Pak Ade,
Terima kasih atas komentarnya. Section tomography dari Jan Spaakman dan Pak
Sri Widyantoro kelihatannya menunjukkan bahwa yang terjadi adalah thrusting ke
arah timur, forearc dari Sangihe menganjak forearc dari Halmahera. Mungkin
benar juga bahwa gerak Philippine Sea Plate ke barat yang menyebabkan
penganjakan ini, terutama di bagian utara, tetapi tidak berjalan untuk
sepanjang collision zone sampai ke selatan sebab di selatan tetap terbuka
(zipper-type collision). Yang selatan mungkin tetap sebagai akibat slab-pull
dari sisi Sangihe dan Halmahera.
Melihat dimensinya, saya pikir oceanic thrusting ini ini tak akan terjadi
pengembangan ke oceanic magmatism sebab yang terlibat dalam thrusting sekarang
adalah forearc accreted crust-nya, bukan oceanic crust hasil spreading dari
tengah Laut Maluku. Arc magmatism oceanic crust yang masuk ke kedalaman lebih
dari 100 km telah terjadi di dua sisi, baik di busur Sangihe maupun di busur
Halmahera.
Dalam paper saya itu, khusus collision sekitar Banda Sea termasuk
Timor-Tanimbar dan Seram, saya membahas sejarah tektonik Banda Sea yang
kompleks, dan agar penilaiannya berimbang saya menggunakan publikasi2 dari
semua schools of thought yang ada yang pernah membahas sejarah Banda Sea :
Jepang (Kaneko et al. 2007 - termasuk yang Pak Ade tulis di bawah), Prancis
(Honthaas, Rehault, Maury, Belllon, Malod, Villeneuve, dkk.), dan Amerika (Ron
Harris). Karena setiap pandangan mereka berbeda, sulit mengkompilasinya.
Papernya nanti saya kirimkan via ja-pri.
salam,
awang
Ade Kadarusman <[EMAIL PROTECTED]> wrote:
Pak Awang,
Pulau di tengah2 Maluku Sea plate (pulau Mayu?), adalah bukti dari intra
oceanic thrusting, dulunya adalah MOR Maluku Sea plate yang membentuk double
subduction, kemudian MOR tsb terhenti memproduksi oceanic crust ke arah timur
dan barat, karena oceanic crust di arah timur berbalik arah gayanya karena gaya
dari Philiphine Sea plate. Bukti dari MOR tsb bisa dilihat dari seismic
tomography Pak Sri Widiantoro.
Intra oceanic thrusting ini cikal bakal dari island arc kalau salah satu
oceanic slab terus menunjam kebawah terhadap oceanic chust yang lainnya, dan
jika mencapai kedalaman sekitar 100km slab tsb akan melting menciptakan arc
magmatism.
Boleh saya dikirimi paper Pak Awang tsb, walaupun beberapa statement yang
disampaikan oleh paper tsb tidak sependapat dengan apa yang saya ketahui
berdasarkan data-data yang saya punya yang berbeda dengan pendapat paper
sebelumnya.
just FYI, tahun ini tiga paper saya terbit di jurnal internasional, dua old
study as co-author dan yang di Karang sambung is a new fresh data. Yang
berminat silahkan kontak ke Japri.
Masih ada dua lagi yang belum kelar, satu paper masih tertahan di Editor Jurnal
karena dianggap tidak mengikuti saran dari reviewer (Ron Harris dan Robert
Hall) untuk revisi.
Dan satu lagi masih diselesaikan, sudah dikejar-kejar oleh Theo van Leeuwen,
lanjutan dari paper yg publish di Jurnal asian Earth Sciences thn 2006 (malino
met complex N Sulawesi),
Judul paper tsb:
Multiple generations of forearc mafic.ultramafic rocks in theTimor.Tanimbar
ophiolite, eastern Indonesia. Akira Ishikawa, Yoshiyuki Kaneko, Ade Kadarusman
, Tsutomu Ota, Gondwana Research 11 (2007) 200-217.
On-going orogeny in the outer-arc of the TimorTanimbar region, eastern
Indonesia. Yoshiyuki Kaneko, Shigenori Maruyama, Ade Kadarusman , Tsutomu Ota,
et al, Gondwana Research 11 (2007) 218233
P-T Evolution of Eclogites and Blueschists from the Luk Ulo Complex of Central
Java, Indonesia, Ade Kadarusman et al, International Geology Review, Volume 49,
Number 4, April 2007, pp. 329-356.
