TECTONICS, VOL. 17, NO.3, PAGES, 458-479 JUNE 1998
Collisional melange development: Geologic associations of active melange-forming processes with e~humed melange facies in the western Banda orogen, Ind~nesia
R. A. Hauis 1 Department of Geology, West Virginia Univesity, Morgantown
R. K. Sawyer 2 Texaco Energia, S.p.A., Rome
M. G. Audley-Charles 3 Department of Geological Sciences, Univer.;ity College London, London
Abstract. Analysis of block assemblages, matrix clay structurally underthrust Australian continental margin minerai composition and microfauna, and offshore seismic sequences during the late Miocene to early Pliocene. The reflection profiles reveal that the Bobonaro melange in the thickest accumulations of mixed block-in-clay melange in the Timor region is sourced from mud-rich Australian continental Timor region are found at the southern edge (outlet) of the margin sequences that are remobilized during accretion and SDZ, near the Central and Viqueque synorogenic basins. The form various facies depending on the structural conditions of extent of block dispersion and mixing in the SDZ is emplacement. Melange facies include broken formation, indicative of intense shear strains perhaps induced by an matrix-rich mud injections, and classic mixed block-in-clay oversupply of accretable material when the suture zone was facies. Each of these are distinguished by varying degrees of clogged by underthrusting of the Australian continental remobilization, mixing, and dispersion at different structural margin. positions across the orogenic wedge. The most important structural control is whether melange was generated beneath or in front of upper plate Banda forearc basement (Banda 1. Introduction Terrane). At the present collisional deformation front in the Timor trough. seismic reflection profiles show that melange The Bobonaro melange is a chaotic lithotectonic unit forms mostly by stratal detachment and fluid-assisted composed mostly of broken, clay-rich layers that are mixed . remobilization above a basal decollement propagating to v,arying degrees with structurally and stratigraphically laterally along overpressured Jurassic to Cretaceous clay-rich . overlying units. The melange forms part of an extensive sequences of the distal Australian continental margin. The suture zone between the Asian and Australian plates that is broken, clay-rich material injects upward through faults to traceable for up to 2CXXl km around the Banda arc orogen of form intrusive bodies at the base of slope cover sediment to eastern Indonesia (Figure 1). Like most melanges, the form mud ridges at the surface. Similar patterns of stratal chaotic internal structure of the Bobonaro melange obscures disruption are exposed onshore in the Pliocene Kolbano fold much of its origin. Yet tracking the progressive development and thrust wedge of southern Timor, which is structurally of melange through different parts of the active Banda arc contiguous with the Timor trough deformation front. continent collision, from where it is actively forming to Melange in the Kolbano Mountains is mostly broken where it is uplifted and ,exhumed, provides a unique formation and matrix-rich injections of mud froin Jurassic and opportunity to explore the associations between melange Cretaceous units. Deformation mechanisms include intense products and processes. layer-parallel extension associated with emplacement by moo In this paper we present the results of a systematic field diapirs that' rise from near the decollement upward to the analysis of the Bobonaro melange in some of these exhumed surface along fault conduits. In the hinterland of the orogenic regions of Timor (Figures 2 and 3). The aim of the paper is to wedge (East Timor and northern West Timor), melange is provide structural and stratigraphic links between various dominantly of mixed block-in-clay facies with large blocks facies we mapped in the Bobonaro melange and nearby activ,e ' derived from roof thrust sheets of intermixed Banda Terrane melange-forming processes. The approach involves (1) map and Maubisse Formation units, At the structural base of these and sample analysis of melange subunits throughout West thrust sheets is the Sonnebait Disruption Zone (SDZ ), which Timor, parts of East Timor, and the nearby islands of Semau formed the initial suture between the Banda Terrane and and Rote, (2) analysis of clay mineral and microfaunal characteristics of the melange matrix, (3) comparison of INow at Brigham Young Univer.;ity, Provo, Utah, block types included in the melange, and (4) correlating 2Now at Texaco, Bellaire, Texas. various melange facies along orogenic strike with active )Now at Mont Cuq, France. melange-forming processes. The Bobonaro melange derives its named from the Copyright 1998 by the American Geophysical Union. Bobonaro scaly clay of Audley-Charles [1968] in East Timor Paper number 97TC03083. and the Bobonaro Complex of Rosidi et at. [1979J in West. 0278-7407/98197TC·03083S12.00 Timor. Its composition and structure are consistent with
458 HARRIS ET AL.: COlliSIONAL MELANGE DEVELOPMENT -GEOLOGIC ASSOCIATIONS 459
LEGEND Pacific Plate Magmatic Arc 6·S Accreted Post breakup Cover Sequences of Australian plate Banda Forearc Basement (Banda Terrane) Accreted Gondwana Sequence
JAVA
12·S
Argo Abyssal Plain Scott Plateau 100 Km I
Figure 1. Simplified tectonic map of the western Banda arc orogen and NW Australian continental margin (see inset for location map and plate convergence vectors). The Java Trench and Timor Trough separate the NNE moving Australian plate to the south from the Asian plate to the north. Indian oceanic lithosphere subducts beneath the Java Trench and carries Australian continental lithosphere into the Timor Trough. Australian plate material accreted to· the Asian plate is shown in stripes (frontal accretionary wedge of postbreakup sedimentary cover sequences) and shading (subcretionary prebreakup continental cover sequences). Accreted units of the Australian plate are structurally stacked beneath and in front of parts of the Banda forearc (Banda Terrane, dark stripes). The Banda magmatic arc (solid) is in the incipient stages of subduction polarity reversal along the Wetar and Flores thrusts. Two diapir fields are shown at the deformation front south of Sumba and between Timor and Rote islands.
Raymond's [1984] definition of melange as a mappable body Broken formation facies were distinguished by their relict of rock characterized by a lack of internal continuity of stratigraphy, which consists of a homogenous assortment of contacts or strata and by the inclusion of fragments and tabular blocks that can be readily correlated to adjacent, blocks of varied size. unbroken source lithologies. This contrasts with matrix-rich We were able to subdivide the Bobonaro melange into three facies, which have fewer blocks, no structural attitude and distinct end-member facies from detailed mapping in the common intrusive, cross-cutting relations. Mixed block-in MutislMollo region, which is along the eastern fringe of clay facies are the most common and correspond to Kekneno structural window (Figure 3). In this region a descriptions of the Bobonaro scaly clay of East Timor combination of rapid rates of rock uplift (0.5 to 1.0 cm/yr) [Audley-Charles, 1968]. This facies is distinguished by a and erosional denudation provide extensive exposure of the poorly sorted array of diverse block compositions, sizes, and Bobonaro melange at several different structural levels. shapes that are embedded in a scaly clay matrix mostly of Within the melange in this region we distinguish variations mixed composition and age. by different degrees of mixing (heterogeneity), Active melange-forming p~ocesses observed to the SW remobilization, and dispersion. The following three end along orogenic strike from the MutislMollo region and in member facies were consistently recognized (Table 1): other parts of the Timor region include (1) cataclasis 460 HARRIS ET AL.: CDWSIONAL MELANGE DEVELOPMENT - GEOLCGIC ASSOCIATIONS
Australian affinity, which are emergent in the Timor region (Figures 2-4). Geologic relations and tectonic reconstructions indicate that subduction initiated in the Timor region of the Banda arc during the middle Miocene [e.g. Hamilton, 1979; Bowin et al., 1980; Abbot and Chamalaun, 1981]. At this time, Cretaceous age Indian oceanic lithosphere attached to Australia subducted beneath the Banda forearc or Banda Terrane [Audley-Charles and Harris, 1990; Harris, 1992). By late Miocene, the distal reaches of the Australian continental margin arrived at the trench in the Timor region. which began a transition from subduction to collision that has propagated SW from Timor to its present position south of Sumba Island (Figure 1). In a manner that is still poorly constrained, some of the sedimentary cover of the subducting oceanic plate and eventually most of the sedimentary rocks covering the NW Australian continental margin accreted to the base and front of the Banda Terrane to form most of the present Banda arc-continent collisional suture zone. We recognize five distinct lithotectonic units in the suture zone (Figure 5); (1) the Banda Terrane of Asian affinity, which is composed of fragmented basement blocks derived
