Evolution of an active metamorphic core complex, Suckling-Dayman Massif, eastern PNG
thesisposted on 28.03.2022, 19:19 by Peter Caffi
A shallow-dipping ductile mylonitic shear zone is the dominant geological structure that controls the orientation of dip slopes on the flanks of Mount Dayman, eastern Papuan Peninsula, Papua New Guinea. The dip slopes dip in all directions from the peak of Mount Dayman and form a domed landform that is much less dissected by streams compared to the adjacent Mount Suckling domed landform. The orientation of megamullions on the domed surface of Mount Dayman is consistent with NNE-directed transport of the hanging wall block. Though previously documented as a thrust surface, the geometry and style of structures and map relations presented in this study indicate an extensional origin for the domed mylonitic foliation (S1) and mineral stretching lineation (L1). The field relationships are consistent with the domed landform comprising the core of a kilometre-scale metamorphic core complex. Observations of dominantly NNE trending regional lineaments in aerial photography and shuttle radar topography mission (SRTM) data correlate with detailed field analysis of mineral stretching lineations (L1) inthe main metamorphic core complex-bounding shear zone. Field relationships show across cutting sequence of structures that includes: (i) ductile S2 folia with ESE-plunging riebeckite mineral stretching lineations; (ii) narrow steeply dipping ductile D2 shear zones; and (iii) semi-brittle to brittle fault zones. S-C’ fabrics, mantled porphyroclasts and fault drag indicate a top down to the NNE sense-of-shear for most structures. Kinematic vorticity analysis of the highest-grade ductile deformation indicates a kinematic vorticity number (Wk) between 0.34 and 0.56, suggesting general shear for the early stage of deformation (D1). Metamorphic mineral assemblages of the metabasite ductile mylonitic rocks record green schist facies conditions. The presence of pumpellyite-actinolite facies assemblages in the core of the complex indicates peak metamorphic pressures of 6-9.5 kbar, demonstrating exhumation of the core from 20-30 km depth. The NNE-directed structural lineaments and L1 mineral stretching lineations are consistent with the Australia-Woodlark Eulerian pole for periods between 0.52-3.6 Ma. This observation is consistent with 3.3 Ma granite and monzonite intrusions that cut the mylonitic fabrics and limit the age of the mylonitic fabrics to older than 3.3 Ma. A SE dipping sedimentary sequence (Gwoira Conglomerate) characterises the hanging wall of the metamorphic core complex. Petrography of the clasts within the sedimentary rocks indicates that metabasite rocks were the dominant source. The unit is in fault contact with the metabasite footwall across prehnite-bearing D3 brittle fault zones. The rotation direction of bedding is consistent with NW-directed extension as predicted by the 0-0.52 Ma Australia-Woodlark Eulerian pole.