Fluids in extensional detachment systems



The data and the idea:

A survey of detachment systems in the North American Cordillera indicates that the footwall of detachment faults consists of a detachment shear zone that ranges in thickness from 100 m to 1 km. A systematic analysis of stable isotopes in synkinematic hydrous minerals (most commonly muscovite fish) within detachment mylonite zones shows that deltaD (Deuterium over Hydrogen ratio) has very low values. The figure below shows deltaD values across the km-thick Columbia River detachment shear zone that bounds the Thor-Odin dome in the Shuswap metamorphic core complex (British Columbia) as well as the 100 m thick shear zone that defines the Northern Snake Range metamorphic core complex. 



Mica in these mylonite zones has very low deltaD values consistent with interaction with meteoric fluids. In fact, values are so low that the catchment region of surface water was likely at high elevation; these values have been used as a paleoaltimetry tool (see list of publications below).



Application to Himalaya (Everest region):

This same analysis was conducted in the Mt Everest region. Although this area is an example of contractional tectonics, there is good evidence that a Miocene orogen-scale detachment shear zone developed along the Himalayan range (the South Tibetan Detachment, or STD). Analyses of synkinematic mica (dated at ca 15 Ma) and hornblende in the STD reveal extremely low deltaD values. This means that meteoric water reached the STD during Miocene time. When compared to published values of isotopic water composition in sedimentary rocks that formed near sea level (Siwalik formation), the Everest data suggest that the catchment area for this water was at an average elevation similar to or higher than today’s elevation. The figure below shows the present-day section across the Himalaya and the interpreted paleoelevation, presumed topography, and position of the STD in Miocene time. 




Continental and oceanic core complexes:



As the ocean floor is better mapped and spreading centers are better understood, there is growing evidence that core complexes that have been described on continents may also be well represented in the oceans. Detachment zones akin to extensional shear zones on continents have been documented in regions of slow and ultraslow spreading. The detachment system clearly forms by the rolling-hinge process, resulting in flattening, or even rollover of the detachment fault as it is exhumed. The fault is exposed on the ocean floor and is typically grooved, defining deep corrugations. Hydrothermal systems associated with oceanic core complexes develop to significant depth and therefore hydrate the crust and mantle, forming serpentinite.
This figure shows a schematic comparison between continental and oceanic core complexes with their main structural and fluid flow patterns (Whitney et al., 2013)







Publications by our group:

Gébelin, A., Mulch, A., Teyssier, C., Jessup, M.J., Law, R.D., Brunel, M. (2013) The Miocene elevation of Mount Everest. Geology, 41, 7, 799-802.
Whitney, D.L., Teyssier, C., Rey, P.F., Buck, W.R. (2013) Continental and oceanic core complexes. Geological Society of America Bulletin, 26 p., doi:10.1130/B30754.1.
Gottardi, R., Kao, P.-H., Saar, M.O., Teyssier, C. (2013) Effects of permeability fields on fluid, heat, and oxygen isotope transport in extensional detachment systems. G-Cubed, 14, doi:10.1002/ggge.20100.
Gébelin, A., Mulch, A., Teyssier, C., Chamberlain, C.P., Heizler, M. (2012). Coupled basin-detachment systems as paleoaltimetry archives of the western North American Cordillera. Earth and Planetary Science Letters, 335-336, 36-47.
Gébelin, A., Mulch, A., Teyssier, C., Heizler, M., Vennemann, T., Seaton, N.C.A. (2011) Oligo-Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada. Tectonics, 30, TC5010   DOI: 10.1029/2010TC002797.
Gottardi, R., Teyssier, C., Mulch, A., Vennemann, T.W., Wells, M.L. (2011) Preservation of an extreme transient geotherm in the Raft River detachment shear zone, Geology, 39, 759-762.
Mulch, A., Teyssier, C., and Chamberlain, C.P. (2007) Stable isotope paleoaltimetry of Eocene core complexes in the North American Cordillera. Tectonics, 26, TC4001, doi: 10.29/2006TC0011995.
Person, M., Mulch, A., Teyssier, C.; Gao, Y. (2007) Isotope transport and exchange within metamorphic core complexes. American Journal of Science, 307, no.3, pp.555-589.
Mulch, A., Teyssier, C., Cosca, M.A., and Vennemann, T.W. (2006) Thermomechanical analysis of strain localization in a ductile detachment zone. Journal of Geophysical Research, 111, B12405, doi: 10.1029/2005JB004032.
Mulch, A., Teyssier, C., Cosca, M., Vanderhaeghe, O., and Venneman, T. (2004) Reconstructing paleoelevation in eroded orogens.  Geology, 32, 6, 525-528.