Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - DEEP TIME (Dynamic Earth Evolution and Paleogeography through Tomographic Imaging of the Mantle)

Teaser

DEEP TIME will unearth a record of geological time that is buried thousands of kilometres deep. The seafloor that covers two-thirds of the earth\'s surface is a tiny fraction of all seafloor created during its history – the rest has sunk back into the viscous mantle. Slabs of...

Summary

DEEP TIME will unearth a record of geological time that is buried thousands of kilometres deep. The seafloor that covers two-thirds of the earth\'s surface is a tiny fraction of all seafloor created during its history – the rest has sunk back into the viscous mantle. Slabs of subducted seafloor carry a record of surface history: how continents and oceans were configured over time and where their tectonic plate boundaries lay. DEEP TIME will follow former surface oceans as far back in time as the convecting mantle system will permit, by imaging subducted slabs down to the core with cutting-edge seismological techniques. Current tectonic plate reconstructions incorporate little if any of this deep structural information, which probably reaches back 300+ million years; they are based on present-day seafloor, which constrains only the past 100-150 million years.

DEEP TIME will match deep slab structure to the geological surface record of subduction – volcanic arcs and other crustal slivers that stayed afloat, survived collisions, and form the world’s largest mountain belts. Integrating these two direct records of subduction, the project will:

• Add paleo-trenches to existing plate reconstructions and extend them 2-3 times longer into the past.
• Produce a 3-D atlas of the mantle that matches subducted seafloor with paleo-oceans inferred by land geology.
• Rigorously test the hypothesis of vertical slab sinking, which may yield an absolute mantle reference frame.

Tomographic models and geological land records will be synthesized into quantitative and testable paleogeographic reconstructions that complement and extend existing ones, especially in paleo-oceanic areas. This is likely to transform our understanding of the earth’s physical surface environment and biosphere during Mesozoic times, as well as the formation of natural resources. It also will put observational constraints on elusive mantle rheologies. Nearly every subdiscipline of the earth sciences could benefit.

Work performed

Work on demonstrating an alternative paleogeography for North America on Jura-Cretaceous times resulted in a major publication. Work on a digital paleo-GIS model for this region’s evolution over the past 200 million years is close to submission and will be the first to include the many microplates that form large tracts of western North America. And additional publication on latitudinal translations of these microplates is underway.

Work on a new digital plate reconstruction for the western half of the Tethys Oceans has started and will be the first to systematically include subsurface information about subducted seafloor. A student thesis project initiated a similar effort for the Indian segments of the Tethys mountain belts.

Work is in an advanced stage for the first global tomographic model to include a large data set of broadband, P-diffracted waveform measurements. These should yield much better resolution of subducted seafloor in the lower third of the mantle that existing models. For DEEPTIME, this will be crucial because it permits to access the entire inventory of subducted slabs in the mantle and should enable identifying and dating them by comparison to the global orogenic record. As a technical prerequisite for this type of waveform tomography, we published the first systematic assessment of uncertainties in the inversion for source time functions (which describe the rupture processes of earthquakes that generate the seismic waves used).

We have also started work on the data portal that will make tomographic models, plate reconstructions, and subduction events available in a digital and integrated form for the first time.

Final results

Progress beyond the state of the art:
* Demonstrating an alternative paleogeography for North America on Jura-Cretaceous times (accepted for publication).
* The most detailed digital (GPlates) model of the evolution of an orogenic belt that exists to date (in progress).
* The first global tomography model that uses broadband, core-diffracted waves, for superior resolution of lowermost mantle structure.
* First systematic assessment of uncertainties in the earthquake source time functions used for global body-waveform tomography.
* A new plate reconstruction of the western Tethys (in progress).

Potential socio-economic and societal impact: This is basic research, we are not expecting direct economic gains anywhere near the time frame of this work. However, this general type of inquiry, i.e., an integrated, generalist approach to paleogeography, informs the exploration for natural resources.

Website & more info

More info: http://deeptime.earth.ox.ac.uk.