Chris Goldfinger

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OCE 661 Plate Tectonics
Chris Goldfinger Burt 282 7-5214 gold_at_coas.oregon
state.edu
Course notes at http//activetectonics.coas.orego
nstate.edu Readings Hall, 2001 Newman et al.,
1999
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Stay tuned, Bill gates doesnt want us to watch
Quicktime movies..
Geodynamics
Plates
Heat flow
Motions
Quakes
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Red River Fault
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Red River Fault
ITRF 2000 No Net Rotation Vectors (GPS)
The collision of Australia and Indonesia is
similar to the early stages of the India-Eurasia
collision. In some sense, it is a late stage of
the SAME collision. The equivalent arcs,
marginal seas and continental blocks are spread
out over a wide region, and will most likely wind
up compressed into a suture zone between
Australia and Eurasia as they are in the Himalaya
between India and Eurasia.
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New subduction zone
New subduction, possibly related to the accretion
of the Finisterre arc, has initiated at the
eastern end of the Caroline plate. The new
subduction zone initiated within the Caroline
plate, but is sometimes referred to as the
Pacific-Caroline plate boundary. This new zone
has had 10 km of shortening, and is
doubly-vergent. The north part has Pacific plate
subducting beneath the Caroline Plate, the
southern part is the reverse.
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As marginal seas close during the collision, many
small plate fragments collide or are subducted.
The Molucca Sea plate is squeezed between the
Singihe plate and the Halmahera plate, pushed by
the Pacific plate on the east, and Eurasia on the
north and west. To the north, these two
subduction zones and arcs have completed
collision, suturing the two arcs together on the
Phillippine island of Mindanao. This suture zone
becomes the strike-slip Phillippine fault further
north, bisecting the island of Luzon.
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The last stage of the destruction of the Molucca
Sea plate is a pair of opposed subduction zones,
with very little (if any) of the Molucca Sea
plate remaining to be subducted. In this way it
is possible to have an arc-arc collision, and a
final configuration with two arcs sutured
together.
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The northern Molucca Sea shows the incipient
subduction of a composite oceanic and arc
volcanic block, the Snellius-Halmahera block
(SHB). Multi-beam, reflectivity, seismic and
gravity data obtained during the MODEC marine
survey showed that the SHB disappears beneath the
accretionary wedge and the outer ridge of the
Sangihe arc. To the north in Mindanao island,
ongoing convergence generated shortening of the
forearc basin and the backthrusting of the SHB,
meanwhile a classical system of paired subduction
(Philippine Trench) and strike-slip fault
(Philippine Fault) was installed. The transition
from lithospheric subduction to crustal
overthrusting is located where the Philippine
Trench sensu stricto begins, and also coincides
with the off-shore extension of the Philippine
Fault. We observe a reversal of the thrusts from
an eastward vergence in the Molucca Sea to a
westward vergence in Mindanao island. This
reversal takes place at the latitude where the
forearc area emerges by uplift, and the downgoing
crust (SHB) deepens resulting in a strong gravity
low centered above the accretionary wedge. The
Philippine Fault initiated in a place where the
crust was sliced off by a transfer zone which
marked the northern termination of the Molucca
Sea, and drags northward a sliver of the
previously accreted SHB. The northward drifting
of this sliver created an extension, which
however cannot account for the gravity low. We
propose that the shortening and the uplift of the
upper plate was induced by the buoyancy of the
subducted unit (SHB), and triggered the thrust
reversal. UPPER PLATE DEFORMATION INDUCED BY
SUBDUCTION OF A VOLCANIC ARC THE SNELLIUS
PLATEAU (MOLUCCA SEA, INDONESIA AND MINDANAO,
PHILIPPINES) Manuel PUBELLIER , Anne Gaelle
BADER , RANGIN Claude , Benoit DEFFONTAINES ,
Ray QUEBRAL .
