Conditions of deformation, metamorphism and fluid flow in an oceanic detachment fault

(Proposal approved by NERC, Feb 2002)

See HERE for some preliminary SEM results from this project

One of the most significant discoveries of the BRIDGE programme was the existence of corrugated surfaces at inside corner ridge-transform intersections along the mid-Atlantic ridge(Cann et al., 1997; Tucholke et al., 1998). These surfaces have been interpreted as detachment faults and compared with continental extensional core complexes, and a range of models have been published to account for their origin (e.g. Tucholke et al., 1998; Karson, 1999). The geometry of the exposed portion of the surfaces is well constrained by multibeam and sidescan sonar, but little is known about their continuation below the surface, the way in which they interact with steeper faults bounding the axial valley, or their relationship to magmatism and fluid circulation. Outstanding questions include (see fig. 1):

Do the faults steepen downwards towards the ridge crest, or do they cut through the magmatic zone?
Is slip accommodated by magmatic extension in the footwall, or can it occur entirely amagmatically?
Do the detachments separate an upper plate with vigorous convective fluid circulation from a lower plate with only localised fluid flow, as often seems to be the case on the continents (McCaig, 1997)?
How important is serpentinisation in controlling rheology and tectonics, fluid flow patterns and alteration around detachments?

These questions can only be answered by combining the "remote sensing" data collected up to now with study of spatially well-constrained samples from detachments.

Cartoon showing possible geometries of a detachment fault at a ridge crest

Until this year, sampling of detachment surfaces has been confined to dredge and limited submersible sampling. In May 2001, a coherent, spatially constrained, sample set was collected from a corrugated surface north of the Fifteen-Twenty Fracture Zone in the equatorial Atlantic using the BGS/BRIDGE wireline drill (cruise JR63 led by Chris MacLeod, funded by GR3/11767 - Andrew McCaig was responsible for characterising cores and dredge samples on this cruise). More than 30 oriented cores up to 70 cm in length of variably deformed serpentinite, gabbro and dolerite were recovered. The importance of this unique sample set for the study of oceanic detachment faults cannot be overstated. Key observations are:

Corrugated surfaces in this area are not confined to inside corner positions (Fujiwara et al., 1999), and appear to reflect a more general extensional regime in an area of restricted magma supply up to 50 km from the transform. Gabbro and sheeted dykes are often absent in crustal sections (Escartin and Cannat, 1999).
Outcrops of the map-scale corrugated surface showed smaller-scale slickenlines in sheared serpentinite.
Cross-cutting relationships suggest that deformation, magmatism and hydrothermal alteration went hand in hand. Sheared fabrics are cut by dolerite chilled margins and amphibole veins, and dolerite fragments are incorporated in highly sheared serpentinite.
Dolerites and gabbros from the footwall show varying degrees of hydrothermal alteration in the greenschist to amphibolite facies with dark green amphibole, epidote and secondary plagioclase common in veins and cataclasites.
Very few samples (other than serpentinites) showed macroscopically ductile fabrics, but cataclasis appears to have occurred over a range of conditions possibly up to the lower amphibolite facies.

These observations indicate a close link between deformation, fluid flow and magmatism. One of the main aims of this proposal is to establish the conditions and time relations of deformation and intrusion with more certainty in order to answer questions a) and b) above

This proposal focuses on cataclastic dolerites and gabbros. Our specific objectives are as follows:

To establish the kinematics of fault movement from micro-scale indicators in oriented core.
To establish the conditions of deformation and fluid-rock interaction in the cataclasites and their wall rocks, and in particular whether any sequence of overprinting deformation and intrusion events under changing conditions can be observed. Temperature conditions will be estimated using available geothermometers based on amphibole-plagioclase-oxide assemblages (Holland and Blundy, 1994; Ernst and Liu, 1998) and chlorite composition (Cathelineau and Nieva, 1985). It is unlikely that reliable pressure estimates can be made from mineral assemblages, but if suitable fluid inclusions are found, microthermometry will provide additional P-T constraints. Additional data will come from parallel work on serpentinites by Javier Escartin and others in Paris (cf. Agrinier and Cannat, 1997; Escartin et al., 1997)
To make a preliminary assessment of the extent of fluid flow in the cataclasites and in footwall rocks (questions c) and d) above) using major and trace element geochemistry and stable and Sr isotopes. By comparing fault rocks with less deformed wall-rocks, microsampling of overprinting fabrics and vein-matrix analyses, directions of chemical change will be established. These are key observations in interpreting patterns of advective and diffusive mass-transfer in fault zones (Knipe and McCaig, 1994; McCaig et al., 1995; McCaig, 1997). The combination of oxygen and strontium isotopes is important because many oceanic and ophiolitic gabbros show alteration in the former but not the latter, whereas higher crustal levels are altered in both isotopes (Stakes et al., 1991; Kempton et al., 1991; Bickle and Teagle, 1992). It should also be possible to establish whether fluids altering mafic rocks have previously been involved in serpentinisation reactions, and vice versa, thus constraining flow paths.

