Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough
Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough
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DOI：
10.1016/s0025-3227(02)00265-7
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发表时间：
2002-07-20
期刊：
影响因子：
2.9
通讯作者：
Taira, A
中科院分区：
文献类型：
--
作者：
Ashi, J;Tokuyama, H;Taira, A
在四国和东海外的南海增生棱镜中进行了详细的海底模拟反射层（BSR）测绘，该反射层标志着从反射层上方的甲烷水合物层到下方的游离气的相变。在从趾部到弧前盆地的棱柱体斜坡中，BSR分布广泛。BSR位置为我们提供了区域热流变化，使用压力-温度条件的甲烷水合物稳定性。估计的热流通常显示出比棱柱体的中间斜坡浅约50 mW/m（2）的恒定值，并且在较低棱柱体斜坡中逐渐向海增加。BSR的出现被认为是天然气水合物稳定性场（BGHS）底部下方游离气的聚集和/或BGHS上方甲烷水合物的浓度。这些条件可以通过甲烷气体的上倾迁移来实现，因为这种甲烷浓度不太可能通过包括低总有机碳的沉积物内的原位生物甲烷生成来完成。此外，逆冲断层的沉积、抬升和沉积物堆积导致BGHS向上迁移，甲烷从解离的水合物迁移到新的BGHS。这种甲烷气体的再循环可能活跃地发生在增生棱柱体中。与此相反，有五个地区没有海底反射：南海海槽底部，棱柱趾，斜坡盆地，陡坡，深海峡谷。槽底、棱柱体底部和斜坡盆地的特征是具有低甲烷产量的年轻沉积物和不适合气体和流体运移的近水平地层。陡坡和峡谷的侵蚀导致水化沉积物的移动和BGHS的向下运动。由BSR位置估计的BSR分布和热结构提供了有关增生棱柱体中发生的活动过程的信息。(C)2002 Elsevier Science B. V.保留所有权利。
Detailed mapping of a bottom simulating reflector (BSR), which marks the phase transition from the methane hydrate layer above the reflector to free gas below, was conducted in the Nankai accretionary prism off Shikoku and Tokai. BSRs are widely distributed in the prism slope from the toe region to the forearc basin. BSR positions provide us with the regional heat flow variations using pressure-temperature conditions for methane hydrate stability. Estimated heat flows generally show constant values about 50 mW/m(2) shallower than the middle slope of the prism, and gradually increase seaward in the lower prism slope. Occurrences of BSRs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field (BGHS) and/or concentration of methane hydrate above the BGHS. These conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon. Moreover, sedimentation, uplifting, and sediment stacking by thrust faulting cause upward migration of the BGHS and migration of methane from the dissociated hydrate to new BGHS. Such recycling of methane gas may have actively occurred in accretionary prisms. In contrast, there are five regions of no BSRs: the Nankai Trough floor, prism toe, slope basin, steep slope, and deep-sea canyon. The trough floor, the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub-horizontal strata unsuitable for migration of gases and fluids. Erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the BGHS. BSR distribution and thermal structure estimated from BSR positions offer information about active processes occurring in accretionary prisms. (C) 2002 Elsevier Science B.V. All rights reserved.