Salam dari Sorowako
Ade Kadarusman
just jumping to the other side of coin
Awang Satyana wrote:
Barangkali ada gunanya.
salam,
awang
Awang Satyana wrote:
To: bagus priyanto
,
Geo Unpad
From: Awang Satyana
Date: Tue, 4 Dec 2007 22:29:23 -0800 (PST)
Subject: [Geo_unpad] Re: Busur Magmatik Halmahera
Bagus dan Handito,
Makalah saya dan kawan2 (Satyana et al., 2007) yang baru dipublikasi di
pertemuan gabungan HAGI-IAGI-IATMI di Bali (14 Nov. 2007) atau wawancara
wartawan Tempo dengan saya seputar gempa dan volkanisme di Halmahera (Majalah
tempo edisi September 2007) memuat hal-hal yang ditanyakan.
Busur magmatik Halmahera tetap dikontrol oleh penunjaman kerak samudera Laut
Maluku yang menunjam ke timur di bawah Halmahera dan menghasilkan busur
gunungapi hasil penunjaman kerak samudera (subduction related-island arc
volcanism), misalnya gunungapi2 Gamalama dan Gamkonora. Uniknya, kerak samudera
Laut Maluku pun menunjam ke barat ke bawah busur Sangihe dan Sulawesi Utara, di
sini melahirkan gunungapi2 seperti Soputan dan Lokon. Keunikan lain adalah
bahwa penunjaman ke dua arah ini telah mendekatkan prisma akresi dan cekungan
depan busur di sistem busur Halmahera dan Sangihe. Lalu, pada Pliosen kedua
sistem prisma akresi dan cekungan depan busur ini berbenturan mengangkat suatu
tinggian di tengah2 Laut Maluku yang kita kenal sebagai Punggungan Talaud dan
Mayu. Pulau2 Talaud dan Mayu dibentuk oleh melange atau prisma akresi tersebut.
Keunikan lain adalah bahwa model benturan sistem Halmahera dan Sangihe ini
adalah menutup di utara dan membuka di selatan, seperti sistem
ritsleting di jaket, maka kita sebut saja zipper-shaped collision.
Obduksi di timur Sulawesi tak langsung berhubungan dengan busur magmatik
Halmahera, yang terobduksi di sini adalah kerak tua Banda yang umurnya lebih
tua dari kerak samudera Laut Maluku.
Di bawah ada salinan dari makalah tersebut, silakan dipelajari dulu nanti kita
diskusikan lagi.
salam,
awang
The Molluca Sea Collisional Orogen
The Molucca Sea collision zone lies in the area of complex junction between the
Eurasian, Australian, Pacific, and Philippine Sea plates. Both the Sangihe
volcanic arc on the west and the Halmahera arc on the east are active, and both
face inwards towards the Molucca Sea. The present day geology of the Molucca
Sea region contains a record of the stages in the collision between these two
arcs (Hall, 2000). The Molucca Sea Plate has an inverted U-shaped configuration
and is dipping east under Halmahera and west under the Sangihe Arc. Regional
seismicity suggests that approximately 200-300 km of lithosphere has been
subducted beneath Halmahera. On the opposite side of the Molucca Sea, the
Benioff Zone associated with the west-dipping slab can be identified at least
to a depth of 600 km beneath the Celebes Sea. In the Molucca Sea the two arcs
of Sangihe and Halmahera are in active collision. The appropriate trenches
would be expected to outcrop beneath the Molucca Sea, but
instead there is a broad topographic high, the Talaud-Mayu Ridge, which
apparently marks the site of the collision of the two arc-trench systems,
composed of their collided accretionary wedges and fore-arc basins.
Origin
Westward subduction of the Molucca Sea beneath the Sangihe Arc probably began
in the early Miocene. Eastward subduction of the Molucca Sea Plate beneath
Halmahera began in the middle Miocene. The double subduction zone was initiated
at this time forming a new plate, the Molucca Sea Plate, separate from the
Philippine Sea Plate. The oldest volcanic rocks dated from the Halmahera Arc
are 11 Ma in Obi at its southern end and are younger to the north (Baker and
Malaihollo, 1996). The earliest indications of arc-arc collision are of
Pliocene age. The wide Molucca Sea collisional complex is composed of the
accretionary wedges of both arcs.
