Sharon Mosher

Laurentia‐Kalahari Collision and the Assembly of Rodinia

The Llano Orogenic Belt along the present southern margin of Laurentia, regarded as continuation of the Grenvillian Orogen along the eastern Laurentian margin and exposed in basement uplifts in central and western Texas, records an ∼300‐m.yr. history of orogenesis culminating in arc‐continent and continent‐continent collision between ∼1150 and 1120 Ma and continuing until ∼980 Ma. The shape of the orogen and kinematics of the contractional deformation along the belt, together with the high‐P metamorphic conditions attained, indicate that a previously unidentified craton served as an indentor. It is paleomagnetically acceptable for the Kalahari Craton of southern Africa to have been opposed to this margin and within ∼1500 km of present‐day central Texas at ∼1100 Ma. Moreover, the Kalahari Craton is the correct size, and the structural and metamorphic evolution of the 1200–950 Ma Namaqua‐Natal Orogenic Belt that wraps around its present southern margin is compatible with that craton having been the indentor. The ocean basin that closed between the Laurentia and Kalahari Cratons would have been comparable to the present Pacific, with island arc/terrane accretion occurring during the Mesoproterozoic along opposing active convergent margins. The coeval 1.1 Ga Keeweenawan and Umkondo magmatic provinces of Laurentia and Kalahari, respectively, are associated with rifts at a high angle to the Llano and Namaqua Orogens. The rifts are interpreted as the result of collision‐generated extensional stresses within the two cratons. The voluminous mafic igneous rocks in both provinces, however, may reflect contemporaneous plume activity. Our reconstruction for 1.1 Ga provides a testable model for the Llano Orogenic Belt of Texas and the Namaqua Orogenic Belt of southwestern Africa as opposite sides of a Himalayan‐type collisional orogen, with the Natal Belt of southeastern Africa and the originally continuous Maudheim Belt of East Antarctica as a related Indonesian‐type ocean‐continent convergence zone. This reconstruction leads to a refinement of the paleogeography of Rodinia, with the Kalahari Craton in a position isolated from both the East Antarctic and Rio de la Plata Cratons by oceanic lithosphere. It also provides the first model for the assembly of that hypothetical early Neoproterozoic supercontinent. At least four separate cratonic entities appear to have collided along three discrete segments of the apparently anastomosing global network of “Grenvillian” orogens: the type‐Grenville Belt of eastern North America and counterparts in South America, the Llano‐Namaqua Belt, and the Eastern Ghats‐Albany/Fraser Belt of India‐East Antarctica and Australia. Over the remarkably short interval of ∼200 m.yr., this first‐order composite collisional event resulted in the amalgamation of most of Earth’s continental lithosphere and defined the close of the Mesoproterozoic Era.

Tectonic evolution of the southern Laurentian Grenville orogenic belt

The Grenville orogenic belt along the southern margin of Laurentia records more than 300 m.y. of orogenic activity culminating in arc-continent and continent-continent collision ca. 1150–1120 Ma. Exposures in Texas provide a unique profile across the Grenville orogen from the orogen core to the cratonal margin. In the Llano uplift of central Texas, ca. 1360–1232 Ma upper amphibolite–lower granulite facies, polydeformed supracrustal and plutonic rocks represent the core of the collisional orogen. This exposure contains a suture between a 1326–1275 Ma exotic island-arc terrane and probable Laurentian crust and records A-type subduction. In west Texas, 1380–1327 Ma amphibolite to greenschist facies, polydeformed supracrustal rocks are thrust over ca. 1250 Ma carbonate and volcanic rocks along the cratonal margin. The carbonate and volcanic rocks form a narrow thrust belt with post–1123 Ma synorogenic sedimentary rocks, which grade into undeformed sedimentary rocks northward on the Laurentian craton. The Texas basement reveals a consistent but evolving tectonic setting for the southern margin of Laurentia during Mesoproterozoic time. This paper summarizes recent advances in our knowledge of the Texas basement and proposes plate models to explain the tectonic evolution of this margin during Mesoproterozoic time. The orogenic history is strikingly similar to that of the Canadian Grenville orogen and requires a colliding continent off the southern Laurentian margin during the assembly of Rodinia.