Kekneno Lithotectoni:: Units from the leading edge of the Banda forearc upper plate; (2) the Strueturlll Window D Synorogenic Sequence Gondwana Sequence and (3) the overlying Kolbano Sequence, _ Banda T8n'Bne which combine to form the sedimentary cover of the • 8oboN.ro Melange Australian continental margin; (4) the Bobonaro melange, fI]] Kolbano Sequence which is dominantly composed of stratal disruption of clay o GoodWine: Sequence rich parts of units 2 and 3 along the tectonic boundary A Active Mud Volea10 Mads ...... 6..- Thrust between these units and unit 1; and (5) synorogenic 50 km sedimentary sequences, which lie unconformably over all of the other lithotectonic units and are locally incorporated into the melange. Detailed descriptions of each of these major Figure 2. General geologic map of Timor and Semau lithotectonic units are provided by Sawyer et al. [1993]. islands showing the distribution of the Bobonaro melange in Structural and stratigraphic relationships between each unit relation to other lithotectonic units and the distribution of and the Bobonaro melange are shown here on maps at two active mud volcano fields (open triangles). Location of more different scales (Figures 2 and 3). in cross section (Figure 4), detailed map (Figure 3) is given in box. and stratigraphic columns (Figure 5). Assembly of the lithotectonic units of Timor involved a complex system of detachments that have allowed Australian associated with decollement development and out-of-sequence cover sequences to stack up both under and in front of the fault zones [Reed et ai. 1986; Breen et ai., 1986; Karig et al.• Banda Terrane [Carter et ai., 1976; Charleton et al. 1991; 1987; Harris. 1991], (2) mud diapirism [Rosidi et al., 1979; Harris, 1991]. Only the lower part of the Australian cover Barber et al., 1986; Harris et al.• 1991]. and (3) mass wasting sequences, the Gondwana Sequence, is thrust beneath the [Fortuin et al., 1992]. Correlating these processes with the Banda Terrane. This sequence is also internally shortened types of melange they form is possible by projecting above a detachment at least at a depth of Lower Permian [Bird structural and stratigraphic relations in the direction of and Cook, 1991]. The resulting northern Timor collision propagation. Several lines of evidence indicate that contractional belt of West Timor and the entirety of East the Banda arc orogen propagated SW from Timor. where it Timor that developed above these detachments lift lower began in the late Miocene, toward Sumba where it is presently Gondwana Sequence units to an elevation of nearly 3CQO m initiating [see Bowin et al., 1980; Harris, 1991). This along the Ramelau-Kekneno arch. South of this arch. isolated orogenic configuration provides a way to relate active klippen of the Banda Terrane and the Maubisse Formation are decollements, diapirs, and accretionary wedge adjustments all that remains of the extensive roof thrust that once capped documented in the youngest, submerged part of the orogen to this region and still covers much of the orogenic wedge in compositional and structural associations of melange facies surrounding parts of the island and offshore along orogenic exposed onshore in exhumed segments of the arc-continent strike [Harris, 1992]. collision zone. The Kolbano Sequence, which was deposited over the Gondwana Sequence, is mostly accreted to the front of the 2. Geologic Overview of Timor Banda Terrane (Figures 2-4) to form the Pliocene Kolbano fold and thrust wedge of southern West Timor [Carter et ak., Onshore, the Bobonaro melange is mostly found along a 1976]. The thrust wedge consists exclusively of imbricate complex suture zone between lithotectonic units of Asian and thrust sheets of the Early Cretaceous to Pliocene Kolbano HARRlS ET AL.: COLLISIONAL MELANGE DEVElDPMENf - GEOLOGIC ASSOCIATIONS 461 I KEY FOR FIGURE 3 .! 124°!30'E SInorOlealc Sequ£D.ce Ke,. to 5,..boll i D L.atc MioccK 10 Q_atcrnuy Viqnqu Ponn_tioa
! QI - Qutcra.ary I imelto~ /' Fracnu'CJ ! Q•• QutCnlary .U1:lvium ThrntFn.lt Band. Terrane ,y • BT· UlIdi(fcrclUiatcd Blada TerraDC ? Normal Pult MUT· lwhni. Mcta=oipbic Complex BobonuG Mt!:eie r~:~1 Mixed blod.-ia-chy melt.alc. brakea form.atioD. Paved Roald ~.~"" .ad IIId.ia.jcCtiOil faciCi
Kolbano Seql1f:nce (Post breakup)
~ Early CrctlCC~1 to Plioccne cOlltjoclllal rlopc aAd riac dcp~it.
GoodWin. Sequence (S1Dbreakup) E:::."] Permiaa to h.t.uic clutica and limc_taDc or KckDcaa Gto.p
~ Pcrmiaa to Tti...ic yolcaaiC:1 aad carbonate or M..biJlC Porm,tiOa
10" OO'S
Figure 3. Geologic map ofcentral West Timor and Kekneno structural window ( north ofQuarternary deposits) showing the location ofsamples used for clay mineral and biostratigraphic analyses. Patterns for 5 10 15 20 Km lithotectonic units generally correspond to those shown in I ! figures 2 and 4.
Sequence, which correlate with slope and rise deposits of NW Timor is named here the Sonnebait Disruption Zone (SDZ). It Australian passive continental margin. The Boti-Merah fault is found at the base of Banda Terrane and Maubisse Formation zone [Sawyer et ai" 1993) at the rear (north) of the Kolbano thrust sheets in the northern contractional belt of Timor fold and thrust wedge separates it from the northern (Figure 4). Each facies of melange is represented throughout contractional belt (Figure 4). Delamination of the Kolbano the SDZ with the mixed block-in-clay facies most abundant Sequence from the underlying Gondwana Sequence occurs with most of its constituents derived from Gondwana along a decollement that propagates into thick clay-rich units Sequence units. The term Sonnebait is taken from Simons near the breakup unconformity between the sequences (Figure [1939] for descriptions of complexly "mixed" Permian 5). The depth to detachment in this region is constrained by through Jurassic units in what he named the Sonnebait Series. drilling in southern Timor [Sani et ai., 1995] and identified Surface exposures of many parts of the SDZ were discovered offshore in seismic profiles [Reed et ai., 1986; Karig et al., in deeply incised portions of the northern contractional belt. 1987]. The decollement surfaces at the Timor trough, which such as the Mollo/Mutis region on the SE flank of the is the southernmost expression of the Kolbano fold and thrust Kekneno structural window (Figure 3). In this region. erosion wedge (Figure 6a). has incised Llrrough much of the Banda Terrane and Maubisse Formation thrust nappes, exposing the SDZ at many places 2.1. Sonnebait Disruption Zone (SDZ) along the sale of the thrust sheets. The SDZ is also exposed on Rote Island near the capital city of Baa. The landscape in The initial suture zone between the Ba.lda forearc and cover these regions' is characterized by sharp, angular, high-relief sequences accreted from the Australian continental margin in ridges of the resistant thrust nappes that are surrounded by 462 HARRIS ET AL.: CDWSIONAL MELANGE DEVELOPMENT - GEOLOOIC ASSOCIATIONS
Table 1. Qualitative Field Criteria for Classification of Bobonaro Complex
Bro~en FOIJDatioo Matrix-Rieb Mi;ed Bloc!c-in-qay
Range of bloclc types narrow (1-2 formations) narrow-wide very wide (most uni ts)
Most conunoo source Australian affinity unilS Australian affinity units Australian and Asian affinity
Bloci: size mostly <10m mostly granules typically bimodai and blocks <1 m <1 m Kemeno Series >I m Maubisse Formation and Banda Terrane
Matrix characteristics mostly flow banded well-developed flow well-developed flow and and scaly and scaly fabrics and scaly fabric
Matrix to blade ratio 1:3 to 3:1 > 5:1 1:2 t05:1
Contact relatioos varies but tmIsitional both cross CUlting locally transitional with brolcen formation, with nearby units and concordant intrusive cross-cuning. and fault-related
Bedding mostly retained absent mostly absent
Structural correlation near base of SDZ of noM and central diapirism and mud within SDZ of north and central Timor. distributioo Timor and near decollement and volcanism throughout surrounding high-level nappes of in diapirs of southwest TiiDOi' orogenic wedge ~,.faubisse fOffiiatioii and Banda Terrane
low-relief hills of Bobonaro melange. Randomly scattered Maubisse Formation thrust sheet fragments that are embedded and shaped blocks in the melange commonly protrude in and intruded by melange. Matrix-rich mud injections are through varicolored scaly clay matrix, producing hummocky, found locally intruding the uppermost thrust sheets. The unstable slopes littered with lag deposits of more resistant facies succession in the SDZ can be very laterally blocks. discontinuous over short distances of a few hundred meters. We fIrst recognized the structural signifIcance of the SDZ The greatest accumulations of melange associated with the by correlating the highly disrupted zone of mixed blocks SDZ are documented near the Central Basin of West Timor and encased in scaly clay immediately beneath and surrounding Viqueque Basin of East Timor. These basins formed near the many fragmented thrust sheets of Maubisse Formation and southern terminus of the Banda Terrane and Maubisse Banda Terrane throughout Timor and Rote (Figure 2). The Formation nappe assemblages, which is also where we infer lowest units of the SDZ consist almost entirely of Gondwana .the SDZ originally surfaced. The Suai and Ossulari wells in Sequence broken formation that is transitional with mix~ the Viqueque Basin (Figure 2) drilled though nearly 2000 m of block-in-clay melange facies above. The highest structural scaly clay melange without reaching its base [Audley units of the SDZ are characterized by Banda Terrane and Charles. 1968]. These thick accumulations have been
WEST TIMOR COMPOSITE STRUCTURAL SECTION NW SE CENTRAL BASIN I NORTHERN CONTRACTIONAL BELT KOLBANO FOLD·THRUST BELT Kekneno Arch Oleu Erosional Surtace Diapir Timor Trough Figure SA KMS.
