Mud diapirism: the origin of melanges in accretionary complexes? TONY BARBER & KEVIN BROWN

Chaotic melange deposits, a mixture of blocks in a clay mam'x, have commonly been atm'buted to the mechanism of submarine slumping. I n the oceans at the present day slumping does not occur on a sufficiently large scale to produce the quantities of melange seen in ancient accretiona y complexes. In modern accretionary complexes, massive shale diapirism produces large volumes of melange, and is an entireb adequate mechanism to account for melanges in ancient complexes.

Accretionary complexes are formed from sedi- ments that are scraped off a down-going oceanic plate on the inner wall of a subduction trench (Fig. 1). The assemblage of rock types forming these accretionary complexes reflects this origin, and includes varying proportions of ocean-floor materials such as pelagic sediments (commonly red shales and cherts), pillowed basalts, basaltic dyke rocks, gabbros and serpentinized perido- tite. In addition to these ocean floor materials, many accretionary complexes (e.g. the Southern Uplands of Scotland, the Franciscan of Califor- nia, the Shimanto Belt of Japan and the Crocker Formation of Sabah, North Borneo) include large volumes of turbidite deposits - i.e. alternations of greywacke sandstone and shale of terrigenous origin, either deposited on the ocean floor as turbidite fans (e.g. the Bengal and Ori- noco Fans) or in the subduction trench and in slope basins formed on top of the developing accretionary complex.

In ancient accretionary complexes these different rock units are typically lenticular in form and complexly folded, faulted and thrust.

Melanges

Among assemblages of rock units regarded as the product of ancient accretionary processes are extensive outcrops of melange. This is a chaotic unit, consisting of blocks of lithified rock of various shapes and sizes, from chips to a kilometre or so in extent, enclosed in a fine- grained clay matrix (Fig. 2).

Blocks in melanges usually include rock types that can be matched elsewhere in the complex, such as greywacke sandstone, bedded chert, pil- lowed basalt, gabbro and serpentinite; but rock types like massive limestones, and metamorphic rocks such as greenstones, glaucophane schists, eclogites and jadeite-bearing rocks, for which there is no obvious source within the complex, may also occur. It is these exotic rock types that constitute the major problem in the interpreta- tion of melanges.

Where the melange matrix has not been sub- jected to later deformation or metamorphism it is typically a scaly clay of various colours, with streaks of red, green, brown, grey or black. The

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Pelagic: referring to sediment of the deep ocean as distinct from that derived directly from the land (terrigenous sediment).

Fig. 1. Model of an accretionary complex, modified from Shiki & Misawa (1980). Oceanic crust with a thick cover of sediments is shown being subducted beneath an accretionary complex. The complex is constructed from sediment scraped off the down-going plate, either at the toe or by the addition of material at its base. As the sedimentary sequence passes beneath the accretionary complex, decollements occur at deeper levels within the sequence, and the development of ramps and duplex structures adds material to the base of the complex in a process of underplating. Eventually fragments of Ocean crust and even the upper mantle may be underplated to the base of the complex. Overpressured clays beneath or within the complex may be mobilized to form intrusive mud diapirs and volcanoes (MV) at the surface. Horizontal and vertical scales are approximately equal.

GEOLOGY TODAY MapJune 1988189

Fig. 2. A melange, composed of irregular fragmented blocks of pale sandstone, with a range of sizes, in a fine-grained matrix. This melange forms part of a mid-Tertiary accretionary complex in Sabah, North Borneo, Malaysia.

Turbidite: sediment deposited from a turbidity current, a density current heavily laden with suspended sediment.

Greywacke: a coarse-grained sandstone consisting of poorly sorted angular to subangular fragments in a clayey matrix.

Fig. 3. Barbados Accretionary Complex. The complex is built up from sediments scraped off the westward moving Atlantic Ocean floor. Water-saturated sediments of the Orinoco Fan buried within the complex are overpressured and rise diapirically to build mud volcanoes on the surface. More than 450 mud volcanoes occur in the area marked as the diapir field. Scattered mud volcanoes and diapirs occur in other parts of the complex and on the island of Barbados.

clay usually shows a scaly fabric which pervades the whole matrix, commonly with a random orientation. Sometimes the scaliness is more regularly orientated, tending to be aligned paral- lel to the margins of the melange against the surrounding country rock, or to the margins of the enclosed blocks (see cover photograph). The scaliness then has the appearance of a flow fabric, wrapping around and between the blocks, the random fabric being preserved where the matrix was protected by the blocks.

