scholarly journals Nature of the crust in the northern Gulf of California and Salton Trough

Geosphere ◽  
2019 ◽  
Vol 15 (5) ◽  
pp. 1598-1616 ◽  
Author(s):  
Jolante W. van Wijk ◽  
Samuel P. Heyman ◽  
Gary J. Axen ◽  
Patricia Persaud
Keyword(s):  
Conceptual Model ◽  
Oceanic Crust ◽  
Gulf Of California ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Seismic Velocities ◽  
Potential Field Data ◽  
Serpentinized Peridotite ◽  
Salton Trough

Abstract In the southern Gulf of California, the generation of new oceanic crust has resulted in linear magnetic anomalies and seafloor bathymetry that are characteristic of active seafloor-spreading systems. In the northern Gulf of California and the onshore (southeastern California, USA) Salton Trough region, a thick sedimentary package overlies the crystalline crust, masking its nature, and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or continental) and develop a conceptual model for rifting. Gravity anomalies in the northern Gulf of California and Salton Trough are best fit with crustal densities that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are assumed. Because extensive mafic underplated bodies would produce gravity anomalies that are not in agreement with observed gravity data, we propose, following earlier work, that the anomalies might be due to serpentinized peridotite bodies such as found at magma-poor rifted margins. The density and seismic velocities of such serpentinized peridotite bodies are in agreement with observed gravity and seismic velocities. Our conceptual model for the Salton Trough and northern Gulf of California shows that net crustal thinning here is limited because new crust is formed rapidly from sediment deposition. As a result, continental breakup may be delayed.

scholarly journals Northward migration of the Oregon forearc on the Gales Creek fault

Geosphere ◽  
2020 ◽  
Vol 16 (2) ◽  
pp. 660-684 ◽  
Author(s):  
Ray E. Wells ◽  
Richard J. Blakely ◽  
Sean Bemis
Keyword(s):  
Gravity Data ◽  
Magnetic Anomalies ◽  
Fault System ◽  
Coast Range ◽  
Geologic Mapping ◽  
Willamette Valley ◽  
Potential Field Data ◽  
Columbia River Basalt ◽  
Geophysical Surveys ◽  
Step Over

Abstract The Gales Creek fault (GCF) is a 60-km-long, northwest-striking dextral fault system (west of Portland, Oregon) that accommodates northward motion and uplift of the Oregon Coast Range. New geologic mapping and geophysical models confirm inferred offsets from earlier geophysical surveys and document ∼12 km of right-lateral offset of a basement high in Eocene Siletz River Volcanics since ca. 35 Ma and ∼8.8 km of right-lateral separation of Miocene Columbia River Basalt at Newberg, Oregon, since 15 Ma (∼0.62 ± 0.12 mm/yr, average long-term rate). Relative uplift of Eocene Coast Range basalt basement west of the fault zone is at least 5 km based on depth to basement under the Tualatin Basin from a recent inversion of gravity data. West of the city of Forest Grove, the fault consists of two subparallel strands ∼7 km apart. The westernmost, Parsons Creek strand, forms a linear valley southward to Henry Hagg Lake, where it continues southward to Newberg as a series of en echelon strands forming both extensional and compressive step-overs. Compressive step-overs in the GCF occur at intersections with ESE-striking sinistral faults crossing the Coast Range, suggesting the GCF is the eastern boundary of an R′ Riedel shear domain that could accommodate up to half of the ∼45° of post–40 Ma clockwise rotation of the Coast Range documented by paleomagnetic studies. Gravity and magnetic anomalies suggest the western strands of the GCF extend southward beneath Newberg into the Northern Willamette Valley, where colinear magnetic anomalies have been correlated with the Mount Angel fault, the proposed source of the 1993 M 5.7 Scotts Mills earthquake. The potential-field data and water-well data also indicate the eastern, Gales Creek strand of the fault may link to the NNW-striking Canby fault through the E-W Beaverton fault to form a 30-km-wide compressive step-over along the south side of the Tualatin Basin. LiDAR data reveal right-lateral stream offsets of as much as 1.5 km, shutter ridges, and other youthful geomorphic features for 60 km along the geophysical and geologic trace of the GCF north of Newberg, Oregon. Paleoseismic trenches document Eocene bedrock thrust over 250 ka surficial deposits along a reverse splay of the fault system near Yamhill, Oregon, and Holocene motion has been recently documented on the GCF along Scoggins Creek and Parsons Creek. The GCF could produce earthquakes in excess of Mw 7, if the entire 60 km segment ruptured in one earthquake. The apparent subsurface links of the GCF to other faults in the Northern Willamette Valley suggest that other faults in the system may also be active.

