Forty two new apatite and zircon fission track analyses on the
Kubor-Marum-Bena Bena area of northern Papua New Guinea
severely constrain the timing of tectonic events and hence the
tectonic models for New Guinea, particularly arc-continent
collision. In addition, the data can be analysed regionally in terms
of known and assumed lithospheric fault zones inferred to control
mineralisation (eg. Corbett 1994; Kendrick et al 1995; Hill et al
1996) to determine reactivation of these zones. When combined
with the new kinematic plate models for the evolution of New
Guinea (Hall 1997) there is the potential to predict likely zones
of mineralisation. The low-medium grade? Palaeozoic rocks of the Bena Bena terrane
yielded a zircon fission track age of 17 Ma, indicating the time of
cooling below 200C. The age is consistent with `the shift, at
about 16-18 Ma, from volcanolithic sediments to mixed
provenance sediments rich in quartz and metasedimentary lithic
fragments in the southern Finisterre Ranges (Abbott et al. 1994).
This shift was recorded in the Sukurum Formation, which ranges
in age from Middle Miocene to Pliocene with quartzose and
metasedimentary grains in all samples. Although recording rocks
at temperatures of 200C and hence depths of ---5km, the 17 Ma
age suggests the Bena Dena terrane was rapidly cooling at that
time due to uplift and erosion, supplying metasediments to the
southern flank of the Finisterre Range, currently 100 km to the
east. Significantly, the Middle Miocene Akuna Intrusive Complex,
dated 14-17 Ma by Page (1976), lies almost entirely within or
adjacent to the Bena Bena terrane. Potentially the uplift, erosion
and cooling of the terrane at 17 Ma may have been caused by the
rising plutons. If so the 800 sq km exposure Akuna Intrusive
Complex at the SE end of the Bena Bena terrane may have been
the focus of uplift and supplied the metasedimentary detritus to
the Finisterre Range. This suggests that the Finisterre Range
was not far removed from the Bena Bena terrane, perhaps less
than the 200 - 400 km indicated by Abbott et al (1984). The Akuna Intrusive Complex probably resulted from subduction
of the Solomon Sea Plate beneath the PNG margin. In order for
the subducted slab to reach depths of -100km below the area to
facilitate partial melting and plutonism (Hamilton 1994),
subduction probably commenced 3-5 Ma Ma before plutonism,
ie at or prior to 20 Ma suggesting a significant change in the
tectonic regime at -20 Ma, perhaps related to collision of the
Solomons - Ontong Java Plateau collision (eg. Hall 1997). The main result to emerge from the 28 apatite fission track
analyses is that the whole of the study area underwent rapid cooling
due to uplift and denudation in the Late Miocene, mainly between
7-10 Ma. Results from several samples indicate that they were
not totally overprinted prior to uplift indicating maximum burial
of -3 km in the Middle Miocene, assuming normal temperature
gradients. The burial is less if higher gradients are assumed,
allowing for Middle Miocene plutonism. The Late Miocene
cooling in the Mobile Belt agrees well with the Pliocene to Recent
cooling in the Fold Belt to the SW, interpreted to be due to uplift
and erosion associated with fold and thrust deformation of the
Miocene Limestones (Hill & Gleadow 1989). This timing
relationship suggests, that compressional deformation in the
Mobile Belt occurred in the Late Miocene, at 105 Ma, and that
the deformation migrated towards the undeformed foreland in
the southwest in the Pliocene, at 5-2 Ma. The regional Late Miocene uplift and denudation in the Mobile
Belt is also consistent with that postulated by Crowhurst et al
(1996; 1997) for northwest PNG, although they considered the
event to have occurred mainly from 8-5 Ma. If so, then the uplift
and denudation may have been migrating from east to west as
well as to the south into the Fold Belt. These timing relationships place significant constraints on tectonic
models for the area, indicating that the major compressional pulse
in PNG commenced in the northeast at -10 Ma and migrated to
the southwest and west in the latest Miocene to Pliocene. This
suggests that the initial arc-continent collision and associated
shortening occurred in the Late Miocene around -10-12 Ma. This
immediately followed intrusion and cooling of the Akuna Intrusive
Complex at -17-14 Ma and the Bismarck Intrusive Complex at
-12.5 Ma (Page 1976), indicating a linkage between the end of
plutonism and the start of compressional deformation. The timing
is consistent with the observation of Hamilton (1994) that in areas
of subduction `the common regime in overriding plates is
extensional and leading edges are crumpled only in collisions.
This suggests that the Middle Miocene magmatism in northern
New Guinea occurred during extension above a subducting slab
and was terminated by Late Miocene arc collision and resultant
compression commencing at -10-12 Ma. In general, these data indicate that the acme of denudation and
cooling in the Mobile Belt was in the Late Miocene, with relatively
little denudation there since. In contrast, Hill & Gleadow (1989)
found that the peak of denudation in basement outcrops in the
Fold Belt was in the Pliocene at -4 Ma. However, in this study
some samples from the Mobile Belt yielded Pliocene ages or high