Structure and Evolution of the Austral Basin Fold

Structure and Evolution of the Austral Basin Fold-Thrust
Belt, Southern Patagonian Andes: Insights from 4D
analogue models
1,2
1,3
3
1,2
1,3
Jeremías Likerman , Juan Francisco Burlando , Vanesa Barberón , Ernesto O. Cristallini and Matias Ghiglione
1
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Avda. Rivadavia 1917, CP C1033AAJ, Ciudad de
Buenos Aires, Argentina
2
Laboratorio de Modelado Geológico (LaMoGe), Instituto de Estudios Andinos Don Pablo Groeber. Universidad de Buenos
Aires, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina.
3
Laboratorio de Tectónica Andina, Instituto de Estudios Andinos Don Pablo Groeber, Universidad de Buenos Aires.
Departamento de Ciencias Geológicas, Buenos Aires.
*Contact email:
[email protected]
Abstract.
The models presented in this work were
designed to simulate the development of an asymmetric rift
system undergoing differential extension to study, the role
of distinct factors controlling the final deformation pattern of
the Austral Basin Fold-Thrust Belt. In particular, we
intended to reproduce the structural settings of the studied
region and to contrast the results with the inverted rift
hypotheses suggested by previous studies.
In agreement with the idea that similar variations found in
the Patagonian fold-thrust belt coincide with ancient
transfers zones between the depocenters, strong variations
in width and lateral position of main thrusts during
compressional stage of the model occurs near the induced
transfer zone. Inversion of extensional structures show a
geometry that is consistent with cross-sections from field
and seismic data for the proposed study area.
2 Experimental Settings
Two experiments were carried out: in the first one we
generate an asymmetric rift setting, and in the second one
the extensional stage was followed by the compression and
inversion of the system. The experiments were performed
in a non-boundaries deformation rig with dimensions 53 x
70 x 4 cm (Fig. 2a). The modelling materials were well
sorted dry quartz sand with well-rounded grains smaller
than 500 µm, and a basal of a ductile material, selected to
simulate stretching at the base of the brittle upper crust.
Above the ductile level, sand was mechanical sieved over
the modelling rig in order to get homogeneous layers (with
an average thickness of 0.5 cm). The extensional phase
was run with an offset baseplate to make a difference in
speed between the two sectors of the rifted area, while for
the compressional phase, the baseplates moved together at
the same speed (Fig. 2b).
The extensional phase was generated with the movement
of two backstops on the left side of the experiment with
two motors driven worm screw at different speeds. They
were 2.5 cm apart on x-axis at time zero of the experiment.
The northern one moved with a constant displacement rate
of 3.3 x 10-3 cm.s-1 and the southern one did at 2.3 x 10-3
cm.s-1. Extension was ended when the two walls were in
the same position of the x-axis. As synkinematic
sedimentation was not simulated, accommodation space
generated by extension was filled with blue colored sand.
Then, a post-rift 1 cm thick brown colored sand package
was added over the models. Subsequently the
compressional stage began with a single left wall moving
in the opposite direction to the extension at a constant
displacement rate of 2.4x10-3 cm.s-1 until reaching a final
shortening of 10cm (Fig. 2b).
A fixed high-resolution digital camera was used to image
the top surface of the models every 1 minute. Using a
ESCAN laser scanner a 3D point cloud of the upper
surface of the models was obtained every 2 minutes. At the
end of the deformation, the models were finely sliced
(~5mm) in order to generate a 3D block showing internal
structural arrangements of the final state of the models.
Keywords: analogue models, austral basin, foreland basin,
asymmetric rift
1 Introduction
A series of analogue experiments have been carried out in
order to simulate the structural evolution of the Austral
Basin Fold-Thrust Belt in the Southern Patagonian Andes.
Scaled analogue models have proved to be powerful tools
for the visualization of the progressive evolution of rift
fault systems reactivation (McClay, 1995; Mcclay, 1990;
Yagupsky et al., 2008). The models presented in this work
were designed to simulate the development of an
asymmetric rift system. Previous authors as Arbe (1989),
Kraemer (1998) and Ghiglione et al. (2009) proposed for
the Lago Argentino – Torres del Paine area (Fig. 1), a
structural style and mechanic stratigraphy that reflects the
control of positive inverted extensional depocenters of
Jurassic age during late Cretaceous uplift of the Andes. In
particular, we intended to reproduce the structural settings
of the studied region and compare the modelling results
with the inverted rift hypotheses, in order to understand the
structural controls exerted by the extensional fault system
during the compressional regime that took place since
times in the Austral Basin Fold-Trust Belt of the Southern
Patagonian Andes.
209
3 Model Results
results show that:
1. There is a larger density of structures in the depocenter
sector of maximum stretching. This is evident in the
northern sector of the model and can be compared to
the Río Guanaco and Torres del Paine sectors in the
Patagonian Andes (Fig. 1).
