The Young Marginal Basin as a Key to Understanding the Rift-Drift Transition and Andean Orogenesis: OBS Refraction Profiling for Crustal Structure in Bransfield Strait

该奖项由极地方案办公室南极地质和地球物理方案提供，支持研究布兰斯菲尔德海峡地区的地壳深部结构。布兰斯菲尔德海峡位于南极半岛的北方，是少数几个现代盆地之一，可能对了解古代造山过程至关重要。海峡是一个活跃的边缘盆地在遥远的东南太平洋，南极半岛和南设得兰群岛之间，一个不活跃的火山弧。广泛的地壳伸展，伴随着沿着海峡轴线的火山活动，可能与南设得兰群岛海沟的洋壳缓慢的下冲有关;在侏罗纪-早白垩世期间，类似的“弧后”伸展发生在被称为冈瓦纳大陆的整个太平洋边缘（现在的南美洲西部/南极洲西部）。大约1亿年前，这些盆地的白垩纪中期变形引发了安第斯山脉的隆升。通过了解布兰斯菲尔德裂谷的深层结构和演化，应该可以评估安第斯山脉的地壳前兆，从而更全面地了解这一全球重要山脉的早期演化。（1）它可能是先前存在的南极半岛地壳中的一个年轻（可能约400万年）裂谷;该地壳的持续拉伸导致复杂的断层模式和相关的火山活动。火山活动、高热流和绘制的地壳趋势都与盆地作为裂谷的持续演化相一致;（2）火山活动是最近的和持续的，发生在沿着一个“新火山”带，中心沿着盆地的轴。1991年莫里斯·尤因号考察船收集的多道地震数据说明了布兰斯菲尔德海峡的以下全盆地特征-- a）广泛的伸展和断层作用，B）与花形正断层构造相关的地壳底辟或穹隆的上升，以及c）横跨走向的复杂的断层边界段系统。地球物理学证据也表明，东北向西南传播的裂谷，与最初的地壳膨胀/隆起，随后的通缩/沉降，火山活动，并延长沿着正常future.Although布兰斯菲尔德海峡的地球物理学和地质证据的伸展和火山活动，大陆地壳的破碎似乎并没有去完成在这个“弧后”盆地和海洋地壳尚未生成。相反，布兰斯菲尔德裂谷位于从陆内裂谷到海底扩张的关键过渡附近。 该盆地的不对称性，以及地壳内浅层剥离断层的地震证据表明，它可能是大陆分裂模型的一个端员。因此，该盆地是研究大陆边缘形成过程的“天然实验室”，了解布兰斯菲尔德裂谷的深部地壳结构是解决其演化阶段的关键，也应该为安第斯山脉的造山模型提供起点。这项工作将通过收集和分析海峡走向沿着和两岸的高质量、高密度海底地震仪剖面图来确定深层结构。科学目标如下：（1）为该裂谷建立详细的地震速度模型;（2）校准从多道地震解释推断的与假定的NE-SW裂谷传播有关的速度结构和地壳厚度变化;（3）记录深部速度结构对应于沿着和跨走向地壳分段的程度;以及（4）评估南设得兰群岛“弧”与布兰斯菲尔德裂谷之间的构造关系。建议的OBS数据，结合对尤因剖面和布兰斯菲尔德海峡其他高质量地球物理报道的解释，将补充正在进行的南极半岛地壳西南部的深地震分析和附加的OBS深地震监测，以了解该地区复杂的板块构造演化。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports research to study the deep crustal structure of the Bransfield Strait region. Bransfield Strait, in the northern Antarctic Peninsula, is one of a small number of modern basins that may be critical for understanding ancient mountain-building processes. The Strait is an actively-extending marginal basin in the far southeast Pacific, between the Antarctic Peninsula and the South Shetland Islands, an inactive volcanic arc. Widespread crustal extension, accompanied by volcanism along the Strait's axis, may be associated with slow underthrusting of oceanic crust at the South Shetland Trench; similar "back-arc" extension occurred along the entire Pacific margin (now western South America/West Antarctica) of the supercontinent known as Gondwanaland during the Jurassic-Early Cretaceous. Mid-Cretaceous deformation of these basins some 100 million years ago initiated uplift of the Andes. By understanding the deep structure and evolution of Bransfield rift, it should be possible to evaluate the crustal precursor to the Andes, and thereby understand more fully the early evolution of this globally important mountain chain.Years of international earth sciences research in Bransfield Strait has produced consensus on important aspects of its geologic environment: (1) It is probably a young (probably ~4 million years old) rift in preexisting Antarctic Peninsula crust; continued stretching of this crust results in complex fault patterns and associated volcanism. The volcanism, high heat flow, and mapped crustal trends are all consistent with the basin's continuing evolution as a rift; (2) The volcanism, which is recent and continuing, occurs along a "neovolcanic" zone centralized along the basin's axis. Multichannel seismic data collected aboard R/V Maurice Ewing in 1991 illustrate the following basin-wide characteristics of Bransfield Strait - a) widespread extension and faulting, b) the rise of crustal diapirs or domes associated with flower-shaped normal-fault structures, and c) a complicated system of fault-bounded segments across strike. The geophysical evidence also suggests NE-to-SW propagation of the rift, with initial crustal inflation/doming followed by deflation/subsidence, volcanism, and extension along normal faults.Although Bransfield Strait exhibits geophysical and geologic evidence for extension and volcanism, continental crust fragmentation does not appear to have gone to completion in this "back-arc" basin and ocean crust is not yet being generated. Instead, Bransfield rift lies near the critical transition from intracontinental rifting to seafloor-spreading. The basin's asymmetry, and seismic evidence for shallow intracrustal detachment faulting, suggest that it may be near one end-member of the spectrum of models proposed for continental break-up. Therefore, this basin is a "natural lab" for studying diverse processes involved in forming continental margins.Understanding Bransfield rift's deep crustal structure is the key to resolving its stage of evolution, and should also provide a starting point for models of Andean mountain-building. This work will define the deep structure by collecting and analyzing high-quality, high-density ocean bottom seismometer (OBS) profiles both along and across the Strait's strike. Scientific objectives are as follows: (1) to develop a detailed seismic velocity model for this rift; (2) to calibrate velocity structure and crustal thickness changes associated with presumed NE-to-SW rift propagation, as deduced from the multichannel seismic interpretations; (3) to document the degree to which deep velocity structure corresponds to along- and across-strike crustal segmentation; and (4) to assess structural relationships between the South Shetland Islands "arc" and Bransfield rift.The proposed OBS data, integrated with interpretations of both Ewing profiles and those from other high-quality geophysical coverage in Bransfield Strait, will complement ongoing deep seismic analysis of Antarctic Peninsula crust to the southwest and additional OBS monitoring for deep earthquakes, in order to understand the complex plate tectonic evolution of this region.