·1. Australian Shen
·20 ,. 20 Synorogenic Kolbano Post breakUp Banda Terrane D Deposits Sequence
Gondwana Syn-breakup • Maubisse Formation Bobonaro Melange ~ Sequence (Kekneno Group) Nappe • Figure 4. Composite NW-SE structural cross section along the western edge of West Timor showing major structural features and provinces. Lithotectonic units shown generally correspond to those in Figure 3. Along-strike positions of Figures 6a and 6b are shown in boxes. Actual positions are given in Figure 1. I! HARRlS ET AL.:
!; SYNOROGENIC BANDA DEPOSITS TERRANE STAGE MA AGE NAME P18ST. ,:.~)-S,'v,-I~~ 1.54 +---.-=+----; PQ~t.~":''''' i sa ...J < 3.4 .: 1-~-I-7-a.-~II-Ila----
interpreted as olistostromes [AudIey-CharIes, 1965 and 2.2. Mud Diapirism 1968; Brunschwieler, 1979], but it is also possible that they represent mass flow deposits of a different type than that Zones of diapirism in the Banda arc orogen are well associated with sedimentary processes. documented on the trenchward-dipping submarine slope Stratal disruption within the SDZ involves at least two (Figure 1) south of Sawu and Rote [Breen et a!', 1986; Karig et phases of deformation: a pervasive layer-parallel extension al., 1987; Masson et ai., 1991] and in the Timor region and more localized rotational shear. Layer-parallel extension (Figure 2). Seismic profiles of the deformation front show is characteri..zed by viscous flow of clay-rich layers out from that diapirs rise from a transparent zone that corresponds to between competent layers as in boudinage (Figure 7). The Jurassic and Lower Cretaceous age clay-rich units near the resulting extension is a function of interface friction between breakup unconformity between the Gondwana and Kolbano the layers. Calcite fibers that fill bed normal fractures Sequences (Figure 6a). Wells that penetrate this stratigraphic throughout melange blocks indicate most fractures are interval both in the Kolbano fold and thrust belt [Sani et aI., extensional (mode 1). The pattern of fracture development is 1995] and on the NW Australian margin [Kingborough et al., consistent with coaxial strain about an axis normal to 1991], have encountered very high pore fluid pressures bedding similar to type I melanges described by Cowan requiring repeated reaming at mud weights above 1j specific (1985). The timing and number of events of layer parallel gravity (SG). Triaxial tests of clay stones from the breakout extension is unknown and may predate collision. However, zones show extreme ductility at differential stress of 40-70 results from an investigation of the temporal and geometric MPa. relationship of nonfilled extensional fractures to collision Diapirs are also found near the crest of the orogenic wedge ielated structures indicate that most fractures are parallel to where it is overlain by the Banda Terrane roof thrust and the prevailing (NW-SE) and a secondary direction (NNE-SSW) synorogenic deposits of the Central and Viqueque Basins of shortening [Mikolas and Harris, 1995] (see Figure 7c). (Viqueque Formation). A cross-sectional view of one of these Rotational shear is localized in broken formations near the diapirs is provided by a seismic reflection profile across the base of the SDZ and along the basal thrust of high level Sawu thrust (Figure 6b). The Sawu thrust cuts into the Sumba nappes near the top of the SDZ. Both contractional and Basin. which is roughly equivalent to the Boti-Merah thrust extensional relations are found. A less common occurrence of and Central Basin of West Timor (Figure 4). The stratal disruption associated with rotational shear in northern interpretation of the migrated time section by Reed et aI. Timor is where blocks of Banda Terrane and Gondwana [1986] shows a diapir piercing at least 1 km of synorogenic Sequence are found in structural juxtaposition without sediment equivalent to the Viqueque Formation. The sediment appreciable surrounding matrix material. This type of "block overlies the Sumba Ridge Terrane, which is the western to-block" broken formation shows the effects of intense extension of the Banda Terrane. The remaining unintruded imbrication similar to the "chaos structure" described by sedimentary units are domed upward into a low-amplitude, Noble [1941] in the thin thrust sheet above the Amargosa . broad wavelength anticline. Thrust. The depth and composition of the diapir, and its lateral Injections of matrix-rich melange sourced from the SDZ are extent can be inferred from onshore exposures along orogenic found locally intruding narrow « 100 m diameter) passages strike in Semau (Figure 7) and Timor where the Central Basin through overlying thrust sheets and forming diapirs that is uplifted and incised. Kenyon [1974] mapped several broad intrude along the base and up through synorogenic deposits anticlines cored with Bobonaro melange that have a similar that overlie these thrust sheets. Most active and relict mtrl size and geometry to the diapir in Figures 6b, 7c and 7d. volcanoes and diapirs west of Timor where the SDZ is at depth Although these anticlines were not initially recognized as cluster in linear zones indicative of leakage of fluid-rich diapiric intrusions. we have found many intrusive melange through fractures in the thrust sheets. This relationships, such as dikes and sills of scaly clay as well as observation is consistent with relations throughout the evidence for stratal disruption during intrusion, such as northern mountains of Timor where matrix-rich mixed block abruptly tilted and even overturned units near the contact with in·day melange from the SDZ fills the gaps between klippen melange as documented in diapiric melanges [e.g. Talbot and of the Banda Terrane and Maubisse Formation. Von Brunn, 1987, Orange, 1990].
