Speaker:
Dr. Will Gosnold
Harold Hamm School of Geology and Geological Engineering
University of North Dakota
Title:
A LINK BETWEEN COINCIDENT HEAT FLOW, GRAVITY AND EROSIONAL ANOMALIES IN THE
MID-CONTINENT OF NORTH AMERICA
Abstract:
An 80,000 km2 area of anomalously high heat flow (70 mW m-2 to 140 mW m-2) in the mid-continent
coincides with a large negative isostatic gravity anomaly (-40 mgal), and the entire region has been uplifted
and eroded by at least 400 m during the past several million years. Numerical models of the thermal
structure of the crust from the surface to the Mohorovicic discontinuity were applied to test several
hypotheses that link the heat flow, gravity and uplift of the area.
1) The heat flow anomaly is due to advective heat transport in a gravity-driven, confined aquifer system
that formed when the Black Hills were uplifted during the Laramide Orogeny 50 Ma. The duration of the
thermal anomaly has caused the lower crust to warm sufficiently to induce about 2 km of downward
displacement of the gabbro-to-garnet granulite phase boundary. The reduced density in the lower crust
causes the negative gravity anomaly and vertical stresses generated by the buoyancy of the lower crust
caused isostatic uplift and subsequent erosion of the Upper Cretaceous and Tertiary strata.
Numerical
models of advective heat transport in the aquifer system provide a test of the heat flow element of this
hypothesis. Analysis of the thickness of the crust using receiver functions from the transportable array in
the Earthscope project provides a test of the gravity and uplift elements of this hypothesis.
2) Radioactive heat production in a large granitic pluton having a density lower than the enclosing
crystalline basement generates both the heat flow and gravity anomalies.
Radioactive heat production and
rock density can be coupled by differentiation of the expressions for V and T as follows:
1
Combining the expressions yields
mass term to density yields
′
1
4
converting A’ to volumetric heat production and the
. The size and depth extent of the gravity anomaly can be estimated
using forward modeling methods. Thus, estimates for heat production can be obtained from the density
estimates.
Calculation of the thermal structure of the crust provides a test of this hypothesis.