Stress direction history of the western United States and Mexico since 85 Ma

Abstract

[1] A data set of 369 paleostress direction indicators (sets of dikes, veins, or fault slip vectors) is collected from previous compilations and the geologic literature. Like contemporary data, these stress directions show great variability, even over short distances. Therefore statistical methods are helpful in deciding which apparent variations in space or in time are significant. First, the interpolation technique of Bird and Li [1996] is used to interpolate stress directions to a grid of evenly spaced points in each of seventeen 5-m.y. time steps since 85 Ma. Then, a t test is used to search for stress direction changes between pairs of time windows whose sense can be determined with some minimum confidence. Available data cannot resolve local stress provinces, and only the broadest changes affecting country-sized regions are reasonably certain. During 85 -50 Ma, the most compressive horizontal stress azimuthŝ 1H was fairly constant at $68°(United States) to 75°(Mexico). During 50 -35 Ma, both counterclockwise stress changes (in the Pacific Northwest) and clockwise stress changes (from Nevada to New Mexico) are seen, but only locally and with about 50% confidence. A major stress azimuth change by $90°occurred at 33 ± 2 Ma in Mexico and at 30 ± 2 Ma in the western United States. This was probably an interchange between s 1 andŝ 3 caused by a decrease in horizontal compression and/or an increase in vertical compression. The most likely cause was the rollback of horizontally subducting Farallon slab from under the southwestern United States and northwest Mexico, which was rapid during 35 -25 Ma. After this transition, a clockwise rotation of principal stress axes by 36°-48°occurred more gradually since 22 Ma, affecting the region between latitudes 28°N and 41°N. This occurred as the lengthening Pacific/North America transform boundary gradually added dextral shear on northwest striking planes to the previous stress field of SW-NE extension.

Deep-Injection and Closely Monitored Induced Seismicity at Paradox Valley, Colorado

“…14). The preferred failure orientation is consistent with other regional stress observations (Bird, 2002), oriented core observations, and analysis of borehole breakouts. This orientation is oblique to the dominant Laramide structural grain.…”

“…Assuming reasonable internal friction angles for the Leadville Limestone (l ‫ס‬ 0.6-0.75) suggests a P-axis azimuth of 293-296Њ. This estimate is 27-30Њ from the primary nodal set azimuth, and within ϳ15Њ of the regional P-axis azimuth for western Colorado over the past five Ma estimated by Bird (2002). Hickman and Summers (personal comm., 2003) analyzed televiewer and dipmeter logs, obtained during initial development of the PVU injection well, to estimate borehole breakouts.…”

Possible Involvement of Permian Phosphoria Formation Oil as a Source of REE and Other Metals Associated with Complex U-V Mineralization in the Northern Bighorn Basin?

“…During the Laramide Orogeny , the central Rocky Mountain region experienced a general NE-SW (40 • -55 • ) transpressional strain regime as a result of the low-angle subduction of the Farallon plate at its western continental margin [31,32]. Beginning in the late Cretaceous, Laramide shortening was accommodated by uplift of broad basement arches that eventually isolated the Bighorn Basin ( Figure 3) [31].…”

“…Beginning in the late Cretaceous, Laramide shortening was accommodated by uplift of broad basement arches that eventually isolated the Bighorn Basin ( Figure 3) [31]. The Bighorn Basin has a generally NW-SE sinuous axis [26].…”

“…The Bighorn Basin has a generally NW-SE sinuous axis [26]. Smaller scale folds in the form of anticlines and synclines formed along the basin margins and served to accommodate regional shortening [31][32][33]. These folds host most of the oil fields of the Bighorn Basin [34].…”