Aviva J. Sussman

Classifying curved orogens based on timing relationships between structural development and vertical-axis rotations

Geologists have long recognized the importance and sought the meaning of curved orogenic belts. Over the last few decades, several classifi cation schemes have been proposed for curved orogens that have been largely based on the relationship between, and geometry of, displacement and strain trajectories. However, determination of strain trajectory paths and the complete displacement fi eld of an arcuate orogen is diffi cult at best. It is often possible to measure only one component of the total strain fi eld, but virtually impossible to measure the complete displacement fi eld. In addition, many of these classifi cation schemes have changed or modifi ed original defi nitions for some key terms, most notably the word orocline, which has propagated confusion in the literature (e.g., the Alaskan and Bolivian oroclines). To avoid some of the ambiguity and confusion associated with classifying curved belts, we propose a new classifi cation scheme based on the angular relationship between structural trend or grain (orientation of major thrusts and folds) and secondary imposed curvature (rotations acquired subsequent to initial thrusting and folding). In this manner, classifi cation of curved belts can be simplifi ed into three broad categories: (1) oroclines, (2) progressive arcs, and (3) primary arcs. Oroclines are those orogens that were originally linear and were curved during a subsequent deformation event. Progressive arcs develop their arcuate nature contemporaneously with growth of the belt. Primary arcs are those orogenic systems that inherit curvature during initial deformation and experience no appreciable tightening during subsequent deformation. The one criterion for ascertaining whether a curved orogen developed through a primary, secondary, or progressive mechanism is the temporal and spatial relationship between the deviation in structural trend and the vertical-axis rotation that took place within the belt. At present, the most useful geologic technique for determining such a relationship, and hence the kinematic classifi cation (primary, progressive, or secondary), of a curved orogen is the combination of paleomagnetism and detailed structural analysis. Following identifi cation of the appropriate kinematic classifi cation for a given curved belt, emphasis should be placed on qualifying that classifi cation with the mechanism by which curvature was attained (e.g., indenter, buckling, wrenching, etc.). In this way, the kinematics of deformation is separated from the mechanics of the process, which can often be described by more than one kinematic model.

Reconstructing the kinematic evolution of curved mountain belts: A paleomagnetic study of Triassic red beds from the Wyoming salient, Sevier thrust belt, U.S.A.

Determining the kinematic history and mechanics of curved fold-and-thrust belts is fundamental to understanding the tectonic evolution of mountain systems. To better understand the development of a classic curved fold-and-thrust belt, we completed an integrated paleomagnetic and strain study of the Wyoming salient. Paleomagnetic data are reported here from 154 sites collected from red beds of the Triassic Ankareh Formation in the salient and nine sites collected from the relatively stable foreland. Red beds display three components with distinctly different magnetic behaviors: (1) a near-primary Triassic magnetization carried by hematite that is stable up to 680 °C (Tr component, 91 sites); (2) a Cretaceous chemical remagnetization carried partly by magnetite (K component, 32 sites); and (3) a recent viscous magnetization that is mostly removed by 350 °C. Site mean vectors for the Tr and K components show a high degree of scatter from expected Triassic and Cretaceous reference directions, suggesting significant tilt and rotation subsequent to magnetization acquisition. Restoration of tilt and folding for individual site means results in well-clustered shallow and moderate inclinations for the Tr and K components, respectively, and in variable declinations related to systematic vertical-axis rotations. Statistical analysis of declinations for both components indicates that ∼75% of present-day salient curvature resulted from secondary rotation, and ∼25% of primary curvature was likely related to sedimentary basin architecture. Analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets (∼80%) compared to the frontal thrust sheets (∼65%), suggesting that rotations were concentrated near the leading edge of the propagating fold-and-thrust wedge, with only minor additional rotation of internal sheets. Transfer zones, oblique ramps, and more deformed overturned fold limbs display locally more complex patterns, which can be understood through careful structural analysis. When combined with internal strain data and regional structural relations, paleomagnetic data support a kinematic model of a progressive arc with curved thrust-slip paths and differential shortening that rotated early layer-parallel shortening fabrics and produced minor strike-parallel extension. This kinematic history likely reflects a combination of processes, including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient, presence of a weak basal detachment and fault-zone weakening that favored lower taper, and buttressing by Laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient.

