Abstract Title: Seismic bright spots, deep fluids and EARTHSCOPE

Abstract Author(s): Brown, Larry (Cornell University)  

Abstract: One of the most dramatic observations in seismic exploration of the 
continental lithosphere over the last 25 years has been the discovery of 
seismic reflection Òbright spotsÓ in the middle to deep crust. Most have been 
interpreted as large magma sills, the ÒbrightestÓ being still molten (ÒhotÓ) 
while others have long since frozen (ÒcoldÓ). These seismically mapped sills 
can have lateral dimensions of over 100 km, and range in thickness from tens of 
meters to possibly several kilometers. The brightest (presumably fluid) are 
most commonly observed near the brittle-ductile transition (~350¡C isotherm) at 
midcrustal depths (15-20 km). Whether they consist of water or molten rock, 
these features can profoundly impact crustal rheology, metamorphism, igneous 
evolution, seismic and volcanic risk assessment, and mineral resources. 
EARTHSCOPE, particularly USArray, offers and unparalleled opportunity to 
systematically identify and map seismic bright spots beneath the western U.S., 
to discriminate ÒhotÓ (magma) from ÒcoldÓ (solidified) sills, and to evaluate 
their physical and chemical implications for crustal evolution. ÒBigfootÓ is an 
obvious reconnaissance tool for searching for such bright spots, with the 
flexible array providing the means to subsequently map their extent, physical 
properties (e.g. fluid/), and tectonic relationships (feeders to surface 
volcanics). The geodetic tools of the PBO and INSAR offer complementary means 
of searching for surface deformation that may be associated with deep bright 
spots, either those identified by seismic studies or those, which should be 
targeted for seismic surveys. The Socorro bright spot in particular is the 
archetype of such features and the obvious choice for a calibration program of 
seismic, geodetic and magnetotelluric studies. The results of this program 
should impact virtually all aspects of lithospheric evolution and thus 
constitute a valuable general resource for EARTHSCOPE. Sills serve as primary 
evidence (past and present) of thermal modification of the lithosphere, as 
markers for unraveling subsequent tectonic deformation and as guides to stress 
and rheological conditions at depth. Understanding the links between sills as 
ore deposits could lead to new approaches to mineral exploration, and a basic 
explanation for the very different regional abundances of environmentally 
sensitive elements.The techniques developed to detect and evaluate intracrustal 
bright spots should hone USArrayÕs general capabilities to resolve more subtle 
intra-lithospheric complexity.