The formation of new oceanic crust

1965 ◽  
Vol 258 (1088) ◽  
pp. 123-136 ◽  
Keyword(s):  
Oceanic Crust ◽  
Red Sea ◽  
Gulf Of California ◽  
Large Scale ◽  
Continental Drift ◽  
Active Regions ◽  
Seismic Exploration ◽  
Seismic Velocities ◽  
Crustal Structures ◽  
Rift Zones

Seismic exploration at sea has established that the oceanic crust is completely different from that of the continents. If we accept continental drift, it is therefore necessary to invoke a mechanism for the evolution of new oceanic crust. An attempt is made to locate regions where new oceanic crust may be forming and it is suggested that these regions are related to regions of uprising convection in the mantle. The crustal structures beneath the Red Sea and the Gulf of California are very similar and closer to oceanic than continental. As these are seismically active regions of extension, it seems reasonable to suppose that they represent areas where new oceanic crust is evolving in regions of continental break-up. These rift zones are in continuity with the seismically active oceanic rifts where similar seismic velocities (about 7 km/s) have been found and it is inferred that the oceanic rifts also represent regions where new oceanic crust is evolving. These are generally near the centres of the oceans. The tensional rift zones which are regions of shallow seismicity help to locate regions of rising convection currents in the mantle. It is further suggested that regions of deep and intermediate focus earthquakes locate regions of descending convection currents and maps of earthquake distributions are used to reveal a possible large-scale pattern of mantle convection. It is supposed that new oceanic crust evolves over the rising convection currents. A study is therefore made of the crustal sections for the Red Sea, Gulf of California and mid-oceanic rift regions and these are compared with typical continental and oceanic crusts. A possible mechanism for the evolution of new oceanic crust is given based on the isostatic equilibrium of oceans and continents.

A method for inversion of 1D vertical soundings of gravity anomalies

Geophysics ◽  
2018 ◽  
Vol 83 (2) ◽  
pp. G15-G23
Author(s):  
Andrea Vitale ◽  
Domenico Di Massa ◽  
Maurizio Fedi ◽  
Giovanni Florio
Keyword(s):  
Potential Field ◽  
Field Data ◽  
Gravity Data ◽  
Finite Size ◽  
Gravity Anomalies ◽  
Real Data ◽  
Gravity Inversion ◽  
Chain Process ◽  
Potential Field Data ◽  
Gamma Density

We have developed a method to interpret potential fields, which obtains 1D models by inverting vertical soundings of potential field data. The vertical soundings are built through upward continuation of potential field data, measured on either a profile or a surface. The method assumes a forward problem consisting of a volume partitioned in layers, each of them homogeneous and horizontally finite, but with the density changing versus depth. The continuation errors, increasing with the altitude, are automatically handled by determining the coefficients of a third-order polynomial function of the altitude. Due to the finite size of the source volume, we need a priori information about the total horizontal extent of the volume, which is estimated by boundary analysis and optimized by a Markov chain process. For each sounding, a 1D inverse problem is independently solved by a nonnegative least-squares algorithm. Merging of the several inverted models finally yields approximate 2D or 3D models that are, however, shown to generate a good fit to the measured data. The method is applied to synthetic models, producing good results for either perfect or continued data. Even for real data, i.e., the gravity data of a sedimentary basin in Nevada, the results are interesting, and they are consistent with previous interpretation, based on 3D gravity inversion constrained by two gamma-gamma density logs.

3D lithospheric structure and density of the NE Atlantic

2020 ◽  
Author(s):  
Maria Laura Gomez Dacal ◽  
Jan Inge Faleide ◽  
Mansour Abdelmalak ◽  
Magdalena Scheck-Wenderoth ◽  
Denis Anikiev ◽  
...  
Keyword(s):  
Gravity Data ◽  
Thickness Distribution ◽  
Tectonic Setting ◽  
Gravity Anomalies ◽  
Density Variation ◽  
Scale Model ◽  
Western Coast ◽  
Seismic Velocities ◽  
Ne Atlantic ◽  
Seismic Profiles

