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T.G. Hildenbrand

Publications and source records attributed to T.G. Hildenbrand.

At least 37 records · Page 2Linked to original sources
USGS1997

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.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Commerce geophysical lineament - Its source, geometry, and relation to the Reelfoot rift and New Madrid seismic zone

The Commerce geophysical lineament is a northeast-trending magnetic and gravity feature that extends from central Arkansas to southern Illinois over a distance of ≈400 km. It is parallel to the trend of the Reelfoot graben, but offset ≈40 km to the northwest of the western margin of the rift floor. Modeling indicates that the source of the aeromagnetic and gravity anomalies is probably a mafic dike swarm. The age of the source of the Commerce geophysical lineament is not known, but the linearity and trend of the anomalies suggest a relationship with the Reelfoot rift, which has undergone episodic igneous activity. The Commerce geophysical lineament coincides with several topographic lineaments, movement on associated faults at least as young as Quaternary, and intrusions of various ages. Several earthquakes (M b > 3) coincide with the Commerce geophysical lineament, but the diversity of associated focal mechanisms and the variety of surface structural features along the length of the Commerce geophysical lineament obscure its relation to the release of present-day strain. With the available seismicity data, it is difficult to attribute individual earthquakes to a specific structural lineament such as the Commerce geophysical lineament. However, the close correspondence between Quaternary faulting and present-day seismicity along the Commerce geophysical lineament is intriguing and warrants further study.

Geological Society of America Bulletin↗
USGS1997

Structural underpinnings and neotectonics of the southern Illinois Basin: An overview

The southern end of the Illinois Basin is one of the most structurally complex regions in the Midcontinent United States. Two major structural elements characterize this part of the basin: (1) A broad southwestward-plunging cratonic depression extends across central Illinois and southwestern Indiana. Investigations of historical and prehistorical earthquakes in this part of the basin indicate that moderate to high earthquake potential exists; the seismogenic sources, however, remain enigmatic. (2) The southernmost part of the basin is underlain by the Reelfoot Rift and Rough Creek Graben, a rift system that formed during late Precambrian to Middle Cambrian time. Geodynamic processes operating within the rift since the late Precambrian have had a major influence on the tectonic, diagenetic, and depositional history of the region. In addition, tectonic compressive stress appears to be reactivating ancient faults within the Reelfoot Rift, resulting in coherent linear segments of earthquake epicenters called the New Madrid Seismic Zone. Geological and geophysical information suggests that the cause of earthquakes in the New Madrid Seismic Zone is unrelated to that in the region north of the rift system.

Illinois, Indiana, Kentucky, Missouri↗
USGS1997

Geophysical setting of the Wabash Valley fault system

Interpretation of existing regional magnetic and gravity data and new local high-resolution aeromagnetic data provides new insights on the tectonic history and structural development of the Wabash Valley Fault System in Illinois and Indiana. Enhancement of short-wavelength magnetic anomalies reveal numerous NW- to NNE-trending ultramafic dikes and six intrusive complexes (including those at Hicks Dome and Omaha Dome). Inversion models indicate that the interpreted dikes are narrow (≤3 m), lie at shallow depths (<200 m) and are steeply dipping. Some of the interpreted dikes closely follow mapped faults; their abundance suggests that the Wabash Valley Fault System contains many more faults than those mapped. Both the interpreted dike pattern and mapped Wabash Valley Fault System terminate near the Reelfoot-Rough Creek-Rome rift system. Based on the interpretation of both the regional magnetic and gravity data and the high-resolution magnetic data, we propose that the shallow faults and deep-seated rift structures in the Wabash Valley terminate at or near the Rough Creek-Shawneetown Fault System. The Grayville Graben (∼20 km wide, ∼700 m maximum basement relief, and <40 km long [Bear et al. , this volume]) underlying the Wabash Valley developed during rifting, perhaps in response to stress concentrations generated by a bend in the Reelfoot-Rough Creek-Rome rift system. We therefore hypothesize that although the Reelfoot Rift and Rough Creek Graben represent tectonic intraplate structures of large areal extent (>500 km long and generally >50 km wide) and with deep basins (locally >3 km thick), the ancestral Wabash Valley faults express, in comparison, minor tectonic structures and probably do not represent a failed rift arm. There is a lack of any obvious relation between the Wabash Valley Fault System and the epicenters of historic and prehistoric earthquakes. Five prehistoric earthquakes lie conspicuously near structures associated with the Commerce geophysical lineament, a NE-trending magnetic and gravity lineament lying oblique to the Wabash Valley Fault System and possibly extending over 600 km from NE Arkansas to central Indiana.

Arkansas, Illinois, Indiana, Kentucky, Missouri, T↗
USGS1996

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. Copyright 1996 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Rationale and preliminary operational plan for a high-altitude magnetic survey over the United States

A proposed high-altitude survey of the U.S. with an ER-2 to collect radar data offers an exciting and cost-effective opportunity to collect magnetic anomaly data. At this workshop, a group of magnetic specialists addressed this opportunity by discussing the need for high-altitude magnetic data and by formulating a preliminary operational plan to acquire such data. The high-altitude aeromagnetic survey would provide critical data needed to expand our knowledge of the geomagnetic field, with applications to a variety of earth science issues including geology, tectonics, core-processes, and rock-magnetism. Test flights with a cesium magnetometer indicate that the ER-2 has the potential to measure the magnetic field at an accuracy of 2 nT or better along flight lines. If the national ER-2 survey is carried out, the successful collection of high-altitude magnetic data hinges on establishing a consortium of federal and state agencies, private industry, and academic institutions to provide the identified resources.

Open-File Report↗
USGS1995

Geophysical setting of the Reelfoot Rift and relations between rift structures and the New Madrid seismic zone

In the winter of 1811-12, three of the largest historic earthquakes in the United States occurred near New Madrid, Missouri. Seismicity continues to the present day throughout a tightly clustered pattern of epicenters centered on the bootheel of Missouri, including parts of northeastern Arkansas, northwestern Tennessee, western Kentucky, and southern Illinois. In 1990, the New Madrid seismic zone/Central United States became the first seismically active region east of the Rocky Mountains to be designated a priority research area within the National Earthquake Hazards Reduction Program (NEHRP). This Professional Paper is a collection of papers, some published separately, presenting results of the newly intensified research program in this area. Major components of this research program include tectonic framework studies, seismicity and deformation monitoring and modeling, improved seismic hazard and risk assessments, and cooperative hazard mitigation studies.

Professional Paper↗