high resolution correlation between high latitude climate events and arabian sea biogeochemistry ...

44 Pages·2001·0.27 MB·English

HIGH RESOLUTION CORRELATION BETWEEN HIGH
LATITUDE CLIMATE EVENTS AND ARABIAN SEA
BIOGEOCHEMISTRY OVER THE LAST GLACIAL CYCLE
M.A Altabet1, M.J. Higginson1, ,and D.W. Murray2,
1School of Marine Science and Technology, U. Massachusetts Dartmouth, 706 S.
Rodney French Blvd., New Bedford, MA 02744-1221 United States.
2Center for Environmental Studies, Brown University Box 1943, Providence, RI
02912-1943 United States.
It is now well known that suborbital-scale climate events recorded in Greenland ice-
cores also have manifestations in low latitude marine records. In the Arabian Sea, this
linkage has appeared as productivity variations recorded by sediment organic carbon
content. Plausible forcing is likely via the intensity of the southwest monsoon and
resulting upwelling of nutrient-rich water. We have examined high accumulation rate
cores from the Oman Margin for productivity proxies and nitrogen isotopic ratio which
records denitrification intensity within the oxygen minimum zone (OMZ). Denitrification
requires suboxic intermediate waters, which can arise from intense local productivity.
With temporal resolution approaching 50 yr, nitrogen isotopic ratio was found to vary
with remarkable similarity (duration and morphology) to the oxygen isotopic record from
the GISP2 ice-core. Given a general temporal framework known from foraminiferal
isotope measurements, a series of tie points were used to correlate the Arabian Sea
nitrogen isotope and ice-core records. The resulting correspondence is striking and
statistical comparison shows a very high index of similarity. Chronology for the first c.
20 kyr is confirmed by radiocarbon dating. Warm interstadial D-O events are marked by
denitrification as intense as found at present. In contrast, Heinrich Events correspond to
intervals of little or no denitrification. Transition to intense denitrification often occurs
over only a few data points, indicating a response time on the order of a century or less.
Arabian Sea denitrification is evidently very closely coupled to high latitude climate at the
decadal/centennial time-scale and is amongst the best comparisons between ice-core and
marine records. The high quality of the nitrogen isotopic record holds much potential for
refining our understanding of high- and low-latitude linkages. A poorer comparison with
productivity indices may result from their extrinsic nature, influenced by sediment
accumulation rate and possible diagenetic alteration. Changing denitrification intensity
may produce a global feedback through variations in global ocean nitrate inventory and
productivity in non-HNLC regions. Such feedbacks would occur, though, on time scales
similar to the residence time of marine nitrate, about 3 kyr at present. Filtering the
nitrogen isotopic record to examine variability on time scales of 3 kyr and greater
produces a series of oscillations between 20 and 70 kyr that are well correlated with a
recent high-resolution atmospheric CO record from Taylor Dome, Antarctica. A possible
2
mechanism relating high frequency N. Hemisphere climate change to lower frequency
global change is suggested.
REGIONAL TECTONIC AND CLIMATIC EVENTS RECORDED
IN THE STRATIGRAPHY OF THE MALDIVES CARBONATE
PLATFORM
Andrei V. Belopolsky* and André W. Droxler**
*BP America Inc., Houston, Texas, USA
** Rice University, Houston, TX, USA
Interpretation of the extensive 2-D seismic dataset and well data from the Inner Sea
of Maldives, equatorial Indian Ocean, showed that the evolution of this isolated Tertiary
platform was controlled by a combination of tectonic, eustatic and climatic factors. The
volcanic basement of the Maldives is thought to be created by the hot spot activity. The
late Paleocene basement is dissected by two deep and narrow graben that have
northeast-southwest orientation. The origin of the extension that created the graben is
not clear but must be related to the collision between India and Asia. Neritic carbonate
production was established in the early Eocene on top of volcanic basement highs while
graben served as deeper seaways. The movement along the graben faults continued
until the early Oligocene.
Evolution of the carbonate platform was controlled by the variations in
accommodation space and sediment supply. Climate played major role in controlling the
formation of carbonate material. The middle Miocene time in the Maldives is marked by
a pronounced bi-directional progradation of bank margins. Bank margins prograded
from both east and west towards the central trough. It has been shown previously in the
Bahamas that the difference between the leeward and windward margins was related to
dominating wind patterns. The, the leeward margin prograded while windward margin
aggraded. The apparent symmetry of prograding margins in the Maldives allows to
speculate that a seasonal switch in the wind patterns was responsible for the bi-
directional progradation. Such switch occurs presently in South East Asia due to the
monsoons. The origin of monsoons is attributed to the elevation of the Tibet Plateau
and the timing of monsoon initiation is not well constrained at this time. If symmetrical
progradation in the Maldives was indeed driven by the monsoons it would place their
initiation in the early Middle Miocene time.
