Extensional Strain Heterogeneity in the Northern Basin and Range Continental Rift Recorded in Low-Temperature Thermochronology

The Basin and Range extensional province of western North America is perhaps the greatest natural laboratory for studying protracted continental extension in the world, but our ability to test geodynamic models of continental rifting remains limited by a lack of data constraining the spatial distribution of strain on 1-10 Myr timescales. In parts of the Basin and Range, low-temperature apatite and zircon thermochronology can be used to constrain the timing and magnitude of extension, but the carbonate-dominated miogeocline of the northeastern Basin and Range lacks suitable apatite and zircon bearing rocks except for an understudied east-west trending suite of Jurassic (pre-extensional) plutonic stocks whose small sizes limit the usage of typical low-temperature thermochronometric sampling techniques. We overcome this limitation by applying three thermochronometers with different closure temperatures in each stock (apatite (U-Th)/He, T_closure~70ºC; zircon (U-Th)/He, T_closure~190ºC; and apatite fission track, T_closure~120ºC). Inverse thermal modelling of our data in QTQt resolves two characteristic time-temperature histories along the transect: domains which cooled below 100º C before 65 Ma (limiting the amount of possible later extensional exhumation), and domains which remained above 200º C until after 20 Ma (indicating they experienced significant Miocene extensional exhumation). Alternating 50-100 km wide domains of enhanced and diminished extension match observations in studies to the south and west (Colgan et al., 2009; Long, 2018). The pattern of extensional domains does not appear to correlate with factors hypothesized to influence later extension, such as the locations of major pre-extension contractional structures, reconstructed Paleogene crustal thickness, Paleogene subcrop geology, the distribution of Miocene volcanism, or the locations of metamorphic core complexes. Instead, alternation between extended and unextended domains may be an expression of periodic necking instability of a strong upper mantle (Fletcher and Hallet, 1983; Zuber et al., 1986), evidence that instability in a layered lithosphere may exert significant control over strain localization during wide continental rifting.