Paleomagnetic declinations in the Gale Hills area, southern Nevada, superimposed on a geologic base (Longwell et al., 1965) over shaded topography (illumination from 340°). White background pie wedges are for the Thumb member of the Horse Spring Formation and pink background from the younger Red Sandstone (both within the brown Tertiary sediments). This figure includes some data analyzed after publication of the Sonder et al. manuscript, and a high resolution JPEG version is available.
Test of maximum size of rigid disks in the Gale Hills obtained by comparing fraction of pairs of localities that are statistically indistinguishable vs. interlocality distance. Curves are for a simple theoretical construct where rotations outside disk are random with a probability Q of being indistinguishable due to declination uncertainty. Because any systematic variation of declinations with distance would make the disk sizes smaller than the curves indicate, this is a conservative upper bound on disk sizes. Some new data has been added from the equivalent figure in the Sonder et al. paper, and the 3 km wide bins have been added. A high resolution JPEG version is available.
Paleomagnetic rotations vs. distance from the Las Vegas Valley Shear Zone, with best estimate thin viscous sheet fits superimposed; data from Nelson and Jones, 1987, and Sonder et al., 1994. Note increase in D (halfdisplacement across LVVSZ) and decrease in n (stress exponent) with increasing distance westward; the increase in displacement westward is compatible with geologic observations, but is totally different from "fault-termination folds." This figure includes some data analyzed after publication of the Sonder et al. manuscript, and a high resolution JPEG version is available.
Tetreault, J., C. H. Jones, E. Erslev, M. Hudson, S. Larson, and S. Holdaway, Paleomagnetic and structural evidence for oblique-slip in a fault-related fold, Grayback Monocline, Colorado, Geol. Soc. Am. Bull., 120 (7/8); p. 877–892; doi: 10.1130/B26178.1, 2008.
Significant fold-axis-parallel slip is accommodated in the folded strata of the Grayback monocline, northeastern Front Range, Colorado, without visible large strike-slip displacement on the fold surface. In many cases, oblique-slip deformation is partitioned; fold- axis-normal slip is accommodated within folds, and fold-axis-parallel slip is resolved onto adjacent strike-slipfaults.Unlike partitioning strike-parallel slip onto adjacent strike-slip faults, fold-axis-parallel slip has deformed the forelimb of the Grayback monocline. Mean compressive paleostress orientations in the forelimb are deflected 15°–37° clockwise from the regional paleostress orientation of the northeastern Front Range. Paleomagnetic directions from the Permian Ingleside Formation in the forelimb are rotated 16°–42° clockwise about a bedding- normal axis relative to the North American Permian reference direction. The paleostress and paleomagnetic rotations increase with the bedding dip angle and decrease along strike toward the fold tip. These measurements allow for 50–120 m of fold-axis-parallel slip within the forelimb, depending on the kinematics of strike-slip shear. This resolved horizontal slip is nearly equal in magnitude to the ~180 m vertical throw across the fold. For 200 m of oblique-slip displacement (120 m
of strike slip and 180 m of reverse slip), the true shortening direction across the fold is N90°E, indistinguishable from the regionally inferred direction of N90°E and quite different from the S53°E fold-normal direction. Recognition of this deformational style means that significant amounts of strike slip can be accommodated within folds without axis-parallel surficial faulting.
