Terminology about detachments and decollements is rather confusing, and much
more confusing is the practice of using this terminology in the field. As a
matter of fact, detachment is an almost exact English translation of the
French decollement (unsticking). The terms are explained by Ramsay & Huber
(1987) based on Wernicke & Burchfiel (1982) and other authors as follows
(pp. 516-518):
".... The low angle major base to the complex normally faulted terrain (in
the hanging wall, my remark) runs close to, but not absolutely parallel
with, the contact between a crystalline metamorphic basement complex and an
overlying sedimentary cover. These large low angle normal or extensional
faults have been termed detachment faults. Although this term has been
connected with the historically well established French term decollement
fault (German Abscherung) the prtesent usage of these terms is not quite
synonymous. The term decollement (unsticking) is generally related to some
special stratal control of the sole fault in a particular soft or
incompetent horizon ... . In contrast, the detachment fault is not exactly
parallel to any one incompetent horizon, even though the fault surface is
controlled by the competencies and the overall orientation of the boundaries
between the differing rock units ...".
As usual, both terms have been used in quite different sense by many
scientists. Lister & Davies (1989) and many other authors describe
detachments as low-angle normal faults that have undergone enormous
extension, with almost compulsory exhumation of high-grade metamorphic rocks
from the lower crust and/or upper mantle in the footwall. This very popular
idea, when implemented to a given terrain, have been very often
(unfortunately) expanded, and this with considerable distortion of the field
evidence (consult the Codes of Ethics, please!). Field examples may be found
at http://www.geology.bas.bg/rgh/lowangle.html
and http://www.geology.bas.bg/rgh/detachhoax.html!
Quite recently, Westaway (1998, 1999) has demonstrated that such kind of
low-angle normal faults with exhumation of very deep crustal levels are
feasible only in a very special type of stress field that could be found
possibly only at major plate boundaries.
Several points may arise about the use of the terms:
* THE PROBLEM OF SCALE: "detachments" and "decollements" if considered only
from the structural viewpoint (detachment or unsticking), may be found at
any scale: from the microscopic to the regional. Boundaries between media
with contrasting rheological properties are particularly favourable for
initiation of parallel or slightly inclined faults of that kind during
folding, thrusting or normal faulting with block tilting and/or rotation.
Such structures are called differently as, e.g., "intraformational glides",
etc. Implicitly, most of the scientists use "detachment" and "decollement"
only for the large (how large?) structures; in such a case, there should be
a name for the small-scale phenomena.
** THE PROBLEM OF SENSE OF MOVEMENT: "detachment" is usually used for
low-angle normal fault whereas "decollement" is usually referred to
thrusting or similar shear zones at the base of nappe structures. How about
fault surfaces with multiphase movements with different kinematics?
*** THE PROBLEM OF THE ROCKS IN THE FOOTWALL: is exhumation of "metamorphic
core complexes in the footwall" a "must" for to have a detachment? And is
such an exhumation that easy??
Hence,**** THE MOVEMENT VELOCITY AND THE TOTAL DISPLACEMENT ALONG
DETACHMENTS: in case you exhume eclogites along a detachment dipping at an
angle of 30 degree, you must have (for a crustal thickness of 40 km) a
horizontal displacement of nearly 70 km, and a displacement along the
low-angle fault (detachment) plane of 80 km. In many cases of alleged
detachments, the footwall metamorphics are directly covered by basal
conglomerates that contain rounded and weathered pebbles from the same
metamorphic basement. The metamorphic basement bears no traces of special
metamorphism related to the particular contact (detachment surface). Thus,
you have 1) contradiction to the basic principles of geology, and 2)
contradiction to the physical laws (s. Westaway, 1999).
***** Florian's question
>is there a relation to something like a basement and how is this relation
>characterized?"
should be considered very seriously. Try to think as geologists, and not as
physicists dealing with formulas and graphs; and try to make a real
geological mapping of a region with "detachments" for to see how many of
these "detachments" turn to be hoaxes!
Ivan
At 13:13 14.12.2001 'ã.'²U¸œI +01, you wrote:
>hi everybody!
>out of a discussion i began to wonder about the exact definition of the term
>"detachment"? what is the difference between "detachment" and "décollement"?
>does any of these terms tell me necessarily anything about the movement
>direction on the plane (is a detachment necessarily a normal fault)? is
>there a relation to something like a basement and how is this relation
>characterized? can the term "detachment" stand for its own, or is it better
>combined with either "thrust" or "fault"?
>regards,
>florian fusseis
>
>***************************************
>Freie Universität Berlin
>Fachbereich Geowissenschaften
>B304, Malteserstr. 74-100, 12249 Berlin
>email: [log in to unmask]
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>
Ivan Zagorchev, Res. Prof., D. Sc., Ph. D.
Vice-President of the Bulgarian National Committee of Geology
Chief Editor of Geologica Balcanica
Geological Institute, Bulgarian Academy of Sciences
Acad.G.Bonchev str. bldg. 24, 1113 Sofia
fax (00359-2) 724638
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