GP43B-0799
A Quantitative Method to Identify Lithology Beneath Cover
Geophysical terranes (map areas of similar potential field data response) can be used in the estimation of geological map units beneath cover (bedrock, alluvium, or tectonic block). Potential field data over nearby bedrock terranes defines "candidate terranes". Geophysical anomaly dimensions, shapes, amplitudes, trends/structural grain, and fractal measures yield a vector of measures characterizing the terrane. To compare candidate terranes fields with those for covered areas, the effect of depth of cover must be taken into account. Gravity anomaly data yields depth estimates by which the aeromagnetic data of candidate terranes are then upward continued. Comparison of characteristics of the upward continued fields from the candidate terranes to those of covered areas rank the candidates. Because of signal loss in upward continuation and overlap of physical properties, the vectors of measures for the candidate terranes are usually not unique. Possibility theory offers a relatively objective and robust method that can be used to rank terrane types that includes uncertainty. The strategy is to prepare membership functions for each measure of each candidate terrane and the covered area, based on observed values and degree of knowledge, and then form the fuzzy-logical combination of these to estimate the possibility and its uncertainty for each candidate terrane. Membership functions include uncertainty by the degree of membership for various possibility values. With no other information, uncertainty is based on information content from survey specifications and geologic features dimensions. Geologic data can also be included, such as structural trends, proximity, and tectonic history. Little knowledge implies wide membership functions; perfect knowledge, a delta function. This and the combination rules in fuzzy logic yield a robust estimation method. An uncertain membership function of a characteristic contributes much less to the possibility than a precise one. The final result for each covered area is a ranked possibility function for each candidate terrane as the underlying bedrock of the covered area that honors the aeromagnetic field and the geologic constraints that have been included. An example of the application of this method is presented for an area in south central Arizona.
GP43B-0800
Drainage-Divide Approach to Finding Boundaries of Geologic Bodies Using Gradients of 2D Potential Field Anomalies and 3D Tomographic Velocity Anomalies
The locations of steep horizontal gradients of gravity and magnetic anomalies often approximate the locations of edges of geologic source bodies, especially shallow sources. Typically, colored or contoured plots of horizontal gradient values display curvilinear high ridges that separate regions of low gradient. Various approaches have been suggested for locating ridge points and for connecting them to form continuous boundaries. A common approach for locating ridges is to examine variously oriented cross- sections at each point in the gradient field for parabolic-shaped cross-sections indicative of a ridge, or to attempt to fit a paraboloid to a small grid of points surrounding the point of interest. Although the resulting collection of ridge points determined by these methods yields a set of curvilinear lines, these lines are often disconnected and do not form continuous boundaries around the intervening low-gradient areas, which would be useful if one wished to "terrace" (as with rice fields) the potential field anomalies (e.g., Cordell and McCafferty, 1989), thereby delineating the "bodies" inferred from the ridge lines of steepest gradient. Mother nature has provided a convenient way for discovering ridges using the flow of raindrops. Gradient ridges form "drainage divides" separating "drainage basins" in the gradient field. Our approach is to identify the lows in the gridded gradient field, and then to determine into which low a raindrop falling at each gridpoint will ultimately come to rest, so that each low corresponds to a drainage basin. This can result in a great many drainage basins for noisy data, but consolidating nearby lows or putting "lakes" in low areas can simplify the pattern of basins. Ridge points can also be tagged with the value of the gradient at the point, offering some quality assessment of boundary lines. One advantage is that drainage basins translate directly into terrace regions. The approach can be easily extended to 3D data sets such as tomographic velocity models where 3D anomalies in velocity may, in some cases, be caused by geologic bodies with sharp contacts that it would be desirable to locate.
GP43B-0801
A Cubic B-Spline Approach for Inter-Transformation Between Potential Field and Gradient Data
Traditionally, algorithms involving Fast Fourier Transforms (FFT) are used to calculate gradients from field data and vise versa. Because the popular FFT differentiation algorithms are prone to noise, expensive field campaigns are increasingly utilized to obtain gradient data. In areas with both field and gradient data, transformation facilitates comparison. In areas with only one kind of data, transformation facilitates interpretation by transforming the measured data into another form of data. We advance unified formulae for interpolation, differentiation and integration using cubic B-splines, and propose new space-domain approaches for 2D and 3D transformations from potential field data to potential-field gradient data and vice versa. We also advance spline-based continuation techniques. In the spline-based algorithms, the spacing can be either regular or irregular. Analyses using synthetic and real gravity and magnetic data show that the new algorithms have higher accuracy, are more noise-tolerant and thus provide better insights into understanding the nature of the sources than the traditional FFT techniques.
GP43B-0802
Potential-Field Forward Modeling and Inversion Using 3D Fast Fourier Transforms
Although 1D and 2D fast Fourier transforms (FFTs) have long been used for the filtering, interpretation, and
modeling of potential-field data, 3D FFTs have not enjoyed similar popularity. This may change with the
recent discovery (Caratori Tontini et al., in press, JGR) that simple 3D FFT filters can be used to transform
distributions of density (or magnetization) within a box-shaped 3D volume into gravity (or magnetic) fields
within the same volume. For example, the continuous 3D Fourier transform of the vertical gravity anomaly
Δgz(x,y,z) in a volume is related to the 3D Fourier transform of the density ρ(x,y,z) in the
volume by
ℑ[Δgz] = i4πG(kz/ | k | 2)ℑ[ρ]; | k | ≠0,
(1)
where G is the gravitational constant, kx, ky, kz are wavenumbers, and
| k | 2 = kx2+ ky2+ kz2.