The development of the collision complex may be elaborated as follows
(Hutchison, 1989; Hall, 2000 ) : each of the Sangihe and Halmahera systems was
previously active and farther apart than now. They constituted each of an
active volcanic arc, subduction complex, and fore-arc basin. Westwards
subduction beneath Sangihe was probably active longer than eastwards subduction
beneath Halmahera because of the deeper Benioff Zone of the former. Hamilton
(1979) envisioned that subducting lithosphere falls under its own weight into
the asthenosphere. As it is pulled down, the arc-trench system migrates
forwards; the two opposed systems migrate forwards and eventually collide. The
first contact between the two arcs probably occurred in the late Pliocene (Hall
and Wilson, 2000). The accretion of the subduction complexes must have ceased
when the two complexes began to collide. Further convergence resulted in
upbuilding the combined accretionary wedges. Silver and Moore (1981)
inferred that the uplift occurred by thickening of the collision complex
through folding and movement along reverse faults. Convergence was also
accommodated by thrusting upwards over the volcanic arc aprons. Ophiolitic
material was upthrust in the collision complex. The collision complex did not
have sufficient strength to maintain steep slopes, so there was periodic
gravitational flow outwards to overlap onto the volcanic arcs. The complex
material flowed downslope from the ridge, then outwards and upwards onto the
arc aprons. By this process a two-sided symmetrical collision zone resulted.
The complex material flowed downslope from the ridge, then outwards and upwards
onto the arc aprons. By this process a two-sided symmetrical collision zone
resulted.
Suturing is at different stages in different places in the Molucca Sea region.
It is most advanced in the north at the latitude of Talaud where most of the
Halmahera forearc and arc has been subducted. Further south and on the west
side of the present arc, the Halmahera forearc is being thrust eastwards over
the Halmahera Arc. Collision between the two arcs in the Molucca Sea region is
still continuing. .
Anatomy
The Molucca Sea collision zone has a remarkable structural symmetry. The
anatomy of the collision orogen from Halmahera to Sulawesi consists of :
arc crust of Halmahera
forearc basin of Halmahera
collided accretionary prisms of Halmahera-Sangihe (Talaud-Mayu Ridge)
forearc basin of Sangihe
arc crust of Sangihe
Ophiolitic rocks form the basement of east Halmahera (Hall and Wilson, 2000).
They are dismembered and formed in an early Mesozoic intra-oceanic arc. The
ophiolitic rocks are overlain by Cretaceous, Eocene and Oligocene arc volcanic
rocks. In the western arms, Oligocene arc volcanic rocks form the basement.
Miocene carbonates unconformably overlie all the older rocks. The Neogene
Halmahera Arc became active at approximately 11 Ma. Volcanism began earliest in
the south and extended northwards producing a volcanic arc similar in position
and extent to the present Halmahera Arc. To the west of the arc, turbidites and
debris flows were deposited below steep west facing submarine slopes containing
material derived from a region of volcanic arc rocks and reef limestones.
Forearc basins of Halmahera and Sangihe develop in southern sector of the
collisional orogen. At the north, forearc basin of Halmahera and its forearc
crust have subducted and been thrust by collided accretionary prisms of
Sangihe-Halmahera. Where the forearc basins develop, they receive sediments
from adjacent high areas (collided accretionary prism and/or accreted crust).
Collided accretionary prisms at the middle of the Molucca Sea form the orogenic
core of the collisional orogen (high central ridge). The uplifted orogen forms
the Talaud, Mayu, and Tifore islands, hence forming the Talaud-Mayu Ridge
(Simandjuntak and Barber, 1996). The Talaud-Mayu Ridge records the collision of
the Sangihe and Halmahera forearcs. This central zone is marked by intense
shallow seismicity and a low gravity. Previous workers (Silver and Moore, 1978;
Hamilton, 1979) called this as mélange wedge or collision complex. Sukamto et
al. (1981) reported the occurrence of deformed Tertiary sedimentary rocks and,
in part, of mélange, in Talaud Island containing blocks of peridotite,
serpentinite, gabbro, and volcanic and Tertiary sedimentary rocks within a
sheared matrix. However, although mélanges may have formed during collision
they are not yet exposed onshore. Those on Talaud and Mayu were not formed
during the present collision but are part of the pre-Neogene
basement of the Sangihe forearc (Hall and Wilson, 2000). The presumed mélanges
of the modern collision complex are all submarine and constitute part of the
bathymetrically shallow and seismically incoherent volume of sediment in the
central Molucca Sea. Accretionary material from both forearcs has contributed
to this melange wedge. On the Sangihe side the sediments in the accretionary
complex date from the middle Miocene. On the Halmahera side they probably date
from the late Miocene.
