Crustal Exhumation of Plutonic and Metamorphic Rocks: Constraints from Fission-Track Thermochronology

The thermal evolution of plutonic and metamorphic rocks in the upper crust may be revealed using fission-track (FT) analyses and other low-temperature thermochronologic methods. The segment of pressure–temperature–time–deformation (P-T-t-D) rock paths potentially constrained by FT data corresponds to the lower greenschist facies, prehnite–pumpellyite, and zeolite facies of metamorphic rocks and also includes regions where diagenetic alteration occurs. When plutonic and metamorphic rocks are exhumed, thermal perturbations caused by fluid alteration, and crystallisation below relevant closure/annealing temperatures at relatively shallow crustal depths, may preclude a simplistic interpretation of thermochronologic ages in terms of monotonic cooling. However, FT ages and track-length measurements provide kinetic data that allow interpretation of T-t paths, even in cases where assumptions based on bulk closure temperatures are violated. We show that geologically well-constrained sampling strategies, and application of multiple thermochronologic methods on cogenetic minerals from plutonic and metamorphic rocks, may provide the most promising means to document the timing, rates, and mechanisms of crustal processes. Case studies are presented for: (1) (ultra)high-pressure (U)HP metamorphic terranes (e.g., Papua New Guinea, Western Alps, Western Gneiss Region, Dabie–Sulu), (2) an extensional orogen (Transantarctic Mountains), (3) a compressional orogen (Pyrenees), and (4) a transpressional plate boundary zone (Alpine fault zone, New Zealand).