Himalayan Nursery: Inception and Tectonic Evolution of the
Active Banda Arc-Continent Collision
Ron Harris
Brigham Young University, Utah, USA
The Alpine-Himalayan convergent margin is reborn in the active Banda arccontinent collision (Figure1). This infant orogen provides many details about the earliest
formative stages of continent-continent collision that are missing or overprinted in more
advanced orogens. Like the Eocene Himalaya, the Pliocene to present Banda Orogen
involves the collision of a large fragment of Gondwana (Australia) with an Asian Plate
convergent margin (Sunda Arc). The transition from subduction to collision in the eastern
Sunda Arc reveals key tectonic processes associated with the inception of mountain
building, such as: 1) the influence of structural variation in the lower plate on the shape of
the orogen, 2) lithospheric vs. crustal scale processes, 3) onset of subduction polarity
reversal, and 4) accretion and demise of arc-forearc terranes.
Figure 1. Banda Arc region (box). Red triangles- active volcanoes. Arrows - plate motion vectors.
Until recently, only reconnaissance-style geological and geophysical investigations
were conducted of the Banda Orogen. However, over the past decade this has changed.
Several new detailed investigations conducted by my students of various parts of the active
Banda arc-continent collision include: structural analyses (Zobell and Harris, 2007; Standley
and Harris, in press; Harris et al., in press), uplift rate and pattern studies using synorogenic
sediments (Roosmawati and Harris, in press) and coral terraces (Cox et al., 2005), P-T-t
studies of hinterland metamorphism and exhumation (Prasetyadi and Harris, 1996, Standley
and Harris , in press), and geodetic studies (Figure 2) of velocity fields (Nugroho et al., in
press).
Figure 2. GPS velocity field relative to Eurasia of transition from subduction in Java Trench to
collision in Timor. Velocities cluster into three major arc-forearc blocks that move in the same
direction as the Indo-Australian Plate, but with increasing velocities toward the most advanced parts
of the collision in Timor. (From Nugroho et al., in press).
These new data provide the constraints needed to address key questions about the
geodynamics of transition from subduction to collision. Questions include: 1) How is
strain partitioned in the upper and lower plates? 2) What happens to the arc-forearc? 3)
What controls vertical motions? 4) How does plate boundary reorganization evolve? and
5) what is the human impact?
The GPS velocity field indicates that as the Australian continental margin enters
the trench at around 120º E, coupling between the upper and lower plates increases.
Increased coupling strengthens the subduction zone, which increases horizontal stresses
and the distribution of strain away from the uplifted trench into weak parts of the upper
and lower plates. In the upper plate arc-directed thrusts develop that eventually close the
forearc, and uplift and extinguish the arc. In the lower plate increased horizontal stress
reactivates faults causing rift basin inversion and renewed extension from bending
stresses.
Uplift is caused by increased positive buoyancy and crustal shortening of the
subducting continental margin. Synorogenic units deposited at depths of 3-4 km at
around 2-3 Ma are incorporated into the infant fold and thrust belt to form peaks up to 2
km in elevation. Quaternary coral terraces are warped and show mostly short
deformational wavelengths consistent with crustal versus lithospheric deformation. Rock
uplift rates constrained from metamorphic cooling history studies are estimated at 4-6
mm/yr. Many apatite fission tracks have a zero age, and only short track-lengths are
found.
NNW
SSE
Figure 3. Cross-sections through transition from subduction (D) to collision (A) in the Banda Arc. Black –
arc-forearc basement. Grey – accreted Australian continental margin units. Bottom – Crustal section of
initiation of collision, with GPS velocities relative to SE Asia and outline of Benioff zone.
The obliquity of collision in the Banda Arc region demonstrates the progressive
development of a major suture zone. Even though various units found in the suture zone
can be tracked to their original tectonic settings, many are so similar in composition and
age that U/Pb detrital zircon age analysis is required to sort out tectonic affinity. Units of
SE Asian affinity have young zircon populations (35-650 Ma) versus old populations
(300-2700 Ma) in units of Australian affinity.
