Dear Richard,

We have recently been looking at displacement gradients and differential exhumation on a set of normal faults in the Gulf of California rift. Maximum finite displacement was not as high as in your case, but we did see increases in fault displacement from <1-2km to ~7-9km over an along-strike distance of about 10-15km, and this too is accompanied by a change from high- to low-angle normal faulting (although there are no mylonites exposed). In our case, the faults are highly curviplanar and exhibit a series of antiformal/synformal megamullions with different "fold" limbs showing different fault kinematics. We attributed both the megamullions and the changes in fault kinematics to the constrictional strain regime of the PA-NA plate boundary.

Have a look at the following papers (sorry for the shameless advertising) for additional details on the structural and thermochronological evolution (and stay tuned for a paper on the stratigraphic evolution of the supra-detachment basin currently in review):

Seiler, C., Fletcher, J.M., Quigley, M.C., Gleadow, A.J.W., and Kohn, B.P., 2010, Neogene structural evolution of the Sierra San Felipe, Baja  California: Evidence for proto-gulf transtension in the Gulf Extensional Province? Tectonophysics, v. 488, p. 87-109.
Seiler, C., Fletcher, J.M., Kohn, B.P., Gleadow, A.J.W., and Raza, A., 2011, Low-temperature thermochronology of northern Baja California, Mexico: Decoupled slip-exhumation gradients and delayed onset of oblique rifting across the Gulf of California. Tectonics, v. 30, p. TC3004.

King regards,
Christian



On 13/03/2012 12:27 PM, Richard Styron wrote:
[log in to unmask]" type="cite"> Assembled tectonicists, 

In a field area of mine in SW Tibet, I have structural and thermochronological evidence for an increase in extension (heave) of a normal fault from ~2-3 km to ~15-18 km over an along-strike distance of about 15 km; this accompanies a change from high- to low-angle normal faulting and core-complex-like features (e.g., a well-developed normal-sense mylonitic shear zone).  There is no obvious sign of transfer of extension to other structures; at this latitude, the fault seems to be the only game in town (other faults do play a role at greater distances though).  The fault itself is >50 km along strike, but the zone of highest extension and low-angle normal faulting is restricted to near the middle of the fault.  I have had trouble finding analogs in the literature for this magnitude of increase in extension over such a short distance.  Does anybody know of other systems like this?

I am also interested in what this might mean for crustal rheology.  The upper plate shows little sign of deformation, although within 10 km of the fault, the hanging wall is covered in a thick pile of moraine and alluvium.  But that magnitude of extensional gradient over that distance intuitively suggests to me that the elastic thickness of the crust is very thin, and probably rather weak.  The hanging wall might be deforming via distributed shear some how.  There is a bit of literature on this (e.g., Ebinger et al., 1999, Phil. Trans. Royal Soc. London) but I am not finding a whole lot.  Are there any key studies or insights out there that I might borrow?

thanks,
Richard Styron
University of Kansas

-- 
Dr Christian Seiler
School of Earth Sciences 
University of Melbourne
Victoria 3010, Australia

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