October Geology and GSA Today media ...(rmise that range front faulting began th...)

Date:9/25/2007

rmise that range front faulting began throughout the Sierra Nevada 10 million years ago as well, and that "delamination" was initiated then, or earlier. A later episode of volcanic eruption and range uplift, beginning 3.5 million years ago, may have been triggered by a later accelerated phase of delamination, range-front faulting, or both.

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Dual role of seawater and hydrothermal fluids in early Archean chert formation: Evidence from silicon isotopes
Sander H.J.M. van den Boorn, Utrecht University, Earth Sciences, Budpastlaan 4, Utrecht 3584 CD, Netherlands; et al. pages 939-942.

Our knowledge of Earths earliest surface heavily relies on information from scarce sediments that have survived since their formation billions of years ago. Rock sequences in the Archean (the period between 4.0 and 2.5 billion years ago) are unusually rich in chert. This rock type, with very high silica content, is a key element in reconstructing the conditions of the early Earth and of the environments where the earliest forms of life developed. Not only are the oldest fossil remains intimately associated with cherts, the properties of cherts have also been used as evidence that Archean ocean water had higher temperatures than later in the geological history.

However, how these abundant cherts formed has remained controversial. Van der Boorn et al. tackle the problem from a new angle by using silicon isotopes to unravel their origin. Their data from 3.5- to 3.0-billion-year-old chert deposits from the Pilbara Craton (Western Australia) demonstrate that silicon isotopes (together with other geological and geochemical data) are diagnostic for the origin of chert. Van der Boorn et al. argue that Archean seawater is the dominant source of silica for one group of cherts and that hydrothermal fluids venting at the seafloor produced a different set of chert deposits. These findings reconcile competing hypotheses, and have important implications for t
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Contact: Ann Cairns acairns@geosociety.org 303-357-1056 Geological Society of America Source:Eurekalert

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