Figure S. Lithotectonic units and fluid pressure profile of the Banda arc orogen. The Banda Terrane is part of the southern edge of the Banda forearc and is composed of high- to low-grade to pelitic metamorphic rocks, amphibolite, and greenschist facies to nonmetarnorphosed intermediate volcanic rocks and cover sediment. The Gondwana Sequence mostly consists of clastic units with some intervals of limestone that accumulated in an intracratonic basin. The Maubisse Formation is a facies variation of the lower Gondwana Sequence. The Kolbano Sequence was deposited on the slope and rise of the NW Australian passive continental margin. which formed during Late Jurassic breakup of Gondwana. The synorogenic Viqueque Formation was deposited in deep water over the Miocene accretionary wedge that formed when subduction initiated in the Banda arc and was deposited in progressively more shallow water as the accretionary wedge was uplifted by collision with the Australian passive margin. The pore fluid pressure profJ.1e after Kingborough et al. [1991] shows fluid overpressures in clay-rich units near the base of the Kolbano Sequence and within the Wai Luli Formation of the Gondwana Sequence, which is the site of the decollement at the base of the Kolbano fold and thrust wedge. HARRlS ET AL.: COLLISIONAL MELANGE DEVELOPMENT - GEOLOOIC ASSOCIATIONS 465
Nort h b
a Sou lh Deformallon Fronl DiaD irs ~ \...-/;..,..:...... _..,;r-".~~~1.//i ' ~ ~'.; ,f~- ~ J VE:3. i x "j/- -i'";.. I2 a :::::- = --7 r Basal DecoJ(c.mcnt VF. = ".~ 15 itm _L:=====~l J SEC S :2~""-~!!~. "'3 I~_t_f ~ ! 'l=~\:'£f;~~+~~t~-;1 1
6
7
8 SEC
Figure 6. Seismic reflection profiles and interpretive line drawings across the (a) front and (b) rear of the active Kolbano fold-thrust wedge west of Timor (line locations are shown in Figure I). These lines provide a modem analog for the Pliocene Kolbano fold-thrust wedge exposed in southern West Timor (Figure 4). Figure 6a illustrates line 2 of Breen et al. [1986] (Reproduced [or modified] with permission of the publisher. the Geological Society of America, Boulder, Colorado USA. Copywrite @ 1986 Geological Society of America.) showing several transparent zones associated with mud ridge development south of the thrust front and above a detachment near the breakup unconformity of the Australian continental slope. Sequence I correlates with the Gondwana Sequence of Timor. Sequence 2 corresponds with acoustically transparent clay rich units at the base of the Kolbano Sequence of Timor (N~nu Formation). Sequence 3 is a homogenous series of pelagic carbonate and marl similar to the Menu and Ofu Formations of the Kolbano Sequence. Figure 6b illustrates line 80 of Reed et al. [1986] (Reproduced [or modified] with permission of the publisher, the Geological Society of America, Boulder. Colorado USA. Copywrite @ 1986 Geological Society of America.) showing a diapir rising from near the edge of the Banda Terrane (Sumba Ridge Terrane of Reed et al., [1986]) into synorogenic sedimentary cover. Compositional data from diapirs and mud volcanoes exposed along orogenic strike in the Central Basin of Timor document a source of melange from the SDZ.
Diapiric features, although not as aerially extensive as 3. Composition of the Bobonaro Melange other facies of the Bobonaro melange, are commonly associated with the matrix-rich facies. This facies is Exposures of melange at various structural levels provide a distinguished by mostly granule-sized fragments (less than 1 rare opportunity to compare its compositional and structural cm in diameter) that at mud volcano vents are found suspended characteristics to potential sources and emplacement in a soupy mud matrix (Figure 7d). In addition to a common mechanisms found along orogenic strike. To further constrain association with mud volcanoes and diapirs, the matrix-rich melange source characteristics, we have analyzed block types facies occurs (l) where it locally fills fracture zones at the and matrix clay mineralogy and microfauna throughout the base and around the edges of the Banda Terrane, (2) near the Bobonaro melange and associated lithotectonic units at unconformity at the base of the Viqueque Formation, and (3) locations throughout the western Banda orogen where in association with broken formation in the Kolbano fold relationships are least ambiguous. thrust wedge. The distribution of matrix-rich facies melange suggests that it may be ubiquitously associated with intrusion 3.1. Block Size and Composition of mud-rich slurries sourced from broken formation or mixed block-in-clay facies melange from within, and even below the Most blocks of the Bobonaro melange are matrix-supported SDZ. that locally pierces the Banda Terrane roof thrust and can be readily identified as fragments of one of the 466 HARRIS ET AL.: COllISIONAL MELANGE DEVElDPMENT - GEOLOOIC ASSOCIAnONS
Figure 7. Photographs of the broken formation and matrix-rich facies of the Bobonaro melange. (a) Layer normal fracrures in block of broken sandstone and siltstone layers interlayered with flowed red and green shale. Fracture array is extensional and propagates from the surface downward, indicative of surface tension. (b) Two layer-parallel extensional phases. An early vein phase is fIlled with fracture-normal calcite fibers. Later clay-fIlled fractures form normal to the earlier phase and show pure extensional offset of veins. (c) Landsat image of the Semau diapir field (north is up, see Figures I and 2 for reference). Circular features, such as the peninsula of NW Semau and the small island between Semau and Timor (Palau Kambing) are diapirs 1-3 km in diameter. Note NW-SE trending fractures that cut raised coral reef terraces in upper right of photograph. These are extensional fractures that have formed parallel to the direction of ma~imum shortening and may correspond to those shown in Figure 7a. (d) Mud volcanoes of matrix-rich melange extruded from the Palau Kambing mud diapir. which is at a similar structural position to the diapir in Figure 6b.
various lithotectonic units of Timor by those with a working underthrust and fluidized units of the Kekneno Group at the knowledge of the distinguishing features of these units (Table base of the SDZ, which locally mix with thrust sheet 2). Truly exotic blocks are rare and found only near the top of fragments of the upper SDZ. the SDZ and at the rear of the Kolbano fold-thrust wedge. We Estimates of block size variations show an abrupt increase found that documenting variations in block size and in maximum block diameter near the top of the SDZ where composition provides a way to constrain the degree of block fractured thrust sheets are incorporated into the Bobonaro dispersal and mixing, which relates to the different melange. These large blocks of the SDZ hanging wall are conditions of melange emplacement. embedded in melange along their base and sides as if they Block counts were conducted at mud volcanoes, diapirs, and have sunk or slumped into the weaker clay-rich substrate [see exposures of the SDZ in Timor, Semau, and Rote (Table 2). Audley-Charles, 1968, Plates 4 and 5]. A similar relationship General trends show that the mixed block-in-clay facies is has been shown by Engelen [1963] in the Alps and been most common. In all facies the most common block types suggested for diacreme development by Woolsey et aI. are fragments of Kekneno Group sandstone and limestone < 3 [1975]. m in diameter (Table 2). However, included in the mix are South of the northern contractional belt of West Timor, larger blocks of the Maubisse Formation and Banda Terrane melange is restricted in distribution and composed of blocks units that are transitional with fragmented thrust sheets up to that are smaller. more homogenous in composition, and lack several kilometers in diameter. We interpret these relations to contributions from the Gondwana Sequence and Banda Terrane indicate that the bulk of the melange is derived from (Table 2). For example, the Oleu diapir has blocks that are a Table 2. Relative Percentage of Blocks in Mud Volcanoes, Diapirs, and Melange
Locality (Area)' Kollbano Seguenceb Kekneno Seriesb Milubisse Founatjonb TC' Mutis· Uncertain· ~' Undifferentiated Blocks Maximum Block Sch· Um Vol Sed Lithosphere Counted Diameter, meters i Ch Ls Cl Ss Ls Fe s Vol Ls Ls ~
