Arlo Brandon Weil

Lithospheric delamination in the core of Pangea: Sm-Nd insights from the Iberian mantle

Delamination of continental lithosphere in the core of active collisional orogens is a wellestablished process; however, evidence for its occurrence in ancient orogenic belts is less obvious. The contrasting Sm-Nd isotopic signature between pre– and post–Middle Permian mantle-derived mafi c rocks from under the Iberian Massif suggests that most, but not all, of the subcontinental lithospheric mantle (SCLM) was replaced in latest Carboniferous to Permian time. Mantle replacement happened during and after the bending of the Variscan orogenic belt into the horseshoe-shaped Iberian-Armorican orocline. Delamination of thickened continental lithosphere in the core of the orocline triggered replacement of the ancient SCLM, thereby providing an explanation for the contrasting Sm-Nd isotopic characteristics of pre– and post–Middle Permian mafi c rocks.

Orocline triggered lithospheric delamination

Lateto post-orogenic oroclinal bending in conjunction with thinning of the lithospheric mantle is potentially an important component of the waning stages of plate convergence in collisional orogenies. This paper addresses possible and hitherto unexplored cause-effect relationships between oroclinal bending of an originally linear orogenic belt and lithospheric thinning and delamination based on an example from the Western European Variscan Belt. We suggest that lateto post-orogenic bending of the lithosphere around a vertical axis may cause thickening and eventual detachment of the lithospheric root of orogenic belts such as the Western European Variscan Belt. The proposed hypothesis is consistent with the chronology of tectonic, metamorphic, magmatic, and hydrothermal events recorded in the Western European Variscan Belt. Moreover, this hypothesis could explain the preservation of the lithospheric root in linear orogens, like the Urals, which lack signifi cant modifi cation following the initial phase of “orthogonal” lithospheric thickening.

New time constraints on lithospheric-scale oroclinal bending of the Ibero-Armorican Arc: a palaeomagnetic study of earliest Permian rocks from Iberia

Abstract: The Palaeozoic Variscan orogeny was a large-scale collisional event that involved amalgamation of multiple continents and microcontinents. Previous palaeomagnetic and structural analyses of the western Variscan orogen, notably the Ibero-Armorican Arc, suggest that this region underwent oroclinal bending of an originally near-linear orogen during the latest stages of Variscan deformation in the late Palaeozoic. These analyses necessitate a two-stage tectonic history with east–west compression forming a linear belt in the Carboniferous, followed by a change in regional compression directions resulting in crustal block rotation and formation of the present-day arc. This study presents new palaeomagnetic analyses of Early Permian samples from the Sotres and Cabranes formations (41.3°N, 222.5°W, α95 = 13.1°) in the northern inner limb and the Viar Basin (51.8°N, 199.2°W, α95 = 6.7°) in the southern outer limb of the arc. Both locations yield expected Early Permian palaeomagnetic pole positions for stable Iberia, implying that measured magnetizations have undergone no significant vertical-axis rotation since acquisition, and thus were acquired subsequent to oroclinal bending. This new result places a time constraint of about 10 Ma for oroclinal bending of the Ibero-Armorican Arc, which agrees well with recent geodynamical models that relate oroclinal bending to lithospheric delamination in this region.

Reconstructing the kinematic evolution of curved mountain belts: Internal strain patterns in the Wyoming salient, Sevier thrust belt, U.S.A.

Analyses of mesoscopic structural and strain patterns in red beds of the Triassic Ankareh Formation and limestones of the Jurassic Twin Creek Formation, in concert with complementary paleomagnetic studies, constrain the three-dimensional kinematic evolution of curved fold-and-thrust systems in the Wyoming salient of the Sevier thrust belt. Spaced cleavage, fracture and vein networks, minor folds, and minor faults in limestones and red beds accommodated early layer-parallel shortening (LPS) concentrated in front of the growing thrust wedge, along with minor strike-parallel extension and wrench shear related to the development of orogenic curvature. Strain, estimated using mass balance relations for cleavage seams, crinoid ossicles in bioclastic limestone, and reduction spots in red beds, displays systematic regional patterns. Principal shortening directions are subperpendicular to structural trend around the salient, reflecting a combination of primary curvature and secondary rotation of early LPS fabrics. LPS magnitudes vary from <5% in central parts of the frontal Hogsback thrust system, where cleavage is absent, to 10%–30% in the more interior Crawford thrust system, where cleavage intensity is moderate to strong; strain also increases toward the salient ends. Internal strain is a significant component of total deformation and should be considered when restoring cross sections. Strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting.

