Abstract
The Earth’s cratons are traditionally regarded as tectonically stable cores that were episodically buried by thin sedimentary covers. Cratonic crust in southern Finland holds seven post-1.7 Ga tiered unconformities, with remnants of former sedimentary covers. We use the geometries of the tiered unconformities, along with previously dated impact structures and kimberlite and carbonatite pipes, to reconstruct the erosion and burial history of the craton and to derive estimates of depths of erosion in basement and former sedimentary rocks. The close vertical spacing (<200 m) of the unconformities and the survival of small (D ≤ 5 km) Neoproterozoic and Early Palaeozoic impact structures indicate minor later erosion. Average erosion rates (<2.5 m/Ma) in basement and cover are amongst the lowest reported on Earth. Ultra-slow erosion has allowed the persistence in basement fractures of Phanerozoic fracture coatings and Palaeogene groundwater and microbiomes. Maximum thicknesses of foreland basin sediments in Finland during the Sveconorwegian and Caledonide orogenies are estimated as ~1.0 km and <0.68–1.0 km, respectively. Estimated losses of sedimentary cover derived from apatite fission track thermochronology are higher by factors of at least 2 to 4. A dynamic epeirogenic history of the craton in Finland, with kilometre-scale burial and exhumation, proposed in recent thermochronological models is not supported by other geological proxies. Ultra-slow erosion rates in southern Finland reflect long term tectonic stability and burial of the craton surface for a total of ~1.0 Ga beneath generally thin sedimentary cover.
Highlights
Cratons hold the Earth’s oldest rocks and landscapes
We examine the erosion and burial history of the craton in southern Finland
The sedimentary rocks preserved in impact structures (IMPs) indicate that (i) western and southern Finland retained Early Palaeozoic cover through the Mesozoic, whereas (ii) in eastern Finland, Early Palaeozoic cover persisted in the Triassic, but the basement was re-exposed by the Late Cretaceous
Summary
Cratons hold the Earth’s oldest rocks and landscapes. Yet cratonic denudation rates are poorly known over geological timescales because even slow denudation, if sustained, eventually removes evidence of its surface operation. There is a pressing need to test thermochronological models for cratons against independent geological evidence (Green et al, 2020). Finland is exceptional as it retains inherited base ment landforms, traces of sedimentary cover rocks, volcaniclastic kimberlite dykes and many eroded impact structures (IMPs) (Fig. 1A). These features indicate that the craton has experienced multiple megacycles of uplift, erosion, relief reduction and burial since 1.7 Ga. We derive high-resolution estimates of depths and rates of erosion in base ment and cover rocks from basement unconformities and the geometries of previously dated IMPs. The revealed erosion rates are amongst the lowest reported on Earth and indicate exceptional stability and pro longed, shallow burial of the craton.
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