Mervin J. Bartholomew

Late Quaternary faults and their relationship to tectonism in the Olympic Peninsula, Washington

Late Quaternary movement on three reverse faults and the presence of one possible strike-slip fault have been recognized in the southeastern part of the Olympic Peninsula of Washington. Movement along all four faults occurred during or after a pre-Fraser glaciation, and the last movement on the Saddle Mountain East fault appears to have occurred about 1,240 to 1,235 yr B.P. Displacements along these faults correlate well with other workers9 recent geophysical analyses of tectonic events in the adjacent Puget Lowland. The Dow Mountain and Cushman Valley (?) faults each approximate an orthogonal plane derived by focal-mechanism solutions for two groups of recent shallow-focus earthquakes in the Puget Lowland. The Saddle Mountain faults do not correlate with recent earthquake data, but the Saddle Mountain East fault, when plotted with its orthogonal plane, is compatible with the modern regional stress field determined by other workers. The Saddle Mountain faults are believed to be Holocene features developed within an older northeast-trending zone of fracturing. This older zone possibly developed during a late stage of complex folding and doming of the Olympic Mountains, before the development of the northwest-southeast compressive stress system that has characterized this region during the Holocene.

Northern ancestry for the Goochland terrane as a displaced fragment of Laurentia

The ancestral position of the Goochland terrane, an isolated block containing Mesoproterozoic crust in the Appalachian Piedmont of eastern Virginia, during Grenvillian orogenesis has direct bearing on the breakup of Rodinia. Ages, lithology, geochemistry, and Pb and Nd isotope compositions of the Montpelier Anorthosite and State Farm Gneiss of the Goochland terrane allow correlation with the Grenvillian Roseland Anorthosite and associated igneous suites of the Blue Ridge farther west in Virginia. In contrast, extensive metapelites, which underwent Devonian metamorphism and intrusion, and amphibolites of the Goochland terrane lack equivalents in the Blue Ridge. Dextral slip faults, which bound the Goochland terrane, as well as dextral faults farther north, suggest large-scale translation prior to late Paleozoic reaccretion. Ages of extension-related A-type granitoids within the Virginia Blue Ridge (735-680 Ma) suggest physical separation from compositionally similar granitoids in the Goochland terrane (630-588 Ma), Reading Prong (602 Ma), and Manhattan Prong (563 Ma). Pre-Iapetan restoration of the Goochland terrane northeastward of the Blue Ridge and outboard of the Reading Prong accounts for its affinities to the Blue Ridge (unique anorthosites), Manhattan Prong (abundant amphibolites), and Reading Prong (ca. 600 Ma magmatism). Thus similar to the translation of Madagascar out of Africa, the Goochland terrane was rifted from Laurentia, marooned in oceanic crust, and then dextrally translated southward ∼500 km prior to late Paleozoic reaccretion. Documentation of large-scale dextral translation of eastern Laurentia relative to its fragments and other cratons during the Neoproterozoic and Paleozoic may assist in locating far-traveled pieces of the Grenville orogen for reconstruction of Rodinia.

Chemical diversity and origin of Precambrian charnockitic rocks of the central Pedlar massif, Grenvillian Blue Ridge Terrane, Virginia

Abstract Middle Proterozoic rocks of the Pedlar massif are comprised of varied charnockitic and granulitic lithologies representing ∼ 1150-1000 Ma Grenvillian metamorphic and igneous events in the central Appalachians of Virginia. Chemically and mineralogically diverse units include metaluminous to peraluminous charnockites, enderbites, jotunites and granulite gneisses ranging in SiO2 from 47 to 79 wt.%. Relative to bulk continental crust, typical charnockites are enriched by ∼ 3–6 × in Rb, K, Ba, LREE, Zr and Hf, less enriched in Ta, Sr, P and HREE, and depleted in Cs, Th, Ti and Sc, although low-SiO2 members have relatively high Ti and P and some high-SiO2 units are depleted in K, Rb and Ba. The Pedlar River Charnockite Suite (PRCS), Vesuvius megacrystic charnockite (VMC), Nellysford Granulite Gneiss (NGG) and the Lady Slipper Granulite Gneiss (LSGG) exhibit common chemical signatures and trends, notably evident in uniform LREE-enriched patterns with variable Eu anomalies, which reflect their derivation from a mixed protolith of volcanics and reworked volcaniclastic sediments dated at ∼ 1130–1150 Ma. Pedlar River charnockites represent portions of deep-seated granulite gneisses that were mobilized by granulite facies dehydration melting and emplaced en masse as plutons into overlying and surrounding gneisses during the Grenville episode. Divergent REE patterns in the PRCS are attributed to crystallization of quartz, feldspar ± garnet, or HREE-compatible mafic and accessory phases leading to complementary chemical signatures in mafic and felsic layers. Depletion of Cs throughout the Pedlar massif is attributed to its incompatibility with major mineral phases during either protolith evolution or granulite facies dehydration, while dispersions in K, Rb, Sr, Ba and Eu are due to the relative proportions of the major phases quartz, K-feldspar, plagioclase and biotite. Mobility of insoluble trace elements Zr, Hf, Ta and Th require partial melting and crystallization of accessory mineral phases, whereas Sc, Cr and REE mobility depends on the proportions of pyroxene and garnet. Several leucocharnockites and other felsic rocks of the PRCS show markedly elevated Th values ranging from ∼ 10 to 83 ppm which complement depleted Th in typical PRCS units. These charnockitic rocks represent low-degree partial melts (∼ 1–5%) from normal PRCS or NGG protoliths to produce high-silica magmas which separated from the main charnockitic body.