Neotectonics of the Macquarie Ridge Complex, Australia-Pacific plate boundary

New marine geophysical data along the Macquarie Ridge Complex, the Australia-Pacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a ∼5–10 km wide Macquarie Fault Zone between the two plates along a bathymetrie high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific strike-slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends ∼50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crust in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas

The Llano uplift of central Texas, United States, exposes the core of a Mesoproterozoic orogenic belt that formed along the southern margin of Laurentia during Grenville time. A new collisional model is proposed that reconciles differences in structural stacking, apparent tectonic transport, and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high-pressure eclogitic rocks, emplacement of ophiolitic rocks, and subsequent late-stage to postcollisional plutonism. Our model proposes that subduction with southward polarity resulted in collision of an exotic arc with Laurentia, emplacement of ophiolitic rocks, and telescoping of the intervening basinal sediments, followed by overriding of the arc and margin of Laurentia by a southern continent with transport toward Laurentia. The model further proposes that convergence led to subduction of the Laurentian margin, resulting in high-pressure metamorphism, but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from Laurentia, in a manner similar to that proposed for the Alpine orogeny. Slab breakoff resulted in upwelling of the asthenosphere, leading to intrusion of juvenile granitic plutons. Subduction along strike caused continued contraction that waned with time. The eastern uplift records continent-arc-continent collision, whereas the western uplift records continent-continent collision; the two regions also expose different crustal levels in the orogen. The striking similarity with Phanerozoic orogens, including emplacement of ophiolites and formation of high-pressure rocks, implies that plate tectonic processes including subduction were active prior to the Neoproterozoic.

Physical models of regional fold superposition: the role of competence contrast

Abstract Physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference. Rheologic contrast affects strain partitioning between layer-parallel shortening and buckling and affects the development of structural anisotropies parallel to first-generation folds. Laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins (Type 1 interference). In addition to folding, the layers accommodate shortening by layer-parallel thickening, preferentially in the fold culminations. Laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces (Type 2 interference). The less competent layers thicken in the hinges; the more competent layers maintain their initial thickness. In plan view, the F 1 ) hingelines are refolded in a lobate—cuspate to box style, whose axial traces form conjugate pairs. The models are dynamically scaled to represent folds with km-scale wavelengths. The evolution of surface structures, map patterns at different depths, and equal-area projections synthesized from structure contour maps were used for analysis, comparable to regional analysis common to field studies. Gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds, thereby enhancing the formation of Type 2 interference.

Extension along the Australian‐Pacific transpressional transform plate boundary near Macquarie Island

The Australian-Pacific transform plate boundary fault zone along the Macquarie and McDougall segments of the Macquarie Ridge Complex (MRC), south of New Zealand, is characterized by dominantly normal faults and pull-apart basins, in apparent conflict with the regional transpressional tectonic setting. We propose that present-day curvature of the transform is inherited from a preexisting divergent plate boundary and that the overall extensional kinematics shown by faults along the main plate boundary trace and exposed on Macquarie Island result from local stresses related to right-lateral, right stepping, en echelon plate boundary faults and not to the current transpressional setting. Transpression along the Australian-Pacific transform plate boundary has resulted in uplift along the ?1500 km long Macquarie Ridge Complex. Macquarie Island, the only subaerial exposure of the complex, sits atop a ?5 km high, ?50 km wide submarine ridge of oceanic crust and lies ?4.5 km east of the major active plate boundary fault zone. Thus Macquarie Island and the surrounding seafloor provide a unique opportunity to study an active oceanic transform fault using complementary marine geophysical and land-based geological data. Mapping of recent faults affecting the topography of Macquarie Island shows that the island is extensively cut by high-angle normal faults forming pull-apart basins. Furthermore, evidence for reverse motion is rare. Using marine geophysical data, including swath bathymetry, reflectivity, and seismic reflection data, collected along the Australian-Pacific plate boundary north and south of the island, we have defined a 5–15 km wide plate boundary zone. A series of right stepping en echelon faults, within this zone, lies along the main plate boundary trace. At the right stepping fault terminations, elongate depressions (?10 km wide and 1.2 km deep) parallel the plate boundary, which we interpret as extensional relay zones or pull-apart basins. We propose that transpression is partitioned into en echelon strike-slip faults at the plate boundary and a convergent component that flexes the crust, causing the anomalous bathymetric ridge and trough morphology of the McDougall and Macquarie segments of the MRC.

