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LITHOSPHERE Highlights: Slab dynamics, the Troodos ophiolite, and the Jurassic Bonanza arc
16 November 2012
Geological Society of America, The (GSA)
The latest LITHOSPHERE articles to go online 26 October through 14 November include studies of slab dynamics both on Earth and on Mars; several discussions of the Troodos ophiolite, Cyprus, as well as other ophiolites; analysis and dating of the Jurassic Bonanza arc, Vancouver Island, Canada; fault system characterization in the central Bhutanese Himalaya; and sandstone dating in northern Russia.
Abstracts are online at http://lithosphere.gsapubs.org/content/early/recent. Representatives of the media may obtain complimentary copies of LITHOSPHERE articles by contacting Kea Giles at the address above.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published. Contact Kea Giles for additional information or assistance.
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Tectonic implications of non-parallel topographic and structural curvature in the higher elevations of an active collision zone, Taiwan
D. Mirakiam et al., University of Connecticut, Center for Integrative Geosciences, 354 Mansfield Rd. U-2045, Storrs, Connecticut 06269, USA. Posted online 26 Oct. 2012; http://dx.doi.org/10.1130/L232.1.
Almost all of the world's great mountain chains exhibit some degree of curvature when viewed on geologic and topographic maps. The processes driving the development of curvature in mountain belts have been of interest to geologists for nearly a century, beginning with the work of W.H. Hobbs in 1914. However, our understanding of curvature has been strongly influenced by studies in ancient mountain belts, where the tectonic processes that built the mountains have either significantly slowed or stopped. In this study, D. Mirakiam and colleagues analyze the Taiwan mountain belt in order to understand the initial formation of topographic curvature in a geologically young and tectonically active region. They present observations of the structure and geomorphology along a topographic break that runs counter to the regional topographic grain of the mountain belt. Their results indicate that the topographic grain of active mountain belts responds to variations in the thickness, composition, and geometry of crust in the subducting plate over relatively short time scales (hundreds of thousands of years) and that curvature in mountain belts is expressed by topographic relief more quickly than by regional-scale geologic features.
Metamorphic constraints on the character and displacement of the South Tibetan fault system, central Bhutanese Himalaya
F. Cooper et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Posted online 26 Oct. 2012; http://dx.doi.org/10.1130/L221.1.
The Himalayan mountain range is Earth's most dramatic example of continent to continent collision. As expected, the majority of faults within the range are contractional, but it also contains a family of extensional faults: the South Tibetan fault system (STFS). Extensional deformation is inconsistent with typical models of contractional mountain building, and the role that the STFS has played in the development of the Himalayas is hotly debated. One of the fundamental questions is how much normal-sense displacement has the system accommodated? In central Bhutan, heavy vegetation cover and rugged relief make direct structural mapping of the main STFS fault zone difficult. Instead, F. Cooper and colleagues looked for discontinuities in peak metamorphic temperature and structural style characteristic of normal fault displacement across the STFS. Peak temperatures in key samples were determined by measuring heat-induced structural reorganization of incorporated organic carbonaceous material. Their results show a drop in peak temperature of about 140 degrees Celsius that coincides with a significant change in observed outcrop-scale deformation 80 km south of the main STFS trace in the direction of fault motion, placing a minimum constraint on STFS displacement of 80 km. This is comparable to displacement estimates on the major thrust faults to the south, suggesting that normal faulting has played a fundamental role, perhaps as important as thrust faulting, in the evolution of the mountain range.
Age, construction, and exhumation of mid-crust of the Jurassic Bonanza arc, Vancouver Island, Canada
D. Canil et al., University of Victoria, School of Earth and Ocean Sciences, 3800 Finnerty Road, Victoria, British Columbia V8W 3P6, Canada http://dx.doi.org/10.1130/L225.1.
Dante Canil and colleagues present new U-Pb zircon ages for heterogeneous mafic-felsic rocks of the West Coast complex, a midcrustal plutonic component of exposed Jurassic arc-crustal section on Vancouver Island. They examine the timing of juvenile plutonic crust production and consanguinity of volcanism and plutonism in this arc. The midcrustal plutons were emplaced between 174 and 193 million years ago, contemporaneous with the upper-crustal (less than 10 km deep) plutonic component of the arc (Island plutonic suite). A 12-km thickness of plutonic arc crust was built as a series of sheets at a rate of about 0.003 cubic kilometers per year. Deformation and emplacement were contemporaneous, and there are no correlations among age, differentiation (peridotite to granite), or structural level of plutons in the arc. The age range and a weak eastward younging age polarity of the Jurassic arc section on Vancouver Island match that of the Talkeetna arc-crustal section in Alaska, suggesting that the two arcs are correlative and evolved by either forearc erosion above of an east-dipping slab, or slab rollback during west-dipping subduction.
