Geology and timing of dextral strike-slip shear zones in Danmarkshavn, North-East Greenland Caledonides

Abstract

The North-East Greenland eclogite province is divided into a western, central and eastern block by the sinistral Storstrømmen shear zone in the west and the dextral Germania Land deformation zone in the east. A family of steep, NNW-striking dextral mylonite zones in the Danmarkshavn area are geometrically and kinematically similar to the ductile Germania Land deformation zone, located 25 km to the east. Amphibolite facies deformation at Danmarkshavn is characterized by boudinage of eclogite bodies within quartzofeldspathic host gneisses, pegmatite emplacement into the boudin necks and subsequent deformation of pegmatites parallel to gneissosity, a widespread component of dextral shear within the gneisses, and localization of strain into 10-50 m thick dextral mylonite zones. The gneisses and concordant mylonite zones are cut by a swarm of weakly to undeformed, steeply dipping, E-W-striking pegmatitic dykes. Oscillatory-zoned zircon cores from two boudin neck pegmatites give weighted mean 206 Pb/ 238 U sensitive, high mass resolution ion microprobe (SHRIMP) ages of 376 ± 5 Ma and 343 ± 7 Ma. Cathodoluminescence images of these zircons reveal complex additional rims, with ages from ranging from c. 360 to 320 Ma. Oscillatory-zoned, prismatic zircons from two late, cross-cutting pegmatites yield weighted mean 206 Pb/ 238 U SHRIMP ages of 343 ± 5 Ma and 332 ± 3 Ma. Zircons from the boudin neck pegmatites record a prolonged growth history, marked by fluid influx, during amphibolite facies metamorphism beginning at c. 375 Ma. The cross-cutting pegmatites show that dextral deformation in the gneisses and ductile mylonite zones had stopped by c. 340 Ma. Ultrahigh-pressure metamorphism in the eastern block at 360 Ma requires that the Greenland Caledonides were in an overall contractional plate tectonic regime. This, combined with 20 % steep amphibolite facies lineations in the eclogites, gneisses and mylonites suggests that dextral transpression may have been responsible for a first stage of eclogite exhumation between 370 and 340 Ma.

The Timing of Strike-Slip Deformation Along the Storstrømmen Shear Zone, Greenland Caledonides: U–Pb Zircon and Titanite Geochronology

“…The boudin neck pegmatites were emplaced during amphibolite-facies metamorphism since both the pegmatites and deformation fabrics along the eclogite margins involve retrograde minerals (e.g. Sartini-Rideout et al 2006, 2009. All of the Caledonian pegmatite samples are interpreted to reflect decompression and retrograde amphibolitefacies metamorphism of the western block between 395 and 390 Ma.…”

“…The Germania Land deformation zone (GLDZ) records dextral displacement and separates the central and eastern blocks . The GLDZ was active between 370 and 340 Ma, and responsible for partial exhumation of the eastern block (Sartini-Rideout et al 2006). Timing of deformation within the SSZ and documentation of the continuity or disparity between both protolith ages and timing of metamorphism across it are required to evaluate the significance of the SSZ and its potential role in sinistral transpression models for the evolution of the Greenland Caledonides (e.g.…”

“…The available age constraints suggest that the SSZ was also synchronous with dextral displacement on the Germania Land deformation zone (GLDZ), which was active between 370 and 340 Ma (Sartini-Rideout et al 2006) and has a conjugate relationship with the SSZ (Fig. 1).…”

Microstructural evolution within a partitioned midcrustal transpression zone, northeast Greenland Caledonides

“…This is not the case even within the most intensely deformed regions of the SSZ, which everywhere contains subhorizontal lineations. Sartini‐Rideout et al [2006] recently proposed that dextral transpression along steep shear zones in Danmarkshavn, approximately 100 km east of Dronning Louise Land, may have been responsible for the initial stages of eclogite exhumation in this area beginning at ca. 370 Ma.…”

Mid/Late Devonian-Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

“…We propose that the SISZ and potential other Caledonian shear zones along the SW Barents Sea margin were inverted as low-angle extensional shear zones during late/post-Caledonian orogenic extension and subsequent collapse. This is based on analog examples in northeast Greenland (Sartini-Rideout et al, 2006;Hallett et al, 2014;McClelland et al, 2016), western Norway (Séranne & Seguret, 1987;1185Séranne et al, 1989Wilks & Cuthbert, 1994;Osmundsen & Andersen, 2001), mid-Norway (Braathen et al, 2000) and Lofoten-Vesterålen Steltenpohl et al, 2004;Osmundsen et al, 2005). Extensional reactivaton of such ductile shear zones along the Barents Sea margin might have initiated in the Early Devonian as in LofotenVesterålen (Steltenpohl et al, 2011), by crustal thinning and orogenic collapse through dominant 1190 top-to-the-NW displacement along the SISZ and later exhumation of the SISZ and underlying basement as a metamorphic core complex.…”