Earthquakes and Plutons in the Midcontinent - Evidence From the Bloomfield Pluton, New Madrid Rift Complex

A three-dimensional gravity and magnetic model of the Bloomfield Pluton near New Madrid, Missouri, has been generated that provides an excellent match to the observed gravity and magnetic anomaly maps. Although this modeling is inherently non-unique, the steep gradients on the flanks of the anomalies constrain the causative body to a relatively shallow depth (≈2 km) and define the outline of the steep-sided (near-vertical) pluton. Comparison of the configuration of the Bloomfield Pluton with concentrated microearthquake seismicity in part of the New Madrid Seismic Zone indicates that earthquakes in the northern part of the zone occur primarily on the southeastern side of the pluton at about 10 km from the edge of the body. Based on the location of hypocenters and earthquake focal mechanisms, there is no evidence that stress concentration associated with the mass excess of the pluton is the cause of earthquakes near the pluton. The seismicity may be along zones of weakness independent of the pluton. Among other explanations investigated to explain the constant distance of earthquakes from the pluton, we favor the possibility of a zone of rigidity contrast caused by the thermal metamorphism of the country rock associated with the pluton intrusion. We have estimated increased temperatures of the country rock using an instantaneous intrusion model. Maximum anomalous country rock temperatures of greater than 100°C dominated the region within 10 km of the pluton edge. This ‘thermally metamorphosed’ aureole may provide sufficient homogeneity in the upper crust to localize anomalous stresses away from the margin and along the most favorable zones of weakness, thus influencing the locations of earthquakes.