Oroclines: Thick and thin

An orocline is a thrust belt or orogen that is curved in map-view due to it having been bent or buckled about a vertical axis of rotation. Two distinct types of oroclines are recognized: progressive and secondary. Progressive oroclines are restricted to the scale of a thrust sheet to thrust belt, are thin-skinned, and develop during thrust sheet emplacement. Secondary oroclines are larger, occurring at the scale of an orogen, and are plate-scale features that affect crust and lithospheric mantle. Unlike progressive oroclines, which develop during initial orogenesis and in response to the same orogen-perpendicular stress responsible for thrust sheet emplacement, secondary oroclines are extra-orogenic, developing after initial orogenesis and in response to an orogen-parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development. We present case studies of the Wyoming Salient, a progressive orocline that characterizes the Sevier thrust belt of the western United States, and the coupled Cantabrian and Central Iberian oroclines, which are linked secondary oroclines affecting the Variscan orogen of Iberia. The vertical-axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis. Detailed three-dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role, if any, of primary curvature in progressive oroclines. The use of tectonic vectors, such as paleocurrent directions, provides a means of recognizing and characterizing the initial geometry of secondary oroclines. Because secondary oroclines involve the entire lithosphere, detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling.

Anisotropy of magnetic susceptibility in weakly deformed red beds from the Wyoming salient, Sevier thrust belt: Relations to layer-parallel shortening and orogenic curvature

Anisotropy of magnetic susceptibility (AMS) and structural studies of red beds in the Wyoming salient were completed to evaluate relations of magnetic fabrics to layer-parallel shortening and vertical-axis rotation in curved fold-thrust systems. The red beds display cleavage, fractures, veins, minor folds, and minor faults that accommodated widespread early layer-parallel shortening and minor strike-parallel extension. Magnetic susceptibility is carried mostly by paramagnetic phyllosilicates and ferromagnetic hematite that have composite fabrics related to sedimentary deposition, diagenesis, and tectonic processes. Anisotropy of magnetic susceptibility fabrics range from distinctly oblate ellipsoids parallel to bedding that reflect dominant sedimentary fabrics (type 1), to moderately oblate ellipsoids with weak magnetic lineations roughly parallel to the intersection of weak layer-parallel shortening fabrics and bedding (type 2), to triaxial and prolate ellipsoids with distinct magnetic lineations parallel to the intersection of moderate layer-parallel shortening fabrics and bedding (type 3). Type 1 sites occur mostly in the central, frontal part of the salient where layer-parallel shortening is 15%. Magnetic lineations are subparallel to structural trend and exhibit a tangential pattern around curved fold-thrust systems. Regional patterns of anisotropy of magnetic susceptibility are broadly similar to patterns of finite strain estimated from reduction spots. Combined with paleomagnetic data, anisotropy of magnetic susceptibility data indicate that early layer-parallel shortening fabrics started with minor primary curvature and then underwent significant vertical-axis rotation during large-scale thrusting. Correlations with finite strain, structural, and paleomagnetic data sets indicate that analysis of anisotropy of magnetic susceptibility in weakly deformed red beds is useful for evaluating kinematic evolution of thrust systems.

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.

The evolution of the paleomagnetic fold test as applied to complex geologic situations, illustrated by a case study from northern Spain

Abstract Paleomagnetic results are most useful if the age of the magnetization can be established with respect to the rock age or the age of specific structural or alteration events. The fold test is a particularly powerful tool; not only can it be used to determine whether magnetizations are pre-, syn- or post-folding, but it can also reassure us that structural corrections need (or need not) be applied to a given magnetization. This study traces the evolution of various fold and tilt tests developed in the 50-some years since the classical test of Graham was published. Syn-deformational magnetizations are a very special case, usually characterized as such by an incremental tilt test. In regions where rotations about (near-) vertical axes are to be expected, a strike test is the best tool for determining them. A case study of syn-deformational magnetizations in the Cantabria-Asturias Arc (CAA) of northern Spain is presented, which illustrates the application of the various tilt and strike tests. One ancient post-deformational and two syn-deformational magnetizations have been recorded in CAA Devonian carbonates, each characterized by different optimal (peak) percentages of unfolding in incremental fold tests. The structural corrections required to bring the individual site-mean magnetization directions into alignment can be used to restore the beds to their attitudes at the times when the magnetizations were acquired. Furthermore, these structural corrections provide robust constraints on the kinematics of the deformation phase that is being removed. In the CAA, removal of late-stage folding about steeply inclined fold axes, due to Permian oroclinal bending, restores the belt to its first folding and thrusting configuration, and produces north–south trending cylindrical folds that formed during the Late Carboniferous. The separate deformations, consisting of earlier folding and thrusting and later oroclinal bending, have implications for the final collisional movements between Gondwana and Laurussia during the end of the Variscan Orogeny. This case history of the use of multiple fold, tilt, and strike tests confirms their value, especially when combined with an understanding of the folding and faulting style in a region.

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Abstract Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, dm=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, dm=7.0°). A new 40Ar/39Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred ∼200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.

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.