Active Continental Extension of Eastern Papua New Guinea: Geologic Studies of Emerged Coral Reefs and Marine Sedimentary Rocks

[00:65 . 18]从狭窄而明确的板块边界的海底扩张到大陆地壳的扩散裂谷的横向转变今天发生在世界范围内的三个地方：红海、亚丁湾和巴布亚新几内亚东部。这三个区域对于理解大陆如何破裂至关重要，因为从海底扩张到大陆裂谷的空间进展提供了对海洋扩张脊到大陆裂谷的时间和结构演变的见解。在像美国盆地和山脉省这样更广泛的大陆内扩张活动带，这种见解并不容易明显，在那里，扩张没有——也可能不会——最终进入海底扩张的地步。在这三个向裂谷过渡的海底扩展区中，只有巴布亚新几内亚在扩展脊的尖端与低角度正断层地震、海底环境中活跃的低角度正断层和晚新近纪变质核杂岩之间表现出密切的空间联系。图特卡斯托群岛和巴布亚半岛处于大洋扩张与大陆裂陷的过渡地带，被晚新近纪沉积岩和晚第四纪珊瑚礁所包围。这些可到达的陆地区域提供了一个理想的地质环境，可以测试几种不同的构造模型，这些模型是基于地震、核复合体的辐射测量和结构研究以及海洋地球物理研究提出的，用于扩展脊裂过渡。这些模型的主要内容包括应用于该大陆地壳区域的海洋传播脊模型、变质核杂岩的局部化和传播脊附近的地形隆起、裂谷位置的横向移动和随后的扩张。为了验证上述构造模型，我们将结合对巴布亚新几内亚东部数百公里海岸线上晚第四纪珊瑚礁和晚新近纪沉积剖面的地质研究，确定晚新近纪到晚第四纪的垂直构造历史模式、变形风格和构造机制。我们在这些研究中要解决的关键问题包括：1)在德恩加斯托群岛和巴布亚半岛，晚第四纪珊瑚礁隆起最大的区域在哪里？哪些结构对它们产生了影响？2)该地区抬升格局是否存在明显的空间偏移？3)在Sanaroa和Fergusson群岛附近的低角度正断层地震区，晚新近纪海相岩和珊瑚阶地本身是否存在低角度正断层？我们的研究结果将有助于解释在红海、东非裂谷和亚丁湾等没有详细珊瑚礁记录的其他活跃裂谷地区，活跃山脊传播或跳跃的构造影响。

0106518MannThe lateral transition from seafloor spreading at a narrow, well defined plate boundary to diffuse rifting of continental crust occurs today in three localities worldwide: the Red Sea, the Gulf of Aden, and eastern Papua New Guinea. These three areas are critical for understanding how continents rupture because the spatial progression from seafloor spreading to continental rifting provides insights into the temporal and structural evolution of an oceanic spreading ridge to a continental rift. Such insights are not readily apparent from the more widespread zones of active intracontinental extension like the U.S. Basin and Range province where extension has not - and may not - eventually proceed to the point of seafloor spreading. Of these three seafloor spreading to rift transition areas, only Papua New Guinea exhibits a close spatial association between the tip of the propagating spreading ridge and low-angle normal fault earthquakes, active, low-angle normal faults in a submarine setting, and late Neogene metamorphic core complexes. The D'Entrecasteaux Islands and Papuan Peninsula, fringed by late Neogene sedimentary rocks and late Quaternary coral reefs, occupy the transition area between oceanic spreading and continental rifting. These accessible land areas provide an ideal geologic setting to test several contrasting tectonic models for spreading ridge-rift transitions proposed on the basis of earthquakes, radiometric and structural studies of core complexes, and marine geophysical studies. The main elements of these previously proposed models include oceanic propagating ridge models applied to this region of continental crust, localization of metamorphic core complexes and topographic uplift adjacent to areas of propagating ridges, and lateral shifts in the location of rifting and subsequent spreading. We are using a combination of geologic studies of late Quaternary coral reefs and late Neogene sedimentary sections along hundreds of kilometers of eastern PNG coastlines to determine the pattern of late Neogene to late Quaternary vertical tectonic history, style of deformation, and tectonic mechanisms in order to test the tectonic models described above. Key questions that we are addressing in these studies include: 1) Where is the area of maximum late Quaternary coral reef uplift in the D'Entrecasteaux Islands and Papuan Peninsula and what structures have affected them? 2) Are there detectable spatial migrations in the uplift pattern of this region? and 3) Are low angle normal faults present in the late Neogene marine rocks and the coral terraces themselves in the area of low angle normal fault earthquakes near Sanaroa and Fergusson Islands? Our results will aid interpretations of the tectonic effects of active ridge propagation or jumping in other active rift areas including the Red Sea, East African rifts, and Gulf of Aden where a detailed reef record such as in Papua New Guinea is not available.