Sub-horizontal axes of folds in sedimentary rocks are often taken to represent the orientation of stresses and strains such that the greatest compressive stress/contractional strain is exactly perpendicular to a fold as it forms. In such an instance, motion of particles on opposite sides of a fold will be directly towards one another, an assumption occasionally critical to quantitative interpretation of fold geometries. This assumption might be unwarranted. In at least two and possibly more instances, published deflections of paleomagnetic declinations increase towards the axis of a contractional, horizontal-axis fold. Additionally, seismological focal mechanisms of some blind thrust faults underlying folds in parts of California indicates a component of strike-slip motion parallel to the fold axis. The net implication of these observations is that these folds simultaneously absorb motion both perpendicular and parallel to each fold's trend, i.e., the fold acts as the center of a strike-slip shear zone. Motion parallel to the fold axis (strike-slip) is absorbed by diffuse deformation responsible for the paleomagnetic rotations. This motion reconciles slip-vector differences between oblique-slip thrusts and their overlying folds. The implications of this include (1) fold axes by themselves might be imperfect recorders of paleostrain orientations, (2) motion in-and-out of sections across folds might exist even without identified strike-slip faults, (3) paleomagnetic deflections of folded material need not reflect passive post-folding rotation of the entire fold. These implications are briefly examined in terms of slip partitioning in the California Coast Ranges and Laramide strain orientations in the Colorado Plateau. A greater survey of rotational deformation in the vicinity of folds is needed to better understand the frequency and maximum magnitude of this style of deformation.
Tetreault, J., and C. H. Jones, Testing Monoclines for Dextral Shear: Paleomagnetic Results From the Nacimiento Uplift, Northern New Mexico, EOS, Fall 2001 AGU Meeting, 2001.
Dextral shear along the eastern margin of the Colorado Plateau has been inferred to have occurred in the Laramide orogeny by many workers. Some shear has been suggested to have occurred on exposed faults, but the limited strike extent of these faults presents a difficulty for advocates of large strike-slip Laramide motion. One possibility is that that shear is expressed as vertical-axis rotations along monoclinal folds that extend beyond the surface traces of the mapped faults. We examine one such monocline bounded to the west by the Nacimiento Fault in northern New Mexico. The Nacimiento Uplift is a north-south trending vertical structure along the eastern boundary of the San Juan Basin formed by Laramide reactivation of a Late Paleozoic uplift. Previous investigations have interpreted Laramide dextral slip along the Nacimiento Fault from minor fault data (Erslev, 2001), Proterozoic piercing lines (Cather, 1999), and related strike slip structures (Woodward, 1987). Paleomagnetic analysis was conducted to determine if the monocline accomodated strike-slip motion through vertical axis rotations. Preliminary results from the Late Cretaceous Mesaverde, Paleocene Nacimiento, and Paleocene Ojo Alamo formations north of the Nacimiento Fault establish an absence of vertical-axis rotation along the fold. The Mesaverde sandstone yields the most favorable results. Antipodal directions in the Mesaverde samples imply that a primary magnetization penecontemporaneous with deposition has been isolated. The tilt-corrected direction is D= 332± 11°, I = 53± 7°, a95 = 8°, N = 24. The results are indistinguishable from the expected Late Cretaceous direction (D= 339± 8°, I = 63± 11°) with a rotation of R = -7 ± 14°. The Nacimiento and Ojo Alamo sandstones produced more scattered directions, yet still consistent with the virtual absence of rotation. Thus either (1) right-lateral shear does not occur along the Nacimiento monocline or (2) it does not produce substantial (>+7°) vertical-axis rotations. Further paleomagnetic studies are underway along monoclines elsewhere in New Mexico to test whether the absence of rotation is a generic occurrence or specific to the Nacimiento Uplift.
Tetreault, J. L., and C. H. Jones, Paleomagnetic constraints on oblique deformation within folds, Colorado Plateau, western USA, Geol. Soc. Abstr. Prog., 35 (5), 13, 2003.
