectonic and magmatic effects of oceanic subduction in the forearc and continental subduction in the backarc. On the western side of the Andes, the residual anomalies are similar to those found at many “Andean”-type subduction margins. In particular, a high-low residual anomaly pair coincident with the coast and the Peru-Chile trench marks the location when the Nazca plate underthrusts beneath western South America. This high-low anomaly couple is mirrored in the backarc by a similar couple that tracks the location of the eastward-vergent Principal Frontal Thrust and results from the westward, antithetic subduction of the Brazilian shield beneath the plateau. The residual high is situated in the Eastern Cordillera of Bolivia, and is caused by a combination of high-density basement rocks near the surface in the hanging wall of the thrust, and by local crustal undercompensation. To the east, the parallel low coincides with the Subandean fold-thrust belt and foreland basin, and is due to a combination of low-density sedimentary rocks in the foreland basin and the locally overcompensated crust of the downflexed foreland lithosphere. The Eastern Cordillera and the Subandean are structural analogs to the uplifted forearc and the Peru-Chile trench on the western side of the Andes. Along-strike variations in the amplitude and width of the Eastern Cordillera and Subandean anomalies indicate that the foreland lithosphere is strongest at the latitude of central Bolivia, but decreases significantly to the south where the eastern margin of the Andes appears to be nearly in a state of local isostatic compensation.

Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north‐south within the magmatic regions and northwest‐southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest‐southeast dextral shear of the Walker Lane belt to the east‐west extension characteristic of the Cascade arc in central Oregon. Faulting along north‐south strikes in the volcanically active areas indicates the east‐west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.

Flank uplift and topography at the central Baikal Rift (SE Siberia): A test of kinematic models for continental extension

Uplifted flanks at intracontinental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of extension, with respect to the asymmetry of rifting and the importance of brittle versus elastic upper crustal deformation. In this paper, I test different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observations from the Baikal rift (SE Siberia). The rift is characterized by prominent flank topography on both sides of lake Baikal. The flanks reach similar elevations but differ in their structure: the tilt of the footwall flank is away from the basin, whereas the basinward part of the hanging wall flank tilts toward the basin center. Fission track data indicate that very little erosion affected the flanks since rifting started; geomorphological and sedimentological observations suggest that prerift relief was minor. Thus, the present‐day topography reflects rift‐related tectonic uplift. Pure‐shear “necking” and pure‐shear/simple‐shear “detachment” models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well. They do not permit to discriminate between different scenarios that have been proposed for the central Baikal rift; that is, half‐graben versus full graben development; rifting at a continuous rate since the Oligocene versus a large increase in extension rate since the Pliocene. The models predict that the kinematics of rifting in Baikal are controlled by a midcrustal (20 km) depth of necking and/or a mid to lower crustal (20–30 km) detachment level; best‐fit elastic thicknesses are in the range 30–50 km. These predictions are in agreement with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath Baikal as inferred from heat flow, seismic refraction and seismological observations. In contrast, a “flexural cantilever” model with low (< 10 km) elastic thickness predicts topographic patterns which are very different from those observed, for a wide range of rifting scenarios. Significant (> 3 km) erosion of the footwall flank is required to fit the topography if a flexural cantilever model is applied; this is incompatible with the fission track data. Thus, the kinematics of extension at deep and narrow intra‐continental rifts such as Baikal appear to be controlled by a strong elastic lithosphere and require significant brittle deformation of the upper crust, as suggested by dynamic models for continental rifting.

Regional study of mineral resources in Nevada: Insights from three-dimensional analysis of gravity and magnetic anomalies

A three-dimensional interpretation of the basins of Nevada was developed based on statewide data bases of digital-gravity, magnetic, geologic, well, and topographic information. An iterative technique was applied to isostatic residual gravity anomalies in Nevada in order to define the location and shape of pre-Tertiary basement and to produce a gravity map that reflects variations of density within the pre-Tertiary basement. The dominant feature of the basement gravity of Nevada is an enormous area of low gravity that spans the entire state between latitudes 37°N and 40.5°N. This regional low strongly correlates with the distribution of middle and late Tertiary volcanic rocks and may reflect silicic intrusions within the mid-crust and upper crust that are the counterparts of volcanic rocks at the surface. Although 80% of Nevada is covered by Cenozoic deposits, these deposits are thicker than 1 km over only about 20% of the state. The remaining 60% of Nevada may have pre-Tertiary basement rocks within reach of current mineral-exploration techniques. Aeromagnetic profiles from the National Uranium Resource Evaluation (NURE) were analyzed in order to produce a map showing the location of shallow magnetic sources in Nevada. This analysis shows that 46% of the state has magnetic sources, generally Mesozoic and Cenozoic igneous rocks, within 1 km of the surface. A linear magnetic anomaly in north-central Nevada has been interpreted by others as a rift zone active during middle Miocene time. The rift also is evident in NURE magnetic profiles, but our interpretation suggests that the magnetic expression of the rift continues south-southeast with similar strike to at least 38°N and perhaps to the amagmatic zone (lat. 37°N). The survival since the middle Miocene of this narrow crustal feature, essentially linear over a distance of 500 km, is difficult to interpret in light of later Basin and Range deformation. Our analysis of gravity anomalies shows that many deep Cenozoic basins are located near the rift, yet only two basins cut across it, and at least five others change strike near the rift, as if to avoid it. The rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation.

