Thursday, July 29, 2010

New core complexes found with Shuttle radar

AZGS Senior Geologist Jon Spencer published the cover article in the August issue of GSA Today, on metapmorphic core complexes and detachment faults in Indonesia, interpreted from Shuttle radar.

Here is a short history of Jon's work on core complexes and the new discoveries just published:

In 1977 Tucson was host to a Geological Society of America Penrose conference on “metamorphic core complexes”. This was a new name to the tectonics community, and a new feature that was not well understood at the time. These tectonic features are characterized by gently to moderately dipping shear zones that formed at sufficient temperature for quartz to shear plastically. The resulting sheared rocks, known as “mylonites”, are on display in the Santa Catalina Mountains north of Tucson and South Mountains south of Phoenix. It is now known that they were produced when the sheared rocks were uplifted from great depth in the Earth (~10-15 km) due to displacement below moderately to gently dipping normal faults known as “detachment” faults.

AZGS geologist Jon Spencer began working at the Arizona Bureau of Geology and Mineral Technology (now the Arizona Geological Survey) in 1982, and with co-worker Steve Reynolds did an extensive study of the Buckskin-Harcuvar core complex in western Arizona. This led to production of Arizona Geological Survey Bulletin 198, published in 1989, and described in detail what turned out to be the largest terrestrial core complex on Earth. First identified in western North America, core complexes are now known in many parts of the world, including Spain, Switzerland, Norway, Greece, Turkey, Iran, Tibet, northern China, Mongolia, Thailand, Vietnam, New Guinea, and New Zealand. Furthermore, large numbers of them have been identified on the sea floor associated with sea-floor spreading at moderate rates, and a feature that may be the largest core complex in the solar system has been identified on Venus due to its distinctive grooved form.

In 2000 the space shuttle Endeavour surveyed much of the Earth with synthetic-aperture radar, yielding a detailed digital elevation model of most land on Earth. The resulting data have been placed on-line by Columbia University, with tools to display and study Earth’s topography ( Spencer studied this data set (for fun he says, and at home during weekends) and identified two previously unidentified core complexes on the island of Sulawesi in Indonesia. One of these was described in an article in recently published AGS Digest 22 . The other, and two previously identified core complexes in Tibet and New Guinea, are described and analyzed in the August issue of GSA Today.

Ref: Structural analysis of three extensional detachment faults with data from the 2000 Space-Shuttle Radar Topography Mission, Jon E. Spencer, GSA Today, August, 2010, pp4-10, DOI: 10.1130/GSATG59A.1

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