THE DIFFERING VIEWPOINTS OF GEOCHRONOLOGY AND GEOSPEEDOMETRY ON THE MATTER OF SHORT-DURATION REGIONAL METAMORPHISM

Short-duration regional metamorphism involves time scales that limit thermal length scales to significantly less than the thickness of the continental crust (in orogenic settings) or oceanic lithosphere (in subduction settings). Thus, by definition, short-duration regional metamorphism records atypical, transient conditions for the tectonic setting to which it is allied. Short-duration regional metamorphism has become a major focus area for metamorphic geology in recent years, in part due to its apparent prevalence, and associated implications for the nature of tectonism and significance of metamorphic facies series.

Geospeedometry applied to regional metamorphic rocks yields significantly (one-to-two orders of magnitude) shorter metamorphic duration estimates than the 10⁶–10⁷ yr that are commonly obtained from high-precision geochronology. Though geospeedometry may suffer from uncertainties in diffusion rates, the discrepancy between durations obtained from the two methods likely also relates to insufficient spatial and temporal resolution, in the geochronology approaches, to resolve time scales < 10⁶ yr. Such limitations may also account for a lack of known examples of Precambrian short-duration metamorphism.

New, very-high-resolution geochronology techniques offer great potential to probe the veracity of the extremely-short metamorphic duration estimates being obtained from geospeedometry. However, perhaps the most promising of these techniques—(1) Sm/Nd or Lu/Hf on microdrilled growth sectors of metamorphic garnet, (2) methods to identify partial isotopic resetting in ⁴⁰Ar/³⁹Ar step-heating spectra, and (3) U/Pb laser ablation split stream ICP–MS petrochronology—are yet to be benchmarked. Application of all three techniques to a common set of young rocks, in combination with multiple approaches in geospeedometry, must be a focus for upcoming work.

If very-high-resolution geochronology confirms findings from geospeedometry of extremely-short-duration (< 10⁶ yr) regional metamorphism, then such time scales need be reconciled with what would appear to be inordinately large thermal length scales (as observed in the field). Explanations may consider sources and tempo for metamorphic heating, and/or alternative metamorphic drivers, such as pressure or fluid pulses.