North Bobomelo mud 0 0 0 71 21 0 0 2 5 0 0 0 0 0 56 0.07 (top) t volcano (North Timor) 0.4 (base) South Bobometo mud 0 0 0 55 31 0 0 3 0 0 0 0 0 11 64 0,07 (lop) ~ volcano (North Timor) 0 Kambing mud volcano 5 2 0 65 25 4 I 0 0 0 1 0 2 0 197 0.15 (top) (near Semau Island) 0.9 (base) ~ Semau mud volcano 0 0 0 90 5 0 0 0 0 0 0 0 0 5 100 0.4 Northeast Role diapir 15 2 6 34 17 6 5 5 1 2 0 0 5 2 133 1 Oleu diapir 5 35 0 0 0 60 0 0 0 0 0 0 0 0 100 1 ; (Southern Timor)
Soe BIC me1an~e 36 0 15 12 0 1 8 3 0 16 1 0 4 3 73 6 (Central basin
Niki Niki mc melange 12 12 26 15 0 7 9 2 0 0 7 8 0 I 85 12 .(South central basin) i~ , South Rote BlC melange 11 0 17 I 1 8 3 56 0 0 0 0 0 2 177 20 @ East Mutis mc melange 0 0 0 8 6 9 2 3 48 0 15 5 2 3 111 150 (SDZ-North Timor) ~ () Bobooaro VilIa",e B1C 10 2 0 20 5 2 30 15 0 15 0 0 1 0 100 500 (Central East imor) ?/;
BIC, block in clay; TC, Tertiary Cablac Formation; Ch, chert; Ls, limestone; 0, calcilutite; Ss, sandstone; Fe s, iron stone; Vol, volcanic; Seh, schist; Urn, Ultramafic; Sed, sedimentary rocks. 'See Figure 2. bAustralian continental margin affinity. ~ "Banda Terrane. 0 enZ
~ 0\ -.I 468 HARRIS ET AL.: ffiLLISIONAL MELANGE DEVELOPMENT - GEOLOOIC ASSOCLA.TIONS
mixture of distinctive iron stones derived from concretions from homogenous gray to alternating, contorted bands of within the Jurassic Wai Luli Formation and competent units maroon, gray, and green varieties. Color variations reflect from the Early Cretaceous portion of the Kolbano Sequence. both the original composition of the clay and that of blocks These broken formations were undoubtedly derived from the entrained in the matrix. basal decollement of the Kolbano fold-thrust wedge. The One of the roost distinctive characteristics of the matrix is a location of the diapir on a major fault zone supports models scaly clay fabric consisting of a pervasive network of for remobilization of tectonic melange along fault conduits microfractures that create shiny, "scaly" partings. The [e.g. Breen et at., 1986]. Unlike many of the diapirs north of occurrence of scaly clay in the Bobonaro melange has been the Kolbano fold-thrust wedge the Oleu diapir does not intrude used as an argument for and against various modes of melange into a roof thrust assemblage or synorogenic sedimentary development in the Timor region [Hamilton, 1979; Barber et cover, which explains its homogeneity. A similar relation is at., 1986; Charlton et ai., 1991]. However, scaly clay fabrics found at the Bobometo diapir of northern Timor (Figure 2) are well documented in melanges of various origins where two separate mud volcano fields mounted on footwall throughout the world [e.g. Raymond, 1984], and in mud units of the SDZ yield more than 85% blocks of Kekneno diapirs [Ray and Grocock, 1974; Williams e/ ai., 1984], Group sandstone and limestone (Table 2). submarine landslides [Phipps, 1989; Brandon, 1989], and in drill cores from submarine accretionary forearcs [Moore et 3.2. Block Sources and Dispersion aI., 1986]. Recent studies of scaly clay fabric development The utility of using block size and compositional show that the poor strain memory of clays only records the variations to develop a melange facies classification scheme last increments of viscous flow during emplacement and is is in linking these variations with regional structural trends not a reliable indicator of melange origin or the deformation and possible sources for the blocks. For example, broken mechanism path of anyone melange-forming process [Jones formation facies represent a near-source, initial stage of and Addis. 1985; Yassir, 1990; Prior and Behrmann, 1990]. block dispersion. Along the base of the SDZ in northern In fact, some of these studies [Nichols, 1980] show that the Timor the broken formation facies represents blocks which low stress conditions required to form the coalescing fractures are sourced from and have a structural position within that make up scaly clay partings prevent the fabrics from deformed Kekneno Group units of the SDZ footwall. In the recording anything but the most recent release of residual Kolbano fold-thrust wedge the source is limited to supra stress during load removal. decollement units of the Wai Luli and Nakfunu Formations. 3.3.1. Clay mineralogy. Audley-Charles (1965] Mixed block-in-clay melange develops where fluidized reported that matrix clays of the Bobonaro melange from East broken formation (matrix-rich facies) is mixed by injection Timor are mostly composed of a bentonitic clay with 35% with the roof thrust assemblage. Trends of increased mixing average smectite. Barber e/ al. [1986] found a dominantly and dispersion of blocks upward through the sm and illite-rich clay assemblage from their analysis of matrix southward toward the central basin of Timor may indicate material in the northern contractional belt of West Timor. viscous flow of disrupted Kekneno Group material upward and These results raise questions about how clay mineralogy outward (southward) within the SDZ. varies throughout the Bobonaro melange and relates to other Exotic blocks are found locally along the base of the Banda clay-rich units. The samples we analyzed from 35 different Terrane and at the rear of the Kolbano fold-thrust wedge. sites in West Timor (Figure 3) represent fresh exposures of Most of these have compositional features common to clay from both Australian-affinity sedimentary successions subduction melanges, which include a serpentinite-rich facies and Bobonaro melange material associated with mixed block with blocks of pillow lavas, manganese nodules, and other in-clay, broken formations, diapirs, and active mud hemipelagic material. This material was initially discovered volcanoes. The clay mineral data yield consistent variations in the Niki Niki region of Timor (near locality 76 in Figure 3) that correlate to stratigraphic and structural differences and in Rote during the 1911-1912 expeditions of Molengraaff between each site (Table 3). and Brouwer [Molengraaff, 1915; Molengraaff and de Clay mineral assemblages form two distinct sample Beaufort, 1921]. The geochemistry and petrography of the populations on the basis of illite/smectite (IIS) content clays were compared by Molengraaff [1915] to similar (Figure 8). Group I samples consist of a mixed illite/smectite deposits documented at the time in Barbados and dredged from (I/S)-rich clay mineral assemblage. Most of these samples the deep sea by the Challenger expeditions. Audley-Charles have more than 70 wt % expandable clay, of which more than [1965] describes similar Cretaceous and Eocene pelagic 65 wt % is smectite (Figure 9). All of these samples are deposits at the rear of the Kolbano fold-thrust wedge along located south of the Boti-Merah fault, and are derived from the south coast of East Timor. The composition and structural stratally intact Lower to Middle Jurassic Kekneno Group, position of these disrupted deep marine deposits suggest that Lower Cretaceous Kolbano Sequence lithologies, and broken they may represent vestiges of the first material accreted to formation and matrix-rich melange facies of the Bobonaro the front of the Banda Terrane from subduction of Eocene to melange. Cretaceous age Indian oceanic lithosphere beneath it Group IT clays are illite, kaolinite and chlorite dominant (smectite poor). These samples were collected from north of 3.3. Melange Matrix the Boti-Merah fault, where the exposed stratigraphic column is composed mostly of Permian through Upper Triassic Fresh exposures of Bobonaro melange are composed of clastic units of the lower part of the Kekneno Group and consistently soft and variegated scaly clay. Color varies mixed block-in-clay melange facies. The similarity between Table 3. Clay Mineral Analyses ofWest Timor Lithotectonic Units