(2)
Translating (1) into a digital FFT filtering operation requires careful consideration of the periodicity of the
density distribution and the gravity field. Nevertheless, (1) provides a highly efficient way to calculate the
vertical gravity anomaly of a 3D density distribution within a few minutes. The calculated gravity anomaly can
be sampled at random points or on an arbitrary surface using tri-linear interpolation. An equivalent space-
domain calculation of the gravity field on an arbitrary surface can take many hours.
The inverse relation to (1) does not appear to offer a practical approach for calculating a reliable density
distribution from observed gravity data. This is because the data would have to cover a substantial portion of
the model volume, and because the transformation is undefined on the plane kz = 0, where both the
denominator of the filter kernel and the Fourier transform of the gravity field are identically zero. Numerical
experiments show that the density distribution resulting from the power on the plane kz = 0 represents a
classic Parker annihilator. This annihilator can be calculated from the density distribution but not from the
gravity field, as would be required for direct inversion.
In forward modeling mode, equations such as (1) permit rapid testing of geological models against observed
potential-field data. The calculation speed of the 3D FFT suggests that a practical iterative inversion
algorithm could be developed from (1) using Markov perturbation of an initial density model. Such an
algorithm would have immediate application to problems of hydrogeology, resource assessment, and tunnel
detection.
GP43B-0803
New Satellite Gravity Solution 'Trident'
In 2004 GETECH, University of Leeds introduced a new satellite gravity model for the continental margins of the Earth, using ERS1 and Geosat altimeter data. This new high resolution satellite gravity model significantly out-performed both the Sandwell and Smith and Danish satellite gravity model solutions. In 2008 both Sandwell and Smith (solution 16.1) and Danish (solution DNSC08) have significantly improved the resolution of their respective solutions such that they are now compatible with the GETECH 2004 solution. The main difference between the solutions appears to be random noise such that by stacking the three solutions there is a measureable overall improvement in resolution. We call this solution 'Trident'. It out performs each of the individual solutions and when compared to a number of high resolution marine gravity surveys its resolution is close to ~3 mGals at 14 km full wavelength (i.e. at 0.5 Coherency).
GP43B-0804
Crustal thickness of Korean Peninsula and neighboring seas from the free-air gravity and topography
Crustal thickness of Korean peninsula and its neighboring seas is estimated using free-air gravity compiled from the near-surface surveys and topography data. Spectral correlation analysis is used to extract the annihilating part in which the correlated features with free-air gravity are removed from the terrain gravity effects. This part is then evaluated for depth variations of the Moho discontinuity. Seismic depth from the data collected from the study area help constrain the inversion. While the crustal thickness is estimated to add the Moho depth to the elevation and bathymetry, the terrain-correlated free-air gravity anomalies, which reflect either components of the free-air gravity that are due to uncompensated or thermally elevated terrain or relatively dense crust, are used to investigate the subsurface structure of Ulleung Basin in East Sea.
GP43B-0805
Using Gravity Data to Constrain the Nature of the Crust Beneath Baffin Bay
The nature of the crystalline basement beneath Baffin Bay, which is covered with over 10 km of sediments in places, remains under debate. Particularly under question is whether it is composed of continental or oceanic crust or both. To the south within the Labrador Sea, seafloor spreading is confirmed to have begun some time between 86 and 61 Ma and ceased around roughly 55 Ma, as determined by well-defined magnetic lineations produced by geomagnetic reversals during spreading, as well as by samples from an ODP hole. In Baffin Bay, rift structures and volcanic materials are found close to the coast both onshore and offshore, indicating that the extensional effects of the Labrador Sea opening continued north to the Bay. Early single- line magnetic profiles collected in sparse locations suggested seafloor spreading patterns, but more recent data filling gaps between these profiles and covering most of Baffin Bay do not resemble seafloor spreading anomalies. Nonetheless, other evidence suggesting Baffin Bay is underlain by ocean crust includes seismic velocities determined by refraction studies (which in some parts of the Bay are near ~5-6 m/s, similar to oceanic layers 2 and 3) and tectonic considerations. However, other tectonic models based on reflection data, including unconformities extending offshore that represent early Cretaceous and Miocene deposition, suggest that seafloor spreading at the time of the Labrador opening could not have occurred. Drill holes have not penetrated basement beyond a few kilometers offshore and are thus not diagnostic. The recent compilation and release of gravity data through the Arctic Gravity Project (ArcGP) provides excellent coverage of Baffin Bay, the Labrador Sea, and surrounding sections of Greenland and Canada. These data allow an investigation into the suite of crustal density and thickness combinations possible within this region. We constrain regional models using onshore and offshore seismic reflection, refraction and large-scale tomography experiments, as well as available constraints on sediment density and layer thickness. Preliminary models suggest that if the crust beneath Baffin Bay is continental, it must be unusually thin, whereas if it is oceanic, its thickness falls within ranges observed over several mid-ocean ridge systems.