Detrital zircon U-Pb geochronology of Mesozoic sandstones from the Lower Yana River, northern Russia
D. Harris et al., West Virginia University, Geology and Geography, 98 Beechurst Ave, 330 Brooks Hall, P.O. Box 6300, Morgantown, West Virginia 26506-6300, USA. Posted online 26 Oct. 2012; http://dx.doi.org/10.1130/L250.1.
The tectonic history of the large basins of the Arctic and of northern Russia remains poorly understood despite a growing body of research. Sandstone dating is quickly becoming a useful tool for determining depositional timing, which can then be used in tectonic reconstructions. D. Harris and colleagues date sandstones from northern Russia in order to determine timing of deposition and thus proximity to their source areas at specific intervals during the tectonic evolution of northern Russia. Their results add to a growing database of sandstone ages in the Arctic, support previous models regarding movement pathways for some of the large landmasses surrounding the Arctic, and suggest that a fault currently interpreted as a suture between two separate terranes is more likely not to be a suture after all.
Stalled slab dynamics
E. Burkett and M. Gurnis, Seismological Laboratory, California Institute of Technology, Pasadena, California 91001, USA. Posted online 26 Oct. 2012; http://dx.doi.org/10.1130/L249.1.
Seismic imaging of the Earth's upper mantle, particularly beneath Western North America where data coverage is unprecedented, has revealed complexities interpreted as structures ranging from plumes to lithospheric drips and subducted slab fragments. Varied interpretations of such structures motivate a better understanding of the geometric, compositional, rheologic, and dynamic conditions inside the upper mantle from which they might originate. In the dynamic numerical models presented here, E. Burkett and M. Gurnis focus in particular on testing conditions that affect how long a stalled subducted piece of a lithospheric plate may remain dangling in the upper mantle before breaking off and sinking, as motivated by interpretations of a structure beneath Idaho as a remnant of the previously subducted Farallon plate left dangling for more than 40 million years.
From basalts to boninites: The geodynamics of volcanic expression during induced subduction initiation
W. Leng et al., Seismological Laboratory, California Institute of Technology, Pasadena 91125, California, USA. Posted online 26 Oct. 2012; http://dx.doi.org/10.1130/L215.1.
Subduction initiation is induced at the interfaces of adjacent plates. This dynamic process is accompanied by different evolutionary pathways, including plate breakup, volcanic eruptions and topography changes. W. Leng and colleagues use numerical models and geochemical data to explain the causes for these different evolutionary pathways, and conclude that the physical properties of the lithosphere are the key point for the process. This result helps people to better understand the divergent history of various subduction zones.
A slab rollback model for the origin of the Tharsis Rise on Mars: Implications for initiation of local plate subduction and final unification of a kinematically linked global plate tectonic network on Earth
A. Yin, UCLA, Earth & Space Sciences, Los Angeles, California 90095-1567, USA. Posted online 14 Nov. 2012; http://dx.doi.org/10.1130/L195.1.
This paper presents geologic evidence from the Tharsis rise of Mars for local operation of plate tectonics processes. Specifically, the temporal and spatial distribution of volcanism, the size of volcanoes, and the formation of faults can all be explained by plate subduction. This process is similar to the formation of "the Ring of Fire" around the Pacific Ocean on Earth.
Earliest Cretaceous Pacificward offset of the Klamath Mountains salient, NW California-SW Oregon
W.G. Ernst, Stanford University, Geological & Environmental Sciences, Building 320, Room 118, Stanford, California 94305-2115, USA. Posted online 14 Nov. 2012; http://dx.doi.org/10.1130/L247.1.
