Boulder, Colo., USA - Rocks within plate boundary scale fault zones become fragmented and altered over the earthquake cycle. They both record and influence the earthquake process. In this new open-access study published in Lithosphere on 4 Feb., Virginia Toy and colleagues document fault rocks surrounding New Zealand's active Alpine Fault, which has very high probability of generating a magnitude 8 or greater earthquake in the near future.
Descriptions already suggest that the complex fault rock sequence results from slip at varying rates during multiple past earthquakes, and even sometimes during aseismic slip. They also characterize this fault before rupture; Toy and colleagues anticipate that repeat observations after the next event will provide a previously undescribed link between changes in fault rocks and the ground shaking response. They write that in the future this sort of data might allow realistic ground shaking predictions based on observations of other "dormant" faults.
Fault rock lithologies and architecture of the central Alpine Fault, New Zealand, revealed by DFDP-1 drilling
V.G. Toy et al., University of Otago, Dunedin, New Zealand. Published online ahead of print on 4 Feb. 2015; http://dx.
Other recently posted LITHOSPHERE articles are listed below.
Abstracts are online at http://lithosphere.
Short-term variations in slip rate and size of pre-historic earthquakes during the past 2000 years on the northern San Jacinto fault zone, a major plate boundary structure in southern California
N.W. Onderdonk et al., California State University, Long Beach, California, USA. Published online ahead of print on 3 Feb. 2015; http://dx.
This study by N.W. Onderdonk and colleagues shows that seismic activity and the rate of slip across one of the major faults in the San Andreas system has changed through time during the past 2,000 years. The northern San Jacinto fault zone has experienced periods of more rapid slip and more frequent and larger earthquakes (as it did from 600 to 200 years ago), and periods of slower slip and less frequent earthquakes (as it has for the past 200 years). This study also provides evidence that some earthquakes on the San Jacinto fault zone can jump across discontinuities between individual fault segments, which creates larger earthquakes.
Slip-partitioning along a continuously curved fault: Quaternary geologic controls on Denali fault system slip-partitioning, growth of the Alaska Range, and the tectonics of south-central Alaska
S.P. Bemis et al., University of Kentucky, Lexington, Kentucky, USA. Published online ahead of print on 3 Feb. 2015; http://dx.
While it is not uncommon to have a major strike-slip fault running through a mountain range, the Denali fault system of Alaska exhibits a unique case of where the strike-slip fault exhibits over 70 degrees of continuous curvature across south-central Alaska with the associated Alaska Range following the same arcuate trace. As a tectonically active fault (the source of a M7.9 earthquake in 2002) that is responding to subduction and collisional processes along the southern Alaska plate boundary, the continuous curvature of the Denali fault helps to illustrate how the strain is transferred from the plate boundary into central Alaska and how it contributes to the continued growth of the Alaska Range.
Upper mantle anisotropy beneath the south Indian shield: influenced by ancient and recent Earth processes
V.P. Kumar et al., Tata Institute of Fundamental Research, Mumbai, India. Published online ahead of print on 5 Jan. 2015; http://dx.
The south Indian shield is primarily a Precambrian terrain, with some parts affected by later tectonic activities at different time periods (e.g., 2.4 Ga, ca. 1.7 Ga, ca. 550 Ma, and ca. 85 Ma). In this study, V.P. Kumar and colleagues measure shear-wave splitting parameters from core refracted phases at 75 digital broadband seismic stations, almost uniformly spread, over the south Indian shield and covering varied geological terrains. This study suggests the observed anisotropy over the south Indian shield as a complex interplay of the architecture of an Archean craton and its subsequent lithospheric deformation in different geological domains due to deep Earth processes.
Seismological mapping of a geosuture in the Southern Granulite Province of India
S. Das Sharma et al., National Geophysical Research Institute, Council of Scientific & Industrial Research (CSIR), Hyderabad, India. Published online ahead of print on 22 Jan. 2015; http://dx.
The Southern Granulite Province of India is one of the most critical segments of the eastern Gondwana configuration. The breakup of east Gondwana started with Madagascar separating from India, and both moved away from Australia and Antarctica. Understanding the tectonic evolution and paleo-position of the Southern Granulite Province with respect to other continental counterparts therefore assumes global significance. Realizing that in a tectonically active terrain of the geological past, seismic images could be of great help; Das Sharma et al. at the CSIR-National Geophysical Research Institute launched a network of seismic stations to unravel the depth images of the Southern Granulite Province using passive seismology. Their results show that the composition and seismic structure of the crust across the eastern and western segments of the Madurai block along its meridional arm as well as across the Namakkal block are distinct. The depth image, which is first of its kind, together with other surface geological features and geochronological results could delineate presence of a definite geosuture in the study region. Seismological results of similar nature accruing from other high-grade terrains of Gondwanaland would enable evaluation of the status/role of continental-scale mega shear zones.
Mixing between enriched lithospheric mantle and crustal components in a short-lived subduction-related magma system, Dry Valleys area, Antarctica: Insights from U-Pb geochronology, Hf isotopes, and whole-rock geochemistry
G. Hagen-Peter et al., University of California, Santa Barbara, California, USA. Published online ahead of print on 5 Jan. 2015; http://dx.
Magmatic arcs that form at convergent plate boundaries may represent new additions of continental crust from mantle-derived melts, "distilling" of preexisting continental crust through re-melting, or some combination of these two processes. Studies of convergent margin magmatism may provide important information about the processes that control the growth and refining of continental crust. The Ross orogen of Antarctica is a belt of deformation, metamorphism, and magmatism that formed along an ancient convergent margin. A combination of uplift, erosion, and glacial processes has exposed an extensive mid-crustal magmatic system. Exceptional exposure in the iconic Dry Valleys area, the largest ice-free area on the continent, provides a "snapshot" of a magmatic system that developed deep in the crust approximately 500 million years ago. In this article, we explore the timescale over which this magmatic system developed and use a combination of geochemistry and hafnium isotopes to explore potential magma sources.
Orogenic paleofluid flow recorded by discordant detrital zircons in the Caledonian foreland basin of northern Greenland
G.A. Morris et al., Geological Survey of Sweden, Uppsala, Sweden. Published online ahead of print on 22 Jan. 2015; http://dx.
Large-scale, long-distance fluid flow is an important process in geology, being responsible for processes such as porosity enhancement in potential hydrocarbon reservoirs, and formation of several types of ore deposit. An important part of understanding fluid flow events is a knowledge of timing. Due to their transient nature, however, there is often no direct evidence of such events. This paper by G.A. Morris and colleagues takes data routinely ignored in detrital zircon studies, coupled with a novel statistical modeling approach, to constrain the timing of one such fluid event associated with the Caledonian Orogeny in Northern Greenland.
Lithological, rheological, and fluid infiltration control on 40Ar/39Ar ages in polydeformed rocks from the West Cycladic detachment system, Greece
É. Cossette et al, University of Ottawa, Ottawa, Ontario, Canada. Published online ahead of print on 3 Feb. 2015; http://dx.
Cossette and colleagues report captivating geochemical and structural data from deformed rocks of Greece that suggest the strength of a rock may control its ability to preserve signatures of older geologic events. In this study, evidence indicates calcite in marbles from a kilometer-scale fault zone accommodated the high strain allowing coexisting mica to remain unmodified and retain older, primary ages. In contrast, the strain is transferred to the mica in quartz-rich rocks, promoting new mica growth and younger mica ages. This study has implications for resolving the age of geologic events preserved in the rock record, as the composition of rock investigated may prejudice the result.