GP43B-0806
Improved Resolution of Moho Geometry From Modelled Internal Crustal Load Variations via a Novel Analytical Solution (ASEP) –- Norwegian Barents Sea
Gravity and magnetic data allow one to image the entire crust from the surface down to the Moho. Seismic, geological and borehole information are generally used to constrain the model, but information about the Moho geometry is rather scarce. The interpretation of the Moho is often based on long wavelength undulations of the gravity field, which is not unique due to other deep sources of density contrast. Moho information, independent of the gravity field, is therefore useful to constrain the modelling. The flexural response of the Earth's crust due to loading can be estimated using a 4th order differential equation describing the flexure of a thin elastic plate. Until recently, the solution for this differential equation analytical for a real topography has been approximated by spectral methods with the disadvantage of low lateral resolution. The analytical solution of an elastic plate (ASEP) is a novel approach which overcomes the drawbacks of the spectral method. The ASEP method allows the analytical calculation of a flexural Moho geometry which directly uses the crustal load. Load is defined as product of density, thickness and gravity acceleration; the crustal load is the sum of basement load, sediment load and load of topography/water above a reference depth. The novel approach, to estimate the Moho geometry from these internal crustal load variations via ASEP, was applied to data from the south-western Barents Sea with emphasis on the southern Nordkapp Basin (NKB) (25-29°E and 71.7-72.7°N). An area of about 230.000 km2 was selected to avoid edge effects in the area of interest (14.000 km2). The factor of enlargement for the area to be calculated is defined by a flexural function that depends on the average rigidity of the Earth's crust. The NKB is a Permian rift basin of more than 10 km sedimentary depths with salt diapirism. Reflection seismic investigations for hydrocarbon exploration had considerable problems to define the base of the salt. In order to overcome this problem, the southern NKB was investigated using high resolution gravity, gravity gradiometry, magnetic and electro-magnetic data. The combination of these data types with 2D and 3D seismic allowed the construction of a full sedimentary model. The top basement was difficult to define due to lack of reflection seismic information as well as small density and magnetic susceptibility contrasts. The basement modelling is directly linked to the topography of the Moho. A few old deep seismic lines exist, but the information about the Moho was sparse and rather uncertain. This NKB density model was transferred into load in order to be used as input for the ASEP Moho estimations. This is superior to depth-density-functions as these would lead to overestimated densities in areas with salt diapirism. Outside of the NKB model, the sediment densities were approximated via absolute sediment thickness data (recent mapping) and a depth-density-function. This function was amended in such a way that the regional density variations roughly match the density distribution in the NKB model. On the poster, the determination of crustal load is described, the advantage of using the crustal load and combination of modelled and approximated sediment loads is demonstrated and the impact of the ASEP Moho on the basement modelling in the NKB is shown.
GP43B-0807
Potential Field Signature of the Eastern North Greenland Transform Margin: Indications of Strong Segmentation and Deep Sedimentary Basins
The eastern North Greenland transform margin formed during Cenozoic opening of the Fram Strait at the transition from the Atlantic Ocean to the Arctic Ocean. Only a narrow zone of outcrops is exposed between the front of the major Greenland Inland Ice and the almost permanently ice covered submerged shelf. Mapping expeditions in the 1980s showed that the coastal geology is defined by a northern extension of mid- late Paleozoic N-S striking Caledonian structures which appears to be abruptly cut by late Paleozoic – Mesozoic NW-SE striking structures of the Wandel Sea strike-slip belt. However, the geology of the area is complicated by Proterozoic structures as well as early Cenozoic (Eurekan) transpressional tectonics. A 50 km wide and shallow shelf characterizes the margin offshore eastern North Greenland. The geology of this ice covered area is very poorly known and has mostly been described in terms of large scale linear structures based on assumed correlation between observed structures at adjacent peninsulas. Substantial uncertainty therefore exists in this area – especially regarding the northward extent of Caledonian structures and the offshore correlation of the NW-SE striking Wandel Sea structures. We present a new interpretation of the onshore and offshore parts of the transform margin based on the latest IBCAO2 physiographic data, DNSC2008 gravity data, and aeromagnetic data acquired by AWI in 1997. Our interpretation adds new information about the previously proposed and simplified plate tectonic setting of the margin. In particular, our results indicate that (1) the margin is characterized by strong segmentation which may be correlated with reactivated Caledonian structures; and (2) deep pull-apart basins exist between the segmented parts. The existence of deep basins is also confirmed by receiver function analysis of new earthquake seismic data from the area.
GP43B-0808
Potential field analysis images Paleoproterozoic terrane boundaries in the unexposed Northern Gawler Craton, Australia
In areas of limited or zero basement exposure, potential field analysis provides an invaluable tool for determining the architecture continental cratons. We present an example from the Northern Gawler Craton, Australia. Here, the basement rocks are almost completely covered by Neoproterozoic and younger sedimentary rocks (with <<1% basement outcrop), so are amongst the least studied on the Australian continent. However, these rocks should preserve the best record of the Gawler Craton's interactions with neighbouring cratons during the amalgamation of Proterozoic Australia. We focus on the basement architecture in the Marla region of the northernmost Gawler Craton. We use geophysical techniques and apply a top-down approach to penetrate the significant thickness of cover and determine the structure of the unexposed northern Gawler Craton. The architecture, density and magnetic susceptibility of the overlying cover sequences are constrained from the surficial geology, borehole data and seismic reflection profiles. The effect of these cover sequences is then removed from gravity and magnetic data highlighting the basement structure. We then determine the architecture of the basement with depth from the potential field data by combining depth-to-source analyses, with forward and inverse modelling techniques constrained by petrophysical data from drill-holes. Results of this analysis include the observation of a major crustal boundary at the NE-SW trending Middle Bore Fault. To the NW of this boundary, crustal scale sources produce large gravity and magnetic anomalies. Whereas to the SE anomalies are sourced in the upper crust and overlie uniform middle to lower crust. We suggest that the Middle Bore Fault represents a boundary between allochthonous terranes that may have accreted to the Gawler craton during the Kimban Orogeny (~1.7 Ga) and the Archaean Gawler Craton that is overlain by Paleoproterozoic metasediments. The basement structure revealed by this approach constrains the processes responsible for the growth and evolution of the northern Gawler Craton and its role in the Proterozoic amalgamation of Australia.
