[S26-1] Exploring connections between heat flow and tectonics I

[S26-1-02] Magma underplating at crust mantle interphase as the source of anomalous heat flow in passive continental margins

^○Valiya Hamza, Fabio Vieira (National Observatory, Rio de Janeiro, Brazil)

Analysis of global heat flow data points to the existence of narrow belts of relatively high heat flow (> 60mW/m2) along several segments of the passive continental margins. These belts are confined between zones of low to normal heat flow, present in adjacent stable segments of continental and oceanic crust. Available data on the width of the heat flow anomaly (range of 100 to 300km) precludes thermal refraction effects. Hence, we consider release of heat by sources located at the base of transitional crust as the likely mechanism. Model studies were carried out to examine the nature of magma underplating. The results indicate that an underplating episode at a depth of 20km, occurring 2Ma back in time, is capable of accounting for the observed heat flow anomaly. It is argued that weak “tectonic bonding" between continental and oceanic segments of the lithosphere and ductile nature of mantle at the contact zones allow lateral flow of magma at the base of the crust. This mechanism seems to be an end-member case of differential stretching, in which extension is limited mostly to the sub-crustal layer. However, these need not necessarily produce intrusive or volcanic activity in overlying strata. We advent the hypothesis that passive flow of mantle material at the base of transitional crust is responsible for much of the micro-seismic activity at shallow depths along continental margins. Results of model studies also reveal that a combination of thermal maturity indices and palinspastic reconstructions of sedimentary strata may be used to constrain estimates of the duration of underplating events. Analysis of data for continental margin of Brazil point to the occurrence of two successive underplating events: a recent short-period one to account for the observed heat flow anomaly and an older long-period one for boosting thermal maturity indices to levels compatible with the occurrence of extensive oil fields.