Later tectonism within the accretionary com- plex may reorientate the fabric in the scaly mat- rix parallel to the strike of the surrounding rocks and to the trend of the complex as a whole. On metamorphism, the scaly fabric becomes a cleav- age or schistosity, brought about by the align- ment of micaceous minerals. Deformation may also elongate the blocks and align them parallel with the schistosity.

Olistostromes Since Beneo described the melanges of Sicily in 1954, most melanges in accretionary complexes throughout the world have been interpreted as products of large-scale submarine gravity slides or slumps. In the discussion following Beneo's presentation, Flores introduced the term o l h - strome to describe these deposits, the enclosed blocks being termed olistoliths. The distin- guishmg feature of olistostromes, which separates them from other sedimentary slumps or debris flows, is the presence of blocks of exotic lithified rocks that are not available from adjacent structural and stratigraphic units. The implication is that these blocks have travelled a considerable distance from their source.

In the years since the concept was introduced, a model has been developed to account for the formation of olistostromes. It is visualized that the exotic rock types have been brought to the surface by faults or thrusts, to be exposed on the sea floor in fault scarps. From time to time, owing to slope instability and perhaps triggered by earthquakes, a mass of material, including large blocks, collapses into an adjacent basin in which deeper water shales are being deposited; these shales constitute the melange matrix. Among other problems, the origin of the charac- teristic scaliness in the melange matrix is not explained by this model; it is necessary to invoke subsequent tectonic deformation to account for the scaly fabric.

During the past few years evidence has been accumulating that many melange deposits are not the result of large-scale submarine land- slides, but are due to massive mud diapirism.

Mud diapirs Mud diapirs and associated mud volcanoes have been described in the geological literature for many years. The first indication that these phenomena might be more than rare curiosities in accretionary complexes was a publication in 1982 by Stride and his colleagues from the Institute of Oceanographic Sciences. This paper featured GLORIA side-scan sonographs of part of the Barbados Accretionary Complex in the West Indies (Fig. 3).

GLORIA images, compiled from side-scan sonar reflections along the track of the survey ship, resemble 'aerial photographs' of the sea floor (Fig. 4). Bathymetric surveys, together with the GLORIA images, show that locally the surface of the Barbados Complex is covered by large numbers of dome-shaped features, up to 5 km across and a few hundred metres high, which are interpreted as mud volcanoes, formed of mud extruded from the underlying complex. Seismic reflection profiles across some of these features show that they are the surface expression of a column of chaotically disturbed

9OIGEOLOGY TODAY May-June 1988

material, extending vertically for several kilo- metres into the complex (Fig. 5). These structures have been interpreted as mud diupirs which acted as conduits for the transfer of mud to the surface.

The Barbados Accretionary Complex is form- ing where the floor-of the Atlantic Ocean is being subducted westwards beneath the Lesser Antilles Volcanic Arc (Fig. 3). Mud diapirism occurs in the southern part of the complex, associated with the accretion of rapidly depo- sited turbidites which form part of the Orinoco Fan. Mud volcanoes cover an area between 40 km and 180 km to the west of the deforma- tion front where fine-grained hemipelagic clays at the base of the fan have been accreted into the complex. In some parts of the complex, mud diapirs and volcanoes form linear chains and ridges and are associated with late thrusts and wrench faults which cut through the complex.

Mud diapirs are also common in the East In- dies. In 1983 one of us (TB) took part in a scientific cruise in Eastern Indonesia in the re- gion where the Indian Ocean floor and the northern margin of the Australian continent are being subducted beneath the Banda Arc (Fig. 6 ) . Mud volcanoes and shale diapirs were identified on SeaMARC imagery (which is ana- logous to GLORIA) and on seismic profiles in the area to the south of Sumba (Fig. 6 ) where they form linear groups up to 10 km in advance of the deformation front. These diapirs have developed at the point where Indian Ocean crust with a thin sedimentary cover gives way east- wards to a thicker sequence of hemipelagic sedi- ment on the north-western margin of the Austra- lian continent. Eastwards, where carbonate sedi- ments of the Australian Continental Shelf are entering the subduction system, no mud volca- noes or diapirs are seen. Evidently the carbon- ates do not behave diapirically. Northwards, however, in the island of Timor, there are 27 active mud volcano fields and abundant evidence of mud diapirism.

Mud volcanoes and mud diapirism in Timor Timor, with the associated islands of the outer Banda Arc, has been formed as a result of colli- sion between the Banda Arc subduction zone and the northern margins of the Australian con- tinent. This collision began about three million years ago and is still in progress.