GP43B-0809
Mapping the depth-extent of crustal magnetic sources in the Great Basin, USA
We revisit the problem of using aeromagnetic data to map depth to the Curie-temperature isotherm in the Great Basin, assuming that the depth-extent of crustal magnetic sources corresponds to the temperature at which rocks lose their spontaneous magnetization (e.g., 580°C for magnetite). The Cenozoic evolution of the Great Basin is a complex product of Basin and Range extension, Yellowstone hotspot migration, and Pacific plate subduction. The geothermal manifestations of this evolution should be reflected in modern-day heat flow. The depth to the bottom of magnetic sources is estimated by assuming that crustal magnetization is a random function of position characterized by a fractal power spectrum. The power spectrum of the magnetization is proportional to the wavenumber raised to -β, where β is related to the geologic terrane. With these assumptions, the shape of the theoretical power spectrum depends on three independent parameters: the depths to the top and bottom of the magnetic source layer, and the fractal exponent β (Maus et al., 1997). We estimate the depth to the bottom of magnetic sources within a sliding window moved across the region. For each window, we first calculate the radial power spectrum of the magnetic anomalies and then search for the set of parameters that provided the best theoretical fit. This method was first validated on synthetic datasets and then applied to newly released aeromagnetic compilations for Nevada and North America. We observe that fixing one of the parameters gives more realistic results, and thus we compute maps of the depth-extent of crustal magnetic sources assuming that the fractal exponent β is constant throughout the study area. There is, of course, some ambiguity when interpreting the resulting spatial variations in depth to the bottom of magnetic sources, which can be attributed either to actual variations in the depth-extent of crustal magnetic sources or to variations in the surface geology affecting the β exponent. The resulting maps show long-wavelength, robust features that, in general, do not depend on the assumed value of β, on the size of the window, or on the data compilation used (Nevada or North America). Furthermore, many of the long-wavelength features of the depth to the bottom of magnetic sources correlate closely with prominent heat-flow anomalies and with the spatial distribution of volcanic rocks. For example, volcanic areas such as the Snake River Plain or the Cascade arc, both characterized by very high heat flow, show very shallow bottoms of magnetic sources, consistent with a shallow Curie-temperature isotherm. We also observe that the thermal structure beneath the modern Cascades arc extends south beneath the ancestral Cascade arc. This might reflect the existence of a temperature anomaly that has persisted since the cessation of subduction or the influence of associated igneous rocks on the β exponent.
GP43B-0810
Geophysical Investigation of the San Juan Mountains Batholith, Southwestern Colorado
One of the largest and most pronounced gravity lows over North America lies over the rugged San Juan Mountains in southwestern Colorado. The mountain range is coincident with the San Juan volcanic field (SJVF), the largest erosional remnant of a widespread Oligocene volcanic field that covered much of the southern Rocky Mountains. A buried, low-density silicic intrusion related to the volcanic field has been the accepted interpretation of the source of the gravity low since the 1970s. However, this interpretation was based on gravity data processed with standard techniques that break down in the SJVF region, due to extreme topographic relief and densities of the volcanic field that are much lower than the standard reduction density (2670 kg/m3) normally assumed for gravity data processing. The combined effects of high-relief topography, low densities of the SJVF, and a reduction density of 2670 kg/m3 result in large-amplitude gravity lows that may mask or interfere with the geophysical signature of a low-density batholith. Here, we apply an unconventional processing procedure that takes into account geologically appropriate densities and digital topography to at least partially remove the effect of the SJVF and derive a gravity map that gives an improved representation of deeper sources, including a reduced-amplitude anomaly attributed to the batholith. We also reinterpret vintage seismic refraction data that appear to image a portion of the batholith as a low-velocity zone. Preliminary results of modeling of the gravity anomaly, along with the seismic refraction data, finds that a smaller batholith than previously thought is allowable, with a minimum thickness on the order of 10 km. The maximum thickness is more difficult to constrain, yet is similar to previous estimates of 20+ km. Further investigation is focusing on the implications of these results, including the relationship between the batholith and SJVF, batholith volume and composition, the geophysical expression of older intrusions in the SJVF area, and regional tectonic control on batholith location.
