HR: 09:45h
AN: T31F-08 [Abstracts]
TI: Amagmatic Extension and Thick Oceanic Crust in the Eastern Black Sea.
AU: * Scott, C L
EM: cls203@noc.soton.ac.uk
AF: National Oceanography Centre, University of Southampton,
European Way, Southampton, SO14 3ZH, United Kingdom
AU: Shillington, D J
AF: National Oceanography Centre, University of Southampton,
European Way, Southampton, SO14 3ZH, United Kingdom
AU: Brown, P J
AF: National Oceanography Centre, University of Southampton,
European Way, Southampton, SO14 3ZH, United Kingdom
AU: Minshull, T A
AF: National Oceanography Centre, University of Southampton,
European Way, Southampton, SO14 3ZH, United Kingdom
AU: Edwards, R A
AF: National Oceanography Centre, University of Southampton,
European Way, Southampton, SO14 3ZH, United Kingdom
AU: White, N J
AF: Bullard Laboratories, University of Cambridge,
Madingley Road, Cambridge, CB3 0EZ, United Kingdom
AB:
Volcanic rifted margins exhibit extensive syn-rift magmatism during extension and produce thick ocean crust
during initial spreading, whilst magma-poor margins are characterised by limited syn-rift magmatism, highly
stretched continental crust and areas of exhumed mantle. Often these two types of rifting can be seen along the
same margin (e.g., off eastern Canada), but the nature of the transition from one to the other is poorly understood.
New seismic refraction data collected in the Eastern Black Sea (EBS) provide provocative new constraints on this
type of transition. Previous studies have revealed a thick sedimentary package (8-9 km) underlain by thin crust in
the centre of the basin, but the nature of the crust has remained controversial. In spring 2005, we collected four
wide-angle seismic profiles in the EBS basin. Lines 2 and 3 are approximately orientated parallel to the inferred
direction of extension and located ~ 200 km apart. Line 1 connects these lines, traversing the centre of the
basin and sampling some of the thinnest crust. Along Line 3, the crust thins abruptly from ~ 32 km to ~
7-9 km in the centre of the basin. Within the thin crust the velocity increases along a shallow velocity gradient of
~ 0.35 s-1 from 5 km/s at the top to 6.5 km/s over a depth of ~ 5 km. The lower crust has a fairly
constant velocity of 6.5-6.75 km/s. This velocity structure indicates that the crust along Line 3 is most likely highly
stretched continental material. Mantle velocities beneath the crust are ~ 8.0 km/s, and there is no evidence
for extensive syn-rift magmatism. Further southeast, the crust along Line 2 is ~ 13 km thick and has two
distinct seismic velocity gradients. The top 4 km has a high velocity gradient from 4.25 km/s to 6.0 km/s. The
velocity gradient gradually shallows to reach seismic velocities of 7.2-7.5 km/s at the base of the crust. This
velocity structure is best interpreted as thick oceanic crust. At the eastern end of Line 1, where it crosses Line 2,
the velocity structure also indicates thick oceanic crust extending for ~ 150 km along the line. A transition
from oceanic to continental crust appears to occur ~ 60 km to the west of the intersection with Line 2 and
coincides with a crustal fault imaged in seismic reflection data and inferred from shipboard gravity data. To the
west of this fault the crust is ~ 6 km thick with a constant velocity gradient of ~ 0.35 s-1. The
thickness of the crust is fairly constant over a distance of ~ 60 km before thickening slightly and decreasing
again to ~ 7 km over a distance of 80 km. Line 3 intersects Line 1 at the western edge of this crustal
thickening. Our data suggest that the extension of the EBS basin is greater in the SE than the NW such that rifting
in the SE culminated with the onset of seafloor spreading. This supports previous hypotheses that the rift opened
by rotation of the Shatsky Ridge away from the Mid Black Sea High. A remarkable feature of the observed crustal
configuration is the existence of highly thinned continental crust immediately adjacent to thick oceanic crust.
These two crustal domains are separated by an interpreted transform fault, implying that rift segmentation exerts
control over the expression of magmatism in the rift or that there were significant along margin changes in mantle
temperature and/or composition at the time of rifting.
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting