Thomas G. Hildenbrand

Stressing of the New Madrid Seismic Zone by a lower crust detachment fault

A new mechanical model for the cause of the New Madrid seismic zone in the central United States is analyzed. The model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust (“rift pillow”). Regional horizontal compression induces slip on the fault, and the slip creates a stress concentration in the upper crust above the rift pillow dome. In the coseismic stage of the model earthquake cycle, where the three largest magnitude 7–8 earthquakes in 1811–1812 are represented by a single model mainshock on a vertical northeast trending fault, the model mainshock has a moment equivalent to a magnitude 8 event. During the interseismic stage, corresponding to the present time, slip on the detachment fault exerts a right-lateral shear stress on the locked vertical fault whose failure produces the model mainshock. The sense of shear is generally consistent with the overall sense of slip of 1811–1812 and later earthquakes. Predicted rates of horizontal strain at the ground surface are about 10−7 year−1 and are comparable to some observed rates. The model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude.

Aeromagnetic study of the Island of Hawaii

An aeromagnetic study of the Island of Hawaii provides new insight on magnetic properties of subsurface rock and geologic structure. On a regional scale, spectral-depth analysis delineates two shallow magnetic zones, each roughly 1.5 km thick, lying at a depth of 1 km. One zone (of unknown origin) lies in the center of the island and correlates with a regional magnetic high. The other zone coincides with pronounced magnetic lows paralleling Kilaueas active east rift zone. These magnetic lows probably depict rocks chemically altered by hydrothermal fluids, in which titanomagnetite has been destroyed. Analysis of magnetic terrain effects indicates that magnetization also decreases with depth within Mauna Kea and Mauna Loa. We estimate that magnetization is reduced by about half at a depth of 1 km. The magnetic method is particularly useful in delineating the lateral extent of local shield structures, such as rifts, summit calderas, pit craters, and vent fissures. Rifts possess characteristic magnetic patterns, primarily long-wavelength linear magnetic low zones. We propose alteration processes reduce magnetizations along the flanks of rifts. On the other hand, along young rifts (e.g., Kilaueas east rift zone), short-wavelength magnetic anomalies probably reflect slowly cooled, unaltered intrusions. Altered rock may also produce magnetic lows that help define buried summit calderas.

GRAVITY ANOMALY MAP OF NORTH AMERICA

The Gravity Anomaly Map of North America is the product of a 12-year international effort to compile, critically edit and merge gravity anomaly data on a continental and global scale. This color‐pixel map, printed on four quadrant sheets at a scale of 1:5 000 000 and including a fifth sheet showing a color index map with data references, is the first such map at this large scale to include several hundreds of thousands of precise surface data of the United States, Canada, Mexico and Central America as well as other high‐quality surface data from neighboring continental and oceanic areas. The map, which shows Bouguer gravity anomalies on land and free‐air gravity anomalies over oceans, is remarkable for its detail. Sixty‐six colors or shadings have been used in a carefully conceived nonlinear scheme to show anomalies at a 5 or 10 mGal interval over a dynamic range from about −300 mGal to +130 mGal.

Quantitative investigations of the Missouri gravity low: A possible expression of a large, Late Precambrian batholith intersecting the New Madrid seismic zone

Analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the Missouri gravity low, an important cratonic feature of substantial width (about 125 km) and length (>600 km). Filtered anomaly maps show that this prominent feature extends NW from the Reelfoot rift to the Midcontinent Rift System. Geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal, 11-km-thick batholith with either near vertical or outward dipping boundaries. Considering the modeled characteristics of the batholith, structural fabric of Missouri, and relations of the batholith with plutons and regions of alteration, a tectonic model for the formation of the batholith is proposed. The model includes a mantle plume that heated the crust during Late Precambrian and melted portions of lower and middle crust, from which the low-density granitic rocks forming the batholith were partly derived. The batholith, called the Missouri batholith, may be currently related to the release of seismic energy in the New Madrid seismic zone (earthquake concentrations occur at the intersection of the Missouri batholith and the New Madrid seismic zone). Three qualitative mechanical models are suggested to explain this relationship with seismicity.

Gravity and Magnetic Expression of the San Leandro Gabbro with Implications for the Geometry and Evolution of the Hayward Fault Zone, Northern California

