“There’s no doubt that there was extensive glaciation. There’s no doubt that there have been sea-level changes throughout time. The question is, was that enough to put the Great Unconformity puzzle together?” Nicolas Flament says, as researchers attempt to resolve a centuries-old geological mystery. New research published in Nature examines how Earth’s mantle convection may have helped create the conditions behind the missing rock record known as the Great Unconformity.
Mantle Convection Simulations Test Dynamic Topography Theory
Geologists have long debated the origins of the Great Unconformity, a massive gap in the geological record. Previous hypotheses pointed toward continental rifting, stretching crust, or global sea-level drops. However, rifting primarily impacts continental edges, and sea-level changes should affect all continents uniformly. To test a fourth idea—that Earth’s mantle moves continents vertically through dynamic topography—Flament and his team ran numerical models.
Deep beneath the surface, solid rock moves in a slow convection cycle comparable to a lava lamp. Cold rock sinks at subduction zones while hot rock rises, exerting physical tugs on the Earth’s surface that raise or lower the ground by hundreds of meters. Dhungana and Flament executed six simulations of mantle convection stretching back 1 billion years. They used plate tectonic reconstructions to map regional elevations and determine where the unconformity formed and survived. Flament noted that the starting configuration of these computer simulations remains uncertain.
Continental-Scale Erosion Shaped by Deep Earth Geodynamics
Research published in Nature highlights how tectonic events influenced preservation across different continental blocks. During the assembly of Pangea, continental margins experienced collisions that either eroded or buried potential evidence of the unconformity. Laurentia possessed a large continental area with limited margin reworking, making it an ideal recorder for the continental-scale feature. By contrast, Baltica experienced repeated orogeny along its eastern margin, leaving the unconformity buried at depth.
North China presented a different puzzle given its small area and four episodes of orogenesis between about 500 Ma and 200 Ma. Early Cretaceous lithospheric delamination could explain the exposure of the unconformity there. Meanwhile, the prominent exposure in the Grand Canyon is linked to the Laramide uplift event from Farallon flat-slab subduction and subsequent Colorado River incision.
Dynamic Topography Alone Fails to Explain Denudation Slopes
Dynamic topography alone produces only gentle continental tilting, which would not create slopes steep enough for the fluvial erosion needed to explain the magnitude of denudation associated with the Great Unconformity. Researchers have therefore debated whether extensive glaciation and sea-level changes were enough to account for the missing rock record.
Frequently Asked Questions About the Great Unconformity
What is the Great Unconformity in classic locations like the Grand Canyon?
The Great Unconformity is a missing span of time in the geological record, prominently exposed in the Grand Canyon.
What processes have been proposed for the Great Unconformity?
Researchers have debated extensive glaciation, sea-level changes, continental rifting, and vertical movements driven by Earth’s mantle.
What role did Laurentia play in preserving this global geological feature?
Laurentia featured a large continental area with minimal edge reworking during supercontinent assembly. This stability favored the preservation of the continental-scale Great Unconformity that scientists study today.
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