B. Bryant | Geology Reports

USGS2012

Implications of low-temperature cooling history on a transect across the Colorado Plateau-Basin and Range boundary, west central Arizona

Fission track ages of apatite and zircon from metamorphic, plutonic, and sedimentary rocks along a 80-km transect across the Colorado Plateau-Basin and Range boundary in west central Arizona show differences in the low-temperature cooling histories between the provinces. The transect extends from Cypress Mountain in the Colorado Plateau transition zone to the eastern Buckskin Mountains in the Basin and Range. Along the northeast margin of the Basin and Range province, metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16–10 Ma. These ages are similar to K-Ar fusion ages of biotite and age minima of K-feldspar on 40 Ar/ 39 Ar age spectra and collectively indicate rapid cooling. One K-feldspar age spectrum has an age maximum of about 22.8 Ma, an age minimum of 11.8 Ma, and a spectrum whose shape is suggestive of reheating, possibly in middle Miocene time. The heating event was probably related to hydrothermal activity during emplacement of Cu and Mn deposits in and above the detachment fault zone. Effects of this heating are only locally detected in rocks above the detachment fault. In the Poachie Range fission track ages of apatite and zircon are 60–50 and 80–70 Ma, respectively. The disparity between the apatite and zircon ages indicates that the rocks cooled slowly in Late Cretaceous and early Tertiary time, probably due to gradual uplift and erosion. Total uplift and denudation in the area of the Poachie Range since Cretaceous time is 6 km or more. To the northeast of the range, fission track ages of apatite and zircon increase and diverge, indicating that apparent uplift decreased in that direction. The apatite ages from the Poachie Range are concordant with early Tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect. The ages represent cooling of crystalline rocks after Cretaceous regional metamorphism and magmatism. Near Bagdad, 20 km northeast of the Poachie Range, 2 km or less of erosion has occurred since intrusion of high-level plutons and dikes and caldera formation in Late Cretaceous time. Remnants of an erosion surface that developed in middle Tertiary time are preserved in the transition zone. Volcanic and sedimentary rocks at least as old as early Miocene were deposited on the erosion surface and filled valleys cut into it. Dissection of these deposits began about 8 Ma. We interpret these data combined with those from other studies to indicate that in Cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust. The Cretaceous igneous rocks at Bagdad are high-level manifestations of this magmatism. Uplift and slow cooling occurred in Late Cretaceous and early Tertiary time. In late Oligocene and early Miocene time during northeast-southwest extension, middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone. Tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern Buckskin and Harcuvar mountains. Middle Miocene reheating occurred locally in the lower plate, along the detachment, and in nearby parts of the upper plate.

Journal of Geophysical Research↗

USGS2000

Preliminary report on the 16 October 1999 M 7.1 Hector mine, California, earthquake

The M w 7.1 Hector Mine, California, earthquake occurred at 9:46 GMT on 16 October 1999. The event caused minimal damage because it was located in a remote, sparsely populated part of the Mojave Desert, approximately 47 miles east-southeast of Barstow, with epicentral coordinates 34.59°N 116.27°W and a hypocentral depth of 5 ± 3 km. Twelve foreshocks, M 1.9-3.8, preceded the mainshock during the previous twelve hours. All of these events were located close to the hypocenter of the mainshock. The Hector Mine earthquake occurred within the Eastern California Shear Zone (ECSZ). By virtue of its remote location, the societal impact of the Hector Mine earthquake was, fortunately, minimal in spite of the event's appreciable size. The ECSZ is characterized by high seismicity, a high tectonic strain rate, and a broad, distributed zone of north-northwest-trending faults (ECSZ; Figure 1 ; Dokka and Travis, 1990 ; Sauber et al., 1986 ; Sauber et al., 1994 ; Sieh et al., 1993 ). Data regarding the slip rates of faults within the ECSZ suggest that on the order of 15% of the Pacific-North American plate motion occurs along this zone ( Sauber et al., 1986 ; Wesnousky, 1986 ). Most of the faults in the ECSZ have low slip rates and long repeat times for major earthquakes, on the order of several thousands to tens of thousands of years. The occurrence of the Hector Mine earthquake within seven years and only about 30 km east of the 1992 M w 7.3 Landers earthquake suggests that the closely spaced surface faults in the ECSZ are mechanically related. The Hector Mine event involved rupture on two previously mapped fault zones—the Bullion Fault and an unnamed, more northerly-trending fault that is informally referred to in this paper as the Lavic Lake Fault (Dibblee, 1966 , 1967a , b ). Traces of the Bullion Fault exhibit evidence of Holocene displacement and were zoned as active in 1988 under California's Mquist-Priolo Earthquake Fault Zoning Act ( Hart and Bryant, 1997 ). The pattern of rupture along more than one named fault was also observed from the 1992 Landers earthquake ( Hauksson et al., 1993 ; Sieh et al., 1994). Much of the fault zone that produced the Hector Mine earthquake had been buried by relatively young stream deposits, and the fault scarps in bedrock have a subdued morphology. It appears that these faults have not experienced significant offset for perhaps 10,000 years or more ( Hart, 1987 ). Planned future investigations will refine the age of the last event on these faults. The portion of the Lavic Lake Fault that ruptured between the northern end of the Bullion Mountains and Lavic Lake had not previously been mapped. However, our field investigations have identified ancient, subdued fault scarps along portions of the 1999 rupture zone in this area. It thus appears that the entire segment of the Lavic Lake Fault that was involved in the 1999 event had ruptured in the past. As is typical for most faults within the Eastern California Shear Zone, the rate of movement along the Lavic Lake Fault may be quite slow (<1 mm/yr) and should produce earthquakes only infrequently. This event is a reminder that faults that have ruptured in late Quaternary time, but that lack evidence of Holocene displacement, can still produce earthquakes in this low-slip-rate tectonic setting. Additionally, the Hector Mine earthquake is noteworthy for a couple of other reasons. First, it clearly produced triggered seismicity over much of southern California, from the rupture zone toward the south-southwest in particular. Second, as we will discuss, the event may provide new data and insight into recently developed paradigms concerning earthquake interactions and the role of static stress changes. Questions such as these will, of course, be the subject of extensive detailed analyses in years to come. Fortunately, the Hector Mine sequence will provide one of the best data sets obtained to date for a significant earthquake in the United States. Because it occurred when major upgrades to both the regional seismic network (TriNet) and the regional geodetic network (SCIGN) were well underway, the Earth science community will have abundant high-quality data with which to explore the important and interesting questions that have been raised. In this paper, we present and discuss the basic data and preliminary results from the Hector Mine earthquake.

California↗

USGS1983

Fission-track ages of apatite in the Wasatch Mountains, Utah: An uplift study

Apatite fission-track ages from basement rocks in the Wasatch Mountains between Ogden and Bountiful, Utah, range from 5 m.y. near the Wasatch fault along the west margin of the mountains to 94 m.y. on the crest of the range and show a correlation with altitude within individual fault blocks. Analysis of these ages yields an uplift rate of 0.4 mm/yr for the last 10 m.y. for the fault blocks having the most sustained rapid uplift. A variety of intermediate ages of apatite from the crest of the range and at lower altitudes is interpreted as representing fault blocks with rocks having mixed ages due to various degrees of tilting and varying but lesser amounts of uplift.

Utah↗