The period between 1.78 and 1.63 billion years ago (Ga) was a time of important southward growth of the North American continent. The rate of growth within this 150-million-year period far exceeds Phanerozoic (542 million years ago to the present) crustal growth rates. As a result of recent work in southern Wyoming and northern Colorado, in a region known as SW Laurentia, the PIs have identified a deformational, magmatic, and thermal event at 1.62-1.58 Ga, which is surprising because the region was thought to be tectonically quiescent. The researchers will combine detailed kinematic histories of shear zones in northern and central Colorado, with precise, texturally-linked dates on deformation, and well-dated metamorphic pressure-temperature paths to evaluate two end-member hypotheses where the enigmatic tectonic 1.6 Ga event may represent either (1) intracratonic deformation of the newly accreted continental crust with stresses transmitted far inboard of the active plate margin or (2) deformation associated with delayed subduction of a long-lived remnant ocean basin. The 1.6 Ga tectonism may reflect the closing of this basin and the final accretion at the southern edge of the North American continent.
The recognition of 1.62 to 1.58 Ga deformation, magmatism, and metamorphism as much as 1200 km away from the continent-ocean plate boundary has important implications on several levels. First, the event may have Phanerozoic analogues in the Laramide (80 million years ago) orogeny or the modern Tibetan Plateau. If correct, this hypothesis would suggest rapid stabilization of, and efficient stress transfer within, newly accreted crust of this age. The present midcrustal level of exposure of rocks affected by the 1.6 Ga event affords an opportunity to investigate the crustal stress transfer in the strongest part of the crust. There are also implications for modern earthquake hazards well away from active plate boundaries. Alternatively, if the event reflects closure of a small ocean basin, then the crustal growth process was not a simple north to south progression and indeed was not completed in some of the northernmost regions until after 1.6 Ga. This might also explain some of the anomalous features on recent deep seismic surveys of the region. In either case, results from this study are likely to result in substantial revision of tectonic models for Paleoproterozoic crustal growth, assembly, and stabilization in SW Laurentia.
The Mazatzal Orogeny, ~1600 million years (m.y.) before present was one of the major events in the growth history of the North American continent. It is interpreted to have involved the addition, through continental collision, of a large swath of new continental crust on the southern margin of the continent. This research investigated the nature and character of tectonic activity in Colorado in the age range 1650-1600 m.y. This tectonism is important because the active continental margin was 100s of kilometers to the south at this time and thus, the tectonism reflects weakness or heterogeneity in the previously assembled continent. Research at the University of Massachusetts under this award was focused on providing new age constraints on tectonic events in the Precambrian basement of Colorado. New results from the Park Range, CO, document that some regions experienced two major phases of tectonism, with ages of approximately 1750 and 1720 m.y. with little evidence of the Mazatzal events. Other regions experienced two additional phases of tectonism (1650, 1600 m.y.). The first phase is directly within the known time range of the Mazatzal orogeny. The second phase however, is outside the range and suggests that Mazatzal-related tectonism persisted, at least in Colorado, well beyond the known range. This event narrows the window known as the Proterozoic tectonic lull (1650-1450 m.y.) in which little tectonic activity is known in any part of North America. Further, the results support the hypothesis that a small ocean basin (remnant basin) may have remained in northern Colorado after the main pre-Mazatzal ocean closed, and may not have closed until the final event at 1600 m.y. The research also served as a test study and development vehicle for new monazite dating techniques at the University of Massachusetts. Geologists have become accomplished at determining the age of many types of rocks. Determining the age of tectonic events is still a challenge. The mineral monazite provides one opportunity, but it is necessary to analyze very small domains (down to several micrometers). A new electron microprobe has been developed at the University of Massachusetts for this purpose. During the course of this research a new procedure for background measurement was developed and tested with great success. The new procedure should be applicable to trace element analysis in general and so, should be widely applicable for other types of research. In addition, the dating technique has been extended to the mineral, xenotime, which provides constraints on the age of the very beginning and ending of tectonism in a mountain belt. Finally, this research provided research experience and analytical experience to several graduate students and several undergraduate students and the results have also been used to enrich undergraduate structural geology and petrology laboratory exercises.
