The isostatic compensation of the Colorado Plateau-Basin and Range transition zone in central Utah

Abstract

The western margin of the Colorado Plateau in central Utah is similar in tectonic style to the adjacent Basin and Range Province and appears to be a transition between the two provinces. It is important to understand the isostatic compensation of the topography, the source of the associated gravity anomalies, and the regional mechanical structure since these variations can be linked to the mechanism controlling the uplift of the Colorado Plateau and adjacent regions. In this paper the isostatic response function (admittance function) technique is used to study the isostatic compensation of the Colorado Plateau-Basin and Range transition zone in central Utah and determine the applicability of this technique in the study of small geologically complex areas. The observed admittance, determined from the topography and free air gravity anomaly, had small errors for the long wavelength estimates, small (nearly zero) phase angles, and high coherence. This suggests that most of the energy in the power spectrum of the free air anomaly is correlated with the gravity fields of the topography and its compensation. The residual gravity anomaly based on the observed admittance is small except in the region of the Wasatch Front where the presence of a fairly symmetric high-low-high trend suggests compensated crustal or lithospheric structures not correlated to the topography. The observed admittance function is compared to three simple isostatic models: a surface loaded continuous elastic plate, a subsurface loaded continuous elastic plate, and a Pratt model of local compensation. It was found that the best fit elastic plate models defined similar results in spite of differing assumptions as to the form of the loading. These models both had best fit effective flexural rigidities of 10 20 Nm and Moho depths of approximately 35 km. The calculated residual gravity anomalies based on these models are nearly identical to the residual anomaly of the Pratt model with a compensation depth of 65 km. The residual anomaly of all three models reproduced the trends and magnitude of the residual anomaly based on the observed admittance. It is suggested that this local compensation is due to subsurface laterally varying thermal density anomalies. These results differ from previous isostatic rebound studies of ancient Lake Bonneville, which give larger magnitude effective flexural rigidities. These differences can possibly be attributed to differing time scales of loading between Lake Bonneville and the long term thermal loading of the Basin and Range extension and associated volcanism or the overall simplicity of the considered models. Care should be exercised in the interpretation of admittance functions determined across transition zones between adjacent tectonic provinces. Depths of compensation and flexural rigidities can represent averaged values. All additional geophysical and geologic information should be considered in the interpretations.