Archean Workshop: Linking Geology and Numerical Models

The Exploration Geodynamics Chapman Conference provided a unique opportunity to bring together numerical modelers and geologists to discuss Archean geology and the use of modeling to constrain Archean processes. A wide number of issues were discussed and this report summarizes a few main conclusions.

The main thread of the discussion centered around the difficulties of applying our understanding of Phanerozoic plate tectonics to the Archean. The most important conclusion reached Phanerozoic-style plate tectonics should not be assumed a priori to underlie Archean geology. This conclusion is based on the impossibility of extrapolating our relatively limited understanding of present-day plate tectonics back in time to the Archean. It was generally felt that Archean studies and related numerical models should not assume plate tectonics as a given, but rather that Archean tectonics should be derived from field observations and tested by means of numerical models.

Some of the key unanswered questions with regards to Archean geodynamics discussed were: (a) the increased role of mantle plumes in the Archean and associated komatiites; (b) the size of Archean plates and their average age at suduction zones; (c) the increased thickness and buoyancy of Archean oceanic crusts and its inhibiting effects on subduction; (d) the weakening effects on continental crust of increased radiogenic heat production and possible increased heat flux in the Archean.

Another issue raised, which may be partly related to weaker continental crust, is that the processes related to the assemblage of Archean terranes are obscured by later internal rearrangement such as gravity-driven overturns (diapirism). There is thus a need to make a clear distinction between continent forming events and internal rearrangement of its components. For example, the present geometry of East Pilbara may reflect poorly the construction of the block, but reflect more clearly subsequent overturn of the buried less-dense granites.

With regards to diapirism in the Archean, there seems to be two important misconceptions which have hampered clarity: a) that vertical tectonics (diapirism) preclude contemporaneous and independent external horizontal tectonics; and b) that all horizontal tectonics is necessarily related to either diapiric granite-gneiss deformation, or conversely, related to plate tectonics. It was felt that diapirism and externally forced horizontal tectonics are independent processes, which may develop simultaneously or otherwise. In this context, it was strongly stressed that externally driven horizontal tectonics is in itself not an indication of Phanerozoic-style plate tectonics.

It was suggested that, because of the relatively poor knowledge of Archean processes and constraints, research projects in general, but particularly numerical models should be directed at investigating simple, well-constrained questions, seeking to provide the general setting of Archean terranes. A tree of numerical models could be built where at each step simple Yes/No answers are derived, thus constraining the problem. It was further suggested that modeling should concentrate on two different approaches: a) models seeking to understand Archean mantle heat transfer (e.g. the increased role mantle plumes); and b) models focusing on continental crustal processes with minimal assumptions regarding tectonic setting and large-scale driving mechanisms. The former approach would provide the basis for improved understanding of the driving mechanisms behind Archean tectonics, and the latter would limit unfounded assumptions, and provide a less biased understanding of Archean crustal processes.

Granitic geochemistry is also potentially subject to interpretation via an uncertain global paradigm. A number of different classifications of granitic rocks have been derived over the years, each reflecting different sources and pressure-temperature conditions at the source (e.g. garnet stable in the source). Because of our relatively extensive knowledge of Phanerozoic tectonic systems, it is then possible to relate these conditions to specific locations within the system and derive the origin of the melts (e.g. slab-derived melts or mantle-wedge melts). The geometry and P-T conditions of Phanerozoic systems are relatively well understood, but this is clearly not the case in the Archean, suggesting that such inferences may be a leap of faith.

In summary, the most important conclusion reached was that it is unwise to approach Archean geology and processes, using plate tectonics as an underlying paradigm. Plate tectonics must instead be derived from observations. Numerical models designed to understand Archean processes should either limit the assumptions related to tectonic setting, or seek to investigate and constrain different possible tectonic scenarios for the Archean.

Roberto Weinberg, UWA
Peter Sorjonen-Ward, CSIRO