The MW 6.9 14 April 2010 Yushu Earthquake and a 10,000-Year Record of Paleoseismicity along the Guoqiong Segment of the Yushu Fault, Qinghai Province, China

in the central region of the Tibetan Plateau, within the Banyan Har Mountain Range along the Ganzi-Yushu fault system. The focal mechanism for the main shock indicated left-lateral, strike-slip movement along a WNW-ESEstriking, near-vertical fault. Near the village of Guoqiong (30km-NW of Jiegu town), a maximum of ~1.8m leftlateral surface displacement occurred along the Guoqiong Segment of the Yushu fault. In October 2011, we located a trench (CUG2011-1) along this segment across a LatePleistocene alluvial fan perpendicular to the surface rupture. A Holocene stream channel, incised 3m into the fan, was deflected left-laterally ~6m indicating substantial strike-slip displacement since incision. The trench-location was specifically selected to ensure excellent preservation of the Holocene sedimentary record. We then used both C and OSL dating-techniques to determine that only four major earthquakes produced surface ruptures along the Guoqiong Segment during the last 10,000 years. Within the trench, three buried A-soil horizons were preserved along the downthrown side of the fault. Line-length balancing and progressive retro-deformation for the 2010 event and three previous surface ruptures indicate ~2m of horizontal shortening perpendicular to the fault and ~1.2m of vertical displacement. If the 2010 event, with ~1.5m left-lateral displacement, was similar to the three earlier events, these 4 events could account for the ~6m of left-lateral displacement in the incised Holocene stream channel. Four events in 10,000-years indicates a much longer recurrence interval than previous estimates.

Structural characteristics of the Late Proterozoic (post-Grenville) continental margin of the Laurentian craton

Distribution and orientation data of 2676 segments of 2119 Late Proterozoic to Early Cambrian felsic and mafic dikes were compared with other Late Proterozoic to Early Cambrian features associated with two stages of Iapetan extension, including: (1) the trend of the Robertson River rift zone; (2) the distribution of granitoid plutons; (3) the distribution of volcanic fields; (4) trends of previously recognized surface and subsurface extensional faults; (5) kinematic indicators from previous studies; and (6) some Paleozoic foliation data. This comparison suggests the following: Felsic dikes in northern Virginia are spatially related to the N25°E-trending rift zone filled with Robertson River granitoids and have a principal relict trend of N25°E. Mafic dikes, identified as Catoctin dikes in the same area, have a principal relict orientation of N35°E but exhibit a wider range of orientations than the dikes associated with the rift — many locally bimodal or trimodal. Other mafic dikes in the same area have a N25°E principal trend. Previous studies indicate chemical similarity between these dikes and Catoctin dikes. These other mafic dikes could be either associated with the earlier stage of extension, when the Robertson River rift zone developed but are, coincidently, from chemically similar magmas as the Catoctin volcanic rocks, or they could be associated with the younger stage of Catoctin volcanism but intruded along fractures developed during the earlier stage. Many amphibolite dikes in the eastern part of the Blue Ridge where Paleozoic metamorphic grade was higher (commonly garnet) have preferred orientations parallel to regional Paleozoic foliation trends and generally lack diverse secondary or minor trends. These amphibolites are interpreted to have been partially transposed (≈ 10°-20° rotation) during Paleozoic orogenesis, but they could reflect original trends that coincidentally are parallel to younger foliation. Overall, preferred relict orientations for all dikes suggest Late Proterozoic-Cambrian extension was primarily along an axis oriented ≈ N55°-85°W — nearly perpendicular to documented Late Proterozoic subsurface faults in the southern Appalachians and to the reconstructed Laurentian margin during Iapetan extension.

Shallow seismic detection of the fault zone associated with a high scarp in southwestern Montana

AbstractWe collected shallow reflection data in southwestern Montana, USA, across a 5.4-m-high tectonic scarp. The goal was to image the normal fault associated with the scarp, observed in an adjacent trench. Processing of the data was challenging because the height of the scarp was comparable to the depths of the reflectors of interest. To find out how to proceed, we processed synthetic data generated using velocity models derived in part from actual shot gathers. The actual data are dominated by large-amplitude low-frequency surface waves, but clear high-frequency reflections are seen in the more distant geophones. Common-offset gathers for the raw and high-pass filtered data reveal sharp discontinuities in arrival times and a strong decrease in amplitudes, respectively, under the scarp. These changes in the wavefield are indicative of lateral variations in elastic properties and are consistent with the presence of a fault zone seen in the trench. The actual data were stacked after the surface waves wer...