Structural measurements throughout the collision zone demonstrate the primary
importance of structural inheritance in shaping the young fold-thrust belt. The structural
grain of the pre-collisional Australian continental margin is also imprinted on the
accretionary wedge. Multiple levels of detachment are also found within the accretionary
wedge, which are controlled by differences in mechanical strengths of various continental
margin cover units (Figure 5). Duplexes of accreted Australian continental margin units
form beneath the edge of the forearc and detach it from its roots, which are mostly
removed by subduction erosion. Thick sections of mélange are found beneath and in
front of the detached thrust nappes of the forearc. The mélange matrix is mostly derived
from thick Cretaceous and Jurassic mudstone units of the Australian continental margin.
Where these units enter into the trench the deformation front, and much of the
accretionary wedge is overprinted by mud diapirism.
Figure 4. Regions of high surface uplift rates measured from foraminifera in uplifted synorogenic
deposits and U/Th ages of uplifted coral terraces (From Cox et al, 2006).
Many propose that seismic quiescence and high uplift rates in eastern-most Timor
result from delamination of the oceanic and continental parts of the subducted plate.
However, patterns of uplifted coral terrace deformation above the region relate more to
active fold growth than long wave-length lithospheric deformation patterns.
Finite element models of the transition from subduction to collision were
designed to test various geodynamic models, such as increased coupling of upper and
lower plates towards most advanced parts of the collision (Yang, et al., 2006). These
models are constrained by fault plane solutions, GPS velocity fields (Figure 2) and
surface uplift rate measurements (Figure 4) throughout the region. demonstrate the
primary importance of lower plate coupling as the major driver of deformational
processes, such as: partitioning of strain away from the deformation front into sites of
intra-arc and forearc thrusts, , complete closure of the forearc basin, extinction of the arc
and its accretion to the continent, and plate boundary reorganization.
Querying 500 years of historical records maintained by mostly Dutch colonists
indicates that there were at least 30 major earthquakes (intensity ≥ IX) in the region
between 1629 and 1841. Since this time only minor tectonic quakes have occurred.
During last 180 years of relative seismic quiescence vulnerability to seismic and tsunami
hazards have drastically increased due to rapid population growth and coastal
urbanization in the region. Could what happened in Sumatra in 2004-2007 happen in the
Banda Arc next.
Figure 6. A) Finite element
mesh and boundary conditions
of 3D power-law viscous flow
model. B) predicted horizontal
(arrows) and vertical (red is
high) motions with weak plate
interface. C) Same model as
‘B’, but with stronger coupling
associated with continental
crust. Strain is distributed into
arc-forearc.
References:
Cox, N. L., Harris, R., Merritts, D., 2006, Quaternary uplift of coral terraces from
active folding and thrusting along the northern coast of Timor-Leste: Eos Trans.
AGU, V. 87, no. 52, T51D—1564.
Harris, R., Vorkink, M.W. and Prasetyadi, C., in review, Geological and neotectonic
evolution of Savu, Indonesia: transition from subduction to incipient arc-continent
collision, submitted to Geosphere.
Roosmawati, Nova and Harris, Ron, in review, Surface uplift history of the
incipient Banda arc-continent collision: depth and age analyses of foraminifera
from Rote and Savu islands, Indonesia, Tectonophysics, in review.
Nugroho, H. Harris R. Amin W. Lestariya and Bilal Maruf, in press, Active plate
boundary reorganization in the Banda arc-continent collision: insights from new
GPS measurements, Tectonophysics, in press.
Standley, Carl and Harris, Ron, in press, Banda forearc basement accreted to the
NW Australian continental margin: a geochemical, age and structural analysis of
the Lolotoi metamorphic complex of East Timor, Tectonophysics, in press.
Yang, Youqing, Liu, Mian, and Harris, Ron, 2006, From subduction to arccontinent collision: Geodynamic modeling of strain partitioning and mountain
building in the Indonesia archipelago, Eos Trans. AGU, V. 87, no. 52, T41C-1589.
Zobell, Elizabeth and Harris, Ron, 2007. New insights into the stratigraphic and
structural evolution of the active Banda Orogen, Geological Society of America, Abstr.
Prog., V. 39, no. 5, p. 45.