Thermal evolution of the Sierra Nevada: Tectonic implications of new heat flow data

Eight new heat flow measurements in the southern Sierra Nevada constrain models of thermal evolution and lithospheric structure. Low reduced heat flows (18 to 21 mW/m²) in the southwest Sierra Nevada are consistent with previous results from the northwestern and central Sierra Nevada and extend the known region of linear heat flow‐heat production correlation an additional 150 km to the south. Isostatic residual gravity and measured rock densities are also linearly correlated in the central Sierra Nevada, suggesting a general association between the upper crustal distribution of density and heat production. The linear residual gravity‐density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of Cenozoic Sierra Nevada uplift. New reduced heat flows measured in the southeast Sierra Nevada are relatively high (32 to 57 mW/m²) and correlate spatially with a region of high seismicity that includes extensional earthquake swarms; this correlation supports the view that Basin and Range extensional tectonics and associated magmatic processes are encroaching on the eastern Sierra Nevada. In the stable southwest Sierra Nevada, as in the central and northwestern Sierra Nevada, the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the Cenozoic uplift of the Sierra Nevada, provided the conductive lithosphere is at least 60 to 90 km thick. Simple order‐of‐magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere, advection of basaltic magma and heat into the crust, simple thermal expansion, and eclogite‐to‐basalt phase conversion can account for the timing and amount of Cenozoic Sierra Nevada uplift. Thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for Cenozoic uplift driven by a Mesozoic crustal root.

Interpretation of reconnaissance gravity and aeromagnetic surveys of the Antarctic Peninsula

Two‐dimensional models are presented for the crustal structure of the Antarctic Peninsula, a Mesozoic‐Cenozoic magmatic arc on the southeast Pacific margin. The models are constrained by measured rock properties. The West Coast Magnetic Anomaly (WCMA) is caused by a zone of mafic plutons which is 70–150 km wide and over 1500 km long. In modelling of Bouguer anomalies, isostatic compensation is maintained by crustal thickness variations. The model Moho is deepest (35 km) beneath the mid‐Cretaceous axis of the arc and shallowest (<20 km) beneath a Late Cenozoic marginal basin. Positive isostatic residual gravity anomalies are associated with the WCMA and a mafic terrane in the Mesozoic accretionary complex. Three broad areas of consistent crustal structure are identified. The junctions of these areas are sinistral fault zones which displace the curvilinear magnetic, physiographic, and gravity patterns by several tens of kilometers.

A new isostatic residual gravity map of the conterminous United States with a discussion on the significance of isostatic residual anomalies

To display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust, a new colored isostatic residual gravity map of the conterminous United States has been prepared using the gravity data set compiled for the Gravity Anomaly Map of the United States (Society of Exploration Geophysicists, 1982). The new isostatic residual gravity map is based on an Airy‐Heiskanen model of local compensation, in which the surface load requiring compensation is defined by 5‐min topographic and bathymetric data sets. A colored first‐vertical‐derivative map of isostatic residual gravity further enhances the short‐wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field. For the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance, the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort. Most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features. Anomalies on isostatic residual gravity maps should not be casually interpreted in terms of “undercompensation” or “overcompensation” because large‐amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium. Many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies. We present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment. In general, highs correlate with intruded or accreted mafic material or with upthrusted crustal sections, whereas lows occur over low‐density sedimentary or volcanic sections, felsic intrusive bodies, or down‐warped crustal sections. Although some longer‐wavelength anomalies, such as the broad gravity high centered over Montana, could be manifestations of density contrasts deep in the mantle, many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers, so that their source bodies need not be deep. The fact that certain of these broader anomalies have well‐defined boundaries which correlate with near‐surface geologic features increases the likelihood that their sources lie entirely within the lithosphere. If so, then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions. We have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns. Contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the United States. These Fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories.

Subsurface structural features of the Saline Range and adjacent regions of eastern California as interpreted from isostatic residual gravity anomalies

New gravity data indicate a large unexpected anomaly in the Saline Range and other anomalies in neighboring ranges and valleys of eastern California. Residual gravity anomalies, calculated by subtracting the field due to an Airy model of isostatic compensation, show the effects of lateral density contrasts in the upper crust. A well-defined negative gravity anomaly is centered over the Saline Range and connects the Eureka Valley and Saline Valley gravity minima. Apparently, the Saline Range is a thin veneer of young volcanic material lying on a major alluvium-filled basin connected on the north with Eureka Valley and on the south with Saline Valley. We suggest that the basin formed in a pull-apart zone that filled with low-density alluvium and became the site of voluminous Pliocene volcanic eruptions. The age of this depression predates volcanism of the Saline Range dated at about 4 Ma. Other significant anomalies are as follows: (1) An offset in the Owens Valley fault at lat 37°01′N appears in the isostatic residual gravity data and marks the western end of a major gravity depression that continues across the Inyo Mountains at Papoose Flat. The gravity depression suggests that the Papoose Flat pluton is larger than suggested by its surface exposure and may extend on the order of 10 km below the topographic surface. (2) A density boundary trends west-northwest across the Inyo batholith and Proterozoic and Paleozoic rocks of the White Mountains. This trend is anomalous to exposed geologic structures and may reflect a structural boundary buried at relatively shallow depth. (3) A large gravity low, unexpected on the basis of exposed geologic features, is in the eastern Sierra Nevada southwest of the town of Lone Pine.