During the collision, the northern margin of the Australian continent has been thrust beneath the accretionary complex and the Banda forearc. The latter consisted of serpentinized peridotites, high-grade metamorphic rocks of continental origin and imbricated Jurassic to Cretaceous ocean floor cherts and greywackes, overlain un- conformably by Oligo-Miocene reefs. These

units are now uplifted to form the tops of the highest mountains in Timor, nearly 3000 m in Mt Mutis. The underlying Australian continen- tal margin sequence, ranging in age from Permian to Pliocene, has been imbricated and thrust southwards towards the Australian con- tinent to build the collision complex, which is undergoing rapid uplift.

Recent geological maps of West Timor show an extensive melange unit, the Bobonaro Complex, which in the field consists of vari- coloured scaly clays enclosing variously sized blocks of lithified rock units, derived from either the Australian margin sequence or the overthrust fore-arc units. With colleagues, we have studied outcrops of scaly clay melange in the island of Semau off the south-western extremity of Timor (Fig. 7), where they are associated with active mud volcanoes. Semau is composed of upraised coral reef terraces punctuated by circular outcrops of clay with blocks, many of which support mud volcanoes. These outcrops are interpreted as the surface expression of cylindrical mud diapirs that have risen through the underlying collision complex, incorporating blocks of lithified units and carrying them to the surface. Active mud volcanism is restricted to small superficial structures 2-3 m in height, resembling miniature volcanoes (Fig. 8) which may occupy a central crater (Fig. 7c). These small-scale

Fig. 4. GLORIA sonograph of part of the Barbados Accretionary Complex, Lesser Antilles Arc. ‘Tram lines’ through the centre of the picture mark the ship’s track and the imagery represents sonar reflec- tions from the surface of the Ocean floor for about 10 km on either side of the track. Bright areas are reflecting surfaces, dark areas are poor reflectors or surfaces that slope away from the ship’s track from which no reflections were received. The imagery shows a series of slightly curved ridges interpreted as folded and thrust slices of sedimentary units that have been scraped off the down- going plate; the troughs represent the location of the thrust traces. Local bright patches are inter- preted as mud volcanoes on the surface, fed by diapiric conduits within the complex.

GEOLOGY TODAY May-June 1988191

Fig. 5. (a) Seismic reflection profie across a mud volcano and an intrusive diapir in the Barbados Trough, West Indies, after Bijou-Duval and others (1982). Depth of profile about 2 km. (b) An interpretative line drawing showing the diapiric plug becoming more intrusive along the layering of bedding in the surrounding sediments as the surface is approached, and the extruded material which builds the topographic form at the surface. The dip of the reflectors on either side of the conduit indicate that the diapir was intruded into the core of a syncline.

Fig. 6. Banda Arc Accretionary and Collision Complex, Eastern Indonesia. Mud diapirs develop where Australian marginal sediments approach the deformation front in the area to the south of Sumba. Active mud volcano fields and melange with a scaly clay matrix (Bobonaro Scaly Clay), providing evidence of massive mud diapirism, occur in all the outer arc islands between Savu and Kai, a distance of 1200 km. Overpressured clays deep in the Australian continental margin sequence give rise to the diapirs. (Submarine contours off northern Australia are in metres.)

volcanoes continually emit liquid mud, gas and oil, representing the escape of the mobile com- ponents from the underlying mud diapir. The topographic form of the diapirs on Semau resembles very closely the 'mud volcanoes' seen on GLORIA and SeaMARC imagery in the Barbados Accretionary Complex and in the area south of Sumba.

Similar circular outcrops of the Bobonaro Complex have been mapped in post-collision Plio-Pleistocene clastic sediments and raised coral reefs elsewhere on Timor, where, since they contain blocks derived from the underlying collision complex, their diapiric origin is obvious. In older rock units they are less easily recognized.

Diapirs occur near the tops of the highest mountains and in the bottoms of the deepest valleys, over a vertical distance of nearly 3000 m y and may have risen from much greater depths within the complex. They cut every stratigraphic and structural unit, from the

Australian margin sequence to the overthrust sheets and the recently uplifted Plio-Pleistocene clastic sediments and coral reefs. The abundance of active mud volcanism on Timor indicates that diapirism is still in progress.

In the areas we have mapped, diapirs fre- quently occur as linear groups aligned along wrench faults, which cut through the thrust and imbricated rocks of the collision complex. Some of these wrench faults are known to have moved during recent earthquakes. There is probably a genetic relationship between earthquake activity, movement along wrench faults and the intrusion of diapirs.