The Rocky Mountains rose in the Laramide in response to shortening, but the shortening direction remains controversial. Lying between the Rockies and the plate margin, shortening directions across the Colorado Plateau are important to determining the cause of the Laramide Orogeny. The curvilinear axial trends and varying dip angles of the monoclines of the Colorado Plateau would suggest that oblique slip has occurred during fold development in the Laramide Orogeny. Recent structural work suggests that oblique deformation has occurred along some of the monoclines of the Colorado Plateau, which can be used to constrain the shortening direction on the Plateau (Tinsdall and Davis, 1999; Bump and Davis, 2003). If these folds are localizing oblique deformation, then lateral strain should occur within these folds. As discrete faults with large slip are not generally observed along monoclines, a more distributed deformation seems likely. Such distributed deformation might be expected to produce paleomagnetic rotations about a near vertical axis. Reconnaissance paleomagnetic sampling of the Cretaceous Mesaverde Formation in the Nacimiento Uplift, where dextral slip has been documented by several authors (e.g. Cather, 1999), produced insignificant rotations of only -7.0 ± 13.0°. The East Kaibab and San Rafael monoclines show -1.3 ± 6.0° and 5.5 ± 9.9° of rotation, respectively, from sampling of the Chinle, Kayenta, and Moenave formations. The absence of convincing rotations on these folds indicates that either (1) rotation is focused in a different part of the fold, or (2) these particular folds lacked oblique slip on their underlying faults, or (3) oblique slip is absorbed by an irrotational mechanism. Preliminary results from ongoing work on the Defiance, Hogback, and the Grand Hogback monoclines will be presented; these are expected to help resolve between these possibilities. Sampling of these folds will extend to the fold hinges as well as the forelimb.
The monoclines of the Colorado Plateau, formed over reactivated Precambrian faults during the Laramide Orogen, locally absorb oblique deformation (e.g. Tinsdall and Davis, 1999; Bump and Davis, 2003). Thus it can be assumed that the distributed deformation in the overlying, folded strata from oblique slip would be in the form of a strike-slip shear zone with some unknown amount of vertical axis rotation. Paleomagnetic vertical axis rotations in the monoclines of the Colorado Plateau can constrain such a rotation from strike-slip motion on the underlying thrust-faults. This study focuses on new paleomagnetic work performed on the anticlinal hinges of the San Rafael monocline, central Utah, and the Grand Hogback monocline, Colorado. The San Rafael monocline is a large, curvilinear structure with oblique deformation from right lateral slip proposed on the southern, northeast-southwest oriented limb (Bump and Davis, 2003). The Grand Hogback monocline is a large, highly sinuous feature, with left-lateral strike-slip deformation proposed along the central, east-west oriented limb (Murray, 1966). Three sites sampled within the anticlinal hinge of the north and south limbs of the San Rafael monocline, yielded no vertical-axis rotations. Two sites were sampled along the northern, north-south oriented limb and the central limb of the Grand Hogback monocline. A minimum rotation of -15°± 8° was determined from Upper Triassic strata sampled on the anticlinal hinge of the central limb (39.5 N, 107.4 W). The tilt-corrected direction for this site is D = -23.6°, I = 24.3°, a95 = 9.6°, N = 16. Correlative strata sampled on the north limb (39.8 N, 107.9 W) contain a clockwise rotation of 8.8° ± 4.5°. This paleomagnetic result provides a minimum constraint of 23° of differential tectonic rotation absorbed in this Laramide structure. Further work in the overlying pre-Laramide strata will provide further information on the amount and distribution of rotation, and thus constrain the mechanism of deformation for the Grand Hogback monocline. The presence of rotations at this monocline and the absence of rotations on the San Rafael monocline must be further investigated to understand how the Laramide Orogen formed the monoclines of the Colorado Plateau.
Tétreault, Joya L., and Craig H. Jones, Three-dimensional studies of oblique deformation within Laramide folds, Geol. Soc. Am. National Meeting, Denver Colorado, 2004.
