Major hydrocarbon discoveries have been made in eastern and westernmost New Guinea, and there is great potential for additional discoveries. Although the island is a type locality for arc-continent collision during the Cenozoic, the age, number, and plate kinematics of the events that formed the island are vigorously argued. The northern part of the island is underlain by rocks with oceanic island arc affinities, and the southern part is underlain by the Australian continental crust. Based on regional sedimentation patterns, it is argued herein that the Cenozoic tectonic history of the island involves two distinct collisional orogenic events.The first Cenozoic event, the Peninsular orogeny of Oligocene age (35–30 Ma), was restricted to easternmost New Guinea. Emergent uplifts that shed abundant detritus resulted from the subduction of the northeastern corner of the Australian continent beneath part of the Inner Melanesian arc. This collision uplifted the Papuan ophiolite and formed the associated mountainous uplift that was the primary source of siliciclastic sediments that largely filled the Aure trough. Between the Oligocene and Miocene, the paleogeography of the region was similar to present-day New Caledonia. The continental crust under central and western New Guinea remained a passive margin.The second event, the Central Range orogeny, began in the latest middle Miocene, when the bulldozing of Australian passive-margin strata first created emergent uplifts above a north-dipping subduction zone beneath the western part of the Outer Melanesian arc. The cessation of carbonate shelf sedimentation and widespread initiation of siliciclastic sedimentation on top of the Australian continental basement is dated at about 12 Ma. This collision emplaced the Irian ophiolite and created the present mountainous topography forming the spine of the island.
Research Article| June 01, 1993 Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts MARK CLOOS MARK CLOOS 1Department of Geological Sciences and Institute for Geophysics, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Author and Article Information MARK CLOOS 1Department of Geological Sciences and Institute for Geophysics, University of Texas at Austin, Austin, Texas 78712 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (6): 715–737. https://doi.org/10.1130/0016-7606(1993)105<0715:LBACOS>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation MARK CLOOS; Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts. GSA Bulletin 1993;; 105 (6): 715–737. doi: https://doi.org/10.1130/0016-7606(1993)105<0715:LBACOS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The sizes of continental blocks, basaltic oceanic plateaus, and island arcs that would cause collisional orogenesis when they enter a subduction zone are calculated in an analysis based upon the assumption of local isostasy and the assumption that plate subduction is primarily driven by the negative buoyancy of the lithosphere. Buoyancy analysis indicates that the bulk density contrast between 80-m.y.-old oceanic lithosphere capped by a 7-km-thick basaltic crust and the less dense underlying asthenosphere is on the order of 0.04 gm/cm3. Oceanic lithosphere that is ∼10 m.y. old is the youngest that is more dense than the asthenosphere and hence inherently susceptible to subduction. Subduction zone metamorphism causes the crustal layer of basalt/gabbro to transform into more dense amphibolite and eclogite. Where eclogite formation is extensive, the descending oceanic lithosphere increases in bulk density by as much as 0.04 gm/cm3. Lithosphere that is 100 km thick with a 30-km-thick granitic continental crust resists Subduction because it is ∼0.09 gm/cm3 less dense than the asthenosphere. Contrasts in lithospheric bulk density (crust + mantle) of <0.10 gm/cm3 are the difference between whether subduction is nearly inevitable (as for normal ocean crust) or greatly resisted (as for thick, ancient continents).Collisional orogenesis is defined as a plate interaction of the sort that causes a rearrangement of plate motions, generally with the initiation of a new subduction zone and the creation of mountains. Buoyancy analysis indicates that only bodies of continental and oceanic island are crust that are > ∼15 km thick make the lithosphere buoyant enough to jam a subduction zone. Oceanic island arc complexes built upon ocean crust typically must be active for more than ∼20 m.y. to attain crustal thicknesses so that their attempted subduction causes collisional orogenesis. Oceanic plateaus where basaltic crust as much as ∼17 km thick caps 100-km-thick lithosphere are inherenty subductable and actually less buoyant than normal oceanic lithosphere following subduction metamorphism. Basaltic plateaus must have crustal thicknesses >∼30 km to typically cause collisional orogenesis during subduction. Short subducting seamounts (<1-2 km tall) typically cause only temporary dents, but taller seamounts locally cause permanent distortions; as they bulldoze the front of the fore-arc block The direct tectonic effect resulting from the subduction of most bathymetric highs is only a temporary isostatic uplift of the fore-arc region of as much as several kilometers, followed by subsidence to original elevations. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Modeling of heat flow through a wedge‐shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of Ar‐isotopic ages and metamorphic progressions across blueschist belts. If convergence rates are maintained at rates of a few centimeters per year or more, material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where Ar‐loss by diffusion and recrystallization effectively ceases. Maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider. Because of this difference in thermal history, K‐Ar and 40 Ar/ 39 Ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth. Application of this thermal modeling to the Franciscan subduction complex of northern California indicates that the coherent blueschists in the Eastern Belt (typically with 115 to 125 m.y. Ar‐isotopic ages) and the high‐grade blueschist blocks in the mud matrix melanges of the western portion of the Central Belt (typically with 140 to 150 m.y. Ar‐isotopic ages) could both have formed during a single, prolonged period of continuous convergence.
High-pressure/low-temperature metamorphic rocks in the Franciscan Complex of western California are primarily found as small blocks in mud-matrix melange and as extensive coherent, bedded sheets or slabs. The highest-pressure rocks, the high-grade sodic and/or calcic amphibole-epidote-garnet-omphacitic pyroxene-bearing blueschists, amphibolites, or eclogites, are found as blocks (typically meters to a few tens of meters across) in the melanges. Low-grade schistose blueschists are found as small blocks in melanges and in extensive coherent belts. Many Franciscan greenstones and metagray-wackes in both the melanges and coherent tracts locally contain sodic pyroxene + quartz, lawsonite, and/or aragonite but are neither blue nor strongly schistose....
Theoretical modeling of the subduction channel (shear zone) at convergent plate margins quantifies the processes of sediment subduction, offscraping, underplating and formation of subduction melange by upwelling. Although bedding anisotropy and variations in lithology and pore-fluid pressure control the details of the deformation near the inlet to the subduction channel, the theory shows there are only five basic kinematic patterns which can result in the development of a distinctive type of margin (Types A-E). All incoming sediment is subducted and subduction erosion can occur at Type A margins. All sediment is subducted but a thick, narrow accretionary prism grows by underplating of subducted sediment at Type B margins. Offscraping leads to the development of a broad, tapering prism at Type C, D, and E margins. Incoming sediment is offscraped and subducted sediment is underplated at Type C margins. Melange upwells from depth and is offscraped and underplated at Type D and E margins. Incoming sediment is also offscraped at Type D margins. The structural and metamorphic histories of the fundamental tectonostratigraphic units within the accretionary prism are distinct during steady-state subduction. The bedded slope cover is not metamorphosed and not intensely tectonized upslope from the inlet. During final dewatering andmore » accretion, offscraped materials undergo a subhorizontally-directed compression whereas underplated materials undergo a simple-shear-style of deformation. The metamorphic changes in subducted sediment or upwelled melange depend upon the depth of maximum burial and the thermal structure of the margin. Various episodic factors, such as seamount or ridge subduction, can modify the structural and metamorphic contrasts.« less
