Astronomy and Astrophysics – Astronomy
Scientific paper
May 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998geoji.133..419s&link_type=abstract
Geophysical Journal International, Volume 133, Issue 2, pp. 419-434.
Astronomy and Astrophysics
Astronomy
8
Deep Seismic Reflection, Electrical Conductivity, Lower Crust, Porosity
Scientific paper
It has been postulated that deep crustal zones of both enhanced electrical conductivity and seismic layering may be ascribed a single physical explanation, and more specifically that both result from the presence of free, interconnected fluids around the brittle-ductile transition zone. We analyse three complementary geophysical data sets-broadband (10-2-104s) magnetotelluric (MT) and P-wave and S-wave deep seismic reflection-from coincident profiles traversing the Weardale granite in northeast England and demonstrate that the lowermost crust in this tectonically stable region contains insufficient free fluid to explain the high VP /VS ratios associated with very bright, subhorizontal lamellae seen within the lower crust (below 22km depth) beneath the granite. Temperature-depth profiles are presented, and the apparent global correlation between the depths to the tops of zones of enhanced conductivity, seismic reflectivity and inferred brittle-ductile temperatures of 300-450°C is also re-examined. The seismic layering demonstrably corresponds to higher temperatures than those usually attributed to the brittle-ductile transition zone. A priori information of different scalelengths is applied to the problem of static-shift correction of the MT data. Gravity models constrain the depth extent of the granite batholith, whilst pre-existing DC measurements and a borehole drilled into the granite provide information concerning the resistivity structure of the uppermost 500m. A 3-D model study investigates the significance of possible 3-D induction effects on the MT data, as implied following application of decomposition. Our results challenge the widespread and little-tested assumption that deep crustal conductors and seismic layering are physically interrelated and occur at the same depth. With static shift and 3-D effects in the MT data accounted for, we demonstrate that below the Weardale granite the onset of any enhanced conductivity is shallower than the top of the seismic layering, and furthermore that the observed deep crustal conductance of less than 200S is significantly less than that to be expected (>2000S) if the high VP /VS ratios calculated from the seismic data are to be explained entirely by fluids. The total conductance of the mid to lower crustal region combined is too low to allow for the significant quantities of free fluids required to explain lower crustal seismic layering through this medium without challenging current hypotheses concerning pore geometries and interconnectivity.
Simpson
Warner
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