GASEOUS HYDROCARBONS IN SEDIMENTS AND OCEAN
WATERS OF THE NE ARABIAN SEA
Ulrich Berner1, Jürgen Poggenburg1, Ulrich von Rad1, Bodo Harazim1, Hartmut
Schulz2, Tim Jennerjahn3, Peter Dietrich4, Peter Linke5 and Nico von Mirbach5
1 Federal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30631
Hannover, Germany
2 Institute of Baltic Sea Research, University of Rostock, Seestrasse 15, D-18119
Rostock-Warnemünde, Germany
3 Institute of Biogeochemistry and Marine Chemistry, University of Hamburg,
Grabenstrasse 27, D-20357 Hamburg, Germany
4 Geological Institute, Technical University and Mining Academy of Freiberg, G.-
Zeuner-Strasse 12, D-09596 Freiberg, Germany
5 GEOMAR, Wischhofstrasse 1-3, D-24148 Kiel, Germany
During cruise SO90 (Sept. 1993) and SO130 (April 1998) of the German research
vessel "SONNE", we detected areas of active gas seeps at the front of the Makran
accretionary complex using TV-sled observations. The carbon and hydrogen isotopes of
methane from sediments and seeps at Makran point to bacterial generation via CO2-
reduction. Associated higher hydrocarbons suggest a mixture between bacterial and
thermal gases. The cold seeps discharge into the ocean waters and large plumes of
methane were observed in the deep water and may extent horizontally over more than
20km from the Makran accretion into the open ocean. We suggest that these plumes are
related to fluids emanating from the accretionary prism, rather than resemble in situ
generation of methane from decay of organic particles. However, no significant vertical
flux of methane across the salinity unconformity between surface water and "North
Indian Intermediate Water" was detected. Carbon isotope ratios of methane suggest that
methane is generated from microbial methanogenesis in the oxygen-rich surface water of
the Arabian Sea. Low to moderate rates of sea-air flux of methane are observed at sites of
the NE Arabian Sea. We suggest that differences of methane-flux of the different areas is
related to differences in primary biological production of the surface water of the Arabian
Sea. However, the estimates of methane fluxes are highly variable and are largely
dependent on wind speed and observed concentration changes in the surface waters.
THE AGE AND ORIGIN OF THE INDUS FAN
Peter Clift1, Christoph Gaedicke2, Nobumichi Shimizu1, Jae Il Lee1, Shahid
Amjad3 and Hans-Ulrich Schlüter2
1 Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods
Hole, MA 02543, USA
2 Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg-2, D-3095
Hannover, Germany
3 National Institute of Oceanography, St-47, Block-1, Clifton, Karachi, Pakistan. E-mail:
[email protected]
The Indus is one of the world’s largest river systems, with a discharge equivalent to
the Mississippi. It currently rises in western Tibet and flows through the Indus Suture
Zone before cutting orthogonally through the Himalaya on its way to the Arabian Sea.
Although much smaller than the Bengal Fan the Indus Fan is still >9 km thick under the
Pakistani Shelf. Moreover, age control derived from petroleum wells on the Pakistani
Shelf and from ties to the Murray Ridge, whose uplift is considered to be principally
Early Miocene, ~20 Ma, demonstrate that around 1/3 of the total fan volume is Paleogene,
i.e., the fan pre-dates unroofing of the High Himalaya. There is a significant increase in
the rates of sedimentation during the Middle Miocene, manifest as the start of major
channel-levee structures on the upper mid fan. These pre-date the apparent strengthening
of the SW Monsoon at 8.5 Ma, but postdate the rapid exhumation of the High Himalaya
at ~23 Ma. This mis-match between the sedimentation patterns and major tectonic and
climatic events probably reflects the drainage area of the Indus River. Calculations based
on Neogene unroofing rates, and map areas suggest that only ~40% of the sediment
reaching the Arabian Sea now is derived from Indian Plate sources, compared with >85%
in the Bengal Fan. This estimate is corroborated by bulk Nd isotopic data from
Pleistocene Indus Fan mud. Instead much of the sediment comes from the Hindu Kush,
Karakoram and western Tibet. This is because the High Himalaya are less elevated in the
western Himalaya compared to the east and moreover the Indus cuts orthogonally through
these ranges, reducing the potential for them to significantly contribute to the bed load of
the river. This contrasts with the range-parallel course of the Ganges. If the High
Himalayas are not eroded much by the Indus it is not surprising that their tectonic
unroofing did not cause a major flux of sediment into the Arabian Sea. Since the Hindu
Kush, Karakoram and western Tibet are not as strongly affected by Monsoonal rains as
the Himalaya it might be expected that the strengthening of this climatic system during the
Late Miocene would have a minor effect on the erosion record of the fan. Instead the flux
of sediment may reflect uplift of the Karakoram at this time, or even in western Tibet.
