Measuring extensional paleorifting in the Scottsville Basin

The Scottsville Basin is a small Mesozoic rift basin formed by extensional faulting in eastern North America. This basin is a very small geologic structure located in central Virginia between the Blue Ridge and Piedmont provinces.  It is bounded by a normal fault on its western border and a shallow normal fault on the eastern border; this segmented half-graben structure is generally indicative of extensional rifting. The Scottsville basin is roughly 130 km², with strata across the surface increasing in thickness from west to east. This is likely indicative of syndepositional rifting, which can be seen in Fig. 1 below. The half-grabens that make up the deeper structure of the basin pass through isothermal closure temperatures that can be measured using isotopic ratios.

 

syndep rifting

Fig. 1: Dipping strata in a normal fault, indicating simultaneous rifting and erosional deposition.

 

Of the Mesozoic basins located in North America, the Scottsville Basin has gotten very little attention. The goal of this project is to quantify the formation rate of this basin using Uranium/Thorium-Helium ratios within sampled rocks. This method is used for rocks with relatively low closure temperatures, which will allow the surficial rifting to be measured.  The relative slip and uplift of the hanging wall and foot wall, respectively, can be calculated by measuring the distance between sample points and time elapsed since passage of the isothermal closure temperature. Given the measured isotopic ratio and the known specific closure temperature of the measured minerals, an extension rate can be calculated, and thus the rifting rate of the Scottsville Basin.

 

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