Thrust belts and foreland Basins – Decembre 2005
Morpho-tectonic and sedimentary processes of Pakistani Makran accretionary prism
Raymi Castilla*, Nicolas Mouchot**, Nadine Ellouz*, Geoffroy Mahieux***, Pascale Leturmy**,
* Institut Français du Petrole, France.
** Univ. of Cergy-Pontoise, France.
*** Univ. of Picardie, France.
The Makran accretionary prism is located in the Northwestern part of the Indian ocean, in
South of Iran and Pakistan (fig 1.). It results of the subduction of the Indian plate under Eurasian
plate since the late Cretaceous (White, 1982; Harms et al., 1982; Platt et al., 1985). Following the
CHAMAK data acquisition cruise, along the Pakistani active margin (fig.1), several studies have
been undertaken as parts of an integral project in the aim of understanding the functioning of the
Makran accretionary prism.. Among all the activities, the analysis of gas samples end cores, the
treatment and interpretation of multichannel seismic-reflection lines, 3;5 kHz profiles and the
morphotectonic analysis of multibeam bathymetry data represent the main lines of investigation.
The purpose of this poster is to present a preliminary interpretation of certain seismic lines and
results (also preliminar) from the analysis of echo-sounder data.
fig. 1. A: Regional tectonic map of Northwest Indian Ocean. Modified after Kukowski et al., 2000);
B: Bathymetric map covered by Chamak survey.
SEDIMENTARY PROCESSES AND TRANSFERT
The analyse of 3,5 kHz profiles allowed to determine 7 echo-facies families. Each echo
describe sedimentary processes and/or sediments occurring on the three provinces highlighted by
bathymetric map (fig 1), i.e the accretionary prism in the north, the Indus deep-sea fan in the south
and the abyssal plain wedged between the two others.
The accretionary prism is characterised by perturbed area (ridge, mass wasting, relief/erosion
effects) and calm area (piggy-back basin) where hemipelagic sedimentation occurs (fig. 2) The
abyssal plain is much more complex with an important sediment transport in the west part,
described by a progressive transition of echo-facies from canyon number 4 (fig. 2). The east part,
near the discharge system of canyon 5 and 6, is an erosive area characterised by instabilities and
erosive substratum or coarse sediments. The northern border of abyssal plain, near the front of the
prism, is represented by the same echo-facies than piggy-back basins but the difference between the
two is that turbiditic sedimentation occurs in the abyssal plain contrary to a hemipelagic
sedimentation. A bathymetric relief of the “Little Murray Ridge” appears in abyssal plain like a
perturbed area (relief/erosion effects) (fig. 2).
fig.2. Echo-facies distribution map in the oriental Makran
The third province is a non active part of the Indus deep-sea fan which collapse southward in
the proximity of the transtensive system of the Murray Ridge (fig. 2). Hemipelagic sedimentation is
the main process occurring in this area, but it is complicated by relief effects link to normal faults
along its southern border. The northern limit, between deep-sea fan and abyssal plain, is a transition
area (combination of hemipelagites/turbidites and coarse sediment echo-facies) where slope failures
Multi-channel seismic reflection lines show the deformation of ocean floor sediments forming
a, sometimes complex, network of ridges with a conspicuous surface expression. Such deformation
results from the activity of north dipping thrusts arranged in an imbricate fashion allowing for the
thickening of the sediment pile toward the continental platform (north). An irregular deformation
front is identified on both seismic lines and bathymetry as well as important lateral variations on
thrusts continuity. Piggy-back basins have developed northerly of most of the ridges showing
multiple sedimentation-deformation events.
A north-south seismic line has been chosen among all to illustrate several aspects observed in
the Makran accretionary prism (Fig. 3). A north-dipping regional unconformity, marked by the
onlap terminations of the overlaying sequence, can be seen near the southern limit of the line. The
tilting of this surface is a consequence of the flexure due to the charge imposed by the accretionary
prism. Above the unconformity, a sedimentary sequence is composed of packages of high
amplitude and highly continuous reflectors (hemipelagic sediments?) and medium to high
amplitude and poor continuity reflectors (distal turbiditic facies?). This sequence shows little or no
flexure and can be recognized to the north deformed by the imbricate thrusts described above. A
lateral facies variation of the supposed distal turbiditic sequences is inferred from the presence of
channel-like geometries deformed inside the folds on the imbricate thrusts sheets.
Fig. 3. seismic line through Makran accretionary prism and abyssal plain.
At the foot of the deformation front an active channel can be recognized (on both seismic and
bathymetry data) showing an onlap filling over an erosive surface. The deformation front, highly
variable laterally, shows in this section an asymmetric fold that can be modelled as a structure of
dominant northward vergence. The imbricate system observed north of the front allows for the
formation of several piggy-back basins. To the south, these basins are small and of limited depth.
On the contrary, to the north two wide bassins are present showing a history of multiple
deformation pulses with several onlap surfaces indicating their progressive tilting.
As in other sectors of the prism, the imbricate system accounts for the important structural
relief (~ 2 sec. TWT visible in this section) created as a consequence of the off scrapped sediments
from the oceanic crust.
In addition to the work on seismic interpretation, a series of sandbox experiments are being
carried out to model several aspects of the internal structure observed in the prism. Further steps
shall include a more complete definition of the structural setting and the kinematic evolution of this
part of the prism as well as the systematic study of several parameters controlling the external and
internal geometry of accretionary prisms.
The analysis of sedimentary processes and morphological data allow us to predict that flows in
provenance of Makran’s coastal rivers are not negligible. Despite this observation, no quantification
of Makran sediment flux prevents to discuss on an quantification of fluvial sediment supply.
The seismic reflection lines of the Makran accretionary prism show a wide diversity of
processes expected to occur in active margins. Among these processes the tilting of oceanic
lithosphere supporting the prism, the deposition of turbiditic systems and their “rapid” incorporation
in the compressive deformation and the formation of deep piggy-back bassins showing deformed
internal sequences are common to all the seismic lines interpreted so far.
Harms, J.C.C., Cappel, H.N., Francis, D.C., 1982. Geology and petroleum potentialof Makran
coast, Pakistan. Offshore South Asia-82 Conference. Singapore.
Platt, J.P., Legett, J.K., Youg, J., Raza, H., Alam, S., 1985. Large-scale sediment underplating in
the Makran accretionary prism, Southwest Pakistan. Geology 13, 507-511.
White, R.S., 1982. Deformation of the Makran accretionary sediment prism in the Gulf of Oman
(North-West Indian Ocean). In: Leggett, J.K. (Ed). Trench-forearc Geology: Sedimentation and
tectonics on modern and ancient active plate margins. Geological Society Special Publication