Curved Andes: Geoid, forebulge, and flexure

Using geoid anomalies determined from satellite observations of the South American plate, we demonstrate the existence of a lithospheric flexural forebulge east of the High Andes. Using the planform and location of the geoid anomalies and accounting for the curvature of the Andean orogen, we can successfully model plate flexure using a uniform elastic thickness. Topography above 3 km elevation between −5° and −30° latitude in the Andes loads the margin of the western side of the Precambrian shield of the continental plate and drives bending of the cratonic plate. Removal of horizontal wavelengths greater than 4500 km from the geoid anomaly reveals a 5–7 m positive anomaly paralleling the trend of the orogen some 400 km east of the mountain front. We interpret the secondary geoid high as a flexural forebulge that developed in response to topographic loading of the South American plate by the Andes. While the topographic expression of this forebulge is hidden by the alluvium shed from the Andes and the vegetative cover of the Amazon jungle, our filtered geoid anomalies and a three-dimensional, single-plate flexural model in spherical geometry are both well fit by a single model with ~50 km effective elastic thickness.

Chemical Explosion Experiments to Improve Nuclear Test Monitoring

A series of chemical explosions, called the Source Physics Experiments (SPE), is being conducted under the auspices of the U.S. Department of Energy’s National Nuclear Security Administration (NNSA) to develop a new more physics-based paradigm for nuclear test monitoring. Currently, monitoring relies on semi-empirical models to discriminate explosions from earthquakes and to estimate key parameters such as yield. While these models have been highly successful monitoring established test sites, there is concern that future tests could occur in media and at scale depths of burial outside of our empirical experience. This is highlighted by North Korean tests, which exhibit poor performance of a reliable discriminant, mb:Ms (Selby et al., 2012), possibly due to source emplacement and differences in seismic responses for nascent and established test sites. The goal of SPE is to replace these semi-empirical relationships with numerical techniques grounded in a physical basis and thus applicable to any geologic setting or depth.

The impact of vertical-axis rotations on shortening estimates

The total amount of deformation between two converging bodies is described by the three components of the displacement field: translation, rotation, and strain. Translations along faults and folding strain are the most common elements of the displacement field incorporated into estimates of tectonic shortening across orogenic systems. Determinations of vertical-axis rotations through paleomagnetic and structural analyses are keys for deciphering the rotational component of shortening within an orogenic system, and they can have a substantial effect on the amount of tectonic shortening in such systems. Accommodation structures observed in orogenic systems are typically noncoaxial and/or noncylindrical geometries (e.g., oblique and lateral ramps, superposed folding). These structures suggest that vertical-axis rotations have taken place, can aid in determining the relative timing of rotation with respect to translation, and may help constrain the location of the rotation axis. In this paper, we define the components of the total displacement field, describe the diagnostic and suggestive features associated with vertical-axis rotations, and apply trigonometric map-view calculations to estimate the amount of shortening contributed by such rotations. An error function relating shortening with vertical-axis rotation has been calculated and predicts values up to 50% for a 60° rotation if the rotation is not taken into account. Finally, we apply our approach to the Wyoming salient and show that previous estimates of shortening there may contain up to 14% error.

Palaeomagnetism in fold and thrust belts: use with caution

Abstract The application of palaeomagnetism in fold and thrust belts is a unique way to obtain kinematic information regarding the evolution of these systems. However, since many potential problems can affect the reliability of palaeomagnetic datasets and their interpretations, such data should be used with caution. In this paper, we thoroughly review the sources of error from palaeomagnetism with a particular focus on deciphering vertical-axis rotations and the assumptions behind the method. Recent investigations have demonstrated that the age of the magnetization and syn-folding results from the fold test must also be carefully examined: factors such as internal deformation, deficient isolation of components (i.e. overlapping) or incorrect restoration procedures may produce apparent syn-folding results. In fact, the restoration procedure used to return the palaeomagnetic signal to the palaeogeographic coordinate system may itself inhibit accurate estimations of vertical-axis rotations when complex deformation histories induce different, non-coaxial, deformation axes. We recommend the auxiliary use of the inclination v. dip diagram as an efficient tool for identifying many errors. Finally, to determine accurate vertical axis rotations, the reference direction should honour standard reliability criteria and would ideally be measured within the undeformed foreland of the thrust system. In this paper, we review five decades of palaeomagnetic research in fold and thrust belts by concentrating on maximizing standard reliability criteria procedures to reduce uncertainty and increase confidence when applying palaeomagnetic data to unravel the tectonic evolution of fold and thrust belts.

