McDannell published his outstanding dissertation (from Lehigh University) in mid-2017. Based on much of his earlier work, he posited that 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. McDannell utilized 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.
He concluded that the material presented in this dissertation has discussed applying thermochronology to old, slowly cooled settings and some of the problems that are encountered during its implementation. Recent advances in understanding the coupling of atmospheric, lithospheric, and surficial processes, along with innovative developments in geochronology and thermochronology over the past few decades, have allowed better integration of temporal records to reconstruct past tectonic environments, orogenic evolution, and erosional processes. Slowly-cooled terranes present challenges for the
interpretation of thermochronologic and geologic records, mainly in the face of the long timescales innately associated with their development, and the detrimental effect of subsequent tectonic activity on primary thermal information.
Elucidating upper-crustal histories also presents challenges for the employed analytical techniques themselves, namely in apatite (U-Th)/He thermochronometry, where unexplained age dispersion is common in slowly-cooled settings. The Continuous Ramped Heating (CRH) method presented here is an example of an analytical improvement that with further work has the potential to offer greater understanding of the contributing mechanisms of cooling age dispersion.
The other presented chapters demonstrate that low-temperature thermochronometers contain much valuable information on constraining topographic histories in settings where post-orogenic activity has been minimal. Central Mongolia experienced typical rapid decay following major continental assembly and mountain-building during the Late Paleozoic-Early Mesozoic. The topographic relief of the Hangay Mountains of central Mongolia were originally comparable to nearby modern, active ranges such as the Chinese Tian Shan. Since the Cretaceous, this region has experienced very slow erosion on the order of 10s of meters per million years. Far-field effects of the India-Asia collision, ongoing uplift of the Mongolian Plateau, local lithosphere-asthenosphere interactions, and recent glaciations have introduced geodynamic complexity in the form of surface uplift and valley-scale incision. However, due to sustained high aridity the regional erosion has remained low. This setting is an important example for highlighting the antiquity and longevity of some of Earth’s landscapes, which are generally seen as youthful or undergoing relatively continuous development at most active plate boundaries.
The billion-year timescale common in cratons is fraught with the same challenges as previously mentioned with thermochronometric data interpretation, just on a grander scale. The southern Canadian Shield is a region where long-term stability since the Precambrian has been inferred mainly due to a lack of information rather than geologic evidence. The evidence that does exist is commonly sporadic and records a much younger, progression of events over the last ~500 million years. The Athabasca region in the northwest craton is highly constrained from sedimentary and geochronologic data, suggesting a rapid, and prolonged nearly two billion year history of stability – in line with conventional views. Thermal-history data from the southern Canadian Shield indicate
that currently exposed rocks were still at temperatures of ~200 °C at 1 Ga. Similar histories are observed in other bordering Baffin Island terranes to the north.
Collectively, regardless of monotonic or reheating histories these areas were still at elevated temperatures at 1 Ga and suggest unroofing was initiated in the Neoproterozoic after a long period of slow cooling. This prior history ties into the later Paleozoic-Mesozoic evidence of mantle-induced dynamic topography and sea-level fluctuations that affected the entire continent and caused widespread, episodic unroofing and burial of the craton to
modest levels of ≤ 2 km.
The Mongolian Plateau and the southern Canadian Shield have some broad similarities. Both regions are continental interiors that have been affected by peripheral tectonism, albeit vastly separated in time. Throughout central Asia there is clear evidence that deformation continues to progress northward from India colliding with Asia. The physiographic expressions of this tectonism are separated by over 2500 km, with the locus of tectonic activity at the high-elevation Himalaya and Tibetan Plateau, while the other end-member is the Baikal rift abutting the Siberian craton at approximate sea-level elevation. The Grenville orogeny at ~1 Ga was of similar scale to the modern Himalaya and may have produced similar far-field effects. Isostatic considerations from a thickened crust may lend support for high-elevation topography in both Mongolia and the ancient craton. Studies over the past 40 years have suggested that mafic magmatic additions to the lower crust are very important for crustal growth and thickening over long timescales. This mechanism can explain the reactivation of the craton and renewal of inferred exhumation after an ~500 m.y. period of mid-crustal holding for southern Canadian Shield rocks. Magmatic underplating due to Mid-Continent mantle plume impingement on the lithosphere could have driven uplift and unroofing of the craton (see Nelson, 1991 for an overview in cratons). Similar underplating processes have been proposed for central Mongolia (e.g. Chen et al., 2015; Petit et al., 2002; Stosch et al., 1995).
The Earth system is very dynamic and complex, however there are certainly exceptions to this – many locations around the globe are characterized by minimal tectonic activity but impressive topography or landscapes persist, like the Appalachian Mountains or central Mongolia. Regions such as central Mongolia suggest that in the face of minimal external forcing, landscapes can endure for extremely long timescales. Ultimately given enough time a lot can happen geologically, nonetheless we often think of continental margins and interiors as being distinctly separate instead of a continuum. Perhaps the Mongolian interior and cratonic interiors are not so different after all and these areas are just in different stages of their tectonic lifecycle.
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