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Structure
and Kinematics of a Syndepositional Paired Detachment System,
Triassic Ischigualasto
Basin, NW Argentina*
By
Brian
S. Currie1 and Tony Albrecht2
Search
and Discovery Article #30059 (2008)
Posted July 15, 2008
*Adapted
from oral presentation at AAPG Annual Convention,
San Antonio, Texas,
April 20-23, 2008
1Department of Geology, Miami
University,
Oxford, OH ([email protected])
2North America
Gas Reserves and Renewal, BP America, Inc.,
Houston, TX
Abstract
In the Ischigualasto basin of
NW Argentina, lacustrine/fluvial
deposits exhibit both extensional and contractional deformation features.
Extensional structures consist of north-striking listric normal faults that are
concentrated in an ~100 meter-wide zone at the eastern edge of the deformed
area. Contractional structures are concentrated along an ~250 m wide zone at
the western side of the deformed area.
The primary structure in this area is a
west-vergent thrust system that contains two primary imbricates and carries
upright to overturned folds with fault bend, fault propagation, and detachment
geometries. Faults in both eastern and western areas have similar magnitudes of
displacement (~80 m) and sole into a sub-horizontal detachment surface, below
which deposits are undeformed. All deformed strata are overlain by undeformed
fluvial deposits. Observed structures are consistent with deformation
associated with syndepositional gravity-induced displacement above an
overpressured detachment horizon.
The contractional frontal part of the system
was driven by extension produced by gravity spreading / depositional loading of
lacustrine delta platform and fluvial deposits, with lateral translation
occurring above an overpressured shale décollement. Deformation occurred at
shallow depths (<30 m) in relatively shallow water (<15 m) environments.
Although 2-3 orders of magnitude smaller, the overall geometry of the
Ischigualasto detachment system is comparable to passive margin detachment
systems. Based on these similarities, the deformation features, internal
geometry, and reconstructed kinematics of the Ischigualasto detachment system
may serve as a small-scale analogue for passive-margin paired
extensional-contractional systems.
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Selected Figures
Significance
-
Deformation features associated with
unrecognized lacustrine paired extensional-contractional detachment
systems may result in misinterpretations of basin tectonic histories.
- Fluid expulsion may produce elevated
pore-fluid pressures that facilitate the development of shallow paired
detachment systems.
- Scale-independent nature of
compressional wedge may produce early development of reservoir-scale traps
in rocks that contain both source and reservoir lithologies.
Conclusions
- Triassic-aged deformation near Agua de
la Peña canyon exhibits the characteristics of a paired extensional-contractional
detachment system.
- Deformation was driven by lacustrine
delta/fluvial channel deposition and resulting gravity spreading above a
shale detachment horizon.
- The Agua de la Peña detachment system
may serve as a small-scale analogue to paired extensional-contractional
deformation systems developed along passive margins.
References
Bilotti,  Frank , and John H. Shaw,
2005, Deep-water Niger
Delta fold and thrust belt modeled as a critical-taper wedge: The influence of
elevated basal fluid pressure on structural styles: AAPG Bulletin, v. 89, p.
1475-1491.
Bhattacharya,
Janok P., and Russell K. Davies, 2004, Sedimentology and
Structure of Growth Faults at the Base of the Ferron Sandstone Member Along
Muddy Creek, Utah, in AAPG Studies in Geology No. 50, p. 278 – 304.
Acknowledgements
Appreciation is
expressed to Carina Colombi, Museo de Ciencias Naturales, San
Juan, and to the Staff of the Ischigualasto
Provincial Park,
San Juan.
Appreciation is also
expressed for grants from:
AAPG Foundation Grants-in-Aid Program
GSA Student Research Grant Program
National Geographic Society
Miami University Department of Geology
Miami University
Hampton Fund
for International Studies
SOLE Environmental Research Fund.
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