RHEOLOGICAL CONTROLS ON THE FOCUSING OF DEFORMATION IN ACTIVE OROGENS

Alaska has been formed through the collision and accretion of numerous terranes onto its southern margin. The currently active oblique collision of the Yakutat terrane, an oceanic plateau, with southeastern Alaska has uplifted the highest coastal ranges on Earth; the St. Elias Mountains. This terrane accretion has created an incipient plate corner tectonic aneurysm, similar to the Himalayan Eastern Syntaxis (Tsang Po- Namche Barwa region). The tectonic aneurysm forms as a weak region within an active orogen where non-linear coupling between deformation and erosion focuses horizontal and vertical strain into the most rapidly eroding region. Signatures of a tectonic aneurysm may include areas of abnormally high heat flow, extraordinary uplift rates, high erosion rates, and steep gradients in topography. In the geologic record, tectonic aneurysms may be recognized by isolated zones of relatively higher metamorphic grade, such as gneiss domes.

Additional rheological non-linearities relevant to geological evolution of terrane accretion arise through mechanisms of strain softening that significantly alter both outcrop and finite strain patterns. The combination of oblique collision and strain-dependent crustal rheology conspire to develop weakened areas that are prone to further focused weakening and deformation. During terrane accretion, these weakened areas step discontinuously through the orogen such that deformation across the area is asynchronous.

The Appalachian Mountains have evolved through a series of terrane accretion events for which the current Yakutat terrane accretion may be used as a modern analog. The Acadian-aged rocks in Maine record a period of dextral transpression that was most clearly recorded within the Norumbega Fault System during the Middle Devonian. The nature of transition of lateral to convergent strain identified in 3D numerical models of Yakutat accretion provides a strain template that can aid in deciphering the local kinematics of the Acadian Orogen.