Lithotectonic Expandable Illite Sample Location Lithology Unit Description Illite, % Expandable, % K+C,% Phase Crystallinity Group I 30 SDZ of North flow banded scaly clay block-in-clay melange SDZ footwall near Banda terrane/ 10 80 10 1-30/501-70 0.65 Timor i 76 Ko\llano red mudstone Kolbano Seqnence broken Nakfunu FOffilation near 10 75 15 1-10/501-90 0.7 ~ Mountains Boti-Merah fault 39b SDZ of North red scaly clay matrix-rich melange SDZ footwall near structural base 15 80 5 1-20/sm-80 0.9 r. Timor of Banda Terrane IS 83 SDZofNorth flow banded scaly clay matrix-rich melange Matrix of SDZ near Banda Terrane 75 10 1-20/sm-80 0.55 .....~ Timor contact 0 51 SDZofNorth red calcareous upper Gondwana Sequence bedded clastics in thrust sheet in SDZ 20 80 0 sm-lOO 0.65 ~ Timor mudstone 55 SDZofNorth black scaly clay block-in-clay melange Matrix of SDZ under Viqueque Formation 20 70 10 1-60/sm-40 0.8 Timor ~ 58 SDZ of North varicolored scaly clay block-in-clay melange Matrix of SDZ at base of Banda Terrane 20 70 10 1-10/501-90 0.7 fd Timor t:J \Tl 82 Central Basin varicolored scaly clay Kolbano Sequence broken Kolbano Sequence in SDZ 20 80 0 1-20/sm-80 0.8 35 SDZofNorth scaly clay in fault Banda Terrane SDZ ffilltriX at base of Banda Terrane 30 70 0 501-100 0.3 ~ Timor gouge ~ Groupll ~ la Kekneno grey scaly clay Triassic Kekneno Series SDZ broken fOffilation at structural base 35 35 30 1-85/501-15 0.8 0 Fenster of of Banda TerranelMaubisse Nappe fH North Timor 0 4b Kekneno grey shale Triassic Kekneno Series SDZ broken fOffilation at structural base of 35 30 35 1-70/sm-30 0.65 § Fenster of Banda TerranelMaubisse Nappe () North Timor ~ Ib Kekneno grey shale Triassic Kekneno Series SDZ broken fOffilation at structural base of 40 30 30 1-80/sm-20 0.6 'R Feusterof Banda TerranelMaubisse Nappe ~ North Timor 0 Z 2 Kekneno grey scaly clay Triassic Kekneno Series SDZ broken formation at structural base of 40 30 30 1-70/sm-30 0.6 til Fenster of Banda TerraneIMaubisse Nappe North Timor 33 SDZofNorth varicolored scaly clay undifferentiated Kekneno SDZ at base of Banda terrane nappe 40 55 5 1-65/sm-35 0.75 Timor series
.to. 0\ \0 470 HARRIS ET AL.: CX>lllSIONAL MELANGE DEVEIDPMENT - GEOLOGIC ASSOCIATIONS
the clay mineralogy of the melange matrix and that of nearby stratally intact lithologies strongly supports a Lower
00 00 Kekneno Group source for the clay component of the ~ ~~s ~ ci ci ;::::::: ~ 0 Bobonaro melange in this region. U No significant difference was found between clay mineral compositions from various melange facies within a similar ..!! 0 .0 c ~ systematic decrease in expandable clay phases with 00 o o It''l o stratigraphic depth argue that burial depth is most likely the +- - - major control of compositional differences between Groups I ~ and II as opposed to sediment type and origin. This trend is supported by vitrinite reflectance values [Kaiser and Harris. 1995]. values from samples of group I lithologies range o o o R. V"l V"l c 120-r------, with independent evidence from drilling, seismic reflection profiles, and field mapping (referenced above) that all GROUP 1 indicate that melange genesis is limited to accreted units ~100 above basal decollements as is the case in southern Timor C3 where the basal decollement is at a Jurassic stratigraphic :0'" .:g 80 level. Data from microfauna are also consistent with trends c l! exhibited by the distribution of group I and group II clay )( UJ mineral assemblages, which indicate that some melange clay ~ 60 in the northern part of Timor is derived from deeper .E stratigraphic intervals. Anomalously young biostratigraphic ~ GROUP 2 ti 40 0 ages found north of the Boti-Merah fault zone (samples 69, Ql 0 E 75, and 122) demonstrate the admixture of clays from the en 0 G G ~ Kekneno Group with overlying units of the Banda Terrane and 20 0 0 G 0 Viqueque Formation, which are both intruded by matrix-rich melange diapirs. O+---r--..,.-,..-.--r--.--,.-,-,..-.--r--.-,---\ 20 30 40 50 60 70 80 90 4. Age of Melange Genesis "10 Expandable Clays Figure 8. Expandable clay mineral groups of West Timor A direct method of dating the age of the Bobonaro melange (see text). is not possible because most, if not all, fossils are inherited from preexisting strata that is either disrupted to form the melange or incorporated during melange flow. The youngest Two other important trends are also observed that warrant fossil assemblages found therefore can only provide a further discussion. First, mud volcanoes exhibit the widest maximum age estimate of melange flowage. Age analysis of microfauna! age range of all sample types (Table 5), with the the stratigraphic and structural associations of the Bobonaro oldest taxa observed corresponding to Kekneno Group melange provide much narrower age constraints. For lithologies. Limited faunal ranges were found at sites entirely' example, the youngest microfossils found in the Oleu diapir within the Kekneno Group footwall of the SDZ as in the of the Kolbano region are Early Cretaceous palynoflora. Yet, Halilulik mud volcano (Figure 2). A wide range of fauna, as in the diapir rose along a fault that cuts Kolbano Sequence units samples from the Semau and Besikama mud volcanoes, is of Early Pliocene Zanclian stage. Zanclian ages are also suggestive of a fairly complete subsurface section of Kekneno found in matrix samples from several of the active moo Group, Kolbano Sequence and synorogenic sequence volcanoes in Timor (Table 5). 'The maximum age of lithologies. In the latter, microfauna are believed to be decollement-related stratal disruption can be bracketed derived from active mud injection into capping Neogene and between the late Miocene age for nappe emplacement in Quaternary units. Central Timor [Audley-Charles, 1986] and the early Pliocene Second, all of the Bobonaro melange samples that yield age of Viqueque Formation units that are stratally disrupted by ages as old as Early Triassic are located north of the Boti melange intrusions [Sawyer et al., 1993]. These data Merah fault. South of the fault the oldest taxa observed indicate an age progression of melange genesis across the correspond to the Early Jurassic. These trends are consistent orogen of Late Miocene-early Pliocene in East Timor and the Wt % Clay Phase Cumulative Wt % Of Expandable Clay Phase o Unit o 40 60 60 100 40 80 120 200 Kolbano Sequence Broken Formation Block.ln-day % Mixed Mellnge % Expandable 115 Mixed 115 Matrlx·rlch Melange uppo< KeI Keknono Serl•• Broken FormltJon Figure 9. Clay mineral phase plotted against relative structural level of sample locations. Major shifts in clay mineral composition correspond to increases in expandable phases near and above the breakup unconformity between the Kolbano and Gondwana Sequences. ~ -...l tv Table 4. Biostratigraphic Assemblages of Melange,Forming Fonnations Total ~~..r.3 Range of FOffilation Sam Dominant Pal noflora sitional Environments Comments Cribas 7 Permian Krallse/isporites sp. samples often barren with Entylissa cymbatllS shallow marine poor preservation Veryhachium Umitisporites cf. i L. moersensis ~ r. Niof 23 Early-Middle Triassic Stallrosaccites qlladriflldlls continental-deltaic- samples often barren with Nammalian-Ladinian Ba/tispaeridillm spp. shallow marine- poor preservation Veryhacillm trispillOslim sublinoral ~ V. bromidense Falcisporites allStralis i Mychrystridillm spp. Aratrisporites tenwspinoslIS ~ Injernopo/lenites clallstratus fcl Aitutu 16 Middle-Late Triassic Staurosaccites quadrifudus Cmcirhabus primus marine, sublinoral samples often barren Ladinian-Rhaetian DllfJlexisporites problemati ArchaeozygodiscllS k. some entactinid radiolaria ~ Aratrisporites parvispillOsllS Conllsphaera z/ambachensis Enzonalosporites vigens ~ ~ 0 Babulu 8 Middle-Lale Triassic Staurosaccites quadrifiu1lls deltaic-shallow marine- assemblages often aparse fcl Ladinian-Carnian Shublikodinium wigginsi sublinoral Samaropo/lenites cf. § Ceratosporites he/idonesis n >en Vl WaiLulu 20 Early-Middle Triassic Callialasporites tllrbatllS Parhabdolithlls liasicus marine, shallow typically abundant diverse l-Ianangian-Bathonian C. dampieti Crepidolithis crasslls sublinoral assemblages ~ C/assopo/lis toroslls Schizosphaere/la ~ Vl Ctenidodillm ct. Mitrolit/lIIs elegans Nannoceratopsis spp. Crucirhabdus prilllllius Caddospaera ha/osa C. crasSlls Dapcodinillm priscum C. p/iensbachiellsis Dissiliodinillm sp. Lot/lOring/liS sp. " _.,.... "'''''~ Table 4. (continued) Total Range of Formation Sample Age Range ~omi~ant Pll1}'I1~o,!__ . _ r>omillllI1.!Na[lJlofiora _---.!