THE GRAVITATIONAL POTENTIAL ENERGY CONSTRUCT: STRENGTHS AND WEAKNESSES IN UNDERSTANDING OROGENESIS

Gravity is the fundamental driver of deformation on the Earth, but the rigidity of plates allows transfer of gravitational force from one area to another, a concept well captured by the essence of plate tectonics. However in orogenesis local variations in body force potential can be important or even dominant. The relatively simple construction of the gravitational potential energy (GPE, the integral of density times gravitational acceleration times height above a datum) has allowed examination of the role of local variations in lithospheric structure in controlling the location and magnitude of deformation. GPE can be directly connected to strain rate under some simple assumptions, and it is readily calculated for hypothetical scenarios. It can be derived from seismological studies, from analysis of the geoid, and petrologically developed lithospheric profiles. With the advent of continent-scale seismological networks, GPE can be mapped with uniform assumptions across the U.S. While the GPE construct can provide insight into the balance between far-field and local stresses, it can only be interpreted with understanding of its limitations. GPE is only meaningfully related to strain rate over length scales where flexural rigidity of the lithosphere is unimportant. GPE is tied to lithospheric deformation rates; vertical variations in deformation are not easily considered in this framework. Most GPE estimates sit atop other assumptions; for instance, seismologically derived estimates require estimating density from seismic wave speeds, a relationship that is not single valued, while geoid-based estimates require removal of signals from the deeper Earth. Examples of how GPE calculations can help to form and test hypotheses for the drivers of deformation will illustrate the utility of this construct.