Eric A. Erslev

How Laramide-Age Hydration of North American Lithosphere by the Farallon Slab Controlled Subsequent Activity in the Western United States

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existing explanations for uplift or magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly beneath the Rocky Mountains of Colorado and New Mexico, and high in velocity to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the lowvelocity volume is partially molten Precambrian North America mantle. We suggest, as others have, that the Farallon slab was in contact with the lithosphere beneath most of the western U.S. during the Laramide orogeny. We further suggest that slab de-watering under the increasingly cool conditions of slab contact with North America hydrated the base of the continental lithosphere, causing a steady regional uplift of the western U.S. during the Laramide orogeny. Imaged low-velocity upper mantle is attributed to hydration-induced lithospheric melting beneath much of the southern Rocky Mountains. Laramide-age magmatic ascent heated and weakened the lithosphere, which in turn allowed horizontal shortening to occur in the mantle beneath the region of Laramide thrusting in the southern Rocky Mountains. Subsequent Farallon slab removal resulted in additional uplift through unloading. It also triggered vigorous magmatism, especially where asthenosphere made contact with the hydrated and relatively thin and fertile lithosphere of what now is the Basin and Range. This mantle now is dry, depleted of basaltic components, hot, buoyant, and weak.

Normalized center-to-center strain analysis of packed aggregates

Abstract The resolution of conventional techniques of center-to-center strain analysis is limited by the degree of original anticlustering of centers on the analyzed plane. However, the three-dimensional anticlustering of packed objects does not result in equivalent anticlustering on two-dimensional planes through these aggregates. Size variations due to imperfect sorting further decrease the anticlustering of natural aggregates. For the Fry all-object-object separations method, these problems are manifested in vague point-density distributions and ambiguously defined strain ellipses. Normalization of center-to-center distances allows more precise determination of small initial and tectonic anisotropies in packed aggregates. On planes through packed aggregates, object spacing is a function of object size, shape and the distance between object margins. Dividing the center-to-center distance between two objects by the sum of their average radii eliminates variations due to object size and sorting. Analyses of synthetic aggregates of packed spheres and statically recrystallized iron show that normalized Fry diagrams form better-defined vacancy fields and sharper rims of maximum point density regardless of the original sorting and anticlustering in the aggregate. Normalized strain analyses of deformed aggregates also show greatly increased resolution, with variable initial and tectonic ellipticity resulting in a wider ring of high point-density.

The Archean-Proterozoic transition: Evidence from the geochemistry of metasedimentary rocks of Guyana and Montana☆

Abstract Chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first-order changes yin the composition and extent of continental crust, from the Archean to the Proterozoic. We reversed the typical sampling pattern, comparing metapelites and carbonates from Proterozoic greenstone belts with those from an Archean continental margin sequence, and found the reverse of the typical age-composition relations. Metapelites and volcaniclastic metagreywackes from the Early Proterozoic greenstone belts of northern Guyana resemble those of many Archean greenstone belts in REE, trace, and major element compositions (mean ratios: La Sm 5.0, La Th 5.1, Eu Eu ∗ 0.80, Th U 3.4; median K 2 O Na 2 O 0.8). Schists from an Archean continental marginal sequence in Montana have compositions like many post-Archean metapelites derived from continental sources (mean ratios: La Sm 6.8, La Th 2.7, Eu Eu ∗ 0.67, Th U 5.2; median K 2 O Na 2 O 1.7). Other Archean continental basin sediments also resemble their post-Archean counterparts, implying that upper continental crusts of these eras may not have differed significantly in these respects. However, temporal change may have occurred in upper crustal contents of Ni and Cr many Archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post-Archean equivalents. Dolomitic limestone from a greenstone belt of northern Guyana has a very low 87 Sr 86 Sr ratio of 0.7007. Such low values may reflect the local influence of mantle-derived volcanic rocks during carbonate deposition, diagenesis, or metamorphism and they may not be reliable indicators of the evolution of Sr in seawater. Archean dolomitic marbles from the continental marginal sequence of Montana have 87 Sr 86 Sr ratios of 0.7065 and higher, significantly more radiogenic than many accepted values for carbonates of Archean greenstone belts.

Basement balancing of Rocky Mountain foreland uplifts

Restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks. Cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts; local zones of shortening and extension occur in hinge zones above transitions in fault curvature. High-curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover. 20 references, 4 figures.

