COEVAL SHALLOW EXTENSION AND DEEP CONTRACTION DURING OROGENIC COLLAPSE: COLLIDING CHANNELS AND DOUBLE DOMES

Gneiss (migmatite) domes such as the one on the island of Naxos in the Aegean Sea form in a variety of tectonic regimes, from contraction to wrenching to extension, and in some cases form primarily by buoyancy forces. These domes are typically made of a core of variably deformed migmatitic and granitic rocks surrounded by a metamorphic envelope. Physical experiments on diapirism have produced mushroom-shaped domes and finite strain fields that are characterized by vertical stretch (constriction strain) in the stem of the diapir, sub-horizontal flattening in the roof of the dome, and near simple shear (plane strain) at the dome margins. Although dome shape and strain distribution are modified when extension or contraction are involved, deciphering fold-dominated, detachment-dominated, or gravity-dominated domes from their shapes and finite strain fields remains elusive. Many domes reveal an internal architecture composed of two cores separated by a narrow, steeply dipping high strain zone. Examples of double domes include the Montagne Noire (southern France), Naxos (Cyclades, Aegean Sea, Greece), Entia (central Australia), Warwick (N Appalachians, USA), and possibly Thor-Odin (BC, Canada). In some domes, the steeply dipping central high strain zone has been interpreted either in terms of diapiric ascent of migmatites or horizontal shortening and folding.

A series of 2D numerical experiments were carried out to explore the flow of deep crust during extension. These models focus on the strain field within and around domes and build on earlier work that examined the thermal and mechanical implications of the role of partial melting in the development of metamorphic core complexes. The model is 360 km long and 120 km deep. It includes a 60 km thick crust above 45 km of mantle and below a 15 km thick layer of low density, low viscosity material simulating air. To facilitate comparison among experiments, we include a fault-shaped weak element in the upper crust to force the development of a dome at the center of the model. Without this anomaly, stretching and localized strain still occur, but the location changes from one experiment to the next.

Modeling results show that double domes form in an extensional setting as a result of the collision of two channels of laterally flowing, partially molten lower crust that converge below the site of upper crustal necking or faulting. Double domes initiate as recumbent structures by passive shear folding during channel flow and are subsequently rotated into a vertical position into the final dome as the flowing crust fills the space opened by upper crustal extension. A steeply dipping high strain zone marks the contact between the two diverted channels, forming a double dome. Isostasy and buoyancy forces combine to drive upward flow into the dome, which is fed by horizontal Poiseuille flow in the deeper crust; this process is self sustained until crustal extension ceases or the lower crustal channel becomes too thin and depleted.

Although a steady extension rate is applied to the far field in the 2D models, strain regimes in the localized extended region are highly variable in space and time, with coeval extension in the upper crust and contraction in the lower crust. Strain paths are complex as the material is transferred upward from contractional to extensional regions. These features are complicated by 3D flow, oblique tectonics, and lateral heterogeneities in crustal thickness and/or channel topography. Nevertheless, the coeval development of upper crustal extension and lower crustal contraction owing to colliding channels may be a common first-order process of material redistribution and heat transfer during the collapse of orogens.

In Naxos, the N-S elongate migmatite dome consists of two main lobes separated by a subvertical, N-S oriented high strain zone. Foliation outlines subdomes within these lobes, and lineation determined by the anisotropy of magnetic susceptibility is in general close to horizontal and N-S oriented, except in the overturned flanks of subdomes in the eastern lobe, where lineation plunges steeply. The eastern lobe has been dragged by the top-to-N shear that characterizes the Naxos detachment. Results from thermomechanical modeling inspire a new interpretation for the Naxos dome: Extension in a N-S elongate zone in the upper crust, which may have been generated as a jog on strike-slip transfer zones, attracted the rise of channels of partially molten crust at depth. These channels collided beneath the extension zone, ascended, and formed domes of foliation, with a N-S maximum extension and E-W contraction, separated by a subvertical high strain zone. As the domes rose in the extension system, they started responding to top-to-N kinematics, particularly the eastern lobe which developed subdomes overturned to the north.