Methods and application of deep-time thermochronology: Insights from slowly-cooled terranes of Mongolia and the North American craton

Continental interiors are an underappreciated facet of plate tectonics due to the perception that they are often static over long timescales. Salient tectonic margins receive more attention, owing to their comparatively dynamic state during the creation and destruction of continents and ocean basins. I utilize low-temperature (U-Th)/He and 40Ar/39Ar thermochronology to address questions regarding the spatial and temporal thermal evolution, and by proxy, the exhumation and burial histories of these slowly-cooled terranes through deep time. Chapter One is focused on the topographic evolution of the Hangay Mountains of central Mongolia, where apatite (U- Th)/He data and thermal models suggest that the post-orogenic landscape experienced rapid relief loss of a few hundred meters in the mid-Mesozoic. The Hangay are now characterized by a relict landscape that has undergone slow exhumation on the order of ~10 m/Ma since the Cretaceous (~100 Ma), analogous to other old landscapes such as the Appalachians. The central Mongolian landscape remains in a state of topographic disequilibrium, while modest surface uplift since the Oligocene and recent glaciation have had little effect on erosion rates due to the fact that there has been minor tectonism and a very dry climate during the Cenozoic.Chapter Two confronts the problem of dispersed apatite (U-Th)/He cooling ages that often afflict slowly-cooled terranes, such as the Hangay Mountains. Conventional total-gas analysis offers little explanation or remedy for He age scatter that has been typically attributed to many factors, such as isotopic zonation, crystal lattice defects, and radiation damage. Unlike conventional analysis, the continuous ramped heating (CRH) technique exploits incremental 4He release during a continuous, controlled heating rate under static extraction line conditions. This approach allows the measurement of the cumulative gas released from apatite grains and assessment of the characteristic sigmoidal release curve shape as a means to distinguish between expected (radiogenic) and anomalous volume-diffusion behavior. Screening results for multiple apatite suites show that the CRH method can discriminate between the simple, smooth release of apatites exhibiting expected behavior and well-replicated ages, and grains that do not replicate well with more complicated 4He release patterns – and offers a means to correct these ages.Chapter Three is focused on understanding the assumed long-term stability of the southern Canadian Shield. Craton stability over billion-year timescales is often inferred due to the lack of geologic records to suggest otherwise. For the Proterozoic (2.5-0.54 Ga) there is little or no intermediate temperature thermal-history information for many locations, however K-feldspar 40Ar/39Ar MDD data and modeled thermal histories linked to published high- and low- temperature data from the Canadian Shield suggest the southern craton experienced unroofing delayed until ~1 Ga, coeval with the formation of the supercontinent Rodinia. K-feldspar data suggest a prolonged period of near-isothermal cooling of <0.5°C/Ma in the late Proterozoic where rocks were positioned at cratonic depths in the middle crust for up to ~500 million years at temperatures of ~150-200°C and subsequently exhumed to the surface in the Neoproterozoic. Thermal history solutions and geophysical evidence of underplating and crustal thickening at the Mid-Continental Rift and adjacent regions suggest uplift and a previously unrecognized phase of cratonic unroofing that began in the Neoproterozoic, which ultimately contributed to the development of the Great Unconformity of North America.

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