Gravity models of Archean crust and implications for the geodynamics of Early continents

Details Gravity models of Archean crust and implications for the geodynamics of Early continents Gravity models of Archean crust and implications for the geodynamics of Early continents

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....11756p&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #11756

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The nature of Archean continental geodynamics continues to be a subject of much debate. One of the principal questions regarding the evolution of the early continents is whether large-scale vertical displacements (e.g. sagduction), driven by gravity instabilities, were predominant tectonic processes, or whether the Archean continents were accreted by successive collisions at subduction zones as in the present-day plate tectonic paradigm. In fact, a proper understanding of the evolution of Archean continental geodynamics must not be based on end-member models of vertical crustal displacements or of Phanerozoic-style plate tectonics. There was necessarily a progressive shift in the processes of craton formation and reworking with the secular cooling of the planet. This work investigates the evolution of continental geodynamics, from the Middle Archean to the Late Archean, by way of gravity models of selected cratonic regions that range in age from ca. 3.5 Ga to ca. 2.7 Ga. Three major cratons are compared; the Pilbara Craton, Australia (3.5-3.2 Ga), the Yilgarn Craton, Australia (2.7Ga) and the Superior Province, Canada (2.7Ga). The modeled regions include large tonalite-granodiorite batholiths hosted within greenstones. The underlying premise is that the subsurface shapes of batholiths and of the host greenstone belts reveal first-order crustal structures recording the Archean tectonic processes. Each region is modeled using several 2D-gravity profiles to obtain a 3D picture of the structure of the upper to middle crust. Magnetics data have been used to constrain the major faulting and dyking events. In the 3.5 Ga Pilbara craton, some modeled batholiths have well defined roots deeper than 10 km whereas others are continuous to less than 6 km depth. The greenstones form V-shaped keels, some as deep as 10 km. The model results from the 2.7 Ga Superior and Yilgarn cratons show no major differences in the structural styles between the two younger terranes. In the Superior and Yilgarn cratons, batholiths share a common elongated thin shape with shallow roots that are not as well defined as those of the Pilbara batholiths; the absence of deep roots and the 5-6 km thicknesses are characteristics of the ca. 2.7 Ga batholiths. The surrounding greenstones show V-shaped keels of 10 km depth as in the modeled regions of the older Pilbara craton. Comparison of the results from the older and younger cratons suggests: (1) greenstones in the Middle and Late Archean terranes form deep keels which are not easily reconciled with the structure of terranes accreted by plate tectonics, (2) the presence (Middle Archean) or absence (Late Archean) of deep batholith roots may indicate a change in the style of geodynamics and in the processes of batholith formation between 3.5 Ga and 2.7 Ga.

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