The Indus Fan is now constrained to the east of the Murray Ridge that appears to have
been uplifted during the Early Miocene, at the same time as uplift of the Owen Ridge.
Prior to the uplift ~50% of the Indus River sediments were sedimented to the west of the
ridge and are now accreted into the Makran Accretionary Complex. The Indus Fan dates
from at least the middle Eocene. Mid Eocene sandstone recovered from the Murray Ridge
contain K-feldspar grains whose Pb isotopic composition require derivation from
ophiolitic and arc units within the Indus Suture Zone. This means that a major river was
already draining the Indus Suture at this time. Since several of the grains are also clearly
not Indian Plate in origin this requires that India-Asia collision had occurred by that time.
Significant thicknesses of seismically similar deposits underlie those drilled at DSDP Site
224 and ODP Site 731 suggesting that the fan may have initiated somewhat earlier, maybe
synchronous with the start of deltaic sedimentation in the Katawaz Basin of Pakistan
close to the Paleocene-Eocene boundary.
The Indus Molasse Group of Ladakh within the Indus Suture Zone of northern India
comprises sediments than record the start of flow of the Indus River through the suture
zone. The oldest fluvial sediments (Chogdo Fmn.) that onlap both sides of the suture
(i.e., Indian and Asian) show derivation from the south and have a provenance indicating
erosion of the platform and its overlying ophiolites. These sediments are overlain by
uppermost Paleocene Nummulitic Limestone before the end of all marine sedimentation.
The following fluvial sequences of Eocene-Oligocene age (Choksti and Nurla Fmns.)
show a west-directed flow and the appearance of K-feldspar grains whose Pb isotopic
composition is consistent with the influx of material of Tibet. The Nd isotopic
composition of the interbedded shales also indicate a change of flow that suggests the
start of a river draining western Tibet shortly after India-Asia collision in the latest
Paleocene. The river and its associated fan appear to be a direct reaction to the initial uplift
of southern Tibet.
Figure 1. Pb isotope discrimination diagram showing the affinity of detrital Eocene and Pleistocene
K-feldspars on the Indus Fan to values characteristic of units within and north of the Indus Suture
Zone rather than the Indian Plate. Crystalline source fields after work of Gariepy et al. (1985) and
Khan et al. (1997).
References
CLIFT, P. D., SHIMIZU, N., LAYNE, G. D., GAEDICKE, C., SCHLÜTER, H. U., CLARK, M. K. &
AMJAD, S. 2000a. Fifty five million years of Tibetan evolution recorded in the Indus Fan. Eos, 81,
277–281.
CLIFT, P. D., SHIMIZU, N., LAYNE, G. D., GAEDICKE, C., SCHLÜTER, H. U., CLARK, M. K. &
AMJAD, S. 2001. Development of the Indus Fan and its significance for the erosional history of the
western Himalaya and Karakoram. Bulletin of the Geological Society of America, in press.
CLIFT, P. D., BLUSZTAJN, J., KROL, M., CARTER, A., KIRBY, E. & GREEN, O. 1999. Timing of
collision and patterns of early uplift along the Indus Suture, Ladakh Himalaya, India. Eos abstract,
80, 313.
GARIEPY, C., ALLÈGRE, C. J., & XU, R.H. 1985. The Pb-isotope geochemistry of granitoids from the
Himalaya-Tibet collision zone: implications for crustal evolution. Earth and Planetary Science
Letters, 74, 220–234.
KHAN, M. A., STERN, R. J., GRIBBLE, R. F., & WINDLEY, B. F. 1997. Geochemical and isotopic
constraints on subduction polarity, magma sources and palaeogeography of the Kohistan intra-oceanic
arc, northern Pakistan Himalayas. Journal of the Geological Society, 154, 935–946.
