METAMORPHIC CONDITIONS OF THE LITTLE WILLOW COMPLEX, UTAH AND THE IMPLICATIONS FOR UTAH'S PROTEROZOIC PALEOGEOGRAPHY

The Little Willow complex (LWC) differs compositionally, in its P-T history and age from other basement complexes exposed in the Wasatch Range. It consists of amphibolite, quartzofeldspathic gneiss, and mica schist. From detailed geologic mapping and structural analysis the LWC exhibits characteristics of a metamorphosed forearc basin perhaps associated with an arc represented by the Santaquin Complex (SC) to the south that collided with a continental margin represented by the Farmington Canyon Complex (FCC) to the north.


Thermobarometric analysis of various LWC units yield peak metamorphic conditions of 520-610°C and 2-3 kbar. These conditions are significantly lower than those of the SC and FCC, which may have been more deeply eroded when they were exposed on the flanks of the Uinta graben during the Late Proterozoic. The LWC forms the basement of Uinta graben, which was buried under several thousand meters of Late Proterozoic basin fill. This implies that the LWC did not experience as much erosion during the Late Proterozoic and may preserve higher structural levels that are less metamorphosed. Exhumation of the LWC was initiated during Sevier thrusting and Tertiary inversion of the Uinta graben to form the Uinta/Cottonwood Arch. However, it is not overlain by Cretaceous and Tertiary deposits, such as the SC and FCC. This additional erosional phase of the SC and FCC may also account for the exposure of deeper structural levels and higher grade metamorphic units than the LWC. Oligocene intrusion of the Little Cottonwood stock (LCS) and Miocene to Holocene extension along the Wasatch Fault exhumed the LWC only within the past few million years.

From detailed structural analysis, six deformational events have been identified and interpreted: 1) vertical compression, 2) Precambrian regional metamorphism, 3) east-west shortening from the Sevier Orogeny, 4) north-northeast shortening from the Laramide Orogeny, 5) uplift and metamorphism from intrusion of the LCS, and 6) exhumation along the Wasatch Fault.

A post-kinematic metamorphic overprint is observed in parts of the LWC, which may be associated with intrusion of the Oligocene LCS. This thermal overprint may also explain our failed attempts thus far to determine the age of the LWC. We are currently analyzing U-Pb in zircons to constrain the age of peak metamorphism.