Beltrando et al., 2010

Beltrando et al., 2010 - From passive margins to orogens:...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
559 ABSTRACT A lithostratigraphic association consisting of serpentinized man- tle rocks, continent-derived allochthons, mid-oceanic ridge gabbros of Jurassic age and post-rift sediments, typical of an ocean-continent transition, is found in the eclogitic Piemonte units, in the Western Alps. In situ U-Pb geochronology was performed on zircons from an ortho- gneiss sampled at the bottom of a sliver of continental basement, in contact with serpentinites. Primary magmatic zircons of Permian age were overgrown by a second generation of zircon at ca. 166–150 Ma, likely related to melt inF ltration associated with the intrusion of the underlying gabbroic body. This indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of hyper-extended crust related to the opening of the Tethys. Therefore, the complex lithological asso- ciation described here, which is also characteristic of several (ultra) high-pressure mélange zones worldwide, was acquired prior to the orogenic event, during which it was only partly reworked. Ocean-con- tinent transitions are in positions favorable to reach (ultra)high-pres- sure conditions, following negatively buoyant oceanic lithosphere into subduction, and then being accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction. The ocean-continent transition is now found in the immediate footwall of a 500-m-thick shear zone, which accommodated multiple episodes of deformation during Eocene–Oligocene time, suggesting an impor- tant link between Alpine deformation and rift-related structures. INTRODUCTION Alpine-type orogens sample oceanic domains and passive margins that formed during the breakup of Pangea. As studies of present-day analogues show, rifted margins comprise wide zones of exhumed crustal and mantle rocks that separate typical continental and oceanic crust (e.g., Manatschal., 2004). These zones, referred to as ocean-continent transi- tions, have been drilled along the Iberia and Newfoundland margins, and reF ection and refraction seismic studies reveal that they may extend over more than 50% of the present-day rifted margins (Péron-Pinvidic and Manatschal, 2009). Ocean-continent transitions are characterized by a complex lithospheric architecture, which results from multistage rift- related lithospheric thinning. In the case of magma-starved margins, such as the Alpine Tethys, rifting results in the exhumation at the sea F oor of serpentinized subcontinental mantle along top-basement detachment faults. Slivers of continental basement, occasionally associated with pre- rift and syn-rift sediments, locally overlie the detachment faults (e.g., extensional allochthons of Manatschal [2004]) and are sealed by post-rift sediments. Variable amounts of mid-oceanic ridge gabbros and volcanics can also be found associated with the serpentinites (Manatschal, 2004).
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

Page1 / 4

Beltrando et al., 2010 - From passive margins to orogens:...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online