>e.Jl<>sitional Environments Comments Nakfunu 29 Early Cretaceous Chamydopholera ambiglla ConllspllOua mexicallO marine, sub-outer abundant indeterminate Berriasian-Aptian Sysremarophhora areolora Microsraunts chiasrillS littoral radiolaria E/lipsagelospaera spp. NanlloconllS spp. i ~ Urhrahidites carniolensis Muderongia sp. r Menu and Lower 17 Early-Late Cretaceous similar Early Cretaceous marine, sub-outer abundant indetenninate ~ Palelo Group Berriasian-Campanian taxa as Nakfunu. littoral radiolaria en EprolithllS floralis ~ Lucianorhabdus malefor Stradneria coenulata EiffelithllS eximillS ~ Nucina spp. g Chastozygus platyrrhlls ~ Ofu and Upper 17 late Paleocene Rericulofenestra hillae marine, subliltoral common microfossils and Palelo Group early Pliocene Dicryococcites bisecta bathyal abWldant reworked Thanetian-Zandian SphenolithllS pseudora Cretaceous taxa Fasciculithus spp. II Spenolithus moriformis RetiCillofenestra umbili Calcidiscus macinryrei ~ Discoasrer brouweri ~ CyciiscargolithllS floridallllS en Chiasmolithlls sp. R Zygrablithus bijugatlls ~ Rericlofenestra hampdenesis ~ en Discoasrer borbadiensis Sphenolitll1lS abies S. heteromorphlls llelicosphaera /camprneri ~ -.lw .p.. -l .p.. Biostratigraphic Analyses of Bobonaro Complex Melange Location Lithology__ Facies . ~sl:riJ>tion_~_..n_.._._~n~ge_ Biostrati~ ·43 Haliluliuk, hydrated clay matrix rich matrix of active mud volcano Early-Middle Jurassic laxa indicative of Wai Luli Formation central Timor' in Kekneno Series lithologies with reworked Middle-Upper Triassic fauna ~ ~ PS-l Semau island, hydrated clay matrix rich matrix of active mud volcano Early Triassic - wide range ofEarly-Middle Triassic, west of Timor' PliocenelPleistocene Early-Middle Jurassic, Cretaceous, Eocene, r and Neogene taxa consistent with Table 3 ~ ~ BS-47 Besikama, south hydrated clay matrix rich matrix of active mud volcano Early Triassic - wide range ofEarly-Middle Triassic, 0 coast Timor' in Kekneno series lithologies PliocenelPleistocene Early-Middle Jurassic, Cretaceous, Eocene, and Neogene taxa consistent with Table 3 ~ 150 South of Central Basin, unconsolidated block in clay matrix of dormant mud volcano Middle Triassic- wide range of Middle-Late Triassic, ~ north of BOli-Merah fault clay stone along BOli-Mecall fault ? Pleistocene Late Jurassic, Early Cretaceous, and fri between Kekneno Sequence Pleistocene taxa coilStstent with Table 3 t:I and Banda Terrane ~ 254 Central Basin, north tectonized broken matrix of SDZ between Kekneno Late Triassic P. triassica, A. koessenensis. T. geometrica, of Boti-Merah fault clay stone formal ion Series and Banda Terrane and C. minutus ~ , 248 as above Kolbano Sequence broken melange block in SDZ Middle-Lale Eocene common Sphenolithll5 radians. S. pseudoradia1lS lH 0 calcilutite formation and S. obtusw. reworked § Cretaceous Globatroncatids () ~ 122 near Kapan north of tectonized block in clay matrix of SDZ between Kekneno Triassic and early common Globigerinoides rober. t/) Central Basin clay stone Series and Maubisse nappe Pliocene or younger Globorotalia cf.. G. twnida. R reworked Triassic pollen § ~ t/) BN-72 SE of KUpangb Kekneno Sequence broken melange block from melange UpperTriassic Suessia swabiana and siltstone formation injected into Viqueque Duple.xisporites problematicll5 formalion near rear of KolbwlO fold-thrust belt Table s. (continued) Melange Sample Location Lithology Facies Description Age Biostratigraphy_ ~__~~__ 86A Deleu Diapir, unconsolidated matrix rich shale diapir above SDZ in Early-Middle Jurassic low-diversity assemblage of NamlOcera/opsis Kolbano mountains clay stone Kolbano Sequence gracilis, N. spicula/a, and frequent Nannocera/opsis sp. I f:,1 (an as yet undescribed species) r 86B as above unconsolidated matrix rich shale diapir above SDZ in Early-Middle Jurassic Nannocera/opsis spp., Callialaspori/es clay stone Kolbano Sequence /urba/us, and ~ Meiollragonyml1a.x sp. 86C as above Kolbano Sequence matrix rich melange block in shale diapir Late Jurassic - sample barren of microfossils/nalmoflora, ~ sideritised mudstone within Kolbano Sequence Early Cretaceous poorly present palynoflora including Egmon/odinium /orynum, ~ common terrestrial palynomorphs ~ tJ 69A south of Cenlml Basin, tectonized siltstone block in clay matrix of SDZ along Boti-Merah middle-late Eocene common Discoas/er saipanensis, D. barbadiensis, north of Boti-Merah fault fault between Kekneno reworking ofLate Jurassic ~ Sequence and Banda Terrane and Early Cretaceous dinocysts, and Early Paleocene taxa ~ I 69B south of Central Basin, tectonized clay stone block in clay matrix of SDZ along Boti-Merah late Paleocene FasciculirhllS sp., Sphenoli/hllS prinulS and fH north of Boti-Merah fault fault between Kekneno PlacozygllS sigmoides, reworked Triassic, Sequence and Banda Terrane Middle Cretaceous pollen, spores and dinocysts ~ () ~ 75A south of Cenlral Basin, tectonized clay stone block in clay matrix of SDZ along Boti-Merah late Paleocene FasciculirhllS sp. and Sphenoli/hllS prinulS en north of Boti-Merah fault fault between Kekneno with reworked Cretaceous R Sequence and Banda Terrane dinocysts and pollen § 0 Z en 75B south of Centml Basin, Banda Termne block in clay melange block at thrust contact late Paleocene sample barren, classification based on lithology north of Boti-Merah fault calcilutite of Kekneno Series with Banda Termne injected into Viqueque Sequence ~ .....:l VI ~$' ~-~-~ .. - p - -- - - .. - • '--,.. • ------476 HARRlS Ef AL.: COllISIONAL MELANGE DEVELOPMENT -GEOLOOIC ASSOCIATIONS northern contractional belt of West Timor to post-Zanclian stage in the Kolbano fold-thrust wedge of southern West Timor to present on the southern slope and at the deformation front. A similar age progression is also inferred toward the SW along orogenic strike. s. Gen'esis and Emplacement of the Bobonaio Melange Our findings suggest the principle mode of Bobonaro melange genesis is stratal disruption in decollement zones and layer-parallel extension of overpressured clay-rich units above these zones. The low density and ductility of matrix source material, and the development of fracture conduits m the accretionary wedge promote emplacement by viscous flow, which results in mixing and dispersion of melange materials and diapirism. However, the channels through which melange flows differ across the orogen. In the southern part of the orogen, represented by the Kolbano fold U lU ";; and thrust wedge, flow channels include many fault conduits -< E in overlying accreted units and limited flowage along the ::l ...., basal decollement. These conditions produce a very "5 .", homogenous melange (limited mixing and dispersion) that ~ intrudes through much of the same material from which it is derived, with the exception of synorogenic deposits. Conditions in East Timor and the northern contractional belt of West Timor are different in two major ways. First., there are multiple decollements, the deepest of which is at the base of the Permian. Therefore the amount of clay-rich accreted material available to source melange is much greater in these regions. Second, the Banda Terrane roof thrust, which acts as a rigid, impermeable lid above the SDZ, creates a major flow channel with limited access of melange to the surface. This condition is likely to cause mixing of melange with roof thrust units, increased dispersion via lateral flow along the SDZ, and accumulation of melange near the channel outlet. '5 '5 'J: 'J: 'J: 'J: 5.1. Decollement Propagation .,'" ;;'" 8 8 At the initial stages of collision, stratal disruption, and diapirism are essentially simultaneous and associated with decollement propagation into zones of high fluid pressures as documented by Breen et al. [1986) at the deformation front south of Sumba. Seismic profiles (Figure 6a) and side-scan sonar images of this location where Australian cover sequences first arrive at a subduction zone show stratal disruption and the development of diapiric ridges by updip expulsion of overpressured material. Diapirs rise to form a series of mud ridges up to 300 m high, 20 km long, and 2 km wide that are oriented parallel to the strike of the thrust fron t and are bounded by a series of normal faults. The decollement propagates up to 15 km south of the thrust front into the Scott Plateau along an overpressured horizon of Jurassic mudstones, which is equivalent to the low-permeability Wai Luli Formation of Timor. Where this pre-breakup, clay-rich unit is missing, no diapirs