ABSTRACT

Various plate reconstructions have been proposed for break up of Rodinia and for the associated rifting history of western Laurentia, but models remain controversial. To address this problem, a research team from Idaho State University, Utah State University, and Weber State University is investigating Neoproterozoic (between 1,100 and 542 million years ago) deposits in Utah and southern Idaho in order to test different models for rift initiation and development: 1) protracted rifting at approximately 750 million years ago associated with an initial breakout of a magmatic plume; 2) continental separation between 720 and 670 million years ago; or 3) rift initiation near the beginning of the Cambrian (about 540 million years ago). The team will employ an integrated field, stratigraphic, structural, zircon provenance, and geochronologic study of Neoproterozoic strata. Specific approaches include: detailed sequence stratigraphic, sedimentologic, and petrologic studies of critical sections; geochemical trace-element analysis of associated volcanic rocks to determine tectono-magmatic setting; U-Pb dating of primary volcanic and detrital zircons using a combination of laser ablation-multicollector-inductively coupled plasma mass spectrometry, sensitive high-resolution ion microprobe, and thermal ion mass spectrometry methods; Ar-Ar dating of hornblende from basalt flows; Hf isotopic analysis of zircons; and modeling of regional subsidence histories. U-Pb dating and Hf isotopic analysis of detrital zircon populations will shed light on source areas and paleogeography, with ages of youngest reworked volcanic zircons providing important brackets for maximum depositional ages.

The Neoproterozoic marks some of the most profound geologic changes in Earth history, including break up of the supercontinent Rodinia, extreme climate changes, and rapid biologic diversification. Various plate reconstructions have been proposed for break up of Rodinia and for the associated rifting history of western Laurentia, but models remain controversial. Evidence important for the resolution of this controversy lies in the Neoproterozoic strata of Utah and southeastern Idaho. These strata form a critical, but incompletely understood part of the rifted western margin of Laurentia and subsequent Cordilleran orogenic belt. This research project will make important strides in understanding the history of these beds and lead to discrimination between opposing models for the early tectonic development of the western United States.

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