Archean tectonics: Analog and gravity models

Abstract

This work investigates Archean continental geodynamics. Three cratonic areas were studied: the Pilbara Craton, Australia (3.5--3.2 Ga), the Yilgarn Craton, Australia (2.7 Ga) and the Abitibi Subprovince, Canada (2.7 Ga). Each region is modeled using several 2-D gravity profiles. In the 3.5 Ga Pilbara craton, some modeled batholiths have well defined roots that extend to depths greater than 10 km whereas others have roots extending to less than 6 km depth. The model results from the 2.7 Ga Yilgarn craton and the Abitibi Subprovince show no major differences between the two terranes. The rarity of deep roots and a thickness of 5 to 6 km are characteristics of the ca. 2.7 Ga batholiths. The surrounding greenstones form keels of up to 10 km depth, in the three modeled regions. Compassion of the results from the older and younger cratons suggests: (1) the presence (Middle Archean) or quasi-absence (Late Archean) of deep batholith roots may indicate changes in continental geodynamics from 3.5 Ga to 2.8 Ga. (2) greenstones in the Middle and Late Archean terranes form deep keels, which are consistent with the gravity driven, diapiric model in the case of the older greenstones, and may be explained by crustal folding in the younger cases. A more detailed gravity study was done on the Abitibi Subprovince using modeled gravity profiles and a wavelet based inversion method, leading to a new proposed model to explain the structures of the Abitibi Subprovince. In this model the Abitibi upper-middle crust is folded, the deformations zones interpreted as detachment folds. We used analog experiments to investigate folding of continental crust subjected to different geothermal gradients. In our experiments, folding is the main response to shortening of the analog crust. The middle and lower crust analogs respond to the shortening by buckling. In the upper crust analog, detachment folds, thrust faults and grabens above anticlines are developed. For the lowest thermal gradient, one anticline-syncline pair is formed. For warmer gradients, multiple folds develop that have smaller amplitudes. Based on our models, we interpret that the increase of crustal temperatures may result in a decrease of the amplitude of the crustal folds. Our results suggest that the deformation style preserved within Archean greenstone belts is strongly influenced by the syndeformational thermal regime of the crust.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office.