<p>The NE Atlantic is a tectonically complex region, also interesting in terms of georesources and therefore large areas are well covered by geophysical and geological data. In this work, we present a 3D lithospheric-scale structural and density model of the region including the eastern-most area of Greenland, the western coast of Norway, Iceland and Svalbard. It covers an area of 2000 km in longitude by 2500 km in latitude with a depth of 300 km and a resolution of 10 km. The model was developed by integrating different kinds of data and regional or global previous models, mainly of seismic origin, and constrained by gravity observations.</p><p>The developed model includes the topography, bathymetry and ice thickness obtained from global compilations models. The thickness distribution of sediments was incorporated based on detailed mapping of most areas covered by the model. The structure of the crystalline crust, differentiating between the oceanic and continental areas, is based on seismic information and previous regional models, cross-checked by additional seismic profiles available in the region. The model also includes high velocity/density lower crustal bodies defined by a previous compilation at the Norwegian and Greenland margins and by the analysis of deep seismic profiles in the case of the Iceland area.</p><p>We assigned constant densities to each layer following seismic velocities and literature-suggested values for every lithology. Due to the active tectonic setting of the area and its consequent elevated temperature and thus low density, the portion of mantle included in the model is the only layer with variable density. To obtain the mentioned density variation, we evaluated different seismic tomographic data for the area and converted them into temperatures. To mitigate the poor reliability of the tomographic models at shallow depths and also taking into account that the effect of the temperature in the uppermost mantle is especially important near mid oceanic ridges, we evaluated the thermal effect of this area by running a thermal model. Therefore, we calculated 3D distribution of temperatures for the whole portion of the mantle included in the model to obtain the reduction in density that these temperatures would cause considering the thermal expansivity of mantle rocks. </p><p>The gravity response of the model was calculated and compared to the gravity observations using the 3D interactive software IGMAS+. The developed model includes the latest data and information of the area and, at the same time, reasonably fits the measured gravity anomalies. Comparison of the first-pass 3D gravity model to the observed gravity data detected some residual anomalies that require further differentiation of crustal densities. The new 3D lithosphere-scale model allows us to analyze the structural configuration of the area and interpret its tectonic implications. It also forms the base for thermal and mechanical models to obtain the 3D distribution of physical variables and predict the rheological and dynamic behavior of the wider NE Atlantic region.</p>

scholarly journals GEOPHYSICAL STUDIES OF BASIN STRUCTURES ALONG THE EASTERN FRONT OF THE SIERRA NEVADA, CALIFORNIA

Geophysics ◽  
1964 ◽  
Vol 29 (3) ◽  
pp. 337-359 ◽  
Author(s):  
J. H. Healy ◽  
Frank Press
Keyword(s):  
Sierra Nevada ◽  
Gravity Data ◽  
El Paso ◽  
Gravity Anomalies ◽  
Density Contrast ◽  
Seismic Velocities ◽  
Owens Valley ◽  
Eastern Front ◽  
Garlock Fault ◽  
Mohorovičić Discontinuity

A seismic and gravity survey along the eastern front of the Sierra Nevada, California, between southern Owens Valley and the Garlock fault, outlines a series of basins with maximum depths ranging from 5,000 to 9,000 ft. These basins follow the front of the Sierra Nevada in a continuous chain with one interruption of about 10 miles near Little Lake. The gravity anomalies indicate that the basins are bounded by a series of high‐angle faults rather than a single large fault. The seismic velocities in the basin deposits appear to correlate with the stratigraphy of the section exposed in the El Paso Mountains. A comparison of Bouguer anomalies with seismic depths indicates a density contrast of 0.35 g/cc in basins less than 3,000 ft deep, and an average but widely varying density contrast of 0.25 g/cc in basins 4,000 to 8,000 ft deep. A digital‐computer program for automatic computation of basin depths from gravity anomalies was evaluated and found to be useful in this type of analysis. Changes in the depth to the Mohorovičič discontinuity cannot produce regional gradients as large as the regional gradients observed in the area of the survey. Either structure on an intermediate crustal boundary or lateral changes in crustal densities, or a combination of these, is required to explain the gravity data.