Least-squares center-to-center and mean object ellipse fabric analysis

Abstract The subjectivity of ellipse fitting in many strain techniques has hindered the determination of fabric anisotropy and tectonic strain. However, many sets of x , y co-ordinates can be approximated as an ellipse using a least-squares algorithm to calculate a best-fit ellipse and associated average radial error. For instance, the two dimensional shape of many objects can be approximated as an ellipse by entering digitized co-ordinates of the object margin into the ellipse algorithm. The rim of maximum point density in a normalized Fry diagram is defined by normalized center-to-center distances between touching or nearly touching objects. The enhanced normalized Fry (ENFry) method automates ellipse fitting by entering center-to-center distances between these “touching” objects into the least-squares ellipse algorithm. For homogeneously deformed populations of 200 objects, the ENFry method gives an accurate and precise measure of whole-rock fabric anisotropy, particularly for low ellipticities. When matrix strain exceeds clast strain, manual ellipse fitting of normalized Fry plots gives more accurate matrix anisotropies. The mean object ellipse (MOE) method calculates the best-fit ellipse from the geometry of the objects. Three points from the margin of each object ellipse, centered at the origin and expanded or reduced to unit volume, are used to calculate the best-fit fabric ellipse. The MOE method is very precise for small data sets, making it a good method for mapping heterogenous object strain. However, least-squares calculations maximize the influence of distal and spurious ellipticities, causing the MOE method to overestimate the fabric ellipticity of most aggregates.

Multiple geometries and modes of fault-propagation folding in the Canadian thrust belt

Abstract A multitude of fold models have been proposed to explain the variety of fold geometries which develop in front of thrust faults. Detailed field, fabric, and photogrammetric studies of 4 fault-cored asymmetrical folds in the thin-skinned Canadian thrust belt were used to test models of fault-propagation folding. Fold geometries include combinations of angular and rounded fold surfaces, highly contorted anticlinal hinge areas, and minimal penetrative deformation or changes in bedding thickness. Interlimb angles generally decrease with increasing shortening, indicating progressive fold tightening about fixed anticlinal hinges. Extensive flexural slip thrusting toward the anticlinal axes of angular folds suggests that kink folding in thin-skinned thrust belts is aided by material transfer from both fold limbs into hinge areas. Fold geometries change dramatically along the strike of individual structures, demonstrating the non-uniqueness of fault-propagation fold geometries. No single mode of fault-propagation folding can explain the diverse fold geometries seen in the Canadian thrust belt. This geometric variability can be ascribed to the complex interplay of multiple modes of folding. In strata near the causal thrusts, oblique shear and flexural slip in triangular shear zones distribute thrust displacements into both rounded and angular folds. Simultaneous angular folding of overlying strata commonly occurs by progressive kink folding where folds tighten by flexural slip on all fold limbs until the thrust breaks through the fold. Regional and local differences in the amount of pervasive, top-to-the-craton shear needed for progressive kink folding may be partially responsible for the variability of fault-propagation fold geometries in thin- and thick-skinned orogens world-wide.

Evidence for Proterozoic mylonitization in the northwestern Wyoming province

The 3-km-thick Madison mylonite zone encompasses the contact between the Cherry Creek Metamorphic Suite and the pre-Cherry Creek metamorphic complex in the southern Madison Range of southwest Montana. In the shear zone, greenschist- and epidote-amphibolite-facies assemblages overprint earlier amphibolite- and granulite-facies assemblages. Large-scale metasomatic conversion of granitic gneiss to mylonite gneiss increased siderophile (Fe, Mn, Mg, P, and Ca) elements and decreased K, Al, and Si in the mylonite gneiss, suggesting extensive fluid flux in the Madison mylonite zone. Southeast-dipping foliations adjacent to the Madison mylonite zone are rotated about their northeastern strike through the vertical to northwest-dipping orientations, indicating southeast-directed thrusting of the pre-Cherry Creek metamorphic complex over the Cherry Creek Metamorphic Suite. Complete stratigraphic continuity of Cherry Creek units within the mylonite zone shows the predominance of ductile shearing. Ductile thrusting is also indicated by asymmetric fabrics, stretching lineations, finite strain, and metamorphic gradients within the shear zone. If the foliations were passively rotated during simple shear, a minimum of 10 km of distributed ductile slip occurred in the Madison mylonite zone. 40Ar/39Ar thermochronology of hornblende and muscovite adjacent to the Madison mylonite zone records a Late Archean (2.5 Ga) cooling event. Within the shear zone, a discordant 40Ar/39Ar muscovite spectrum is consistent with initial cooling to argon closure at about 1.9 Ga with subsequent disturbance at about 1.6 Ga. Hornblendes from the Madison mylonite zone show highly discordant argon-loss spectra with temperature steps starting at 1.8 Ga and ending at 2.5 Ga. These argon age spectra indicate Early Proterozoic heating and argon loss in the Madison mylonite zone, resetting and subsequent retrogressing of muscovite but only partial resetting of hornblende due to its higher argon closure temperature. This study strongly suggests Early Proterozoic (1.80-1.90 Ga and possibly later) thrusting in the Madison mylonite zone, which may be correlative with retrograde metamorphism and Early Proterozoic isotopic resetting northwest of the Madison Range. The Madison mylonite zone is probably a foreland thrust zone on the margin of a major compressional orogen of Early Proterozoic age that reworked the Archean basement of the northwestern Wyoming province.