SEISMIC STRATIGRAPHY OF THE OFFSHORE INDUS BASIN
T. Daley, N. Simons, S. Beswetherick
LASMO Oil Pakistan Ltd
In 1997 Lasmo Oil Pakistan Ltd gained a significant acreage position in the
Offshore Indus Basin with the award of the Indus A and B Blocks offshore south of
Karachi. The hydrocarbon play considered to have the most potential is Miocene shelf-
delta sands interbedded with intraformational shale seals and sourced by presumed gas-
prone offshore equivalents.
As a result of this acreage position, Lasmo gained access to approximately 14,000
km of 2D seismic data across the region. This was enhanced by reprocessing 5000km
of the existing data and acquiring 3660 km of new proprietary data. Only four wells
have tested the preferred play type to date and no core or rock data were available from
these to provide insights into facies or age dating. The Oligo-Miocene succession has
conventionally been grouped stratigraphically as ‘Gaj Formation’, but for a unit in
excess of 4 km, this did not provide a sufficient basis for play fairway analysis.
Seismic sequence analysis has now provided the key to defining the internal
stratigraphy of the previously undifferentiated Gaj Formation and to understanding the
hydrocarbon prospectivity of the region.
Log data from two key wells in the offshore Indus area record the initial in-fill of
the basin by shale dominated basinal or outer shelf sediments, followed by stacked thin-
bedded sandstone-shale sequences of a shelf-delta nature. The transition between the
two is marked by a zone of progradational sequences but no other workable divisions
are apparent. Regional seismic correlations quickly established the diachronous nature
of the prograding shelf package and these were matched by distinct bands of seismic
progrades. A series of simple palaeogeographies of the prograding shelf margin were
developed showing initial sediment input from the north and rapid progradation towards
the south and west.
The most obvious seismic features are canyons. These can be dated relative to one
another on the basis of their stratigraphic position. Two main phases of canyon
development are apparent. The earlier phase is interpreted to be Lower Miocene in age.
These early canyons originated in the north and became more widespread. Down
cutting at this time rarely exceeded 300 m. A second phase of canyon development
occurred during the Plio-Pleistocene These later canyons dominate much of the shallow
section, often show down cutting of up to 600m and are characterised by at least 8
phases of cut and fill. Seismic progrades occur along the canyon axes and slumps are
observed along their margins. Where drilled, canyons of both ages are shale prone.
This is probably because they are located on the slope in a zone of erosion and sediment
by-pass and were subsequent filled by fine-grained deposits after abandonment. The
two phases of canyoning are considered to relate to co-eval phases of tectonic activity in
the collision zone between the India-Pakistan and Eurasian plates. The size and
prevalence of the canyons probably reflects either the magnitude of inversion and uplift
in the hinterland, or the proximity of the tectonic activity to the palaeo-shelf.
The implications for the prospectivity of the offshore Indus Basin and for the tectonic
evolution of the Kirthir fold-belt and the Indus hinterland will be discussed in the light
of these new observations.
EVOLUTION AND PROPERTIES OF ON- AND OFFSHORE MUD
VOLCANOES OF THE MAKRAN DESERT
G. Delisle1, U. von Rad1, A. Tabrez2, and A. Inam2
1 Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655
Hannover, Germany
2 National Institute of Oceanography (NIO), St. 47 Clifton Block 1, Karachi, Pakistan
We present results of reconnaissance surveys of two active onshore mud volcano
fields (Chandragup and Jebel-u-Gurab) and of the newly born (March 1999) offshore
mud volcano (Malan Island). The mud volcanoes on land vary in height from a few to
100 m. Malan Island rose in March 1999 for about three months above the water line at
the same location, where in November of 1945 an island had emerged, only to be washed
away by wave action within months. All visited on- and offshore mud volcanoes line up
along one southwest- northeast trending structural lineament (main axis of Dhak-
anticline).
Isotopic data for gas components of the muds, temperature of the rising muds and
discharge rates of gas and mud were measured. In addition, we have searched the
literature for historic descriptions of the locations in an attempt to establish the longtime
changes in shape or activity of these complexes.
Onshore mud volcanoes: The Chandragup volcano complex is the largest known
structure along the coast of the Makran desert. He has existed at least since 1840 without
experiencing any significant modification in shape or mud discharge activity. Our
repeated temperature measurements in 1998 and 1999 point to slight variations in mud
flow discharge, which is in line with the report by locals of a dependence of mud
discharge on the periodicity of monsoon activity. Gas analyses from the Chandragup
complex as well as from the Jebel-u-Gurab volcano field about 6 km to the west yielded
invariably methane of bacterial origin with only traces of higher hydrocarbon compounds.
