Reinterpreting origins, examining skeletal preservation, and understanding volcanics

December 19th, 2013
GSA Bulletin articles posted online ahead of print on 6 and 13 December 2013 cover earthquake hazards of the Santa Barbara suburban area; apatite and the skeletons of early animals; the peculiar geological features of Faial (Azores, Portugal); the nature of Mount Rainier; the origin of Pearya terrane, Canada; a re-interpretation of the Chilhowee Group of the Appalachian Blue Ridge; and more. Authors hale from the U.S., Italy, South Africa, and Canada.

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Tectonic geomorphology of marine terraces: Santa Barbara fold belt, California

Larry D. Gurrola et al., Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA; corresponding author: Edward A. Keller, keller@geol.ucsb.edu. Posted online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/B30211.1.

The Santa Barbara urban area has a more serious earthquake hazard than previously thought. Results from studying the coastal landscape over the past 100,000 years suggest rates of uplift from active earthquake processes has formed a series of elongated hills (growing anticlines that look like half a banana sliced lengthwise flat side down on a table). We informally referred to the set of east - west coastal hills, some of which are several km (mi) long and with over 100 m (320 ft) in local relief (elevation change from base to top), the Santa Barbara Fold Belt. Our research estimates ages of uplifted marine terraces (wave-cut platforms covered with a thin layer of beach or land derived sediments and periodically uplifted by earthquakes) at the coastline at several sites applying multiple age-dating techniques. Marine terraces that range in age from 47,000 to 105,000 years are being uplifted a rate of approximately 1 to 2 mm/year (3 to 6 ft per thousand years). The cumulative uplift that forms the linear hills is the result of multiple earthquakes and fold growth.


Phosphorus sources for phosphatic Cambrian carbonates

Jessica R. Creveling et al., Dept. of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA, jcrevel@gps.caltech.edu. Posted online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/B30819.1.

In lower Cambrian rocks, the skeletons of early animals are commonly replicated by the mineral apatite, a calcium phosphate mineral of similar composition to human teeth and bones. For reasons that remain unknown, however, apatite fossil preservation decreased during the middle of the period, limiting paleontologists' view of Cambrian animal diversification. To understand the loss of this fossil preservational mode, Jessica R. Creveling and colleagues analyzed a drill core of Cambrian carbonate rocks from central Australia that display exceptional apatite fossil preservation. Authors Jessica Creveling and colleagues asked: What was the source of phosphorus for this fossil preservation? Combining geological observations and geochemical data, the authors found that apatite fossil preservation occurred during times when subsurface ocean water masses were anoxic (without oxygen) and rich in iron. This suggests that, in addition to phosphorus delivery by organic matter settling to the seafloor, adsorption to iron minerals that formed in the anoxic water column provided a significant phosphorus shuttle to Cambrian marine sediments. Future research should further explore whether changes in the amount or style of phosphorus delivery to the seafloor, the post-delivery fate within sediments, and/or skeletal evolution drove the observed decline through time of Cambrian phosphate mineral accumulation and exceptional fossil preservation.


Relationships between tectonics and magmatism in a transtensive/transform setting: An example from Faial Island (Azores, Portugal)

D. Trippanera et al., Dipartimento di Scienze, Università Roma Tre, Largo S.L. Murialdo, 1, 00146 Rome, Italy, daniele.trippanera@uniroma3.it. Posted online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/B30758.1.

The Azores archipelago, lying where the plate boundary between the Nubian and Eurasian plates meets the Mid -Atlantic Ridge, is a rare site to investigate relationships between magmatism and tectonics. The peculiar geological features of Faial, among the most active island of the archipelago (e.g. the 1998 earthquake and the 1957-58 Capelinhos eruption), make it the ideal island to focus the study. We analyzed the relationships between tectonics and magmatism, using different field and laboratory analyses. The morphology of the island is dominated by WNW-ESE major faults with oblique motion and NE-SW opening, probably related to the nearby transtensive Terceira Rift. Most of magmatic and tectonic features of the island are parallel to this system. Our field data allow to estimate the mean extension rate (between 3.4 and 8.2 mm/yr), which is similar or slightly larger than that of Terceira Rift. Part of the extension may be magma induced. We suggest that Faial, along with the nearby Pico Island, is a major locus of extension within the Azores and constitutes the offset, westward magmatic continuation of the Terceira Rift. The results of this study highlighted the importance of how extensional deformation along transtensive plate boundaries may influence volcanic activity.


Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff

Heather M. Wright, U.S. Geological Survey, MS 910, 345 Middlefield Road, Menlo Park, California 94025, USA hwright@usgs.gov; and Katharine V. Cashman. Posted online ahead of print on 13 Dec. 2013; http://dx.doi.org/10.1130/B30668.1.