Nonlinear inversion of isostatic residual gravity data from Montage Basin, northern Gulf of California

Geophysics ◽  
2017 ◽  
Vol 82 (3) ◽  
pp. G45-G55 ◽  
Author(s):  
Juan García-Abdeslem
Keyword(s):  
Inverse Problem ◽  
Gravity Anomaly ◽  
Gulf Of California ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Nonlinear Inversion ◽  
Residual Gravity ◽  
Sedimentary Sequences ◽  
Residual Gravity Anomaly ◽  
Northwestern Mexico

The flexural isostatic response to surface loads is used to estimate the crustal thickness in northwestern Mexico and Southwestern USA. This estimate is used to compute an isostatic regional gravity, which, subtracted from Bouguer gravity anomalies, led to the isostatic residual gravity anomaly at Montage Basin. This basin is located between the southern portion of the Mexicali Valley and the northern Gulf of California, it roughly has an extension of [Formula: see text] wide, and it shows a gravity minimum reaching approximately [Formula: see text]. Montage Basin is within the extensional province of the Gulf of California, where rifting is currently an ongoing geologic process, and deep exploratory wells drilled by Petróleos Mexicanos have shown that the basin accommodates thick sedimentary sequences greater than 5 km. The interpretation of the isostatic residual gravity anomaly is considered as a nonlinear inverse problem, constrained using density as a function of depth derived from Gardner’s equation applied to dual time [Formula: see text]-logs, assuming isostatic equilibrium and considering the basin as a subsurface load that is compensated at depth by a mass of unknown shape and density. The outcome of the inverse problem suggests that Montage Basin accommodates as much as 7.5 km thick sedimentary sequences and a compensating mass at a minimum depth of 13 km.

Tectonic significance of gravity and magnetic variations along the Lithoprobe Southern Canadian Cordillera Transect

1995 ◽  
Vol 32 (10) ◽  
pp. 1584-1610 ◽  
Author(s):  
Frederick A. Cook ◽  
John L. Varsek ◽  
Jeffrey B. Thurston
Keyword(s):  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Canadian Cordillera ◽  
Western Coast ◽  
Potential Field Data ◽  
Long Wavelength ◽  
Geological Features ◽  
Gravity And Magnetic ◽  
Thrust Sheets ◽  
Accreted Terranes

Correlation of potential field data to regional geological features within the Lithoprobe southern Canadian Cordillera transect corridor allows characterization of anomaly patterns according to their likely sources. Long-wavelength Bouguer gravity anomalies are attributed to isostatic effects of topography, which in most areas is compensated. Two notable exceptions occur: in the Foreland belt a large positive isostatic anomaly is likely due to mechanical support of topography formed as Cordilleran thrust sheets were emplaced over the thick craton, and on the west coast, isostatic anomalies are related to active subduction. Long-wavelength magnetic anomalies in the Foreland belt are associated with cratonal basement beneath the thrust sheets, and these can be followed westward to near the Omineca belt. A prominent positive magnetic anomaly along the western Coast belt is probably associated with mafic rocks generated during subduction. Elsewhere, relatively short wavelength gravity and magnetic anomalies correlate well with either plutons (both gravity and magnetic), volcanics (primarily magnetics), or faults (magnetics) within the region of accreted terranes.

scholarly journals Joint Gravity and Seismic Reflection Methods to Characterize the Deep Aquifers in Arid Ain El Beidha Plain (Central Tunisia, North Africa)

Water ◽  
2021 ◽  
Vol 13 (9) ◽  
pp. 1310
Author(s):  
Hajer Azaiez ◽  
Hakim Gabtni ◽  
Mourad Bédir
Keyword(s):  
Seismic Reflection ◽  
Large Scale ◽  
Seismic Analysis ◽  
Gravity Data ◽  
Geophysical Methods ◽  
Gravity Anomalies ◽  
Integrated Approach ◽  
Central Tunisia ◽  
Deep Aquifers ◽  
Advanced Analysis

