KEEPING OROGENS HOT: THE IMPORTANCE OF PROTRACTED MANTLE MELTING

A well-known, but little appreciated, feature of many orogenic belts is that peak metamorphic mineral assemblages, and many granite intrusions, formed synchronously with thrust-related foliations. This cannot be readily explained by conventional tectonic models invoking continental collision and clockwise P-T-t paths. Elevated heat fluxes are commonly attributed to the mantle, but a satisfactory model for the geodynamic process has remained elusive, particularly for orogenic belts where mafic rocks are uncommon.

A "hot orogen" is the accretionary, Cambrian-Devonian Lachlan Fold Belt (LFB) of eastern Australia, where silicic igneous rocks comprise up to 50% of exposure, but minor mafic rocks also exist. Enigmatically, primitive Lachlan mafic rocks resemble modern intraoceanic arc/backarc basalts throughout the orogenic history, until the continent stabilisation stage (Late Devonian) when they acquired a continental character. This suggests that the orogen underwent protracted lithospheric extension to produce asthenosphere-derived basaltic magmas, but it was interrupted by short-lived contractional events, when localised orogenic belts formed and voluminous granites were emplaced. Rapid alternation between extension and compression is called tectonic switching .

Tectonic switching occurs when ongoing lithospheric extension and backarc basin development is interrupted by intermittent subduction of buoyant oceanic plateaus. It provides a mechanism for episodic, short-lived orogenic contraction of an otherwise continuously extending upper plate formed by ongoing slab retreat. The protracted extension mode generates basaltic magmas and repositories into which sediment can accumulate and the contraction mode inverts the basins and buries crustal rocks, which become the main source for granite generated during the next extensional phase, when basaltic underplating and consequent melting of the thickened crust occurs. Granite transfer and emplacement refocuses advective heat in the middle-upper crust, producing hot orogens. Tectonic switching is cyclic, depending on the frequency of plateau arrivals, and both tectonic modes are necessary for efficient continental growth.