Offshore mud volcanoes: We have obtained echo-sounder profiles across the location of
former Malan Island. In place of the former island we have identified a 2 m deep
depression of the sea floor, on which remnants of the former island rest. The composition
of the gas emanating vigorously from the sea floor is identical to the gas of the onshore
mud volcanoes.
We conclude that mud volcanoes on land and in the sea are emplaced during short violent
episodes, followed by long periods of subdued gas and mud discharge activity. Mud
volcanoes in the sea are quickly washed away, while mud volcanoes on land can maintain
low level discharge activity for centuries.
GAS HYDRATES ACTING AS CAP ROCK TO FLUID AND GAS
DISCHARGE IN THE MAKRAN ACCRETIONARY PRISM?
G. Delisle, and U. Berner
Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover,
Germany.
A thick and extensive gas hydrate layer caps the Makran accretionary wedge at water
depths of > 800 m. Its presence allows us to estimate heat flow density values from the
depth of observed bottom simulation reflectors (BSR) along reflection seismic transects
from the abyssal plain to near the coast of the Makran desert. These estimated values and
data from in-situ measurements, available from the literature, were compiled and
compared with the results of numerical modeling of the geothermal effect of the
subduction of oceanic crust under the Makran accretionary prism. A comparison of
model-derived with BSR-derived and measured heat flow values suggest a predominance
of conductive heat transport within the accretionary complex. Little evidence is found
which suggests fluid flow across the gas hydrate layer to modify the observed geothermal
field to any great extent. We further studied the geothermal field associated with the decay
of the gas hydrate layer as the accretionary prism and the capping gas hydrates are
tectonically uplifted out of the gas hydrate stability field into shallower and warmer sea
water. Theoretical considerations suggest a complete disappearance of gas hydrates at a
water depth of about 760 m. Extensive sampling of the water column (13 stations) along
three N-S transects across the accretionary prism has revealed the presence of gas plumes
of bacterial methane. They emerge from the sea floor at water depths of less than 800 m
and drift laterally for several km seawards, before they disintegrate due to dilution and
bacterial consumption of the methane. The gas seeps are thus concentrated in the zone of
shallow waters and outside the stability field of gas hydrates. This suggests that gas
hydrate layers in the Makran accretionary prism act as a very effective cap rock. The flow
of fluids containing high amounts of dissolved gas from the deep parts of the prism
(below the BSR) is redirected to the near coastal regions of the accretion, where gas
hydrates of low permeability do not exist.
CRUSTAL STRUCTURE OF THE DALRYMPLE TROUGH AND
MURRAY RIDGE - IMPLICATIONS FOR THE LOCATION OF
THE OCEAN-CONTINENT TRANSITION WEST OF INDIA.
R. A. Edwards1, T. A. Minshull2, C. Kopp3, and E. R. Flueh3
1. Bullard Laboratories, Department of Earth Sciences, University of Cambridge,
Madingley Road, Cambridge, CB3 0EZ, UK;
2. School of Ocean and Earth Sciences, Southampton Oceanography Centre, European
Way, Southampton, UK;
3. GEOMAR, Wischhofstrasse 1-3, D-24148, Kiel, Germany.
The Dalrymple Trough and Murray Ridge represent the northernmost extension of the
Indian-Arabian plate boundary in the Arabian Sea. Seismic reflection profiles and swath
bathymetric data show a significant component of present-day extension across the
predominantly strike-slip plate boundary. The 150 km-long Dalrymple Trough is
bounded by normal faults divided into a series of en-echelon segments, the lengths of
which vary between 30-50 km on the northwestern flank and up to 90 km on the
southeastern flank. The Murray Ridge represents a topographic high immediately to the
southeast of the Dalrymple Trough. Along the crest of the ridge rough acoustic basement
is observed at the sea-floor on reflection profiles. Any magnetic anomalies associated
with the ridge are very weak.
Wide-angle seismic data were collected along a series of four lines in the Dalrymple
Trough and Murray Ridge region as part of FS Sonne cruise 123. Lines 5 and 7 cross the
trough and ridge, line 4 is located along the axis of the trough, while line 6 is parallel to,
and northwest of, the trough. Up to 10 OBH/S (Ocean Bottom Hydrophones-
Seismographs) were deployed along each line. Travel-time picks from the OBH/S have
been modelled using 2-D ray-tracing to produce velocity models of the crustal structure
along the four lines.
