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

从狭窄、明确的板块边界的海底扩展到大陆地壳的弥漫裂谷的横向转变今天发生在世界三个地方：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是如何破裂至关重要的，因为从海底扩张到大陆裂谷的空间进程提供了对大洋扩张脊到大陆裂谷的时间和结构演变的洞察。这种洞察力在更广泛的大陆内活跃伸展区域并不容易显现，比如美国盆地和山脉省，那里的伸展还没有--也可能不会--最终发展到海底扩张点。在这三个海底扩张到裂谷过渡区的区域中，只有巴布亚新几内亚在扩张海脊尖端与低角度正断层地震、海底环境中的活动低角度正断层和新近纪晚期变质核杂岩之间存在密切的空间联系。D‘Entrecasteaux群岛和巴布亚半岛被晚第三纪沉积岩和晚第四纪珊瑚礁环绕，占据了大洋扩张和大陆裂谷的过渡区。这些可接近的陆地地区提供了一个理想的地质环境来测试几种不同的构造模型，这些模型是根据地震、核杂岩的辐射和结构研究以及海洋地球物理研究而提出的扩张脊-裂谷转变的构造模型。这些模型的主要内容包括应用于这一陆壳区域的大洋传播脊模型、变质核杂岩的定位和与传播脊区域相邻的地形隆起，以及裂谷和随后扩张位置的横向移动。我们利用巴新东部数百公里长的晚第四纪珊瑚礁和晚新世沉积剖面的地质研究相结合的方法，确定晚第三纪至晚第四纪的垂直构造历史、变形样式和构造机制，以检验上述构造模式。我们在这些研究中讨论的关键问题包括：1)德恩特雷卡斯托群岛和巴布亚半岛晚第四纪珊瑚礁最大隆起的区域在哪里，什么构造影响了它们？2)在该地区的隆升模式中是否存在可检测到的空间迁移？(3)在萨那罗岛和弗格森群岛附近的低角度正断层地震地区，晚第三纪海相岩和珊瑚阶地中是否存在低角度正断层？我们的结果将有助于解释在红海、东非裂谷和亚丁湾等其他活动裂谷地区活动的脊状扩展或跳跃的构造效应，这些地区没有详细的珊瑚礁记录，如巴布亚新几内亚。

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.