Paleomagnetism of the Quaternary Bandelier Tuff: Implications for the tectonic evolution of the Española Basin, Rio Grande rift

We present newly acquired paleomagnetic data from Bandelier Tuff exposures in the Jemez Mountains (New Mexico) that show no statistically significant tectonic rotation over Quaternary time. Cooling units of the tuff were mapped in detail and correlated using new geochemical data, allowing us to confidently sample isochronous units for paleomagnetic remanence directions. In total, 410 specimens were subjected to step-wise thermal and alternating field demagnetization. Of the 40 accepted site means, 30 have α 95 values ≤5°. Analysis of the geographic distribution of the site-mean declinations of the data set reveals no statistically significant tectonic rotation either across (northwest/southeast) the northeast-striking Jemez fault or across (east/west) the north-striking Pajarito fault zone. Similarly, our data do not record any measurable relative change in declination difference (−1.1° ± 1.6°) that could be interpreted as a rotation over the ∼0.36 m.y. time duration between deposition of the two principal stratigraphic members of the Bandelier Tuff. The step-over discussed in this paper is an area of exceptional structural complexity and, as such, meets the definition of “accommodation zone.” We propose the name “Jemez-Embudo accommodation zone” for this composite of structural and volcanic features in recognition of its regional importance in the evolution of the Rio Grande rift. In this part of the rift, where Proterozoic- and Laramide-age faults have preconditioned the crust, idealized relay ramps, prevalent locally, do not occur at the regional scale. Instead, transfer fault zones have developed between half grabens dominated by preexisting faults. The pattern of faulting and accommodation of strain in the right-relayed step-over of the rift has been more or less invariant since the onset of rifting. From a global perspective, the difference between areas of modest crustal extension dominated by distributed deformation and those regions that develop transfer fault zones may ultimately be diagnostic of crustal conditioning and fault strength, such that weak fault systems focus strain within narrow zones.

Introduction: Palaeomagnetism in fold and thrust belts: new perspectives

E. L. PUEYO1*, F. CIFELLI2, A. J. SUSSMAN3 & B. OLIVA-URCIA4 Instituto Geológico y Minero de España, Unidad de Zaragoza c/Manuel Lasala 44, 98, 50006 Zaragoza, Spain Dipartimento Scienze, Università degli Studi di Roma TRE, Largo San Leonardo Murialdo 1, 00146 Rome, Italy Earth and Environmental Sciences Division, MS-D452, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Departamento Geologı́a y Geoquı́mica Facultad de Ciencias, Universidad Autónoma de Madrid, Campus de Cantoblanco C/Francisco Tomás y Valiente, 7, M-06, 6 planta, 28049 Madrid, Spain

Optical Microscopy Characterization for Borehole U-15n#12 in Support of NCNS Source Physics Experiment

Optical microscopy characterization of thin sections from corehole U-15n#12 is part of a larger material characterization effort for the Source Physics Experiment (SPE). The SPE program was conducted in Nevada with a series of explosive tests designed to study the generation and propagation of seismic waves inside Stock quartz monzonite. Optical microscopy analysis includes the following: 1) imaging of full thin sections (scans and mosaic maps); 2) high magnification imaging of petrographic texture (grain size, foliations, fractures, etc.); and 3) measurement of microfracture density.

SAGE celebrates 25 years of learning geophysics by doing geophysics

The increasing world demand and record-high costs for energy and mineral resources, along with the attendant environmental and climate concerns, have escalated the need for trained geophysicists to unprecedented levels. This is not only a national need; its a critical global need. As Earth scientists and educators we must seriously ask if our geophysics pipeline can adequately address this crisis. One program that has helped to answer this question in the affirmative for 25 years is SAGE (Summer of Applied Geophysical Experience). SAGE continues to develop with new faculty, new collaborations, and additional ways to support student participation during and after SAGE.