Nature of Deformation
The Talaud-Mayu Ridge is part of a broad deformed ridge of clastic sediments
bounded on the east and west by topographic troughs which mark the location of
thrust contacts with the adjacent aprons. Seismic reflection profiles across
the Ridge are interpreted as tectonic mélange and complexely folded rocks. They
indicate that the collision complex is thrust upwards and eastwards over the
Halmahera volcanic arc apron. The western margin is similarly upthrust
westwards (Hutchison, 1989)
In the northern Molucca Sea the Sangihe forearc was thrust eastwards onto the
Halmahera forearc and arc. In the region between Morotai and the Snellius
ridge, parts of the Neogene Halmahera Arc and forearc have disappeared. A cross
section between Morotai and the Sangihe Arc shows the overthrusting of the
Halmahera Arc by its own backarc which occurred at the end of the Pliocene
(Hall, 2000; Hall and Wilson, 2000). Further south this east-vergent thrusting
carried the Halmahera forearc onto the flanks of the active Halmahera Arc, and
now pre-Neogene rocks of the Halmahera forearc basement are exposed in islands
of the Bacan group and off the coast of northwest Halmahera. Where the
Halmahera forearc and arc have been significantly overthrust the Sangihe
forearc has been jacked up. The overthrusting of one forearc by the other has
led to major thickening of the accretionary complex producing the large volume
of low density material and associated gravity low of the central
Molucca Sea. Thickening of the collision complex by the accretion of the
Halmahera accretionary wedge and forearc crust, and by shortening of the
Sangihe Forearc, has caused the uplift of forearc basement in the Talaud
islands (and locally in Mayu) where ophiolites are exposed. The wide Molucca
Sea collisional complex is composed of the accretionary wedges of both arcs.
The forearc basement of the Sangihe Arc is exposed where it thrusts over this
wedge. The present Halmahera trench or trough broadly represents the frontal
thrust of the Sangihe forearc, which is overriding the Halmahera forearc and
arc. Locally there is backthrusting of the Sangihe forearc towards the Sangihe
arc at the Sangihe trench or trough, but this is a relatively minor feature.
SULAWESI Collisional Orogen
Sulawesi Islands in Central Indonesia provides a good place to examine
collision tectonics. The islands were assembled by collision of terranes and
have been modified by post-collision escape tectonics (Satyana, 2006a).
Collision of two microcontinental blocks of Buton-Tukang Besi and Banggai-Sula
with the eastern part of the island initiated the Neogene orogeny in Sulawesi
(Simandjuntak and Barber, 1996).
Origin
There is no significant difference of the interpretations on the origin of
collision of microcontinents (Banggai-Sula and Buton-Tukang Besi) to the east
of Sulawesi with eastern Sulawesi. Generally, workers here believed that these
two microcontinents, having separated from the northern continental margin of
Australia, possibly from the region of the Birds Head, were carried westwards
along the Sorong transcurrent fault zone by the movements of the Philippine Sea
plate and collided with the eastern margin of the ophiolite complex of East
Sulawesi. Difference of interpretations lies on the timing of collision.
Hall (1996) reconstructed the detachment of the microcontinents from the Birds
Head of Papua, their transfer to the west, and their collisions with eastern
Sulawesi. At 20 Ma (Early Miocene), these microcontinents were dismembered from
the Birds Head by the Sorong Fault splay. At 15 Ma, a strand of the Sorong
Fault propagated westward, at 11 Ma Buton-Tukang Besi collided with Sulawesi.
Collision of Buton-Tukang Besi with Sulawesi locked the strand of the Sorong
Fault and requiring a development of a new fault strand which caused the
detachment of Banggai-Sula microcontinent. Banggai-Sula drifted northward and
collided with East Sulawesi ophiolites. Overthrusting of the ophiolites onto
the western edge of Banggai-Sula microcontinent occurred in the latest Miocene
(Davies, 1990) indicating that collision of the Sula platform with East
Sulawesi must have occurred at 5 Ma (end of Miocene). Garrard et al. (1988)
interpreted that collision of Banggai-Sula with eastern
Sulawesi took place initially from middle Miocene.
The collision caused the leading edges of the Buton-Tukang Besi and
Banggai-Sula microcontinents were thrust beneath the ophiolites, obducted the
ophiolites onto the microcontinental blocks. The collision has uplifted the
tightly folded, faulted and imbricated ophiolites and their pelagic covers to
heights more than 3000 meters (Simandjuntak, 1986; Garrard et al., 1988;
Davidson, 1991). Also, as a result of the collision, the metamorphic belt of
Central Sulawesi was thrust westwards over West Sulawesi and uplifted to form
mountain ranges of nearly 3000 meters. These mountain ranges were formed by
collision, hence they called as the Sulawesi Collisional Orogen.