GP43B-0811
Implications for Fault and Basin Geometry in the Central California Coast Ranges from Preliminary Gravity and Magnetic Data
Preliminary aeromagnetic and newly processed gravity data help define block-bounding faults and deep sedimentary basins in the central California Coast Ranges, ranging from the Hosgri fault east to the San Andreas fault and from Monterey Bay south to Pt. Conception. Interpretation of these data results in an improved framework for seismic hazard and groundwater studies. Aeromagnetic data include a new survey with a flight-line spacing of 800 m at a nominal 300 m above ground and covering 15,000 km2. More than 11,500 gravity measurements, reprocessed with terrain corrections calculated from 30-m DEMs, form a roughly 2-km grid over most of the study area. Combined potential-field data and existing geologic mapping, delineate major fault-bounded blocks in the central California Coast Ranges. Main block-bounding faults from west to east include the San Gregorio- Hosgri, San Luis-Willmar-Santa Maria River-Little Pine, Oceanic-West Huasna, Nacimiento, Rinconada-South Cuyama, San Juan-Chimineas-Morales, and San Andreas faults. Most of these faults have evidence of Quaternary activity. Gravity gradients indicate that the reach of the San Andreas fault bounding the Gabilan Range and the northern extension of the Rinconada fault bounding the Santa Lucia Range dip steeply southwestward and have a reverse component of slip. Magnetic and microseismicity data suggest that the northern reach of the Hosgri fault dips eastward. The potential-field data also delineate several deep sedimentary basins, such as the 3-4 km deep Cuyama basin, the Santa Maria basin, and several basins along and possibly offset by the Rinconada fault. Gravity data show that the main west-northwest-striking faults bounding the Cuyama basin dip away from the basin, indicating compression adjacent to the big bend in the San Andreas fault. Prominent gravity and magnetic highs northeast of the San Andreas fault immediately east of Cuyama Valley suggest that there the San Andreas fault dips southwest. Such dip information is important for estimating shaking potential of scenario earthquakes and for calculating geodetic deformation whereas basin shapes and fault locations are critical components for groundwater flow modeling.
GP43B-0812
THE HOSGRI FAULT ZONE, CENTRAL CALIFORNIA: COLLECTION AND PRELIMINARY ANALYSIS OF MARINE MAGNETIC AND SEISMIC REFLECTION DATA
Newly acquired high-resolution marine magnetic and seismic-reflection data collected offshore Point Buchon, California, are being combined with existing regional magnetic, oil industry multichannel seismic (MCS), onshore geology, and seismicity data to investigate the tectonics and earthquake hazards associated with the Hosgri and other nearshore fault zones. This research is part of a broader study of earthquake hazards in the central coastal California region that was most recently illustrated by the 2003 M6.6 San Simeon earthquake. High-resolution marine magnetic and single-channel mini-sparker seismic reflection data were collected in June 2008 aboard the USGS R/V Parke Snavely. These data were collected in the offshore areas between Cayucos and Pismo Beach from the nearshore (6 m depth) to just west of the Hosgri Fault Zone. The seismic reflection data were collected using a mini-sparker source and a 15-meter single-channel hydrophone streamer at a 500 joule power level and shot interval of 1/2 second, generating a signal between 200 and 1500 Hz. The G-882 cesium marine magnetometer was mounted on the opposite side of the stern as the seismic source and streamer and was towed approximately 30 meters behind the vessel. Mini-sparker and marine magnetic data were collected simultaneously along shore-perpendicular tracklines spaced 800 meters apart. Along the Hosgri Fault Zone, marine magnetic data were collected with 400 meter trackline spacing. Marine magnetic data reveal two distinctive magnetic anomaly patterns north and south of Point Buchon. The transition between these anomaly patterns corresponds to the boundary between the Los Osos Valley and the San Luis/Pismo structural block to the south. Within the northern offshore Los Osos Valley block a series of broad magnetic highs extends northwestward from Morro Rock. These anomalies may signify the offshore extension of the Oligocene Morro Rock-Islay Hill igneous complex that appears to have been displaced along previously identified right-lateral strike-slip faults. Short wavelength magnetic anomalies along the offshore extension of the northern Cambria Fault indicate the presence of magnetic volcanic rocks and serpentinite within the Franciscan Complex, comparable to those mapped onshore. Similarly, the short wavelength anomalies in the new marine data along the coastline south of Point Buchon are likely caused by magnetic rocks within an offshore extension of the Franciscan Complex. Maximum horizontal gradient analysis of the marine magnetic data shows that gradients commonly correspond to locations of mapped faults.
GP43B-0813
The San Andreas Fault as a Backstop to Crustal-Scale Folding Revealed by Geologic and Geophysical Modeling Near Parkfield, Central California
Two-dimensional (2-D) geologic and geophysical modeling across the San Andreas Fault (SAF) in central California reveals major structure including an antiformal wedge of magnetic Great Valley basement on the northeast side of the fault that is truncated at the fault. Regional compressional structures, including blind thrust faults associated with the 1983 Coalinga earthquakes, appear to root into the antiform. These relations suggest that regional transpression in the Parkfield-Coalinga region has not resulted in a "flower structure" centered on the San Andreas Fault, but rather in a near vertical almost purely strike-slip San Andreas that serves as a "backstop" to crustal-scale folding and associated thrust and reverse faulting on the east side. We computed 2-D forward models from gravity and magnetic data along four ~ 30 km-long profiles perpendicular to the SAF. High resolution aeromagnetic data were collected in the study area at a nominal 300 m above terrain along NE-SW flightlines normal to the SAF, and gravity data are distributed unevenly but generally spaced ≤ 1.6 km. Initial models along all four profiles, constrained by geologic cross sections and well data from the vicinity of our profiles, were unable to explain the large, regionally extensive magnetic highs observed immediately northeast of the San Andreas Fault. Subsequent modeling of filtered magnetic data along ~ 100 km-long profiles in the same area shows that the Great Valley magnetic high, located ~ 50 km northeast of the SAF and extending along the entire Great Valley for a distance of 700 km, can be modeled here as resulting from a west-dipping wedge of magnetic material 2-10 km thick that extends about 100 km east from the SAF. Immediately northeast of the SAF in the Parkfield area, this magnetic material, previously interpreted to be ophiolite, is upwarped to within several kilometers of the surface, thus explaining the magnetic high in an area with little or no magnetic rock at the surface.
