Instability of the southern Canadian Shield during the late Proterozoic

Abstract

Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40 Ar/ 39 Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (~150-350°C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth's Proterozoic "Middle Age." Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5°C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ~200°Csignaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (<< 60°C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the very least 5 km of exhumation beginning at ca. 1.0 Ga. A possible McDannell et al.-Late Proterozoic Instability of the Canadian Shield 2 explanation may be found in evidence of magmatic underplating that thickened the crust, driving uplift and erosion. The timing of this underplating coincides with Mid-Continent extension, Grenville orogenesis, and assembly of the supercontinent Rodinia. 40 Ar/ 39 Ar MDD data demonstrate that this technique can be successfully applied to older rocks and fill in a large observational gap. These data also raise questions about the evolution of cratons during the Proterozoic and the nature of cratonic stability across deep time.

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…Recent work extends the low-temperature thermal history of cratonic North American into deeper geologic time (Figures 8a and 8c; Ault et al, 2018;DeLucia et al, 2018;McDannell, Zeitler, & Schneider, 2018;Orme et al, 2016). For example, zircon He thermochronometry date-eU patterns from the North American midcontinent cratonic basement reveal~850-to 680-Ma cooling and exhumation associated with the development of the Great Unconformity, an erosion surface representing~100-1,000 Ma of missing geologic time between Precambrian and Phanerozoic rocks (DeLucia et al, 2018).…”

“…Tectonics data suggest prolonged residence in the midcrust through the Proterozoic and exhumation initiating at~1.0 Ga related to the development of the Midcontinent Rift System (McDannell, Zeitler, & Schneider, 2018). Collectively, low-temperature thermochronometry from across the North American craton reveals phases of burial and subsequent unroofing associated with processes operative during two supercontinent cycles, Rodinia and Pangea, raising possible links between cratonic exhumation and dramatic, possibly global, paleoenvironmental change (Figure 8c; Ault et al, 2013;DeLucia et al, 2018;.…”

“…For example, zircon He thermochronometry date‐eU patterns from the North American midcontinent cratonic basement reveal ~850‐ to 680‐Ma cooling and exhumation associated with the development of the Great Unconformity, an erosion surface representing ~100–1,000 Ma of missing geologic time between Precambrian and Phanerozoic rocks (DeLucia et al, ). In addition, 40 Ar– 39 Ar multidiffusion data suggest prolonged residence in the midcrust through the Proterozoic and exhumation initiating at ~1.0 Ga related to the development of the Midcontinent Rift System (McDannell, Zeitler, & Schneider, ). Collectively, low‐temperature thermochronometry from across the North American craton reveals phases of burial and subsequent unroofing associated with processes operative during two supercontinent cycles, Rodinia and Pangea, raising possible links between cratonic exhumation and dramatic, possibly global, paleoenvironmental change (Figure c; Ault et al, ; DeLucia et al, ; Flowers et al, ).…”

Growth, overprinting, and stabilization of Proterozoic provinces in the southern Lake Superior region

“…40 Ar/ 39 Ar analysis of K-feldspar using the MDD method has been utilized in a number of laboratory experiments (Harrison et al, 1991;Lovera et al, 1989Lovera et al, , 1993Lovera et al, , 1997Lovera et al, , 2002 and has recently been successfully applied to link higher and low-temperature thermochronological systems from well-constrained cratonic localities (McDannell et al, 2018). 40 Ar/ 39 Ar K-feldspar MDD analysis is able to determine continuous temperature-time paths over the range ~150 to ~300 °C.…”

“…Inverse thermal history modeling of MDD data was carried out following the methods outlined in McDannell et al (2018), by first using the domains program to invert the laboratory-derived kinetic data and heating schedule for feldspar domain structure. The same approach was taken for each sample when modeling the diffusion domain distribution: (1) use of a slab diffusion geometry; (2) modeling was only performed up to 1050-1100 °C, just before typical K-feldspar melting temperature; (3) the number of diffusion domains were allowed to be between 3 and 10;…”