Line 6 and the northwestern end of line 5 show a velocity structure typical of oceanic
crust; with a 6 km-thick two-layer crust. However, line 4 along the axis of the trough,
and the southeastern part of line 5 show a very different structure. Here the crust has a
thickness of 12–14†km and velocities indicative of oceanic layer three are absent. The
increased crustal thickness and relatively low lower crustal velocities suggest that the
Dalrymple Trough is underlain by thinned continental crust.
The Dalrymple Trough therefore forms the northwest boundary of a region of thinned
continental crust which may extend beneath much of the Indus Fan and over 400 km from
the continental shelf of India. Oceanic crust lies immediately northwest of the Dalrymple
Trough and the ocean-continent transition occurs over a less than 10 km-wide region of
active oblique extension along the northwestern flank of the trough. The Moho under the
northwestern flank of the Dalrymple Trough has a very steep dip of 45¥ , comparable to
the steep Moho dips observed along transform (or sheared) margins.
TECTONIC STYLE OF THE MAKRAN ACCRETIONARY PRISM
Nadine Ellouz
Institut Français du Pétrole (IFP), 1-4 av. de Bois-Préau, 92852 Rueil-Malmaison
Cedex-France
The Makran accretionary prism in Pakistan, has developed in the vicinity of the triple
junction between the Arabian, Eurasian, and Indian plate. This junction fits the connection
between the E-W Makran tectonic accretionary prism, a N-S transform fault system
(Chaman–Ornachnal) and, finally, a WSW-ENE Ridge (Murray Ridge). The western part
of the prism is developed in a frontal convergence setting, accommodating the low-angle
oceanic subduction of Cretaceous oceanic crust. The eastern part of the prism has been
strongly under the control of the left lateral transform motion related to the Northward
movement of India since Paleogene time. Consequently, deformation has adapted to
strain-partitioning processes here. Also, the oblique Murray Ridge, which is a prominent
poly-phased transtensive/transpressive structure linked to the Owen Fracture zone,
introduces a strong heterogeneity in the plunging panel.
All over the prism, the tectonic style as well as the sedimentary filling have registered
strong lateral variations through time. The periodicity, the orientation and the tectonic style
of the structures are guided by the sedimentary thickness variation as well as by the related
depth to decollement levels. Close to Pasni, total sedimentary thickness, deduced from
seismic data is 3.5 twt up to basal detachment. When close to the triple junction, it is more
than 4.5 twt up to the first more shallow regional detachment (depth of the basal
detachment is unknown).
To the west of the prism, deformation propagates along a basal detachment level close
to the top of the oceanic crust (free border), but laterally some oblique ridges (such as
volcanic Little Murray Ridge or Murray Ridge itself) act as rigid blocks. Secondary
decollement levels are inferred, particularly onshore, justifying the imbricate structure type
clearly expressed on satellite images. To the east, offshore, the northern flank of the
Murray Ridge is starting to be incorporated in the prism itself. Onshore, close to the Las
Bela depression, the development of “en échelon” folds, often associated with mud
extrusive processes, gives evidence of deep overpressure regime. These structures seem
to have extended offshore, as expressed on seismic data and by the emergence of the new
Malan island in 1999. The overpressure shales have probably originated from one of the
deep decollement level.
The age of the offshore basal sedimentary layers (inferred from well correlation across
the Murray Ridge, but not yet defined), may introduce uncertainties in the chronology of
the sedimentary distribution and the deformation. These limitations should be raised either
by drilling or by new constraints on dating and analyses of the mud ejected at the surface
(or sea bottom), giving new ideas about nature and age of the deepest part of the
sedimentary pile. However, the oldest sediments incorporated in the inner prism itself are
Late Cretaceous/Paleogene in age. The sedimentary pattern has been driven by the
architecture of the Paleo-Indus input through time, ever since its initiation during
Paleogene. On the offshore part of the prism, the sedimentary nature and thickness varies
from east (channel-levees) to west (distal sands and mudstones), suggesting that
sedimentary transfer (channel pattern) and deposits (sedimentary lobes) have a rough east-
west spatial distribution. The relationship between the compressive episodes, Eocene to
Present along the W Indian paleo-margin, and the Paleogene to Middle Miocene
sedimentary distribution west of the Chaman Fault is the important key point to be
defined, in order to estimate the migration of the Paleo-Indus drainage area. Especially