GP43B-0814
Three-Dimensional Mapping of Magnetic Strata From Aeromagnetic Anomalies: The Deformed Neroly Formation South of Mt. Diablo, Northern California
We apply direct inversion of aeromagnetic anomalies to analyze the subsurface 3D shape of the highly magnetic Miocene Neroly Formation, which consists largely of medium to coarse-grained andesitic sandstones containing abundant magnetite. The Neroly Formation is widespread in the eastern San Francisco Bay region, and locally is tightly folded and disrupted by faulting in the compressional regime related to the left-stepping (restraining) connection between the strike-slip Greenville and Concord Faults. The inversion technique is based on the conversion of the anomalies produced by a magnetic layer to their equivalent magnetic potential (psuedogravity) anomalies, manipulation of these anomalies to produce anomalies that would result from a half-space with a variable-depth top having the shape of the top surface of the layer, and then inverting these pseudogravity anomalies for the shape of that top surface. Assumptions include a constant layer thickness, uniform magnetization which implies a constant pseudodensity contrast, and a surface that is single-valued (no recumbent folds or strata repeated with depth). Constraints on 3D position are applied where the layer crops out or is at a depth known from well or other information. Application of this inversion technique to aeromagnetic anomalies over the Neroly Formation yields a complex top surface characterized by elongate overlapping troughs and structural highs, including the well-known Tassajara anticline and adjacent Sycamore Valley syncline. Troughs are true synclinal lows whereas the structural highs may be fold crests, steep truncated strata, and/or fault duplicated strata. The strongest deformation is confined to within ~7 km of the near-vertical overturned Neroly beds that crop out along the NE margin of the valley, and is characterized by four laterally overlapping, margin parallel structural highs and intervening troughs, each between 10 and 20 km in length. A fifth possible structural high lies farther SW. Separation between the highs increases southwestward across strike away from the valley margin. The gross structure implied by the inferred shape of the Neroly layer is that of a 7 km wide, NW oriented doubly- plunging synform with internal, high-amplitude 'wrinkles'. The technique shows promise for application to deformed magnetic layers in other regions. Elsewhere in California these include the Purisima Formation of the Hollister Valley and Santa Cruz Mountains, the Etchegoin Formation along the San Andreas Fault near Parkfield, and the Coastal Belt of the Franciscan Complex.
GP43B-0815
Using High Resolution Aeromagnetic Data to Map Pervasive Folding in the Lithologically Indistinct Franciscan Coastal Belt
We use high-resolution aeromagnetic data to map magnetic bodies of graywacke of limited exposure that are either interbedded or structurally emplaced within broader areas of non-magnetic graywacke within the Franciscan Complex Coastal belt in northern California, which is bounded by the San Andreas Fault on the west and the Franciscan Complex Central belt on the south and east. Previous work has not extensively subdivided the Coastal belt because of the poor exposure and the fact that the exposed lithology is primarily graywacke indistinguishable in outcrop and hand sample and is thus difficult to map in the field. A hand-held magnetic susceptibility meter, however, in combination with thin-section analysis, reveals that some Coastal belt graywackes are magnetic. The thin-section analysis shows that the magnetic samples have a significant component of andesitic grains, whereas the non-magnetic samples do not. Further, the locations of these magnetic rocks correspond to elongate regions of high magnetic intensity (magnetic anomalies) kilometers to tens of kilometers in length. Previous 2D modeling showed that the bodies of magnetic graywacke can be modeled as a folded sheet, with antiformal limbs near or exposed at the surface and synformal limbs reaching a depth of about 1 km. Locations of edges of magnetic source bodies can be extracted from their magnetic anomalies. Near surface, steeply dipping edges lie beneath local maxima in the horizontal gradient of the magnetic potential surface. The edges are demarcated by locating discrete points along the local maxima. We connected these points, using an algorithm with a specific set of parameters, to delineate the edges of the magnetic graywacke bodies. Together with the previous 2D modeling, the anomalies and their edges show that the Coastal belt contains antiformal structures 5 to 20 km in length and 1.5 km in width, with a wavelength approximately 1.5 km. The modal direction of elongation is oriented approximately 50 degrees NW, sub parallel to the surface trace of the San Andreas Fault (trending 40 degrees NW in the area), which it intersects at an acute angle. The elongated structures in the relatively lithologically indistinct Coastal belt closely resemble structures defined by terrane fault boundaries and magnetic anomalies (produced mainly by tabular ophiolitic sources) in the adjacent lithologically heterogeneous Central belt. The distributions of mean vector direction and dispersion of magnetically-defined edges of the two datasets are very similar, and the global mean direction and dispersion agree to within 2 degrees. Many edges defined by the magnetic data are coincident with or continue the trend of mapped faults that form the structural boundaries between the Coastal belt and the Central belt terranes. The similarity of the pervasive structure in the Coastal belt evident from the aeromagnetic data with structure in the adjacent Central belt suggests that the structures within the two terranes are similar. This not only implies that structure in the Central belt can be used to inform further structural interpretations in the Coastal belt, but also suggests that the structural pattern seen in both terranes is the result of a younger tectonic overprinting that postdates the amalgamation of the two terranes.