Fold-axis parallel shear within fault-related folds is increasingly recognized in many traditional Laramide folds as a mechanism to absorb transpressional stress. Structural and geophysical evidence from Laramide folds in the Rocky Mountain Foreland and the Colorado Plateau supports oblique shear within folds. Vertical-axis rotations are one means by which such shear can be accommodated and so identified by paleomagnetic measurements. Clockwise and counterclockwise rotations on the Grand Hogback and Grayback monoclines of Colorado correlate with oblique deformation. 9 to 24° of clockwise rotation are found in the north-south trending segment of the Grand Hogback Monocline, and 15° ± 7.8° of counterclockwise rotation are found in the east-west trending segment (Tetreault et al., 2003). Clockwise paleomagnetic rotations of 39° ± 14° are found in the steeply dipping forelimb of the Grayback Monocline (Holdaway, 1998). These paleomagnetic rotations in both folds are probably produced as northeast shortening during the Laramide Orogeny acted upon monoclines trending obliquely to this shortening. However, the mechanisms of oblique folding are not well understood; similar folds on the Colorado Plateau inferred to have absorbed oblique shortening have not yielded paleomagnetic rotations. For example, right-lateral shear across the trends of the East Kaibab and Nacimiento monoclines are widely reported, yet preliminary paleomagnetic studies have not found vertical-axis rotations to support large right-lateral slip. In order to understand how oblique deformation is being absorbed in these folds and why they are perhaps being absorbed with very different mechanisms, we are thoroughly investigating two folds to understand the three-dimensional deformation. Research is focused on constraining mechanisms of oblique deformation by making paleomagnetic and structural measurements on the Grayback and Grand Hogback monoclines over varying structural depth and along transects across and along the monoclines. Our goals are to understand how oblique shear is manifested in Laramide folds, how the magnitude and mechanism of shear varies in three-dimensions, and to determine which kinematic or mechanical model of folding best fits these Laramide folds.
Tétreault, J L. and C. H. Jones, Paleomagnetic and Seismologic Evidence for Oblique-Slip Partitioning to the Coalinga Anticline From the San Andreas Fault, Eos Trans. AGU, 88 (52), Fall Mtg. Suppl., Abstract T43A-1099, 2007.
The Coalinga Anticline is a one of a series of fault-related folds in the central Coast Ranges, California, oriented subparallel to the San Andreas Fault (SAF). The development of the Central Coast Range anticlines can be related to the relative strength of the SAF. If positing a weak SAF, fault-normal slip is partitioned to these subparallel compressional folds. If the SAF is strong, these folds rotated to their current orientation during wrenching. Another possibility is that the Coast Range anticlines are accommodating oblique-slip partitioned from the SAF. The 1983 Coalinga earthquake does not have a purely thrusting focal mechanism (rake =100°), reflecting the likelihood that oblique slip is being partitioned to this anticline, even though surface expression of fold-axis-parallel slip has not been identified. Paleomagnetic vertical-axis rotations and focal mechanism strain inversions were used to quantify oblique-slip deformation within the Coalinga Anticline. Clockwise rotations of 10° to 16° are inferred from paleomagnetic sites located in late Miocene to Pliocene beds on the steeply dipping forelimb and backlimb of the fold. Significant vertical-axis rotations are not identified in the paleomagnetic sites within the nose of the anticline. The varying vertical axis rotations conflict with wrench tectonics (strong SAF) as the mechanism of fold development. We use focal mechanisms inversions of earthquakes that occurred between 1983 to 2006 to constrain the seismogenic strain within the fold that presumably help to build it over time. In the upper 7 km, the principal shortening axis is oriented N37E to N40E, statistically indistinguishable from normal to the fold (N45E). The right-lateral shear in the folded strata above the fault tip, evident from the paleomagnetically determined clockwise vertical-axis rotations, is being accommodated aseismically or interseismically. In the region between 7 and 11 km, where the mainshock occurred, the shortening direction ranges from oblique to normal to the fold trend. Our results show that right-lateral slip is resolved along the main fault plane and not distributed to the smaller aftershocks at depths of 7-11 km. The principal strain axes and clockwise paleomagnetic rotations indicate that the Coalinga Anticline is accommodating minor right-lateral shearing and thus shares some of the strike-slip motion of the San Andreas system.
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C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder
Last modified on May 21, 2012