These results will be combined with analysis (by allied researchers MacLeod and Escartin) of bathymetric and TOBI data collected on JR63, to produce a new integrated model for the kinematic evolution of the detachment and its interaction with magmatic and hydrothermal processes at the ridge crest.

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References

Agrinier, P., and M. Cannat, 1997. Oxygen isotopic constraints on serpentinization processes in ultramafic rocks from the Mid-Atlantic Ridge (lat. 23°N). In J. Karson, M. Cannat, J. Miller and D. Elthon (Eds.), Proc. ODP, Sci. Results, 153: College Station, TX, (Ocean Drilling program), 381-388.

Bickle, MJ & Teagle, DAH, (1992) Strontium alteration in the Troodos ophiolite… EPSL 113, 219-237

J.R.Cann, D.K.Blackman, D.K.Smith, E.McAllister, B.Janssen, S.Mello, E.Avgerinos, A.R.Pascoe & J.Escartin, 1997. Corrugated slip surfaces formed at North Atlantic ridge-transform intersections. Nature 385, 329-332

Cannat, M, Mével, C., & Stakes, D. 1991. Normal ductile shear zones at an oceanic spreading ridge: tectonic evolution of Site 735 gabbros (southwest Indian Ocean) Proc. ODP 118, 415-430.

Cathelineau M & Nieva D 1985. A chlorite solid solution geothermometer. Contrib. Min. Pet. 91, 235-244.

Ernst WG & Liu J, 1998. Experimental phase-equilibrium study of Al- and Ti-contents of calcic amphibole in MORB - A semiquantitative thermobarometer. Am Mineral 83, 952-969.

Escartin, J. & Cannat, M., (1999) Ultramafic exposures and and the gracvity signature of the lithosphere near the Fifteen-Trwenty Fracture Zone (Mid-Atlantic Ridge, 14-16 deg. N) EPSL 171, 411-424.

Fujiwara T et al. 1999. Bathymetry, Geomagnetic and Gravity Anomalies of the Mid-Atlantic Ridge between 14 and 16 degrees N. Eos Trans AGU, 80, F955.

Holland T & Blundy J 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contrib. Mineral. Petrol. 116, 433-447

Karson, JA, (1999). Geological investigations of a lineated massif at the Kane Transform Fault: implications for oceanic core complexes. Phil Trans. Roy. Soc A 357, 713-740.

Kempton, PD, Hawkesworth, CJ & Fowler, M (1991) Geochemistry and isotopic composition of gabbros from layer 3 of the Indian ocean crust, Hole 735B Proc. ODP, 118, 127-144

Knipe, R.J & McCaig, A.M., (1994) Microstructural and microchemical consequences of fluid flow in deforming rocks. Geol. Soc. Spec Pap. 78, 99-111

McCaig, A. M., Wayne, D. M., Marshall, J. D., Banks, D. A. & Henderson, I, (1995). Isotopic and fluid inclusion studies of fluid movement along the Gavarnie Thrust, central Pyrenees: Reaction fronts in carbonate mylonites. American Journal of Science, 295, 309-343.

McCaig, A. M. (1997) The Geochemistry of volatile fluid flow in shear zones In: Holness, M. (ed.) Deformation enhanced melt segregation and metamorphic fluid transport. Chapman and Hall, 227-260.

Stakes, D, Mével, C, Cannat, M. & Chaput, T., (1991) Metamorphic stratigraphy of hole 735B Proc. ODP, 118, 153-180.

Tucholke, BE, Lin, J & Kleinrock, MC, (1998). Megamullions and mullion structure defining oceanic core complexes on the Mid-Atlantic Ridge. J Geophys. Res., 103, 9857-986

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