Boulder, CO, USA - The premier issue of The Geological Society of America's newest journal, Lithosphere, is now available. Papers cover Holocene surface-rupture earthquakes on the rocky coast of New Zealand; Earth's fluid factory and the evolution of Earth's surface and near-surface environment; factors involved in erosion of the Wasatch Mountains, Utah, USA; backward stacking of submarine channel fans and fault activity in Japan; and the Ouachita basin as a failed rift contained within the North American craton.
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Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the uplifting of beach ridges on a rocky coast
Timothy A. Little et al., School of Geography, Environment & Earth Sciences, Cotton Building, Room 411, Victoria University of Wellington, P.O. Box 600, Wellington 6040, New Zealand. Pages 4.
The Holocene beach ridges at Turakirae Head, New Zealand, are remarkable because the fault that caused their uplift is accessible to paleoseismic trenching. Little et al. used 14-C samples from eight trenches to identify five surface-rupturing earthquakes that occurred over the past five-thousand years. The paleoearthquake record includes two more events than were recorded by the uplift and stranding of beach ridges at Turakirae Head. They concluded that the beach ridges at Turakirae Head may provide an incomplete record of paleoearthquakes on oblique-reverse faults. Variations in wave climate or sediment supply (or interseismic subsidence) may also influence the number of beach ridges preserved by governing the morphology of the storm berm and controlling its extent of landward retreat. Such retreat may cause a berm to overwhelm, or amalgamate with, the next-highest beach ridge, resulting in the omission of one ridge, as probably happened at Turakirae Head at least once. Their 14-C data support the view that a widespread post-Last Glacial Maximum aggradational terrace in southern North Island, New Zealand, was abandoned approximately 12,000 years ago. From this, they infer that the Wairarapa fault has a late Quaternary slip rate of 11 plus or minus 3 mm per year.
A fluid factory in solid Earth
M. Santosh et al., Department of Natural Environmental Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan. Pages 29.
Earth is a giant fluid factory, according to Santosh and coworkers, researchers from Japan. The authors propose a new model for the nature and distribution of fluids from the core to Earth's based on modern concepts of plate tectonics and argue that fluids within Earth play a critical role in constraining both the interior Earth dynamics and the evolution of the surface and near-surface environment. They are of the opinion that the global material circulation in our planet is controlled by a combination of processes that operate on the surface of Earth (plate tectonics), in the intermediate depths (plume tectonics), and at the core-mantle boundary region (anti-plate tectonics). They envisage that hot, rising plumes act as giant pipes connecting the deeper portions of Earth with the surface. Santosh et al. propose that free fluid circulation within Earth occurs only within restricted zones, such as along regions were tectonic plates are subducted. The water subducted along the plate boundaries reaches up to the 410-660 km boundary termed as the mantle transition zone. Water stored in dense hydrous silicates in this region constitutes a huge water tank with a capacity of nearly five times the volume of the water in modern oceans. The authors also propose that for the major part of Earth's history, the fluid transport was mostly one way -- from the outer core to the surface. The return flow of water probably started 750 million years ago and the penetration of water to deep levels through plate subduction provided adequate lubrication to transport some of the deeply subducted rocks back to the surface in the younger Earth. Such rocks returned from depth are identified from their ultrahigh-pressure mineral assemblages.
Spatial and temporal variations in denudation of the Wasatch Mountains, Utah, USA
Greg M. Stock et al., Yosemite National Park, Division of Resources Management and Science, 5083 Foresta Road, P.O. Box 700, El Portal, CA 95318, USA. Pages 34
Stock and colleagues investigated erosion of the Wasatch Mountains of Utah over the past 10,000 years by measuring concentrations of 10-Be in stream sediment (higher concentrations equate to lower erosion rates and vice versa). The erosion rates, which were determined from 14 drainage basins between the cities of Provo and Ogden, were compared with previously published data on uplift and erosion of the range. Streams draining the range front show uniform erosion rates of about 0.1 mm/yr, while steeper drainage basins in the interior of the range show faster and more variable rates, from 0.2 to 0.8 mm/yr. Stock and colleagues attribute the larger variation in the core of the range to landslides, which likely are occurring in response to erosion by glaciers during the Last Glacial Maximum. The mean erosion rate for all studied drainage basins of 0.2 mm/yr is generally consistent with erosion rates determined over time scales ranging from thousands to millions of years. This suggests that erosion of the Wasatch Mountains has been roughly steady over the past 5 million years. Although 10-Be-based drainage basin erosion rates are sensitive to localized geomorphic processes such as landslides, overall they appear to reflect the larger tectonic forces that drive long-term erosion of the Wasatch Mountains.
Backward stacking of submarine channel-fan successions controlled by strike-slip faulting: The Izumi Group (Cretaceous), southwest Japan
Atsushi Noda, Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan; and Seiichi Toshimitsu. Pages 41.
Deposits in sedimentary basins sometimes have cyclic stratigraphy, which is commonly induced by fluctuation of sea level, climate, sediment supply, or subsidence of the basin. Basin-filling processes in especially fault-bounded basins, such as strike-slip basins, are affected by the regional tectonics. It is, however, difficult to quantify interactions between tectonics and sediment dynamics in such fault-related basins, and thus causes of the stratigraphic cyclicity remain poorly understood. In order to investigate formation of cyclic stratigraphy, we focused on the Izumi Group (Upper Cretaceous, southwest Japan) deposited in a strike-slip basin. Sedimentary facies analysis revealed that the strata in the main part were point-sourced submarine channel-fan successions composed of channel-fill conglomerates, turbiditic lobes, and basin-floor mudstones. Two units of the submarine channel-fan successions are stacked with about 10 km of backward shift. Each unit has a cycle of rapid upward coarsening and thickening in the lower part and gradual upward fining and thinning in the upper part; it is estimated at 5 x 105 years for the offset. Although many processes can control the stratigraphic architecture, the cyclicity observed in this area is associated with the depocenter migration, suggesting the primary control of the fault movement on the stratigraphy. Numerical simulations indicated that episodic changes in fault-slip rate or sediment-supply rate could control the stratigraphic cyclicity. In this paper, Noda et al. propose a model that the cyclicity is ascribed to temporal variations of fault activity controlling accommodation generation, sediment supply, and relative sea level; these variations could generate cyclic stratigraphy associated with depocenter migration in strike-slip basins.
Arkansas crustal xenoliths: Implications for basement rocks of the northern Gulf Coast, USA
Dennis Dunn, 5802 Republic of Texas Blvd., Austin, TX 78735, USA. Pages 60.
Fragments of crustal rocks found within the diamond-bearing volcanics in southwestern Arkansas indicate the age and nature of the North American basement rocks at depth. Two types of deep-seated rocks were dominant: amphibolite and altered granites. K-Ar isotopic age dating revealed the amphibolite to be ~1,410 million years old. This data, when used with published geophysical and drill data, allow the evaluation of two tectonic models for formation of the Ouachita Mountains. One model suggests that the Ouachita basin is a failed rift contained within the North American craton. The second model suggests that the Ouachita basin was located at the margin of the North American craton in the early Paleozoic, with accretion of a late Paleozoic volcanic arc terrane. Existing geological and geophysical data is best explained by the first model with the Ouachita basin consisting of a stretched North American craton associated with a Paleozoic rift system that developed separately from the Appalachian-style continental margin.
View the complete table of contents for the current issue of LITHOSPHERE, or review abstracts for these articles at http://lithosphere.gsapubs.org/current.dtl.
Journal
Lithosphere