 |
| Titel |
Detachments in Shale: Controlling Characteristics on Fold-Thrust Belt Style |
| VerfasserIn |
Rowan Hansberry, Ros King, Alan Collins, Chris Morley |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077778
|
|
|
|
|
|
| Zusammenfassung |
| Fold-thrust belts occur across multiple tectonic settings where thin-skinned deformation is
accommodated by one or more detachment zones, both basal and within the fold-thrust belt.
These fold-thrust belts exhibit considerable variation in structural style and vergence
depending on the characteristics (e.g. strength, thickness, and lithology) and number of
detachment zones. Shale as a detachment lithology is intrinsically weaker than more
competent silts and sands; however, it can be further weakened by high pore pressures,
reducing resistance to sliding and; high temperatures, altering the rheology of the
detachment. Despite the implications for petroleum exploration and natural hazard
assessment the precise nature by which detachments in shale control and are involved
in deformation in fold-thrust belts is poorly understood. Present-day active basal
detachment zones are usually located in inaccessible submarine regions. Therefore,
this project employs field observations and sample analysis of ancient, exhumed
analogues to document the nature of shale detachments (e.g. thickness, lithology, dip
and dip direction, deformational temperature and thrust propagation rates) at field
sites in Thailand, Norway and New Zealand. X-ray diffraction analysis of illite
crystallinity and oxygen stable isotopes analysis are used as a proxy for deformational
temperature whilst electron-backscatter diffraction analysis is used to constrain
microstructural deformational patterns. K-Ar dating of synkinematic clay fault gouges is
being applied to date the final stages of activity on individual faults with a view to
constraining thrust activation sequences. It is not possible to directly measure palaeo-data
for some key detachment parameters, such as pore pressure and coefficients of
friction. However, the use of critical taper wedge theory has been used to successfully
infer internal and basal coefficients of friction and depth-normalized pore pressure
within a wedge and at its base (e.g. Platt, 1986; Bilotti and Shaw, 2005; Morley,
2007). Therefore, through a mixture of field observations, sample analysis and
theoretical analysis it will be possible to determine a full range of shale detachment
parameters and their impact on the structural style of fold-thrust belts across a variety of
settings. Recent work in Muak Lek, central Thailand has focused on a structural
investigation of fold-thrust belt deformation of a passive margin sequence as a result of
continent-continent collision during the Triassic Indosinian Orogeny. Exceptional
outcropping of the detachment lithology is accessible in the Siam City Cement quarry
allowing construction of sections detailing the deformational style across the detachment
itself. The detachment forms complex, 3-dimensional duplex-like structures creating
egg-carton geometries enveloping foliation surfaces in the zones of most intense strain. Up
section strain decreases to discrete thrust imbricates of decametre scale. Samples of
limestone and secondary calcite were collected through the sections for oxygen stable
isotopes analysis which show a distinct pattern of isotopic fractionation across
the main thrust and into the detachment. Results from this study give insights into
the nature of shale detachments and the control on fold-thrust belt development. |
|
|
|
|
|
|