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Sorong Fault
Here is a cutaway view of the Molucca sea doubly
vergent subduction system. At the south end, the
Molucca sea plate terminates in the Sorong Fault
Zone, a major left-lateral fault system
separating Australia from the Philippine Sea
Plate and the Molucca Sea Plate. The fault zone
juxtaposes Mesozoic-Tertiary continental and
arc/ophiolitic rocks. Continental crust was
derived from the Australian margin. Crust of
Philippine Sea Plate origin has a basement of
ophiolitic and/or arc origin. Fragments of
Australian and Philippine Sea Plate origin have a
common stratigraphic history after the early
Miocene. The geology of the region indicates that
arc-continent collision between the Philippine
Sea and Australia occurred at 25 Ma and led to
creation of the left-lateral Sorong Fault Zone.
Subsequent Neogene convergence between East Asia
and the Philippine Sea Plate occurred by
subduction to produce the Halmahera arc. Arc
activity started earliest in the south, in Obi,
and ceased earliest in the south. Neogene
movement of Australia northward has occurred
without subduction although accompanied by
movements of small fragments and local
'collisions'. The Australian-Philippine Sea plate
boundary has been a strike-slip zone since the
early Miocene. This implies northward movement of
the plate boundary in the Sorong Fault Zone
region at a similar rate to that of Australia.
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How the region looks from the perspective of
reference frames on the participating plates
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The reconstruction and kinematics of Indonesia
use global plate models, fault mapping and slip
rate determinations, faunal associations, and now
GPS as well. The region is so complex that GPS
has become instrumental in defining block
boundaries. Bock et al., show that a Sunda Shelf
block can be defined, including Sumatra, Borneo,
and parts of southeast Asia. The method involves
finding an Euler pole that minimizes motion
between stations on the block. There is some
motion between stations, indicating some internal
deformation of the block, but the motion is
relatively coherent, thus such a pole can be
found.
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Figure 1. Bathymetry of the central Indian Ocean
Smith and Sandwell, 1997 showing ridges and
ocean basins.
Figure 2. Tectonic map of the surrounding area of
Ninety-East Ridge with magnetic lineation, fossil
spreading center (solid boxes) and fracture zones
(dashed line) after Krishna et al., 1995.
Tiwari, V. M., M. Diament, and S. C. Singh,
Analysis of satellite gravity and bathymetry data
over Ninety-East Ridge Variation in the
compensation mechanism and implication for
emplacement process, J. Geophys. Res., 108(B2),
2109, doi10.1029/2000JB000047, 2003.
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Bathymetric map of the Indian Ocean showing the
path of ODP Leg 121 and the drill Sites. The
large shallow (light blue) feature in the south
central area is the Kerguelen Plateau. Sites
752-755 are located on the Broken Ridge, 756-758
on the Ninetyeast Ridge.
Nearly three thousand Cretaceous to mid-Eocene
basaltic tephra layers associated with the
Kerguelen Hotspot were recovered during drilling
on the Broken Ridge (Sites 752-755) and the
Ninetyeast Ridge (Sites 757 and 758) in the
eastern Indian Ocean. (Figure 1) These tephra
layers occur as volcaniclastic massflows,
basaltic tuffs, and discrete distal fallout
layers. Tephra-producing volcanism ended in the
mid-Eocene when the Ninetyeast Ridge and the
Broken Ridge were separated from the Kerguelen
Plateau and the Kerguelen Hotspot by the
formation of the South East Indian Rift. Mid- to
upper Cretaceous volcaniclastic massflows are
characterized by coarse grained (lapilli size)
graded beds, typically 10-30 cm in thickness,
with sharp upper and lower contacts. These
layers, often closely spaced in the host
sediment, forming a several meter thick composite
unit, were observed on Broken Ridge and northern
Ninetyeast Ridge. The layers are depleted in fine
material and often contain lithic fragments.
Upper Cretaceous to Paleocene basaltic tuffs in
the central Ninetyeast Ridge at Sites 214 and 757
are primarily composed of altered glass and
contain interbedded angular basalt pebbles and
mussel fragments. The 157 m of tuff at Site 757
is composed of several smaller graded layers some
of which have scoured basal contacts. An
explosive shallow water (lt100 m) environment is
inferred for the formation of these layers.
Dehn, J. 1992. Volcanotectonic evolution and
tephrochronology of the Eastern Indian Ocean.
Doctoral Dissertation, Christian Albrecht
Universität zu Kiel. 210pp.