are found. Examples are the ocean ..,. basin west of where the Scott Plateau enters the Sunda trench, 00 which is underlain by Jurassic and younger oceanic crust. There are also no diapirs or melange reponed from the Swnba Island region, which is a part of the Banda forearc that may HARRIS ET AL.: COLLISIONAL MElANGE DEVELOPMENT - GEOLOOIC ASSOCIATIONS 477 not yet have been underthrust by thick clay-rich continental The inlet of the channel is at the leading edge of the hanging margin strata. wall. In the Timor region this would have been near the Sawu The mud ridges south of Sumba serve as a modem analog for and Boti-Merah thrusts until the channel was clogged by the Oleu diapir and others like it throughout the Kolbano fold underplating of an increasingly thick Australian continental thrust wedge. Like the mud ridges, the Oleu diapir rises from margin cover sequence. TIle model predicts that when the the basal decollement along a thrust fault. Data from the accretionary influx exceeds the channel's flow capacity, the diapir indicate that it bears only blocks and matrix material positive buoyancy of the sediment becomes dominant over from the basal decollement and the lower part of the hanging downward shear stresses. This condition can produce density wall, which the Banli-1 well (Figure 3) encountered at about 1 inversions with pressure gradients sufficient to cause fluid and kID depth on the south coast of West Timor. These blocks are mud expulsion. Under these conditions, pressurized melange pervasively broken by mostly bed-normal mode 1 fractures. from the channel may penetrate through the hanging wall, The origin of these fractures is difficult to constrain as there extend preexisting and resultant cracks, and pry off blocks are many preaccretionary and synaccretionary possibilities. that are dispersed and mixed within the melange. 1be However, it is likely that syncollisonal layer-parallel dispersion and mixing of hanging wall blocks throughout the extension would be limited to sequences that were detached Bobonaro melange and the thick accumulations of mixed from the lower plate during accretion and free to spread block-in-clay melange near the subduction channel inlet both laterally. attest to viscous flow of the Bobonaro melange within the SDZ during emplacement. This mechanism of melange 5.2. Wedge Taper Adjustments emplacement differs from that to the south of the Banda Terrane where expulsion of overpressured broken formation Hanging wall extension can also occur on a much larger and matrix-rich facies is less inhibited, experiences less shear scale as a decollement is weakened by increasing pore fluid strain, and is therefore more homogenous and localized. pressure, such as the region south of Sumba and Sawu where overpressured continental margin units are underthrust. The 6. Conclusion surface slope of the accretionary wedge above this region reduces from around 4 degrees west of Sumba to less than 2 The principle driving force in melange genesis in the Timor degrees south of Sawu. Other indicators of wedge extension region is massive underthrusting of thick, mostly expandable are the development of half-graben slope basins [Harris et aI., clay-rich layers that produce density inversions with pressure 1994; van der WerjJ, 1995] and moo diapirism [Masson et gradients sufficient to cause viscous flow of broken strata. al., 1991). The link of diapirism in time and space with Disruption of fractured layers into broken strata occurs within regional extension is consistent with the findings and detached units in and above a basal decollement. Differences modeling of Jackson and Vendeville (1994). in degree of dispersion and mixing of the broken formation On the basis of the characteristics of fluid interaction with are used to classify various facies within the Bobonaro accretionary systems [e.g. Moore and Vrolijk, 1992), we melange. These facies are mostly a reflection of various infer that extension of the accretionary wedge south of Sawu conditions of melange emplacement, whether at or near the is not only likely associated with widespread stratal deformation front or within a subcretionary channel, such as disruption but also with the opening of many new conduits along the SDZ beneath the Banda Terrane roof thrust. for melange emplacement and expulsion. Extensional In East Timor and the northern Timor contractional belt, fragmentation and dismemberment of the Banda Terrane and blocks and matrix are mostly derived from detached illite-rich Maubisse roof thrusts in Timor, which is apparent in the units of Permian to Upper Triassic Kekneno Group stacked mapped distribution and internal structure of these nappes beneath a crystalline roof thrust assemblage. Overpressured (Figure 2), may also be associated with wedge extension mud from these units, with inclusions of broken sandstone during the transition from subduction to collision. Thick and limestone layers, accumulated within the Sonnebait accumulations of melange between some nappes protrude Disruption Zone (SDZ), which forms at the base of the roof several hundreds of meters above their structural base, which thrusts. The boundary conditions and structural evolution of may indicate that nappe fragmentation was accompanied by the SDZ are similar to that of a subduction channel that is either melange intrusion or nappe slumping. clogged by accretion of increasingly thick cover sequences of the Australian continental margin. As the subduction channel 5.3. Channel Flow in the SDZ reaches its capacity, shear stresses increase, which causes mixing and dispersion of blocks as melange flows upward Many of the characteristics of melange genesis and through the hanging wall and possibly outward through the emplacement beneath the Banda Terrane crystalline roof channel inlet. These processes produce a very distinctive thrust, within the underlying SDZ, and the high degree of mixed block-in-clay facies melange that is pervasive mixing and dispersion that produces the block-in-clay throughout most of the hinterland part of the Banda orogen. melange of East Timor and the northern contractional belt of South of the SDZ hanging wall the basal decollement ramps West Timor can be explained by a subduction-channel model up into the uppermost Kekneno Group Wai Luli Formation. similar to that presented by Cloos and Shreve (1988) or Processes of melange genesis in this region may be similar to Waldron el al. [1988]. The subduction channel of these that in the SDZ, but the mechanism of emplacement differs in models is bounded by a more rigid hanging wall and footwall, that there is no roof thrust to redirect expulsion. The melange which is a condition very similar to the structure of the SDZ. produced in this region is very homogenous and more like a 47& HARRIS ET AL.: CDWSIONAL MELANGE DEVEWPMENT - GEOLOOIC ASSOCIATrONS remobilized broken formation that may mix with Texas Petroleum, Amoseas Indonesia, and the National Science Foundation (EAR-9118151). Field studies in Tunor were spoosored by synorogenic sequences that it intrudes. One of the best the Geological Research and Development Center (GROC) and examples of this type of melange is the Oleu diapir and other University Pembangunan Nasiooal (UPN "Veteran"). Assistance in the diapirs throughout the Bobonaro fold-thrust wedge of field was provided by Sabat Tobing, Carolus Prasetyadi, David Hunter, Michael Dropkin, Ron Adams, and many very helpful and generous southernmost Timor and mud ridges currently forming at the Timorese people. We thank Harry Shaw for clay mineral analyses, deformation front. Corelab, Inc. for microfaunal analyses, Amoseas Indonesia for logistic support, and Jim Ellis and Chevroo Overseas for Landsat image processing. Valuable reviews and discussion were contributed by Acknowledgements. This worle was partially funded by grants from Dewey, Moore, Sarah Roeske, Darrel Cowen, Eli Silver, and Donald the American Chemical Society, Petroleum Research Fund, Unioo Reed.. References Abbott, M.J., and F.H. Chamalaun, Cowan, D.S., Structural styles in Mesozoic and of the late Miocene to Quaternary Geochronology of some Banda Arc Cenozoic melanges in the western deposits of Timor, Ph.D. diss., Univ. of volcanics. 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