Tectonism rather than "snowball Earth" glaciation is responsible for the Great Unconformity

Abstract

The Great Unconformity (GUn)-a widely recognized discontinuity and associated gap in the rock record between Precambrian and Cambrian rocks-represents a globally important interval of continental exposure and erosion that is notable also for the first appearance of all major animal phyla on Earth. However, its origin remains the subject of vigorous debate. Here, we present field relationships, and zircon and monazite U-Pb, biotite and muscovite Rb-Sr, and zircon (U-Th)/He thermochronology data for Precambrian crystalline basement rocks from North China to constrain the exhumation history below the unconformity. Dates from multichronometers and thermal history inversions show that the most substantial cooling of continental basement took place from ~2,100 to 1,600 Ma. Comparison with thermal history data from Laurentia, Baltica, and Amazonia suggests that protracted plate tectonics broadly modulated by supercontinent cycles, and not "snowball Earth" glaciation, is responsible for crustal exhumation below the unconformity. The most pronounced erosion evident in both the thermochronologic record and geochemical indicators of continental weathering is shown to correspond with development of Earth's first true supercontinent (Columbia), rather than with either the Cambrian explosion or the emergence of modern plate tectonics.

Keywords: Cambrian explosion; Great Unconformity; basement exhumation; multiple thermochronology; supercontinent cycle.

Fig. 1.

Tectonic framework of North China showing the distribution of Archean to Paleoproterozoic basement, Meso- to Neoproterozoic and Cambrian strata with the sites of sampled sections. The Qingshuihe, E’hutan, and Xiweikou sections lie in the cratonic interior. Samples Subaigou and Luotuoshan represent the cratonic margin. Geologic and petrologic details for each sample are provided in SI Appendix, Figs. S1–S6.

Fig. 2.

Field relationships of the Great Unconformity (GUn) in North China. (A) The GUn across cratonic interior at the Xiweikou section (GPS: 35°43′42.33″N, 110°43′11.85″E) in the southernmost Lüliang Mountains, with ~520-My-old nearshore sandstone overlying 2,182-My-old granitic basement. (B) The basement nonconformity near cratonic margin at the Subaigou section (GPS: 39°10′13.65″N, 106°57′49.14″E) in the western part of Inner Mongolia (northwest China), with ~1,750 to ~1,600-My-old nearshore sandstone on 2,014-My-old basement rocks of schist.

Fig. 3.

Zircon (U-Th)/He (ZHe) data. (A) ZHe date versus effective uranium concentration (eU) plot for all samples. Uncertainties of eU are conservatively assumed to be 15%. Uncertainties on individual ZHe dates are reported at 2σ, and most are smaller than the datum marker size. (B) Plot of ZHe date versus spherical grain radius.

Fig. 4.

Thermochronologic and thermal history model results. (A) Zircon and monazite U–Pb, biotite/muscovite Rb–Sr, and zircon (U–Th)/He (ZHe) age data obtained from five basement samples. (B–F) QTQt time‐temperature (t–T) inversions of new thermochronologic data showing consistent thermal histories with rapid late Paleoproterozoic cooling. Colors refer to the probability of a thermal history to pass through a given point. Thin and thick black lines show the 95% credible intervals of the expected model and a weighted average of all accepted models (weighted by the posterior probability of each model). All models enforced geologic (orange boxes) and chronologic constraints (red boxes). Episodes of Columbia supercontinent cycle (27) and “snowball Earth” glaciations (28, 29) are highlighted with pale orange and airy blue vertical bands, respectively.

Fig. 5.

Summary of inverse thermal histories of cratonic interiors in the context of Earth systems evolution. (A) Synoptic thermal histories of interior North China, Baltica, Amazonia, and Laurentia show significant cooling during supercontinent cycles of Columbia and Rodinia (pale orange bands) prior to snowball glaciation (airy blue bands). Timeline of supercontinent Columbia and Rodinia highlighted with pale orange vertical bands is from ref. . Thermal histories are shown by expected t–T paths, which represent “preferred” single path within 95% credible intervals of the expected model (68). T–t paths of Baltica and Amazonia are from Paleoproterozoic basement in the Laxemar region of the Fennoscandian Shield (Sweden, 51) and late Paleoproterozoic to early Mesoproterozoic Rio Negro–Juruena basement of the western Guiana Shield (Colombia, 69). T–t paths of Laurentia are from Paleoproterozoic Granite Gorge Metamorphic suite and granitoids of the western Grand Canyon (Arizona, 14, 15), Mesoproterozoic granitic batholith of the St. Francois Mountains of the Ozark Plateau (Missouri, 5), Archean basement of the Laramide ranges (Wyoming, 70), Paleoproterozoic crystalline rocks of the Cookes Range (New Mexico, 71), Mesoproterozoic basement of the Carrizo Mountains (Texas, 71), and late Mesoproterozoic Pike Peak batholith (Colorado, 6) of the southern Rocky Mountains, Neoarchean basement of the central Canadian Shield (Manitoba, Canada, 9), Paleoproterozoic Superior Province and Mesoproterozoic Grenville Province of the southern Canadian Shield (Ontario, Canada, 46). Thermal histories of these regions are depicted to represent Laurentia because thermochronologic data are available for basement rocks directly overlain by Sauk sequence strata. Ca, Carrizo Mountains; Co, Cookes Range; M, Mesoproterozoic Grenville Province; P, Paleoproterozoic Superior Province; Pi, Pikes Peak batholith. QTQt time–temperature inversions of published thermochronologic data showing expected thermal history paths of these regions are in SI Appendix, Figs. S11–S22. (B) The seawater strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) curve (72) and histograms of global detrital zircon age distributions (73) showing several populations in their U-Pb crystallization ages over the course of Earth history that are very similar to supercontinent cycles. Bin width is 25 My. (C) The cadet blue dashed curve shows the global running median of zircon δ¹⁸O data from recent sediment (74). The green solid curve shows a recently improved curve of zircon δ¹⁸O from 1,000 Ma to present after removal of isolated data contributions (75). Yellow and brown lines represent the global running median of εHf for detrital zircons and εNd for whole-rock sediments and granitoids (76), respectively. Gray rectangles show periods of major rises of ⁸⁷Sr/⁸⁶Sr ratios through Earth history.