A great volcanic-plutonic curvilinear arc (similar to the modern Circumpacific Andean, Alaskan-Aleutian, and Kamchatka-Kuril-Japan arcs) began forming along the margin of North America about 170 million years ago, reflecting eastward subduction of the Farallon oceanic lithosphere. The igneous arc stretched from at least the southern extent of Baja California through the Peninsular Ranges of southern California and the mighty Sierra Nevada Range northward into the Klamath Mountains of NW California and SW Oregon; this same arc probably continued on into the Blue Mountains of eastern Oregon. After about 30 million years of sustained arc volcanism and granitic intrusion, the Klamath Mountains province was gradually displaced westward about 200 km from the igneous belt, and while magmatic activity continued on until about 85-80 million years ago both north and south of the Klamath Mountains, such activity did not occur in the westward projecting salient. W.G. Ernst hypothesizes here that the Farallon oceanic plate consisted of old, thick oceanic lithosphere but at the latitude of NW California-SW Oregon, a relatively thin oceanic plate segment impinged against and beneath the Klamath province. Thicker lithosphere both north and south of the Klamaths collided with, and caused contraction in the Blue Mountains and Sierra Nevada, resulting in the relatively westward projection of the Klamaths relative to the rest of the once-continuous arc.
Using field data to constrain a numerical kinematic model for ridge-transform deformation in the Troodos ophiolite, Cyprus
C.P. Scott et al., Carleton College, Geology, One North College St., Northfield, Minnesota 55057, USA. Posted online 14 Nov. 2012; http://dx.doi.org/10.1130/L237.1.
Geoscientists study the boundaries between tectonic plates to improve the understanding of geologic hazards and of Earth in general. Oceanic plate boundaries, such as the Mid-Atlantic Ridge, comprise spreading segments, where new material is created, and fault segments, where two plates slide past one another. The Troodos ophiolite in Cyprus is a special example, where an ancient slice of oceanic plate material, including a spreading segment and a fault, is accessible on land. This paper by Chelsea P. Scott and colleagues uses features of rocks in the Troodos ophiolite to characterize the patterns of rock deformation arising in this preserved oceanic plate boundary.
Pre- to syn-glacial rift-related volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS constraints
J. Keeley et al., 619 N Arthur Ave, Pocatello, Idaho 83204, USA. Posted online 14 Nov. 2012; http://dx.doi.org/10.1130/L226.1.
This paper presents stratigraphic relationships and precise age constraints from the Pocatello Formation of southeast Idaho that bear on the 660- to 715-million-year-old Sturtian glacial phases within a Cryogenian snowball Earth. These U-Pb zircon ages constrain the timing of rifting of the supercontinent Rodinia. Joshua A. Keeley and colleagues employ several U-Pb zircon dating techniques and provide the first ~700-million-year-old zircon lutetium-hafnium isotopic data from western Laurentia. This work is part of a larger body of emerging research on Neoproterozoic stratigraphy in the western U.S. that has implications for Cordillera-wide and global paleogeographic reconstructions.
Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction?
S. Osozawa et al., Dept. of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan. Posted online 14 Nov. 2012; http://dx.doi.org/10.1130/L205.1.
Soichi Osozawa and colleagues present new trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, along with a large body of previously published data, that give new insight into the tectonic history of this storied ophiolite. The data also demonstrate the variability of suprasubduction-zone ophiolites, as well as differences between them and commonly used modern analogs. Post-analysis, Osozawa and colleagues propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, and that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction.
Cold and old: The rock record of subduction initiation beneath a continental margin, Calabria, southern Italy
D. Shimabukuro et al., University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720-4767, USA. Posted online 14 Nov. 2012; http://dx.doi.org/10.1130/L222.1.
David Shimabukuro and colleagues present data from northern Calabria, southern Italy, showing that subduction may have initiated beneath a continental margin east of the Corsica-Sardinia-Calabria block during the Eocene. Calabria lacks ophiolites (oceanic rocks) within the upper plate. Ophiolites are the strongest evidence for intraoceanic subduction initiation. The structurally higher nappes of the Calabrian subduction complex include continental crustal material, a feature that is also inconsistent with intraoceanic subduction initiation. In addition, Calabria lacks an amphibolite-facies metamorphic sole, regarded by some as a consequence of intraoceanic subduction initiation in young oceanic lithosphere. The age of blueschist metamorphism, compared with ages of arc volcanism in Sardinia, indicates preservation of rocks subducted and accreted during or shortly after subduction initiation, precluding significant subduction erosion. Peak subduction-related metamorphism reached blueschist facies, including rocks that apparently accreted early, during or shortly after subduction initiation. The protoliths of these rocks were about 80 m.y. old at the time of subduction, indicating that subduction began in old and cold lithosphere along a continental margin. Subduction initiation here suggests that the serpentinization of the upper mantle, observed in the Tethyan rocks of Calabria, may have been important in weakening the oceanic lithosphere at the continental margin. Alpine subduction may have initiated in a similar manner along other reaches of the orogen.