GP43B-0816
Narrow Sub-basins Along the Margins of the Los Angeles Basin Inferred From Gravity
On the isostatic residual gravity map of the Los Angeles Basin (LAB), southern California, prominent gravity gradients occur over reverse faults that place dense pre-Cenozoic basement rock against middle Miocene to Holocene low-density sedimentary fill. The gravity gradients clearly identify boundary structures of the LAB such as the Santa Monica-Hollywood-Raymond and Palos Verdes fault systems. Although mapped regional faults and local active fault strands generally follow these steep gravity gradients, in places the geophysically-defined range-front faults bounding the LAB deviate from the mapped faults. Enhancement of short-wavelength, shallow-source gravity anomalies clearly shows narrow closed gravity lows reflecting increased sediment thickness lying between the defined range-front faults and intra-basin arches. Inversion models used to determine the geometry of these narrow sub-basins (widths generally 2 to 5 km and lengths 10 km and greater) indicate that the associated ~ 5 mGal gravity lows reflect an additional 0.5 to 1.0 km of near-surface, low-density sediments. Sub-basin widths are too small to be consistent with bending or folding of a uniform crust layer of order 10 km thick. However, the sub-basins' width scale of several km implies a similar space scale for their mechanical origin. Possible features of this length scale are a secondary fault sub-parallel to the main reverse fault, aseismic creep on the upper part of the main reverse fault, a weak detachment fault at shallow depth, and non-elastic deformation of the top few km of LAB material.
GP43B-0817
Interpreting Faults and Fractures in Hydrothermal Basins With High-Resolution Aeromagnetic Data in Yellowstone National Park
Hydrothermal systems in Yellowstone National Park originate at deep structures and manifest on the surface along faults and fractures. Precisely locating the controlling structures is essential to understanding fluid flow, geochemistry, and natural hazards of the hydrothermal systems. Structural interpretation in Yellowstone's hydrothermal basins is problematic, however, due to intense alteration of the country rock, widespread glacial deposits and myriad directions of apparent linear trends. To address this problem, high-resolution aeromagnetic data are examined to identify abrupt changes in magnetization attributed to juxtaposition of hydrothermally altered rocks, in which magnetic minerals have been destroyed, with their unaltered equivalents. These data show that near-surface hydrothermal manifestations within Yellowstone Caldera correlate with structures inferred from aeromagnetic-derived maximum horizontal gradient and minimum point of curvature analyses. The maximum horizontal gradients reflect changes in magnetic susceptibility along shallow (depths <100m) fractures. Minimum point of curvature analysis reveals deeper (depths <500 m) structures interpreted as buried faults and fractures. In turn, the interpreted geologic structures and forward modeling of the magnetic and thermal features data are used to constrain simple fluid flow models
GP43B-0818
GIS Integration of GPS-Positioned Gravity Data With Geologic Maps and Other Images, Well Control, and Seismic Data for Outcrop to Basin-Scale Structural Analysis: Examples from Marsh and Cache Valleys, Idaho and Utah
The Basin and Range Province presents unique challenges for the interpretation of regional to local late Cenozoic structural architecture. A significant amount of geologic mapping has taken place within the ranges, however, mapped Neogene structures are difficult to project into the adjacent basins because younger Quaternary fill generally covers all but the most recent structures. Potential field data has traditionally been used to help with the structural interpretations within the basins, but not with the resolution, accuracy, and speed that can be attained with the use of sub-meter to centimeter-scale Global Positioning System (GPS) equipment and Geographic Information Systems (GIS) software readily available today. Spatial integration of potential field data, reflection seismic sections, well control, geologic maps, earthquake epicenters and other data sets such as digital elevation models (DEMs) digital raster graphic maps (DRGs), and remote sensing images among others, provides a powerful means for continuous 2-D to 3-D structural interpretation between the ranges and basins. This relatively detailed interpretation of structure can be used for a variety of purposes, including energy exploration, groundwater resources evaluation, and earthquake hazards investigations. Gravity data previously acquired by the USGS and other agencies, universities, and companies in and around Marsh and Cache Valleys, Idaho and Utah, have been combined with detailed gravity profiles and more regionally spaced stations acquired and reduced with the aid of sub-meter to centimeter-scale GPS equipment, a hand-held laser rangefinder and various scale DEM's. Interpretation of these gravity data and other data sets mentioned previously show a two to three-phase history of late-Cenozoic extensional deformation within Marsh and Cache Valleys, as well as the surrounding ranges. The earliest pre-detachment phase of extension appears along the western margin of Cache Valley as isolated Eocene to Oligocene basins. The major phase of extension, which began 16 to 13 Ma in Cache Valley and surrounding ranges, and before 10 Ma in Marsh Valley and surrounding ranges, resulted in deposition of the Salt Lake Formation. This phase culminated in widespread top-to-the southwest extension above the Bannock detachment fault in both Cache and Marsh Valleys and surrounding ranges between ca. 10 and 4 Ma, waning or ceasing sometime between 5 and 3 Ma. The northeast tilted basins, formed during and possibly before the detachment phase of extension are preserved in the isolated exposures in the ranges, but are also interpreted from gravity, seismic, and well data to occur as mostly half-graben subbasins beneath the Quaternary fill of both Marsh and Cache Valleys. After 3-4 Ma, predominantly east-west extension along relatively steeper-dipping, widely-spaced north-south Basin and Range block faults cut the detachment fault and earlier formed basins. This phase of faulting resulted in the current physiographic expression of Marsh Valley, Cache Valley, and the surrounding ranges. Gravity and other data suggest that faults formed during this phase of deformation occur not only along the margins of the ranges but also within the more central parts of the basins. Gravity and geologic data also suggest that numerous accommodation zones and transfer zones occur within and between Marsh and Cache Valleys, and that the breakaway zone of the Bannock detachment fault may have formed along or above an earlier thrust ramp.
