Concept of the Exhumed Partial Annealing (Retention) Zone and Age-Elevation Profiles in Thermochronology

Low-temperature thermochronology is commonly applied to constrain upper crustal cooling histories as rocks are exhumed to Earth’s surface via a variety of geological processes. Collecting samples over significant relief (i.e., vertical profiles), and then plotting age versus elevation, is a long-established approach to constrain the timing and rates of exhumation. An exhumed partial annealing zone (PAZ) or partial retention zone (PRZ) with a well-defined break in slope revealed in an age-elevation profile, ideally complemented by kinetic parameters such as confined track lengths, provides robust constraints on the timing of the transition from relative thermal and tectonic stability to rapid cooling and exhumation. The slope above the break, largely a relict of a paleo-PAZ usually with significant age variation with change in elevation, can be used to quantify fault offsets. The slope below the break is steeper and represents an apparent exhumation rate. We discuss attributes and caveats for the interpretation of each part of an age-elevation profile, and provide examples from Denali in the central Alaska Range, the rift-flank Transantarctic Mountains, and the Gold Butte block of southeastern Nevada, where multiple methods reveal exhumed PAZs and PRZs in the footwall of a major detachment fault. Many factors, including exhumation rates, advection of isotherms and topographic effects on near-surface isotherms, may affect the interpretation of data. Sampling steep profiles over short-wavelength topography and parallel to structures minimises misfits between age-elevation slopes and actual exhumation histories.