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GEO-TECTONICS  February 2005

GEO-TECTONICS February 2005

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Subject:

Re: Heterogeneous fault-slip data

From:

Julia Kramer <[log in to unmask]>

Reply-To:

Tectonics & structural geology discussion list <[log in to unmask]>

Date:

Tue, 8 Feb 2005 08:59:36 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (187 lines)

Atsushi, Mark, and others

first, thank you for your replies and thank you Atsushi, for your offer. I've
asked for permission to send the data and I am waiting for a reply. I am
certainly interested in testing the new version of your software. I have used
your current software version on my data and found that in the end, one still
has to manually/ visually select the calculated tensors, a process which might
be biased. Especially in a case, where two tensors have a similar orientation
(my case), one still makes use of additional field evidence to pick the tensors
that best fit to the additional data.

I would therefore like to raise the question if it is sensible at all to treat
the fault-slip data as if no other field observations have been made and if
they were the only information available. I have observed and recorded, for
instance, four superposed sets of fold axes with unequivocal relative age
relationships. Before searching for automatted routines, I have tried to
manually sort the fault-slip data so that they fit to the estimated orientation
of the stress field responsible for the orientation of folds, and thereafter
calculate stress tensors from the pre-sorted data, an approach which is also
chosen by Liesa and Lisle (who, however, don't manually sort their data). This
does of course not take into account possible (and probable) rotations of fold
axes during later deformation phases. It was only after several trials that I
thought that my manual-sorting approach (in lack of sufficient programming
skills) might be very unscientific and extremely biased. But is it really?

Regards, Julia





Quoting Mark Brandon <[log in to unmask]>:

> Julia,
>          Your objective is a difficult one. There remains much uncertainty
> about
> how to decompose fault slip data into events. Johnson (1995) provides a nice
> review about the activity of faults associated with a non-plane strain
> brittle
> deformation. He also discusses how fault orientation varies as a function of
> the state of stress. The issues he raises are important ones to consider
> given
> that they challenge some critical assumptions in the paleostress method.
>          You might try a different approach. Rather than trying to
> immediate decompose
> your data into fault event, you could ask the question what is the integrated
> brittle strain that has resulted from all of the faults. You may find that
> this result
> provides a more robust way to start interpreting your data. In this regard,
> the
> Twiss et al micropolar method may be a useful approach in that it focuses
> on the
> the deformation produced by the fault slip as indicated by the pattern of
> slickensides.
>          I have included references below. Note that historically, this
> issue of how to
> invert fault data for stress or strain has tended to generate many strong
> and divergent
> opinions. This forum may be a good one to hear what the current thinking is.
> Cheers,
> Mark
>
>
> Johnson, A. M., 1995, Orientations of faults determined by premonitory
> shear zones: Tectonophysics, v. 247, p. 161-238.
>
>   Twiss, RJ and MJ Gefell (1990) Curved Slickenfibers - a new Brittle Shear
> Sense Indicator With Application to a Sheared Serpentinite. IN: JOURNAL OF
> STRUCTURAL GEOLOGY. 471-481. PERGAMON-ELSEVIER SCIENCE LTD. OXFORD.
>
>   Twiss, RJ, GM Protzman and SD Hurst (1991) Theory of Slickenline Patterns
> Based on the Velocity-gradient Tensor and Microrotation. IN:
> TECTONOPHYSICS. 215-239. ELSEVIER SCIENCE BV. AMSTERDAM.
>
>   Twiss, RJ, BJ Souter and JR Unruh (1993) The Effect of Block Rotations on
> the Global Seismic Moment Tensor and the Patterns of Seismic P-axes and
> T-axes. IN: JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH. 645-674. AMER
> GEOPHYSICAL UNION. WASHINGTON.
>
>   Unruh, JR, RJ Twiss and E Hauksson (1996) Seismogenic deformation field
> in the Mojave block and implications for tectonics of the eastern
> California shear zone. IN: JOURNAL OF  Twiss, RJ and JR Unruh (1998)
> Analysis of fault slip inversions: Do they constrain stress or strain
> rate?. IN: JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH. 12205-12222. AMER
> GEOPHYSICAL UNION. WASHINGTON.
>
>
>
>
> At 09:22 AM 2/7/2005, you wrote:
> >Dear list members,
> >
> >I currently try to separate stress tensors from heterogeneous fault-slip
> data
> >collected at the eastern margin of the Tibetian Plateau, where I found six
> (or
> >more?) superposed deformation phases (D1 to D6/7/8?) - the first two of
> which
> >record ductile, the succeeding phases brittle deformation.
> >
> >The data set for the brittle faults (D3 to D6/7/8?) comprises the
> >orientation of
> >the faults and associated slickensides or slickofibres on the fault
> >surface and
> >the sense of movement derived thereof. Fault planes frequently exhibit two,
> >seldomly three different sets of lineations. However, overprinting
> >relationships are usually unclear, as lineations have formed on several
> >parallel planes within the 1-3 cm width of the fault ("zone") and the
> >individual layers have weathered off irregularly. Fault planes have
> therefore
> >not only been newly built, but have certainly been reactivated and
> >demonstrably
> >been rotated.
> >
> >There is some control of the approximate orientation of the paleo-
> >stress-field,
> >derived from the orientation of fold axes and large-scale shear zones.
> >For a more precise calculation of the stress tensors from the fault-slip
> data,
> >however, I am in need of some computer software capable of differentiating
> >between tensors of several deformation phases.
> >
> >Does anyone of you know some software that could perform such operation? I
> am
> >aware of the recent publications of
> >
> >Liesa and Lisle: Realibility of methods to separate stress tensors from
> >heterogeneous fault-slip data. J Struc Geol, 26 (2004), 559-572
> >
> >and Yamaji: The multiple inverse method: a new technique to separate
> stresses
> >from heterogeneous fault-slip data. J Struc Geol 22 (2000) 441-452 (not so
> >sucessful with my data)
> >
> >Any hint is highly appreciated,
> >
> >Regards
> >Julia
> >
> >
> >Dr. Julia Kramer
> >Postdoctoral Researcher
> >Economic Geology Research Institute(EGRI)
> >School of Geosciences
> >University of the Witwatersrand
> >Private Bag 3
> >2050 Wits, Johannesburg
> >South Africa
> >
> >Office: +27-11-717 66 11
> >Mobile phone: +27-72-997 50 72
> >Email: [log in to unmask]
> >Or: [log in to unmask]
> >
> >----------------------------------------------------------------
> >This message was sent using IMP, the Internet Messaging Program.
>
>
> ________________________________________________________________________
> Mark Brandon, Professor, Dept. of Geology and Geophysics
> Yale University, P.O. Box 208109, 210 Whitney Avenue, New Haven, CT
> 06520-8109
> e-mail: [log in to unmask]
> wk. phone: +203-432-3135, wk. fax: +203-432-3134
> Dept. Web site: http://www.geology.yale.edu
> Brandon's site: http://www.geology.yale.edu/~brandon
> ________________________________________________________________________
>


Dr. Julia Kramer
Postdoctoral Researcher
Economic Geology Research Institute(EGRI)
School of Geosciences
University of the Witwatersrand
Private Bag 3
2050 Wits, Johannesburg
South Africa

Office: +27-11-717 66 11
Mobile phone: +27-72-997 50 72
Email: [log in to unmask]
Or: [log in to unmask]

----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.

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