2. A strong variations in width and lateral position of
main thrusts during compressional stage of the model
occurs near the transfer zone (Fig. 3c), in agreement
with the idea that similar variations found in the
Patagonian fold-thrust belt coincide with ancient
transfers zones between the depocenters, possibly due
to the development of an asymmetric rift.
3. The inversion of extensional structures show a
geometry that is consistent with cross-sections from
field and seismic data for the proposed study area.
Modelling results were divided in four stages for
description purposes: (1) The first increments of
deformation (1,5 cm), during the extensional stage,
resulted in well-developed straight rift-borders parallel to
the moving backstops walls (Fig. 3.a) . Within the interior
of the model, minor extensional faults inclined in opposite
directions. A central horst constituting a topographic high
formed in the sector of maximum stretching north of the
transfer zone. In the southern area the horst appeared later
after further extension. Subsidence of the system is
illustrated in Fig. 3b showing isopach maps, generated
from laser scanning, highlighting the differential
subsidence between the non-deformation stage and the 1,5
cm of extensional displacement. For this time-lapse
interval the areas where more space is generated are
concentrated in the north-central area of the model, near
the transfer zone, where the contrast between the rates of
extension is greater.
(2) From 1,5 to 2.7 cm of extensional displacement,
straight borders started to lose righteousness. The
topographic high, begins to be dissected by small internal
faults, while in the southern area, this feature is still well
expressed. An important process at this stage is the
generation of incipient stike-slip faults in the coupling
zone between baseplates. The extension ended at 6.20 cm
of displacement, this last stage emphasized the
development of normal faulting in the northern sector.
Normal faults growth closely spaced ending laterally
against the central strike -slip zone. Northern
accommodation areas grew, as evidenced by the study of
isopach maps, much more than in the south. These maps
also show clearly the development of an accommodation
zones between the basement offsets, both the rift border
and the intra-rift faults systems break almost 90° when
reach this area. The southern zone, suffered minor
extensional development of their system, forming a
laterally continuous small number of structures (Fig. 3.c).
(3) The contractional stage began with two N-S trending
thrusts, at the edge of the compressional moving wall,
cutting lengthwise through the transfer zone and a
backthrust developed following the pre-existing rift border
faults and showing a strike turn just in the transfer zone. At
the next stage of shortening, the N-S trending active
deformation front propagated to the E, the critical wedge
increase topographically in order to reach a supercritical
state, which resulted in the generation of thrusts and splays
at the foot of the orogenic front, increasing in longitude
and decreasing the wedge angle. Also, at the transfer zone,
a small backthrust formed a “pop-up” structure with N-S
trending axis. (Fig. 4).
Figure 1. Structural map of the of the Austral Basin Fold-Thrust
Belt, showing structural domains along-strike variations in width
and lateral position; and Lago Viedman, Lago Argentino and
Torres del Paine Transfer faults (modified from Ghiglione et al.,
2009).
3 Conclusions
Analogue modelling of the Austral Basin Fold-Trust Belt
of the Southern Patagonian Andes simulate the extensional
phase and posterior shortening of the basin. The models
Figure 2. Experimental apparatus. a) Sketch of model set-up,
210
McClay, K. (1995). Analogue modelling of orthogonal and oblique
rifting. Marine and Petroleum Geology, 12(2), 137-151.
doi:10.1016/0264-8172(95)92835-K
assembly and initial dimensions for all models. VD indicates the
velocity discontinuity under the ductile layer and between de
mobile basal plates. b) Experimental apparatus and model setting
for the shortening phase after extension..
Mcclay, K. (1990). Extensional fault systems in sedimentary basins: a
review of analogue model studies☆. Marine and Petroleum
Geology, 7(3), 206-233. doi:10.1016/0264-8172(90)90001-W
References
Yagupsky, D., Cristallini, E., Fantin, J., Valcarce, G., Bottesi, G., &
Varade, R. (2008). Oblique half-graben inversion of the Mesozoic
Neuquén Rift in the Malargüe Fold and Thrust Belt, Mendoza,
Argentina: New insights from analogue models. Journal of
Structural Geology, 30(7), 839-853. doi:10.1016/j.jsg.2008.03.007
Ghiglione, M. C., Suarez, F., Ambrosio, A., Da Poian, G., Cristallini, E.
O., Pizzio, M. F., & Reinoso, R. M. (2009). Structure and
evolution of the austral basin fold-thrust belt, Southern Patagonian
Andes. Revista de la Asociación Geológica Argentina, 65(1), 215
- 226. SciELO Argentina. Retrieved from
http://www.scielo.org.ar/scielo.php?pid=S000448222009000500014&script=sci_arttext&tlng=pt
Figure 3. a) Plan view of extensional model at 1,5 cm of of extensional displacement; b) fault interpretation and incremental basin
subsidence calculated from differential laser scans for 1,5 cm stage; c) basin topography at end of experiment (6,20 cm). TZ, transfer
zone.
Figure 4. Interpreted serial section through the orthogonal rift-inverted model.
211