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深部地壳结构的研究。布兰斯菲尔德海峡位于南极半岛北部，是为数不多的现代盆地之一，可能对了解古代造山过程至关重要。海峡是遥远的东南太平洋上的一个活跃伸展的边缘盆地，位于南极半岛和南设得兰群岛之间，南设得兰群岛是一个不活跃的火山弧。广泛的地壳伸展，伴随着沿海峡轴线的火山活动，可能与南设得兰海沟洋壳的缓慢俯冲有关；在侏罗纪-早白垩世，类似的“弧后”伸展发生在被称为冈瓦瓦兰的超大陆的整个太平洋边缘(现在的南美洲西部/西南极)。大约1亿年前，这些盆地在白垩纪中期发生变形，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部结构和演化，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条全球重要山链的早期演化。多年来，国际地球科学对布兰斯菲尔德海峡的地质环境的重要方面达成了共识：(1)它可能是先前存在的南极半岛地壳中的一个年轻的(大约400万年前)裂谷；地壳的持续伸展导致了复杂的断裂模式和相关的火山作用。火山活动、高热流和绘制的地壳趋势图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，沿着盆地轴线集中的“新火山”带发生。1991年在莫里斯·尤因号上收集的多道地震数据显示了布兰斯菲尔德海峡的以下盆地特征：a)广泛的伸展和断裂，b)与花状正断层构造有关的地壳底辟或穹隆的隆起，c)跨走向的复杂的断层段系统。地球物理证据还表明，裂谷由NE向西南扩展，伴随着最初的地壳膨胀/穹隆，随后是收缩/下沉、火山作用和沿正常断层的伸展。尽管布兰斯菲尔德海峡展示了伸展和火山作用的地球物理和地质证据，但在这个“弧后”盆地中，大陆地壳碎裂似乎并未完成，洋壳尚未形成。相反，布兰斯菲尔德裂谷位于从陆内裂谷向海底扩张的关键过渡阶段。盆地的不对称性和地壳内浅层拆离断层的地震证据表明，它可能接近提出的大陆分裂模型的一端成员。因此，该盆地是研究大陆边缘形成的各种过程的“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，也应该为安第斯造山模型提供一个起点。这项工作将通过收集和分析沿海峡走向和跨海峡走向的高质量、高密度海底地震仪(OBS)剖面来确定深部结构。科学目标如下：(1)建立该裂谷的详细地震速度模型；(2)根据多道地震解释，标定与假定的北东向南西裂谷传播有关的速度结构和地壳厚度变化；(3)记录深部速度结构与沿和跨走向地壳分段的对应程度；(4)评估南设得兰群岛“弧”和布兰斯菲尔德裂谷之间的构造关系。拟议的OBS数据与尤文剖面和布兰斯菲尔德海峡其他高质量地球物理覆盖的解释相结合，将补充正在进行的南极半岛西南地壳深地震分析和额外的OBS深部地震监测，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.