Low velocity and low electricalresistivity layers in the middle crust

N I Pavlenkova N I Pavlenkova

doi:10.4401/ag-3268

Abstract

Some Deep Seismic Sounding (DSS) revealed low velocity layers in the upper and middle crust of old platforms. The layers are often characterised by a lower electrical resistivity. It is not clear, however, how reliable the layers recognized from DSS data are, if they are regular or occasional events and how they correlate with other geophysical parameters. To answer these questions the experimental DSS data obtained in the Baltic and Ukranian shields by different institutions were reinterpreted by the author with unified methods. The shield areas are well studied using both the DSS and high-frequency magnetotelluric sounding (MTS) methods. As a result a marked velocity inversion (waveguide) was observed in a 10 to 20 km depth range in the majority of the DSS profiles. An increase in the electrical conductivity is typical for the waveguide. A comparison of the results with the data from other platform regions allow the conclusion that this low velocity and high electrical conductivity layer has a global significance. In the continental crust, the layer is characterised by changes in the reflectivity pattern, earthquakes number and changes in velocity pattern where the block structure is transformed into a subhorizontal layering. These structural features suggest that the layers separate brittle and weak parts of the crust. Usually they play the role of detachment zones at crustal block moving. A possible factor responsible for this phenomenon is an increase in porosity and in the salinity of the waveguide pore water compared with the upper crust. This suggestion is confirmed by the Kola superdeep borehole data. Porosity increasing in the middle crust is explained by the change in rock mechanical properties with depth, by fracturing porosity and by dilatancy effect, at a depth of 10-20 km.

Highlights

At first the low velocity layers were determined in the crust from seismological data in tectonic active regions
The analysis has shown that the observed wave pattern involved in the recognition of low velocity layers is not always clear
The low velocity and high electrical conductivity layer in the middle crust is characterised by some structural peculiarities which are important for understanding the origin of the layer

Summary

Introduction

At first the low velocity layers were determined in the crust from seismological data in tectonic active regions. The velocity inversion zones were determined in the upper and middle crust in many platform regions: in the Baltic Shield (Grad and Luosto, 1987), on the Russian plate (EUROBRIDGE Seismic Working Group, 1999) and in the Siberian craton (Pavlenkova et al, 2002). Often these layers are characterised by higher electrical conductivity (Kovtun et al, 1994; Beljavsky et al, 2001).

Low velocity layers in the crusts of the Baltic and Ukranian shields

Magnetotelluric data

Nature of the low velocity and high conductivity layer in the middle crust

Findings

Conclusions