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| Titel |
Constraints on strain rates during large-scale mid-crustal shearing: An example from the basal Vaddas shear zone, northern Caledonides |
| VerfasserIn |
Deta Gasser, Holger Stünitz, Pritam Nasipuri, Luca Menegon |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075155
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| Zusammenfassung |
| The Caledonian orogen in Scandinavia is characterized by large-scale crustal nappe stacks
which were emplaced east-/southeast-wards onto the Baltica shield. Whereas original thrust
relationships are generally obscured by syn- to post-collisional extensional deformation in the
southern and central Scandinavian Caledonides, several large-scale thrust systems are
well-preserved in the northern Scandinavian Caledonides in Troms and Finnmark. One
example is the mid-crustal Vaddas shear zone, which emplaced the Vaddas nappe on top of
the Kalak nappe complex. In this contribution we present a structural, petrological and
geochronological analysis of the rocks under- and overlying the Vaddas shear zone in
northern Troms, in order to estimate the strain rate associated with thrusting along this major
shear zone.
The Vaddas nappe above the investigated shear zone consists mainly of Upper Ordovician
to Silurian metasediments, which were deposited in a marine environment and which were
intruded by voluminous gabbroic intrusions, before they were sheared off from
their substratum and transported on top of the Kalak nappe complex during the
Caledonian orogeny. PT conditions from one of these gabbroic bodies indicate
that the body intruded the metasediments at ~9 kbar (Getsinger et al., subm to
G3), which corresponds to a depth of ~34 km. U-Pb SIMS dating of zircons from
this gabbro indicate that intrusion occurred at 439±2 Ma. The Vaddas nappe is
separated from the Kalak nappe by an at least ~150 m thick, amphibolite-facies shear
zone with a subhorizontal fabric and top-to-the-SE shear sense. It has developed
within the lowest part of the Vaddas nappe as well as the upper part of the Kalak
nappe complex and PT calculations indicate that final shearing occurred at ~450Ë
C and ~6 kbar (depth of ~23 km). U-Pb TIMS dating of titanites, which grow
parallel to the shear fabric in the Kalak nappe complex, gives 206Pb/238U ages
ranging from 442±1 to 429±1 Ma, indicating that shearing probably commenced
right after intrusion of the Vaddas gabbros and continued over a period of ~10
m.y.
Assuming an original dip of the Vaddas shear zone of 30Ë , the horizontal displacement
would be ~23 km as the most conservative assumption (i.e. for a highest thrust angle) for the
depth difference of 11 km between intrusion of the gabbro and late shear deformation. Given
the thickness of the shear zone of 150 m and the time for the shearing of ~10 m.y., a strain
rate of ~5*10-13 s-1 can be estimated. As suggested, this is a conservative estimate, so that
strain rates may well have been faster. These strain rates appear to be similar to those of
Alpine nappes, so that it is suggested that Caledonian nappe stacking has probably taken
place at the same rates or even faster than Alpine ones. Given the fact that the transport
distances in the Caledonides are far greater than in the Alps, this would suggest that the
Caledonian orogeny has taken place over a longer period of time than in the Alps. |
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