Explosive volcanic eruptions can produce pyroclastic flows, gravity-driven currents of hot volcanic gas and pyroclastic rocks. When these flows stop moving, they form deposits of pyroclastic material (pyroclastic-flow deposits) by losing gas (decreasing pore space) and compacting (densifying). In some cases, the pyroclastic material is sufficiently hot and the thickness of the deposit is great enough that the particles anneal to each other, a process called "welding." Here, Heather Wright of the U.S. Geological Survey and colleagues show how the geometry of pore space controls compaction, gas escape, welding, and post-emplacement gas flow within pyroclastic-flow deposits. Importantly, they present new permeability measurements that demonstrate that lateral gas flow is favored over vertical gas flow if there is a sufficient pressure gradient driving lateral gas migration. Further, Wright and colleagues document how competing processes of gas loss, compaction, cooling, and H2O resorption into volcanic glass control the development of overpressure capable of creating secondary explosions from within pyroclastic-flow deposits.


Petrogenesis of Mount Rainier andesite: Magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

T.W. Sisson et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94925, USA, tsisson@usgs.gov. Posted online ahead of print on 13 Dec. 2013; http://dx.doi.org/10.1130/B30852.1.

Mount Rainier volcano dominates the skyline of the southern Puget Sound region (Washington, USA), towering over the peaks of the surrounding Cascade Range. Radiogenic (Sr, Nd, Pb), and stable (O) isotopes of its andesitic magmas identify modest incorporation (0% to 20%) of materials derived from old continental interior sources, present in southwest Washington as sandstones and shales of the Eocene Puget Group. Other rocks of the region are unsuitable as major assimilants. Most of Mount Rainier's andesites, however, result from a process of in situ differentiation and mixing wherein mantle-derived magmas stall beneath the volcano and crystallize rapidly to advanced degrees, yielding evolved interstitial liquids, some of which are entrained and mixed into subsequent magmas ascending through the intrusive plexus. Chemical effects of accessory mineral (zircon, apatite) growth in magmas too primitive for such minerals to be stable, unaccompanied by isotopic evidence for assimilation of foreign rocks, is the primary signature of the in situ differentiation and mixing process. Long-term magma flux appears to control differentiation style, with high-flux Mount Adams having chemical variations consistent with gradual and progressive growth and separation of crystals from melt, to low-flux Mount St. Helens, with chemical variations indicative of strongly in situ differentiation and mixing.


Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt

Kathryn A. Cutts et al., Centre for Crustal Petrology, Dept. of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa, cuttsk@sun.ac.za. Posted online ahead of print on 13 Dec. 2013; http://dx.doi.org/10.1130/B30807.1.

The Barberton Granite Greenstone Belt (BGGB) of South Africa is a well preserved Meso-Paleoarchean metamorphic supracrustal belt. Two hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a vertical tectonic process driven by sinking of greenstone layers and doming of the underlying granitoid crust. Metamorphic pressure-temperature (P-T) analysis with garnet Lu-Hf and monazite U-Pb geochronology is used to directly date the amphibolite-facies metamorphism within the southern BGGB. A garnet age of 3233 plus or minus 17 million years (Ma) and a monazite age of 3191 plus or minus 9 Ma were obtained for two samples that reached peak P-T conditions of 8.5 kbar and 640 degrees C with a clockwise P-T evolution. The duration of the ca. 3200 Ma event is estimated to be ~50 to 20 million years based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the southern BGGB indicate that metamorphism occurred in response to crustal thickening due to accretionary tectonics.


A pericratonic model for the Pearya terrane as an extension of the Franklinian margin of Laurentia, Canadian Arctic

Thomas Hadlari et al., Geological Survey of Canada, 3303 33rd Street NW, Calgary, Alberta T2L 2A7, Canada, thomas.hadlari@nrcan.gc.ca. Posted online ahead of print on 13 Dec. 2013; http://dx.doi.org/10.1130/B30843.1.

Pearya terrane lies on northernmost Ellesmere Island and has been interpreted as exotic to North America, prior to Silurian juxtaposition at approximately 430 Ma. This hypothesis was tested through detrital zircon analysis that showed a gradational change in provenance between the Pearya terrane and well-defined North American rocks, which is more consistent with a connection in the Cambrian, rather than a large separation. Thomas Hadlari of the Geological Survey of Canada and colleagues propose a return to an older tectonic model of the Pearya terrane wherein convergence recorded by the M'Clintock orogeny began along the Franklinian margin of North America in the mid-Ordovician, at approximately 475 Ma.


Volcanic rift margin model for the rift-to-drift setting of the late Neoproterozoic-early Cambrian eastern margin of Laurentia: Chilhowee Group of the Appalachian Blue Ridge

Joseph P. Smoot, U.S. Geological Survey, M.S. 926A, Reston, Virginia 20192, USA, jpsmoot@usgs.gov; and Scott Southworth. Posted online ahead of print on 13 Dec. 2013; http://dx.doi.org/10.1130/B30875.1.

Sedimentary and volcanic rocks of the Neoproterozoic-Cambrian Chilhowee Group in the Blue Ridge Province of eastern North America are re-interpreted as an analogue for a volcanic rifted margin of the Iapetus Ocean. New data indicates that an older sequence of sediments interbedded with sheets of basalt were deposited in a rapidly subsiding rift margin producing features equivalent to seaward-dipping reflectors (SDRs) noted along modern volcanic rifted margins. These deposits are unconformably overlain by a sequence of shelf deposits that were deposited in relatively low subsidence, analogous to modern shelf deposits that unconformably overlie SDRs. This model is applicable to other areas along the Iapetus Ocean margin and provides insight for viewing cores through modern ocean margins.

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