Geology topics

John H. McBride

Publications and source records attributed to John H. McBride.

5 recordsLinked to original sources

USGS2012

Characterization of intrabasin faulting and deformation for earthquake hazards in southern Utah Valley, Utah, from high-resolution seismic imaging

We conducted active and passive seismic imaging investigations along a 5.6-km-long, east–west transect ending at the mapped trace of the Wasatch fault in southern Utah Valley. Using two-dimensional (2D) P-wave seismic reflection data, we imaged basin deformation and faulting to a depth of 1.4 km and developed a detailed interval velocity model for prestack depth migration and 2D ground-motion simulations. Passive-source microtremor data acquired at two sites along the seismic reflection transect resolve S-wave velocities of approximately 200 m/s at the surface to about 900 m/s at 160 m depth and confirm a substantial thickening of low-velocity material westward into the valley. From the P-wave reflection profile, we interpret shallow (100–600 m) bedrock deformation extending from the surface trace of the Wasatch fault to roughly 1.5 km west into the valley. The bedrock deformation is caused by multiple interpreted fault splays displacing fault blocks downward to the west of the range front. Further west in the valley, the P-wave data reveal subhorizontal horizons from approximately 90 to 900 m depth that vary in thickness and whose dip increases with depth eastward toward the Wasatch fault. Another inferred fault about 4 km west of the mapped Wasatch fault displaces horizons within the valley to as shallow as 100 m depth. The overall deformational pattern imaged in our data is consistent with the Wasatch fault migrating eastward through time and with the abandonment of earlier synextensional faults, as part of the evolution of an inferred 20-km-wide half-graben structure within Utah Valley. Finite-difference 2D modeling suggests the imaged subsurface basin geometry can cause fourfold variation in peak ground velocity over distances of 300 m.

Utah↗

USGS2002

The Commerce geophysical lineament and its possible relation to Mesoproterozoic igneous complexes and large earthquakes in the central Illinois Basin

New inversions of existing magnetic and gravity data lead to insights on upper crustal structures in the central Illinois basin. The results of 2D and 3D inversion techniques suggest that the source of the Commerce geophysical lineament follows the southeast boundary of a dense and magnetic, northeast-trending igneous center, named the Vincennes igneous center. Thus the Commerce geophysical lineament, defining the 5- to 10-km-wide Commerce deformation zone, appears to have influenced the structural development of the Vincennes igneous center. Overlying this igneous center is the Centralia seismic-reflection sequence, expressed as highly coherent reflectors ( McBride and Kolata, 1999 ). We suggest that the buried Vincennes igneous center is the source of inferred volcanic units of the Centralia sequence and is related to a rifted margin or a Proterozoic plate boundary. A comparison of gravity and magnetic fields over the Vincennes igneous center and the St. Francois Mountains igneous center in southeastern Missouri suggests that the associated sources in these regions are similar in composition and perhaps origin. Such a conclusion, although speculative, suggests that ∼1.47 Ga-year-old volcanic fields of high-silica rocks, with more abundant mafic components at depth, characterize basement in the regions of the Vincennes igneous center and the St. Francois Mountains. Further, we conclude that the Commerce deformation zone evolved in the Mesoproterozoic (1.1 to 1.5 Ga) as a major cratonic rheological boundary and was the focus of episodic reactivation related to varying stress regimes throughout its history. Spatial relations of the Commerce deformation zone with large Pleistocene and Holocene earthquakes suggest that the zone represents a major rheological boundary intimately related to both surface and deep structures and to the seismic hazard of the Illinois basin region. Assuming recent right-lateral slip along the Commerce deformation zone, we propose that a jog or left step in the Vincennes area leads to thrusting or a restraining bend, where associated stress accumulations may have resulted in nearby large prehistoric earthquakes and may also be the nucleation sites of future large earthquakes.

Illinois, Indiana, Kentucky↗

USGS2002

Interpreting the earthquake source of the Wabash Valley seismic zone (Illinois, Indiana, and Kentucky) from seismic-reflection, gravity, and magnetic-intensity data

Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ m bLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, m bLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although magnetic intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.

Illinois, Indiana, Kentucky↗

USGS1997

Färoe-Iceland Ridge Experiment: 1. Crustal structure of northeastern Iceland

Results from the Färoe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s −1 ). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Journal of Geophysical Research B: Solid Earth↗

USGS1990

Integration of COCORP deep reflection and magnetic anomaly analysis in the southeastern United States: Implications for origin of the Brunswick and East Coast magnetic anomalies: Alternative interpretation and reply

Integration of magnetic anomaly analysis with COCORP deep reflection data from the southeastern United States provides three new constraints on the interpretation of the Brunswick and East Coast magnetic anomalies, as well as on the reflection data. These are as follows. (1) The source of the Brunswick anomaly lies within the deep crust. This anomaly is not caused by a Mesozoic rift basin, as proposed by some workers. (2) A simple, seaward-dipping, high- susceptibility slab model can explain both the Brunswick and East Coast magnetic anomalies. The along-strike change in character of the two anomalies results largely from a change in azimuth of the source body. (3) Beneath the southeastern United States, this source body dips south, lies immediately on the south flank of the prominent southward-dipping reflective zone revealed on COCORP surveys, and was previously associated with the Alleghanian suture between North America and Africa. These results imply that a dipping, highly magnetized zone in the upper plate of the Alleghanian suture is responsible for both the Brunswick and East Coast magnetic anomalies. The high- susceptibility material responsible for these anomalies might be mafic lower continental or oceanic crust thrust upward during Alleghanian continental collision, or mafic igneous material intruded into the upper plate of the suture zone during subsequent Mesozoic rifting, or both. The latter hypothesis implies that the Alleghanian suture acted, as a zone of weakness (a repository ?) which was reactivated to control the site of ultimate Atlantic rifting and possibly initial sea-floor spreading.

Geological Society of America Bulletin↗