GP43B-0819
Results of co-located gravity and water-level monitoring at alluvial aquifer wells in southern Arizona
Correlation of gravity and water levels at thirty-nine wells in southern Arizona during 1988 to 2002 indicates that water-level change is not always a reliable indicator of aquifer-storage change for alluvial aquifer systems. Gravity change is directly proportional to aquifer storage change where other sources of mass change are minimal. Gravity estimates of aquifer-storage change can be approximated as an infinite slab except near some withdrawal wells and recharge sources. Linear regression of water-level change against one-dimensional estimates of storage change is an estimate of the aquifer storage coefficient. Non-aquifer gravity signals from temporarily stored water that does not percolate to the aquifer do not normally contribute significant noise to the observation record, but can be greater than four microGal (equivalent to three inches of water) when water is held in the root zone for brief periods following extreme rates of precipitation. The primary reason that gravity and water-level correlation was poor at some wells is that the screening of those wells across the heterogeneous alluvial aquifer did not allow monitoring of hydraulic head in the local water table aquifer. Monitoring of storage change using gravity methods at wells improved understanding of local hydrogeologic conditions. Good correlation of gravity and water levels occurred at wells where the local aquifer is primarily unconfined. Unconfined conditions were indicated at fifteen wells where significant water-level and gravity change were well correlated. Good correlations resulted in extremely large specific yield values, greater than 0.35, at seven wells where significant ephemeral streamflow infiltration likely resulted in unsaturated-zone storage change and delayed deep percolation to the water table. Poor correlation indicates confined, multiple, or perched aquifers. Confined aquifer conditions are likely at three wells where large water-level variations were accompanied by little gravity change. Monitoring of a multiple compressible aquifer system at one well resulted in negative correlation of rising water levels and subsidence-corrected gravity change, which suggests that water-level trends at the well are not a good indicator of overall storage change.
GP43B-0820
Linking self-potential and geochemical signatures over a large porphyry mineral deposit in Alaska
Geophysical and geochemical data collected during the 2007 and 2008 field campaigns at Pebble, a large Cu-Au-Mo porphyry deposit in southwest Alaska, are analyzed to help better understand the surface signatures associated with the deposit. In particular, we investigate the possible role of electrochemical transport associated with the "geobattery" created by the mineralized zone within the Earth's natural redox field. Several large self-potential anomalies, on the order of -600 mV and nearly 1 km in diameter, are consistent with the presence of a subsurface electrochemical cell. Source inversion of the self-potential data is used to determine the subsurface distribution of current sources associated with the cell, and is reconciled with the surface geochemical data. Additional constraints to the problem are provided by drill core information from exploratory drill holes, resistivity information from previous geophysical surveys, and information about regional groundwater elevations.
GP43B-0821
Gravity studies at Etna volcano: a comparison between relative and absolute gravity measurements
The INGV has been operating at Mt Etna a discrete gravity network since 1986 and three continuous gravity stations since 1998. The combined use of discrete and continuous gravity measurements has provided, through the detection of phenomena with a wide range of evolution rates (periods ranging from minutes to years), both substantial improvements in the knowledge of the dynamics of the shallow plumbing system at Etna and the identification of any gravity transient before and during the last volcanic eruptions. Recently, with the aim of compare relative microgravity measurements routinely acquired on Etna volcano using spring gravimeters with absolute gravity observations, we performed two surveys in June 2007 and July 2008 by using the new IMGC-02 transportable absolute gravimeter. The IMGC-02 transportable instrument, developed by INRiM – Torino, adopts the absolute ballistic method, which was recognized at international level (Comité International des Poids et Mesures - CIPM) as primary method of measurement of the acceleration due to gravity. Taking into account the logistic situation of Etna, four absolute gravity stations were settled in 2007, while a fifth station was installed in 2008. Four of them were located very close to the active craters at: (i) the Serra la Nave Astrophysical Observatory (1740 m a.s.l.); (ii) the Montagnola (2500 m a.s.l.); (iii) the Pizzi Deneri Volcanological Observatory (2810 m a.s.l.); and the newest (iv) the Caserma Donnavita (1250 m a.s.l.). One absolute station was installed out of the volcanic area, inside the gravity laboratory of INGV - Catania, to be adopted as reference. We present the results obtained by comparing relative and absolute gravity measurements and their implications on the latest Etna eruption started on 13th May 2008.
GP43B-0822
New Experimental High Sensitivity Magnetic Observatory Installation In Mexico
Newly installed magnetic observatory consists of Potassium dIdD and Potassium Supergradiometer. It is a
byproduct of experiments in Earthquake studies that have started in 2005 after installation of Potassium
Supergradiometer in Oaxaca Province of Mexico. Supergrad's sensitivity of about 50fT (0.05pT) is sufficient
for a short base gradiometric studies of Earthquakes.
One of the obstacles of this installation is a minute difference in magnetic field direction at the three
Supergrad sensors. This difference is causing invasion of diurnal changes of magnetic field into gradients
between the sensors of the order of 10-30pT. High sensitivity Potassium dIdD is needed to correct for this
phenomenon. The dIdD was installed May 2008 and the first results are expected shortly thereafter.
Potassium dIdD will measure components of magnetic field with pT sensitivity.
http://www.gemsys.ca