Grenville Orogeny - General Tectonics

General Tectonics

Reconstruction of the events of the orogeny is ongoing, but the generally-accepted view is that the eastern and southern margins of Laurentia were active convergent margins until the beginning of continental collision. This type of subduction (B-type) tends to emplace magmatic arcs on or near the edge of the overriding plate in modern subduction zones, and evidence of contemporary (ca. 1300-1200 Ma) island arcs can be found throughout the Grenville orogen. The Andes of South America are considered a modern analogue. From about ca. 1190-980 Ma (the actual timing varies by locality) two separate continental blocks collided with Laurentia. Both of these collision events are thought to be analogous to the collision driving modern-day growth of the Himalaya range. For some time one of the blocks was believed to be the continent of Amazonia, but paleomagnetic evidence has now proven that this is not the case.

These periods of thrusting and metamorphism were not continuous, but rather interrupted by comparatively quiet periods, during which AMCG (anorthosite/ mangerite/ charnockite/ granite) plutons were intruded into the country rock. Polarities of subduction (which plate overrode which) vary by region and time. Some island arc remnants were emplaced on the Laurentian margin, and some were accreted during orogeny. Timing of these events is constrained by cross-cutting relations observed in the field as well as SHRIMP (sensitive high-resolution ion microprobe) and TIMS (thermal ionization mass spectrometry) uranium- lead dating.

The first period of tectonic activity was the accretion of an island arc at some point during the Elzevirian Orogeny. Before the accretion of the island arc took place, subduction between a continental plate and presumably an oceanic plate was taking place. Slab pull and far-field drivers such as ridge push were aiding in closing the distance between the island arc and the continent. Depending on the angle of subduction deformation of the continental crust was already taking place and thickening the lithosphere. By 1.19 Ga the Elzevir back arc basin was closing.

From 1.18 to 1.14 Ga extension was occurring in the area. Whether due to lithospheric cooling also known as thermal subsidence or the compressional activity in the area reactivated some extensional faults the extension is marked by the isotopic ages of the previously mentioned rocks. Additionally there is the formation of sedimentary basins which means the margin was quiescent enough that sediments could accumulate. However in some areas from 1.16 to 1.13 Ga, coeval with extension, there is evidence there was still thrusting and emplacement of terranes occurring.

According to one model westward thrusting occurred from 1.12 to 1.09 Ga and then extension was the primary tectonic activity until 1.05 Ga. It was at this point that the Central Granulite Terrane was exhumed and minor magmatism occurred.

The precise reason for change from compression to extension is unknown but may be the result of gravitational collapse, mantle delamination, the formation of a plume underneath a supercontinent, changes in far-field drivers on the distribution of stress, or any combination of reasons originating from the fact that our planet is dynamic. The cyclic compression and extension history of this area is similar to the Wilson Cycle. In this area of the world the Wilson Cycle would be creating the basin for the proto-Atlantic Ocean (Iapetus Ocean)