The Hayward Fault, one of the most hazardous faults in northern California, trends north-northwest and extends for about 90 km along the eastern San Francisco Bay region. At numerous locations along its length, distinct and elongate gravity and magnetic anomalies correlate with mapped mafic and ultramafic rocks. The most prominent of these anomalies reflects the 16-km-long San Leandro gabbroic block. Inversion of magnetic and gravity data constrained with physical property measurements is used to define the subsurface extent of the San Leandro gabbro body and to speculate on its origin and relationship to the Hayward Fault Zone. Modeling indicates that the San Leandro gabbro body is about 3 km wide, dips about 75°-80° northeast, and extends to a depth of at least 6 km. One of the most striking results of the modeling, which was performed independently of seismicity data, is that accurately relocated seismicity is concentrated along the western edge or stratigraphically lower bounding surface of the San Leandro gabbro. The western boundary of the San Leandro gabbro block is the base of an incomplete ophiolite sequence and represented at one time, a low-angle roof thrust related to the tectonic wedging of the Franciscan Complex. After repeated episodes of extension and attenuation, the roof thrust of this tectonic wedge was rotated to near vertical, and in places, the strike-slip Hayward Fault probably reactivated or preferentially followed this preexisting feature. Because earthquakes concentrate near the edge of the San Leandro gabbro but tend to avoid its interior, we qualitatively explore mechanical models to explain how this massive igneous block may influence the distribution of stress. The microseismicity cluster along the western flank of the San Leandro gabbro leads us to suggest that this stressed volume may be the site of future moderate to large earthquakes. Improved understanding of the three-dimensional geometry and physical properties along the Hayward Fault will provide additional constraints on seismic hazard probability, earthquake modeling, and fault interactions that are applicable to other major strike-slip faults around the world. Manuscript received 11 January 2002.

Geophysical constraints on understanding the origin of the Illinois basin and its underlying crust

Abstract Interpretation of reprocessed seismic reflection profiles reveals three highly coherent, layered, unconformity-bounded sequences that overlie (or are incorporated within) the Proterozoic “granite–rhyolite province” beneath the Paleozoic Illinois basin and extend down into middle crustal depths. The sequences, which are situated in east–central Illinois and west–central Indiana, are bounded by strong, laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad “basinal” packages that become areally more restricted with depth. Normal-fault reflector offsets progressively disrupt the sequences with depth along their outer margins. We interpret these sequences as being remnants of a Proterozoic rhyolitic caldera complex and/or rift episode related to the original thermal event that produced the granite–rhyolite province. The overall thickness and distribution of the sequences mimic closely those of the overlying Mt. Simon (Late Cambrian) clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post-Cambrian Illinois basin stratigraphic succession, which is centered farther south over the “New Madrid rift system” (i.e., Reelfoot rift and Rough Creek graben). The present configuration of the Illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions, the Proterozoic caldera complex and/or rift in east–central Illinois and west–central Indiana and the New Madrid rift system to the south. Filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any Proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite–rhyolite.

New way of processing near-surface magnetic data: The utility of the Comprehensive Model of the Magnetic Field

In assembling near-surface magnetic surveys (e.g., airborne or marine data) for regional geologic studies, one often has a problem in properly merging different surveys without applying ad-hoc leveling methods or warping the long-wavelengths of individual data sets. A new comprehensive long wavelength and long time span magnetic field model based on satellite and observatory data may, at last, resolve this problem. The model, the Comprehensive Model (CM), is developed by Terry Sabaka of Raytheon ITSS/NASA and Nils Olsen of the Danish Space Research Institute. The present version of the model, CM3, incorporates data from magnetic field satellites POGO (1965–1970), Magsat (1979–1980), Orsted (2000 to present), and CHAMP (2001 to present) and magnetic observatory data from the early 1960s to 2002.

A source-depth separation filter: Using the Euler method on the derivatives of total intensity magnetic anomaly data

Derivatives of potential-field anomalies (or the anomaly gradients) enhance the field associated with shallow features and de-emphasize the field from deeper sources. The derivative approach of separating anomalies of shallow, intermediate, and deep sourves is, however, qualitative.

Aeromagnetic survey over U.S. to advance geomagnetic research

A proposed high-altitude survey of the United States offers an exciting and cost effective opportunity to collect magnetic-anomaly data. Lockheed Martin Missile and Space Company is considering funding a reimbursable ER-2 aircraft (Figure 1) mission to collect synthetic aperture radar (SAR) imagery at an altitude of about 21 km over the conterminous United States and Alaska. The collection of total and vector magnetic field data would be a secondary objective of the flight. Through this “piggyback approach,” the geomagnetic community would inherit invaluable magnetic data at a nominal cost. These data would provide insight on fundamental tectonic and thermal processes and give a new view of the structural and lithologic framework of the crust and upper mantle.

Upgraded gravity anomaly base of the United States

A concerted effort to compile an upgraded digital gravity anomaly database, grid, and map for the United States by the end of 2002 is under way. This joint effort by the geophysics groups at the University of Texas at El Paso (UTEP), U.S. Geological Survey (USGS), and National Oceanic and Atmospheric Administration (NOAA) (with support from the National Imagery and Mapping Agency [NIMA]), is an outgrowth of the new geoscientific community initiative called Geoinformatics (www.geoinformaticsnetwork.org). This dominantly geospatial initiative reflects the realization by Earth scientists that existing information systems and techniques are inadequate to address the complex scientific and societal issues that we must confront. Currently, the lack of standardization and chaotic distribution of available geoscience data, a lack of documentation about them, and the lack of easy-to-use access tools and computer codes for their analysis are major obstacles for scientists, government agencies, and educators alike. ...