July 20. Alice Springs Orogeny

Gondwana

It’s easy to visualize Gondwana as one big unified block with not much going on except around the margins. But in at least some places, there was plenty of action, even though we don’t necessarily understand it too well. Deformation, mountain building, within a tectonic plate is sometimes enigmatic. We know about collisions and subduction and such and what they can do to the earth, but the causes and consequences of intraplate deformation are harder to understand. Today, in central United States in southeastern Missouri and adjacent states, intraplate deformation is occurring, producing the well-known earthquakes in the New Madrid Zone. But the ultimate plate tectonic cause is challenging to pin down with certainty.

During the Pennsylvanian, a long-lived intraplate orogeny was culminating in what is now Central Australia. Australia was firmly attached to the eastern side of Gondwana, but mountain uplifts had been occurring there, well within the tectonic plate, since Devonian time or earlier. It’s called the Alice Springs Orogeny, which appears to represent north-south compression (in modern coordinates) and shortening that rejuvenated some old faults whose heritage dated back to the Cambrian or longer. One of the rejuvenated zones, called the Redbank Shear Zone, may be a really major break that extends all the way to the crust-mantle boundary. It is marked by the most intense gravity gradient in the world, a reflection of the huge contrast in density between the mantle and the crust. Such a prominent weak zone would be the sort of thing that could be activated relatively easily, more easily than a fresh break through unbroken crust. 

It seems that it wasn’t simply a matter of reactivating old faults, though. Central Australia had subsided over time, and the load of sediment eroded into the subsiding basins may have had a role in the tectonic activity as well. The tectonism broke the old basin into several sub-basins.

What was causing the compression? By Pennsylvanian time, Gondwana was rotating in a more-or-less clockwise manner, so that its western parts were moving north to collide with North America. Australia, to the east, would have been moving south. It’s possible that the highly variable thickness of the crust in Australia could have led to differential motion, sort of like one section catching up with another to provide squeezing. Whatever the cause, it seems pretty certain that there was no typical continent-continent collision, so the ultimate cause remains pretty enigmatic.

—Richard I. Gibson

References and Links:
Martin Hand’s abstract

Chris Klootwijk’s “heretical view” 

Modeling the Alice Springs Orogeny 

Cartwright, Buick, Foster, and Lambert (1999), Alice Springs age shear zones from the southeastern Reynolds Range, central Australia: Australian Journal of Earth Sciences: Geological Society of Australia, 46:3, 355-363