Anatomy
The Sulawesi collisional orogen involves regions from the Banggai-Sula
microcontinent, through the Batui Thrust orogen in eastern Sulawesi, Ophiolite
Belt of East Sulawesi, Metamorphic Belt of Central Sulawesi, Magmatic Arc of
Western Sulawesi, and a fold-thrust belt at western Sulawesi to the eastern
side of the Makassar Strait. The Makassar Strait, western Sulawesi and central
Sulawesi are underlain by continental crust of the easternmost part of the
Sundaland (Simandjuntak and Barber, 1996). Therefore, the Sulawesi Orogen
records collision of continent-continent.
Based on the history of collision where the Banggai-Sula microcontinent
collided the eastern part of Sulawesi, the Banggai-Sula microcontinent is on
foreland position (pro-foreland) whereas the East Sulawesi eastwards are on
hinterland position (retro-foreland side). Based on Garzanti et al. (2007)s
classification, the Sulawesi Orogen is from Alpine-type collision orogen where,
like the Meratus Orogen, two continents suture together, resulting in
high-relief, thick-skinned, doubly vergent, push-arc orogen.
The elements of anatomy of the Sulawesi Orogen from foreland to hinterland
positions (from Banggai-Sula to the Makassar Straits comprise :
undeformed or mild-deformed continental Banggai-Sula Platform,
pro-foreland basin of Banggai,
pro-foredeep of the Banggai Basin,
pro-foreland fold and fault/thrust belt of the Banggai Basin,
suture/axial belt/ of East Sulawesi Ophiolites,
internal metamorphic zone of Central Sulawesi,
internal igneous zone of magmatic arc of western Sulawesi,
retro-foreland fold and fault/thrust belt of the Lariang Basin,
retro-foredeep of the Lariang Basin,
retro-foreland of the Lariang Basin,
undeformed North Makassar attenuated continental basement.
The East Sulawesi Ophiolite (ESO) and Central Sulawesi Metamorphics form the
orogenic core where they have been uplifted to the heights of around 3000
meters. The East Sulawesi Ophiolite is one of the three largest ophiolites in
the world (Monnier et al., 1995; Kadarusman et al., 2004). It comprises, from
base to top, residual mantle peridotite (spinel lherzolite, intercalated with
harzburgite and dunite), mafic-ultramafic cumulate through layered to isotropic
gabbro, to sheeted dolerites, and basaltic volcanic rocks (lavas) of normal
mid-oceanic-ridge basalt (MORB) composition. Major and trace element
geochemistry of basalt and dolerite suggests origins of MOR, oceanic plateau
(major), and supra-subduction zone (minor). Based on the chemical similarity
between the ESO lavas and those from the Eocene Celebes Sea back-arc basin
crust together with their identical age, Monnier et al., (1995) suggested that
the ESO was initially generated in a back-arc tectonic environment
representing a fragment of the Eurasian Plate obducted onto the East Sulawesi
basement of Australian origin. However, Kadarusman et al. (2004) based on
published paleolatitude data of lava sequence in the Balantak area
reconstructed using plate trajectory analyses, indicated that the site of
generation of the ESO was somewhere at area located 2000 kms south from the
present position (it is also possible 10,000 kms SW from the present position).
The site of generation is considered in SW Pacific area (?), near the compound
of the Pacific oceanic plateau and seamounts generated by SW Pacific
superplume. Referred to the relationship between time of ophiolite generation
(erupted age) of ESO which is 138 Ma, and time of ophiolite emplacement (ESO
obduction onto Sundaland Craton at 30 Ma), then the ESO has 108 Ma migrating
period. This migration period should be examined by the rate of plates
convergence between Pacific and Eurasia plates during the Late
Cretaceous-Oligocene.
The Central Sulawesi Metamorphic Belt, anatomically, forms an internal
metamorphic zone of the Sulawesi collisional orogen. The belt is called the
Pompangeo Schist Complex (Parkinson, 1991; Parkinson et al., 1998). It is
predominantly composed of high-pressure (HP) phyllitic marble, calcareous
phyllite, graphitic schist, quartzite and metaconglomerate; rocks predominantly
of shallow marine and continental margin origin. The schists are relatively
coherent and increases in metamorphic grade from east to west. K-Ar dating of
phengite from three schist samples yielded ages of around 111 Ma, indicating
that underthrusting of the margin of the continental fragment resulted in the
recrystallization of the supracrustal sediments to become Pompangeo schists
occurred in late Early Cretaceous. Parkinson (1991) reported KAr ages of 3328
Ma from phengites and CaNa amphiboles from central Sulawesi. There, the
sub-ophiolite sole experienced a blueschist overprint after the 3228 Ma
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