The Late Great Unconformity of the Central Canadian Shield

Abstract

The Great Unconformity is a widely distributed surface separating Precambrian rocks from overlying Phanerozoic sedimentary sequences. The causes and implications of this feature, and whether it represents a singular global event, are much debated. Here, we present new apatite (U‐Th)/He (AHe) thermochronologic data from the central Canadian Shield that constrain when the Precambrian basement last cooled to near‐surface temperatures, likely via exhumation, before deposition of overlying early Paleozoic sedimentary sequences that mark the Great Unconformity. AHe data from 11 samples (n = 57) across a broad region define a similar date‐eU pattern, implying a common thermal history. Higher eU (>25 ppm) apatite form distinct flat profiles of reproducible dates at ∼510 ± 49 Ma (mean and 1σ standard deviation), while lower eU (<25 ppm) apatite define a positive date‐eU trend with younger dates. The data patterns, geologic context, and thermal history modeling point toward >3 km of erosion across the entire ∼450,000 km2 study area between 650 and 440 Ma, followed by modest reheating during later burial. Plume activity associated with intracratonic basin formation or continental rifting/breakup may have caused this erosion event. The post‐650 Ma timing of the last major sub‐Great Unconformity exhumation phase in this region implies a late Great Unconformity that is younger than inferred elsewhere in North America. This suggests that this feature is likely the result of multiple temporally distinct erosion events with differing footprints and mechanisms.

Existing thermochronologic data do not constrain Snowball glacial erosion below the Great Unconformities

“… 2 , because, for Athabasca, ref. 9 provides strong evidence for post-650 Ma (likely post-Snowball) basement exhumation in the central Canadian shield with a footprint likely encompassing the Athabasca region and, for Pikes Peak, we consider the Tavakaiv injectite relationships to indicate pre-Snowball exhumation ( 1 ). Left shows thermochronologic dates predicted by these t–T paths with the HeFTy software program using the same kinetic models as in ref.…”

Thermochronologic constraints on the origin of the Great Unconformity

“…, 28]. However, this proposal has not been without controversy [e.g., 29,30,31]. While some of this controversy may be attributable to di↵erences in terminology, significant points of contention remain-primarily, whether Neoproterozoic glaciation did or did not cause significant upper crustal exhumation.…”

“…In principle, thermochronology, which allows us to determine time-temperature (and thus exhumation) histories, is well-suited to resolve such questions. However, recent attempts [29,30,31], taken individually, fall short of truly resolving the critical questions.…”

Supplemental Material: Cryogenian glacial erosion of the central Canadian Shield: The “late” Great Unconformity on thin ice

“…NOTES ON MODELING: We did not impose a Precambrian surface "exploration box" between 800-440 Ma at 0-40 ¶ C, or a small 200-180 ¶ C box from 1100-1000 Ma derived from regional40 Ar/ 39 Ar K-feldspar data presented in McDannell et al(11)-as these are either assumed conditions lacking a physical geological basis and/or have the potential to unduly influence the inversion due to the limited sensitivity of the input AHe data to high (> 100 ¶ C) and low (< 50 ¶ C) temperatures. Instead, a large t-T box was placed between 1200-480 Ma from 250-0 ¶ C. The constraint/exploration boxes were the same as Sturrock et al(1) for the Phanerozoic, except the Ordovician "surface box" was extended to 40 ¶ C (less certain than locations near the Hudson Bay unconformity) and the upper temperature limit was set to 120 ¶ C for the larger boxes. Parameter settings for paths between boxes: segments halved 5 times and randomizer style was "gradual" with an imposed heating-cooling rate limit of 5 ¶ C/Myr.…”

“…Parameter settings for paths between boxes: segments halved 5 times and randomizer style was "gradual" with an imposed heating-cooling rate limit of 5 ¶ C/Myr. The modern surface temperature was set to 2 ± 2 ¶ C. All other HeFTy model settings were the defaults or the same as those in Sturrock et al(1).…”