Seafloor spreading at mid-ocean ridges is accommodated by a combination of magmatic accretion and tectonic stretching of the lithosphere. Recent investigations of mid-ocean ridges show evidence for large-offset normal faults or detachments that accommodate a significant portion of the plate separation. Once thought of as rarities along the global spreading system, detachment faults are now considered to play an integral role in seafloor spreading at slow- and ultra-slow-spreading ridges. Estimates of the proportion and extent of tectonic spreading however vary widely; some authors suggest as much as 50% of the Mid Atlantic Ridge is underlain by active detachment faults. The lack of consensus is in part a consequence of the difficulty in documenting the extent of tectonic stretching and detachment faulting, especially from surface morphology alone. On the modern seafloor tectonic activity may potentially be greatly under-estimated if lava flows blanket and obscure the faulting below. This thesis addresses the question of whether and how such ‘hidden tectonics’ may be accommodated beneath the seafloor by means of a field-based study of the Troodos ophiolite, Cyprus, which is believed to be a slow-spreading ridge analogue. My interdisciplinary approach utilises structural mapping, palaeomagnetism and geochemistry to investigate the spreading structure and mechanism of accretion of the Troodos upper crust. I document significant extensional faulting and rotation within the crust above a detachment fault at the level of the sheeted dyke complex, yet show that the lavas at the surface are sub-horizontal and unaffected by the deformation beneath. Unconformities in the lava section demonstrate progressively greater rotations deeper in the extrusive pile, controlled by tectonic stretching and tilting of the underlying dykes, rather than rotation by loading and/or subsidence within the lava pile as previously proposed. Ponding of late, primitive, low viscosity lava flows against normal faults on the seafloor thickens the extrusive sequence substantially and obscures the faulting below. The study shows that syn-tectonic volcanism is an effective mechanism for filling active half-grabens at slow-spreading ridges. Tens of percent tectonic stretching at depth may be completely masked at the surface by syn-tectonic lava extrusion. Such ‘hidden tectonics’ may be far more common at modern mid-ocean ridges than commonly supposed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:649436 |
| Date | January 2014 |
| Creators | Young, Ella C. |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/73633/ |
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