Pressure Solution as a Deformation Mechanism in Pennsylvanian Conglomerates from Rhode Island

The Pennsylvanian age Purgatory Conglomerate of Rhode Island was deformed under late Paleozoic lower greenschist facies conditions. Two mechanisms are invoked to account for the deformation. Pressure solution resulted in the formation of a highly flattened and elongated pebble fabric. This fabric was enhanced by slip and concurrent pebble rotation on intra-pebble shear fractures. Adjacent quartzite pebbles are molded against each other and usually exhibit distinct pressure shadow overgrowths in the direction of pebble elongation., The matrix between pebbles, exclusive of pressure shadow overgrowths, is predominately micaceous. Where pebbles are indented there is no evidence of adjacent penetrative deformation of the quartz within the pebbles. Internal deformation features in the pebbles exhibit no consistent relation to pebble shape. This supports the contention that there was little if any penetrative strain.

High-Pressure Metamorphism in the Texas Grenville Orogen: Mesoproterozoic Subduction of the Southern Laurentian Continental Margin

Collisional orogenesis in the Llano Uplift of central Texas during the late Mesoproterozoic drove metamorphism that comprised both an initial high-pressure (HP) phase (610-775°C at 1.4-2.4 GPa) and a subsequent moderate-pressure (MP) phase (~700°C at ~0.7 GPa). A later, low-pressure overprint (525-625°C at 0.3 GPa) took place under largely static conditions. Evidence for HP metamorphism is geographically widespread, but confined principally to boudins of mafic eclogite encased in felsic gneisses. The geographic distribution of P-T conditions for HP metamorphism inferred from the eclogites, combined with evidence from the relative degrees of homogenization of growth zoning in garnet from both eclogites and felsic gneisses, suggests a general increase from northeast to southwest in depths of burial during HP metamorphism. Exhumation of HP rocks to shallower depths prior to MP metamorphism appears to have been rapid: ages for the two phases of metamorphism overlap, on the basis of the very limited data now available. The regions early metamorphic history is best explained by southwestward subduction of the Laurentian continental margin during collision with a still-unidentified continental mass, followed by buoyancy-driven uplift to lower crustal levels while collisional contraction continued.

Macquarie Island's Finch-Langdon fault: A ridge-transform inside-corner structure

Macquarie Island consists of uplifted oceanic crust, uniquely situated in the ocean basin where it formed, thus allowing onshore structures to be placed into their regional oceanic tectonic context. The Finch-Langdon fault, the most significant spreading-related structure on the island, juxtaposes upper-crust rocks against lower-crust and upper-mantle rocks. It consists of dominantly oblique strike-slip, northwest-, west-northwest–, and north-northeast–striking fault segments that bear hydrothermal mineralization indicative of faulting during seafloor spreading. Talus breccias and graywackes overlain by volcanic flows proximal to the fault indicate a long-lived submarine fault scarp that exposed diabase dikes and gabbros during volcanism. Swath reflectivity and bathymetry reveal ridge-parallel spreading fabric and perpendicular fracture zones, the closest 7 km east of the island. On the basis of field and swath data, we propose that this fault zone formed near the inside corner of a ridge-transform intersection and that structures on the island are conformable with those in the surrounding seafloor.

Ridge reorientation mechanisms: Macquarie Ridge Complex, Australia-Pacific plate boundary

The Macquarie Ridge Complex portion of the Australia-Pacific plate boundary south of New Zealand provides a unique, complete record of a 60°–90° change in spreading direction since 40 Ma that resulted in the transition from a spreading center to a transform plate boundary. Marine geophysical data show that during reorientation, most ridge segments completely disappeared and all shortened. Additionally, modification of newly created crust caused differences in widths of correlative spreading segment corridors on the two plates. We propose two models for ridge reorientation that explain the observed spreading fabric and arcuate fracture zone relationships. Nonrigid plate deformation was accommodated by failing and propagating spreading ridge segments (rifts) and transfer of crust between plates during the gradual reorientation of the spreading axes.