Mechanism of mud diapirism The development of diapirism on a large scale in both the Barbados and Banda Accretionary Complexes suggests that this phenomenon may be expected to occur whenever thick sedi- mentary sequences containing major uncon- solidated shale units become involved in the accretion process.

Source regions of mud diapirs in the Barbados Complex are hemipelagic clays associated with distal turbidites of the Orinoco Fan. In Sumba and Timor the shale units are hemipelagic clays from the Australian continental margin sequence, of Late Palaeozoic to Mesozoic age. These shales must have remained stable within a passive continental margin for hundreds of millions of years before becoming mobilized when the margin collided with the Banda Arcs, some three million years ago.

Off Barbados, Graham Westbrook and Mike Smith described diapirs in front of the accretion- ary complex, which they attribute to the trans-

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mission of high pore pressures along particular clay horizons, which then burst up to the surface to form mud diapirs. They suggest that these clay horizons define the decollement surface along which thrust slices become detached, to be accreted into the accretionary complex.

Much more commonly, mud diapirs occur within the accretionary complex itself. Here, stacking up of thrust slices exerts an overburden pressure on underconsolidated clay horizons. Renewed thrusting or wrench faulting may trig- ger the release of the overpressured fluids which, with entrained mud, move towards the fault plane, then rise towards the surface as a diapir, piercing all the overlying units and incor- porating blocks of more consolidated units as it goes. Any new fault or thrust movement can generate a new phase of diapirism.

When the diapir reaches the surface, the mud is extruded to build the characteristic dome- shaped forms seen in the GLORIA and Sea- MARC imagery and on Semau. The eruption may be explosive. The pore fluids include large

- volumes of methane gas which is released from solution with the reduction in pressure as the diapir approaches the surface. The sudden ex- pansion of gas in the conduit may blow blocks of rock hundreds of metres into the air. Explosive eruptions, followed by voluminous extrusions of mud, have been recorded from New Zealand, Trinidad in the West Indies and Kamchatka in the USSR.

After the diapir reaches the surface, pore

pressure is gradually reduced over a period of time, perhaps over hundreds of years, by leak- age of water, oil and gas, to build the small superficial mud volcanic cones seen on the dia- pirs of Semau. As the fluids are lost, the clay becomes increasingly viscous and, still driven by pressure from below, may be extruded like toothpaste; again, such extrusions have been recorded from Trinidad.

A further diagnostic feature that we have observed in the melanges resulting from diapir- ism studied in Timor, and also in Sabah (North Borneo), is the form of the enclosed blocks and their relationship to the clay matrix. The blocks are generally highly irregular in shape, with characteristically re-entrant angles. The clay matrix penetrates the blocks along internal fractures and may even break up the blocks, dispersing the fragments into the matrix.

The irregular form of the blocks and injection of clay provide diagnostic features which are commonly preserved even where the melange has been deformed and the scaly clay matrix has been metamorphosed to slate or schist. Melanges resulting from diapirism can be dis- tinguished from ‘broken formations’ (where stratigraphic units have been disrupted in situ as the result of high pore fluid pressures) by the presence of far-travelled blocks unrelated to the local stratigraphy. These features provide criteria for the recognition of diapiric melange, even in highly deformed ancient accretionary complexes.

GEOLOGY TODAY May-June 1988193

Fig. 7. (a) Mud diapirs, composed of clay with blocks (Bobonaro Scaly Clay) cut through raised coral reefs forming the island (Pulau) of Semau to the south-west of Timor, Eastern Indonesia. Some of the diapirs show active mud volcanism. Pulau Kambing, just offshore from the main island, has a central crater (b) in which there are several mud volcanoes 2-3 metres high (c) (see Fig. 8).

Tenigenous: describing sediments derived from a land surface.

Decollernent: surface of detachment separating moved rocks above from unmoved rocks below, usually a bedding surface along which fault movements have taken place.

Duplex structures: obliquely stacked (imbricate) thrust slices, bounded by thrusts above and below.

Overpressuring: where pressure in the pore fluids of a sediment exceeds the pressure that would be due solely to the weight of an overlying water column.

Schistosity: a variety of foliation in coarser-grained metamorphic rocks, due to the parallel arrangement of platy and ellipsoidal mineral grains.

Hemipelagic: fine-grained sediment, partly of pelagic and partly of terrigenous origin, deposited in a deep-water environment.

Fig. 8. A group of mud volcanoes on Pulau Kambing (Fig. 7). Dried mud flows on the flanks of the volcanoes show where fluid mud has recently been erupted from the volcanoes. The crater wall can be seen in the background.

