Details of Award
NERC Reference : NE/D008263/1
Punctuated or continuous deformation during the exhumation of subducted crust? A linked tectono-metamorphic and geochronological study.
Grant Award
- Principal Investigator:
- Dr R Cliff, University of Leeds, School of Earth and Environment
- Co-Investigator:
- Dr J Dixon, University of Edinburgh, Sch of Geosciences
- Co-Investigator:
- Professor R Butler, University of Aberdeen, Sch of Geosciences
- Grant held at:
- University of Leeds, School of Earth and Environment
- Science Area:
- Earth
- Overall Classification:
- Earth
- ENRIs:
- Environmental Risks and Hazards
- Science Topics:
- Properties Of Earth Materials
- Tectonic Processes
- Abstract:
- This project is concerned with dating the metamorphic history of once deeply-buried crust and relating these dates directly to the deformation structures that formed while these rocks returned towards the Earth's surface. These materials (so-called high pressure (HP) metamorphic rocks / eclogites and blueschists) and their equivalents (greenschists, recrystallised under lower pressure conditions) are important recorders of the vertical movements in plate boundary zones and thus underpin many tests of geodynamic models. Important questions include: Are HP conditions experienced at distinct times to lower P (greenschist) conditions? Do different parts of an HP terrain exhume at different times, creating an amalgam of HP fragments? What are the rates of exhumation? Answers to these questions impact directly on a vigorous debate concerning subduction zone processes. Do these zones experience distinct periods of local tectonic divergence within otherwise convergent plate boundary zones with lithosphere-wide necking and exhumation of HP rocks punctuated by periods of renewed burial? Or, is material continuously cycled down and up subduction zones as detached 'pips'. To address the questions it is necessary to date the mineral fabrics that link to deformation and metamorphic conditions. We will use the rubidium-strontium method to date phengite mica growth and recystallisation, a method proven by us in deformation age dating of rocks sheared under greenschist facies conditions in the Alps. It is now timely to extend the approach to track rocks explicitly through decompression / from blueschist into greenschist facies. We will focus on the spectacular blueschists of the Attic-Cyclades belt exposed on the island of Syros. These are highly suitable not only because the tectono-metamorphic history is much more tractable on the outcrop scale than in the Alps but also because peak temperatures did not exceed 500C, the closure temperature for the Rb-Sr system in phengite. Existing models imply punctuated exhumation of the Syros blueschists rocks to greenschist facies conditions (from 20 kbar, ~70km depth to c. 6 kbar, ~20km). However, the models rely on isotopic ages for peak pressure metamorphic minerals (e.g. U-Pb ages on the mineral zircon) and cooling ages (Ar ages on micas, apparently dating the last experience of ~350C for the rocks). Neither approach can readily be related to specific deformation fabrics. The Rb-Sr method can. Consequently our research will establish, for the first time, the rates of decompression and related deformation over a wide range of exhumation conditions charted by single outcrops. Advances in micro-sampling and mass spectrometry now allow the detailed work necessary to date fabrics within specimens that show repeated mineral growth and structural overprint. We will build on the regional expertise and sample collections held in Edinburgh to focus on well-characterised structures. These researches show the Syros blueschists to be ideal for our purpose: the mica chemistries and relationships to metamorphic assemblages are very well characterised showing a wide range of pressure conditions with phengite growth and recrystallisation during deformation and decompression. The chemical composition of phengite is sensitive to the pressure conditions during mineral growth. And phengite commonly defines the deformation fabric in the rocks. By contrasting the dated decompression history of the HP rocks on Syros we will establish whether their exhumation was a pulsed, punctuated process or was continuous, and whether different parts of the HP terrain exhumed at different times and different rates. Not only will these results help to understand the geological evolution of the HP rocks on Syros, they will provide direct tests of the competing models for the exhumation of HP rocks, and provide fundamental data on the rates and dynamics of subduction zone processes.
- NERC Reference:
- NE/D008263/1
- Grant Stage:
- Completed
- Scheme:
- Small Grants (FEC)
- Grant Status:
- Closed
- Programme:
- Small Grants
This grant award has a total value of £43,105
FDAB - Financial Details (Award breakdown by headings)
| DI - Other Costs | Indirect - Indirect Costs | DA - Investigators | DA - Estate Costs | DA - Other Directly Allocated | DI - T&S |
|---|---|---|---|---|---|
| £10,163 | £6,582 | £10,866 | £1,832 | £11,618 | £2,043 |
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