Fig. 1: The BGS/Bridge wireline drill and various members of the shipboard party on the RV James Clark Ross at 150 45' N in the mid-Atlantic, June 2001These SEM images are from gabbro dredged from the footwall block of a low angle extensional detachment fault in the mid-Atlantic at 15o 45'N. The samples, together with oriented drill cores obtained using the BGS/BRIDGE wireline drill (see Fig. 1), were collected on NERC-funded research cruise JR63 using the BAS vessel James Clark Ross in May/June 2001.
Fig. 1: The BGS/Bridge wireline drill and various members of the shipboard party on the RV James Clark Ross at 150 45' N in the mid-Atlantic, June 2001
Further microstructural and geochemical work at Leeds on both dredge and drill samples has been funded by NERC starting 1/4/2002 - see here for updates on work in progress.
The detachment surface is corrugated and striated at scales ranging from the km to the mm, and consists of fault rocks composed of sheared serpentinite and cataclastic dolerite deformed under greenschist-facies conditions. Gabbro has not been found in the detachment fault rocks, but is present as km-scale bodies enclosed in footwall serpentinised peridotite. Cross-cutting relationships suggest that gabbros and associated dolerite dykes were intruded at the same time as movement on the detachment, which probably cut through the axial magmatic zone at a low to moderate angle. Gabbros are generally undeformed but locally show intense deformation and hydrothermal alteration in the greenschist to amphibolite facies as shown below. The orientation of these faults is uncertain since the samples were dredged, and they may or may not be directly related to the detachment fault.
Figure 2 shows a discrete shear zone marked by flattened aggregates of plagioclase (dark) and amphibole (bright). The wall rocks of the shear zone consist of less deformed pseudomorphs afetr original pyroxene and plagioclase, retaining a gabbroic texture with an original grain size of about 7 mm. Pyroxene is no longer present and is replaced by complex intergrowths of hornblendic and actinolitic amphibole. Calcic plagioclase is partially replaced by oligoclase, often along healed crack networks. Both shear zone and wall-rocks are cut by post-tectonic veins of ferroactinolite. Higher power images (Fig. 3,4) show that the amphibole and plagioclase are mixed together in a fine-grained matrix within the shear zone, although areas derived from original coarse feldspar and amphibole/pyroxene grains can be identified. Fluid flow during deformation is suggested by pervasive bright rims on amphibole, high Cl contents in amphibole, and extensive grain boundary porosity. Plagioclase in particular has thoroughly recrystallised after deformation.
Fig. 2: Low power backscatter SEM photo showing shear zone in altered gabbro dredged from the footwall of the detachment
Fig. 3: Hornblendic Cl-rich amphibole in the shear zone. Note pervasive, bright Fe-rich rims around the amphibole grains suggesting pervasive fluid flow, probably during cataclastic fault movement. Note also plagioclase (dark), interpreted to have been physically mixed with the amphibole during cataclasis.
Fig. 4: Plagioclase-rich area. Note uniform sodic plagioclase composition apart from calcic plagioclase porhyroclast at upper left. Bright amphibole is well mixed with the feldspar which has recrystallised to a foam texture. Grain boundary porosity is abundant.