Imbricated: describing a series of stacked thrust sheets separated by thrusts dipping in the same direction.

diapirs may bring blocks of lithified rock to the surface of the complex, where they may become involved in slumping. Of all the mud volcanoes seen on the Barbados Complex, only one gives rise to a submarine slump that flows down into the subduction trench. Clearly submarine slumping is an entirely inadequate mechanism to produce the volumes of melange seen in ancient accretionary complexes.

Suggestions for further reading Barber, A. J., Tjokrosapoetro, S. & Charlton,

T. R. 1986. Mud volcanoes, shale diapirs, wrench faults and melanges in accretionary complexes, Eastern Indonesia. Bulletin of the Amm’can Association of Petroleum Geologists, v. 70(11), pp. 1729-1741.

Bijou-Duval, B., Le Quelle, P., Mascle, A., Renard, V. & Valery, P. 1982. Multibeam bathymetric survey and high resolution seis- mic investigations on the Barbados Edge complex (eastern Caribbean): a key the howledge and interpretation of an accretion- ary wedge. Tectonophysics, V. 86, pp. 275- 304.

Melanges: mud diapirs or submarine landslides? In the Barbados accretionary Complex and in the Australia-Banda Arc collision zone, mud diapir- ism has occurred on a massive scale and has produced an enormous volume of melange. In the Barbados Complex, mud diapirs occur over an area of 100 000 square kilometres (h2)* In part of the complex, more than 450 dapirs occur in an area of 700 h2, an average of one diapir for every 1.5 km2.

In West Timer, melange of the Bobonaro Complex covers 40% of the total area of 21 000 km2, and in East Timer the

and mud dia- Pirs are recorded on the islands of the Outer Banda Arc, from Savu west of Timor to the Kai Islands near New Guinea, a distance of

Obviously, the mechanism of mud diapirism is

deposits seen in ancient accretionary complexes.

Breen, N. A., Silver, E. A. & Hussong, D. M. 1986. Structural styles of an accretionary wedge south of the island of Sumba, Indone- sia, revealed by SeaMARC I1 side scan sonar. Bulletin of the Geological Society of America,

Shiki, T. & Misawa, Y. 1982. Forearc geological structure of the Japanese Islands. In: Trench- Forearc Geology: Sedimentation and Tectonics on Modem and ~~~i~~ ~~~i~~ plate ~ ~ ~ g i ~ ~ (ed. J. K. Leggett). Geological Society of

73.

N. H. 1982. stmctural grain, mud volcanoes

97, pp. 1250-1261.

4000 km2- Mud

1200 km, in a belt 120 km wide (Fig* 6). L o d o n , Special Publication No. 10, pp. 63-

entirely adequate to produce all the melange

On the other hand, no OliStOStrOme deposits Of the type and scale required are being formed in

Stride, A. H., Belderson, R. H. & Kenyon,

and other features of the Barbados Ridge Complex revealed by Gloria long-range side-

association with accretionary complexes on the Scan sonar. M ~ , + , ~ ~ ~ ~ l ~ ~ , v. 49, pp. 187- ocean floors at the present time. Large-scale 196.

complexes* SeaMARC I1 imagery from the decollements and mud volcanoes: evidence south Of Timer shows a Scar lo km from the Barbados Ridge Complex for the across, but only the upper few tens of metres of role of high pore fluid pressure in the unconsolidated sediment are affected. Even the development of an accretionary complex. Bassein Slide, off the Irrawaddy Delta, the ~ ~ ~ l ~ ~ , v. 11, qp. 279-283. largest submarine slide seen in the present-day williams, p. R., plgram, c . J. & D ~ ~ , D. B. oceans, affects only unconsolidated Pleistocene 1984. ~~l~~~~ production and the import- sediments. ance of shale diapirism in accretionary ter-

there rams. Nature, London, v. 309, pp. 145-146. appears to be no mechanism by which lithified rock units may be uplifted along faults to the Tony Barber is Reader in Geology at Royal surface and then affected by large-scale Holloway and Bedford N e w College, University of submarine slumping. The upper, older parts of ~ ~ n d ~ ~ .

slumps are commonly Seen on accretionary Westbrook, G. K. & Smith, M. J. 1983. Long

In modern accretionary

modem accretionary complexes are invariably covered by a veneer of recent sedimmt, indi- cating that this is an area of deposition rather than of erosion. Of course, as we have seen,

Kevin Brown holds a N A T O Post-Doctoral Research Fellowship at the Earth Sciences Board,

Of Califomia,

94IGEOLOGY TODAY Mny-June 1988