Electric resistivity sounding and tomography, as well as electromagnetic sounding, are the classical methods frequently used for hydrogeological studies. In this work, we propose the development and implementation of an original integrated approach using the unconventional hydro–geophysical methods of gravity and seismic reflection for the fast, large–scale characterization of hydrogeological potential using the Ain El Beidha plain (central Tunisia) as an analogue. Extending the values of vintage petroleum seismic reflection profiles and gravity data, in conjunction with available geological and hydrogeological information, we performed an advanced analysis to characterize the geometry of deep tertiary (Oligocene and Eocene) aquifers in this arid area. Residual and tilt angle gravity maps revealed that most gravity anomalies have a short wavelength. The study area was mainly composed of three major areas: the Oued Ben Zitoun and Ain El Beidha basins, which are both related to negative gravity trends corresponding to low–density subsiding depocenters. These basins are separated by an important NE–SW trend called “El Gonna–J. El Mguataa–Kroumet Zemla” gravity high. Evaluation of the superposition of detected lineaments and Euler deconvolution solutions’ maps showed several NE–SW and N–S relay system faults. The 3D density inversion model using a lateral and vertical cutting plane suggested the presence of two different tectonic styles (thin VS thick). Results from the gravity analysis were in concordance with the seismic analysis. The deep Oligocene and Eocene seismic horizons were calibrated to the hydraulic wells and surrounding outcrops. Oligocene and Eocene geological reservoirs appear very fractured and compartmented. The faulting network also plays an important role in enhancing groundwater recharge process of the Oligocene and Eocene aquifers. Finally, generated isochron maps provided an excellent opportunity to develop future comprehensive exploration surveys over smaller and more favorable areas’ sub–basins.

Computer Application in Geosciences: Imaging of the Subsurface by Structural Coupled Inversion of Potential Field Data

2014 ◽  
Vol 644-650 ◽  
pp. 2670-2673
Author(s):  
Jun Wang ◽  
Xiao Hong Meng ◽  
Fang Li ◽  
Jun Jie Zhou
Keyword(s):  
Potential Field ◽  
Field Data ◽  
Gravity Data ◽  
Computer Application ◽  
Magnetic Data ◽  
Imaging Method ◽  
Inversion Algorithm ◽  
Constant Property ◽  
Near Surface ◽  
Potential Field Data

With the continuing growth in influence of near surface geophysics, the research of the subsurface structure is of great significance. Geophysical imaging is one of the efficient computer tools that can be applied. This paper utilize the inversion of potential field data to do the subsurface imaging. Here, gravity data and magnetic data are inverted together with structural coupled inversion algorithm. The subspace (model space) is divided into a set of rectangular cells by an orthogonal 2D mesh and assume a constant property (density and magnetic susceptibility) value within each cell. The inversion matrix equation is solved as an unconstrained optimization problem with conjugate gradient method (CG). This imaging method is applied to synthetic data for typical models of gravity and magnetic anomalies and is tested on field data.

scholarly journals Basin Depth Mapping Beneath Wellington City Based on Residual Gravity Anomalies

2021 ◽  
Author(s):  
◽  
Alistair Stronach
Keyword(s):  
Gravity Anomaly ◽  
Sedimentary Basin ◽  
Gravity Data ◽  
Three Dimensional ◽  
Maximum Amplitude ◽  
Central Business District ◽  
Gravity Anomalies ◽  
Depth Map ◽  
Capital City ◽  
Detailed Knowledge

<p><b>New Zealand’s capital city of Wellington lies in an area of high seismic risk, which is further increased by the sedimentary basin beneath the Central Business District (CBD). Ground motion data and damage patterns from the 2013 Cook Strait and 2016 Kaikōura earthquakes indicate that two- and three-dimensional amplification effects due to the Wellington sedimentary basin may be significant. These effects are not currently accounted for in the New Zealand Building Code. In order for this to be done, three-dimensional simulations of earthquake shaking need to be undertaken, which requires detailed knowledge of basin geometry. This is currently lacking, primarily because of a dearth of deep boreholes in the CBD area, particularly in Thorndon and Pipitea where sediment depths are estimated to be greatest.</b></p> <p>A new basin depth map for the Wellington CBD has been created by conducting a gravity survey using a modern Scintrex CG-6 gravity meter. Across the study area, 519 new high precision gravity measurements were made and a residual anomaly map created, showing a maximum amplitude anomaly of -6.2 mGal with uncertainties better than ±0.1 mGal. Thirteen two-dimensional geological profiles were modelled to fit the anomalies, then combined with existing borehole constraints to construct the basin depth map. </p> <p>Results indicate on average greater depths than in existing models, particularly in Pipitea where depths are interpreted to be as great as 450 m, a difference of 250 m. Within 1 km of shore depths are interpreted to increase further, to 600 m. The recently discovered basin bounding Aotea Fault is resolved in the gravity data, where the basement is offset by up to 13 m, gravity anomaly gradients up to 8 mGal/km are observed, and possible multiple fault strands identified. A secondary strand of the Wellington Fault is also identified in the north of Pipitea, where gravity anomaly gradients up to 18 mGal/km are observed.</p>

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