Year

2021

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric and Life Sciences

Abstract

This thesis aims to establish how the late Cenozoic oblique convergence of the Pacific- Caroline and Australian plates has affected the tectonic history of the eastern Bird’s Head Peninsula in northwestern New Guinea. Emphasis is placed on insights provided by >5 magnitude earthquakes since 1976 and the structural geology of Miocene-Pleistocene sediments in the Manokwari area and Permian to Paleogene units of the northern Lengguru Fold Belt around Mawi Bay. The Bird's Head Peninsula is moving west- southwest relative to the Australian Plate based on GPS data, but slower than the west- southwest movement of the Pacific-Caroline Plate. This accounts for plate convergence between the Pacific-Caroline Plate and the Bird's Head Peninsula consistent with earthquakes caused by gently south dipping thrusts in an arc along the northern Bird’s Head coastline and indicating subduction along the Manokwari Trough. Southwards subduction along the New Guinea Trench to the northeast of Bird’s Head Peninsula is also shown by abundant earthquakes caused by southwest dipping thrusts in northern Papua between Yapen Island and Jayapura. East-west sinistral strike-slip focal mechanism solutions along the Yapen Fault in northern Cendrawasih Bay east of the Bird’s Head Peninsula reflect partitioning of oblique convergence into east-west strike- slip movement and subduction along the New Guinea Trench and subduction along the Manokwari Trough. The Sorong Fault in the northern Bird's Head is a largely an aseismic zone with morphological features consistent with an active fault. The Kemum High in northern Bird's Head Peninsula is an example of a mega pop-up structure formed by a double bend in the Sorong and Yapen faults. West-southwest movement of the Bird’s Head Peninsula is accommodated by sinistral strike slip along the Tarera-Aiduna Fault further south.

In the northeastern Bird's Head, contractional deformation and uplift is shown by folded Miocene to lower Pliocene sediments and Pleistocene raised coral reefs. Provenance indicated by detrital zircon ages and clast types show initial uplift along the Kemum High mega pop-up in the late Miocene. North-northeast convergence in the Manokwari area from partitioning of the plate vector caused buttressing with deformation decreasing to the northeast as shown by interlimb angles of map-scale folds. An angular unconformity with weakly deformed overlying uplifted coral reefs of the Pleistocene Manokwari Limestone limits the deformation to the late Pliocene to Pleistocene. U–Th series ages of 65–250 ka on raised coral reefs shows that uplift continues to the present. Coral-filled fissures are caused by near-surface ground ruptures formed by earthquakes.

Earthquakes form a cluster of 29 earthquakes in the period 1977–2018 with magnitudes of M4.9 to M7.5 around the Ransiki Fault and Yapen Fault intersection. In the 10 October 2002 earthquake, surface ruptures formed and have trends subparallel to the Ransiki Fault (140°E) and some at low-angle to it (120°E–130°E). Liquefaction also occurred as indicated by fissures, and sand blows, in the saturated sediments of the Ransiki delta. Focal mechanism solutions indicate both dextral and sinistral strike-slip motion along the Ransiki Fault, although dextral movement is considered dominant and associated with the largest earthquake (M7.5 10 October 2002). Another significant earthquake (M6.7 event, 21 April 2012) was also associated with surface ruptures and liquefaction and focal mechanism solutions indicate sinistral strike-slip in an east-west direction consistent with movement on the Yapen Fault.

At the northern end of the Lengguru Fold Belt, the Permian Mawi Complex is multiply deformed and the D1 deformation, characterized by recumbent isoclinal to tight folds with an axial planar cleavage, predated the unconformably overlying Upper Triassic – Lower Jurassic Tipuma Formation. D1 developed in the Late Permian to Early Triassic in an East Gondwanan Andean active continental margin setting. A D2 deformation in the Mawi Complex produced open to gentle steeply inclined folds that also caused deformation of the overlying Mesozoic to Paleogene succession and formed the late Miocene to early Pliocene Lengguru Fold Belt. The northern part of the Lengguru Fold Belt curves seamlessly into the Central Bird’s Head Monocline which is responsible for the (?)Pliocene to Quaternary uplift of the Kemum High in the northern Bird’s Head Peninsula.

FoR codes (2008)

0403 GEOLOGY

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.