Multistage, multidirectional Tertiary shortening and compression in north-central New Mexico

The nature of Laramide deformation in the southern Rocky Mountains remains highly debated. Advocates of north-south strike-slip faulting favor transpression and northward displacement of the Colorado Plateau, whereas advocates of east-west shortening by thrust faulting contest evidence for large right-lateral displacements. Minor faults (n = 2552) were measured to determine the Laramide to Holocene structural evolution of north-central New Mexico. Multimodal slickenline and ideal σ1 orientations, as well as consistent crosscutting relationships, indicate multiphase, multidirectional faulting. The oldest set of thrust and strike-slip faults indicates east-northeast– to east-trending shortening and compression. The faults cut rocks as young as the upper Paleocene–lower Eocene Diamond Tail Formation. Strike-slip faults from a second phase of northeast- to north-northeast–trending shortening and compression cut the Eocene Galisteo Formation but do not cut the 27 Ma Galisteo dike. Elsewhere, later north-striking strike-slip faults cut 24 Ma igneous units, indicating north-northeast– to north-trending shortening and compression of mid-Tertiary age. Subsequent north-striking normal faults related to Rio Grande rifting commonly reactivate the mid-Tertiary strike-slip fault planes. These results show the validity of both strike-slip and thrust hypotheses in north-central New Mexico, although neither is adequate in exclusion of the other. Early Laramide east-west thrusting probably formed the north-trending Laramide arches of the region. Later counterclockwise rotation of regional shortening and compression directions may have caused transpression, opening some late Laramide axial basins and causing limited northward displacement of the Colorado Plateau. Subsequent mid-Tertiary strike-slip deformation may be a missing link between Paleogene Laramide shortening and Neogene Rio Grande rifting.

2D Laramide Geometries and Kinematics of the Rocky Mountains, Western U.S.A.

Kinematic hypotheses for 2D, cross-sectional Laramide deformation in the Rocky Mountains include (1) block-tilting models and (2) basement thrust models with (a) subcrustal shear during low-angle subduction, (b) lithospheric buckling, (c) thickened lower crust under Laramide arches, and (d) crustal buckling during detachment of the upper crust. Vertical tectonic models invoking vertical or normal faults are falsified by major Laramide thrust faults and pervasive minor faults indicating NE-SW to E-W horizontal shortening. Seismic tomography documenting 200-km-thick Rocky Mountain lithosphere contradicts models predicting wholesale removal of North American mantle lithosphere by subcrustal shear during low-angle subduction. Preliminary gravity and deep seismic interpretations indicate a lack of correspondence between the Moho and Laramide arch geometries. This apparent lack of major Laramide faulting and folding of the Moho under the Rockies contradicts block-tilting models invoking reverse faults cutting the Moho and lithospheric buckling models. In addition, the maximum distances between Laramide arch culminations are generally smaller than buckle wavelengths expected for cratonic continental lithosphere. Geophysical evidence for rootless Laramide arches is incompatible with models predicting pure shear thickening of the lower crust under individual arches but is consistent with models invoking distributed lower crustal thickening over the entire Rockies. Rocky Mountain crustal geometries are most consistent with initial crustal buckling followed by upper crustal detachment on subhorizontal thrust faults that root to the west. An originally thick Rocky Mountain crust may have facilitated delamination and detachment folding of the upper crust during both the Laramide and the Ancestral Rocky Mountain orogenies.

Non-volatile element and volume flux in coalesced slaty cleavage

Abstract Non-volatile element and volume flux during slaty cleavage formation was determined by mapping major element compositions in hand specimens of slate with variable cleavage intensity due to non-argillaceous interlayers. Intense spaced cleavage zones form where cleavage coalesces within the inner arcs of folded nonargillaceous layers or at the offsets between imbricated non-argillaceous layers. The element distributions in samples from Nova Scotia, Vermont and New Jersey were mapped using an XRFmacroprobe, an energy dispersive XRF with automated spectrum acquisition and a mm-scale collimated beam. Cleavage zones are depleted in quartz and SiO 2 , and enriched in phyllosilicates, Al 2 O 3 , Fe 2 O 3 , K 2 O, MgO and TiO 2 . CaO and MnO show more variable behavior consistent with simultaneous depletion in carbonates and residual enrichment in silicates. Quartz-rich, weakly cleaved areas adjacent to fold outer arcs or between imbricate offsets are enriched in quartz with respect to probable pre-cleavage compositions, with SiO 2 commonly reaching 75–80 wt%. The spaced cleavage zones and their intervening microlithons grade texturally and compositionally into penetrative cleavage with distance from non-argillaceous layering. Average compositions of heterogeneously cleaved beds are quite similar to compositions of homogeneously cleaved beds in the same sample, suggesting balanced, localized element enrichment and depletion adjacent to non-argillaceous layers. The similarities between average macroprobe, slate and shale compositions suggest minimal non-volatile net volume flux during the formation of slaty cleavage. Volume losses of 50% by SiO 2 flux would result in a rock with approximately equal SiO 2 and Al 2 O 3 wt%, not a slate. Most of the non-volatile element and volume flux in the slates occurred on a hand specimen scale, with quartz depletion in cleavage zones balanced by enrichment in adjoining microlithons.