The Andaman and Nicobar Islands are rimmed by discontinuous fringing reef that is in general wider on western margin vs the eastern margin. This study characterizes the facies updip from the modern fringing reefs to the present shoreline of south Andaman and Swaraj Dweep, and describes in detail the coral terraces/carpets within and above the inter-tidal zone representing the Holocene Fringing reef. Field studies, satellite, and drone datasets have been utilized to map different facies, that include: coralgal boundstone, biodetrital-grainstone, beachrock, and coralgal rudstone. Multiple exposed microatolls as well as coral terraces (coral carpets) of Acropora and Porites (dated 8.7-8.4 ka BP) have been identified within the intertidal zone (Radhanagar Beach, Swaraj Dweep) indicating that Holocene fringing reef have down-stepped offshore to the current location of modern fringing reefs owing to either tectonics or eustasy. The eustatic sea-level fluctuations are relatively well established for the Holocene and we compute the tectonic uplift rates utilizing the stream-power-incision and linear-inversion model. A tectonic uplift rate of ~ 0.05 mm/yr (for Swaraj Dweep) during the past 100 ka is estimated, while taking into account a wide range of erodibility indexes and response time intervals. It is identified that the computed uplift rate is an amalgamation of the coseismic deformation along with the interseismic and aseismic surface deformation. Thus, not all exposed coral terraces/microatolls are exposed due to coseismic deformation (for example uplift in parts of Andaman due to earthquake in 2004). The average long-term uplift rates are a magnitude lower than the eustatic sea-level fall rates during Holocene, thus, we suggest that most of the Holocene fringing reefs are exposed due to eustatic sea-level fall and down-stepped to the current location of the modern fringing reefs. This would entail that the eustatic sea-level change rates would play a significant role in determining future of the modern fringing reef (catch-up vs keep up vs give up), and the coastal morphology of south Andaman and Swaraj Dweep, with implications for coastal inundation and stability in the scenario of climate change.

中文翻译：

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安达曼南部和 Swaraj Dweep 海岸线的全新世边缘礁


安达曼群岛和尼科巴群岛被不连续的边缘礁所包围，西缘通常比东缘更宽。本研究描述了从现代边缘礁到现在的南安达曼和 Swaraj Dweep 海岸线的相，并详细描述了代表全新世边缘礁的潮间带内部和上方的珊瑚梯田/地毯。实地研究、卫星和无人机数据集已被用于绘制不同的相，包括：珊瑚界石、生物碎屑颗粒岩、海滩岩和珊瑚赤褐石。在潮间带（Radhanagar 海滩、Swaraj Dweep）内已确定多个裸露的微环礁以及 Acropora 和 Porites 的珊瑚梯田（珊瑚地毯）（年代为 8.7-8.4 ka BP），表明由于构造或收缩作用，全新世边缘礁已下降到现代边缘礁的当前位置。全新世的静压海平面波动相对成熟，我们利用河流功率切割和线性反转模型计算构造抬升速率。在过去 100 ka 期间，构造抬升速率估计为 ~ 0.05 毫米/年（对于 Swaraj Dweep），同时考虑了广泛的可侵蚀性指数和响应时间。结果表明，计算出的抬升速率是同震变形与地震间和地震表面变形的合并。因此，并非所有裸露的珊瑚梯田/微环礁都因同震变形而暴露（例如，2004 年安达曼部分地区因地震而隆起）。 平均长期抬升速率比全新世期间的静压海平面下降速率低一个数量级，因此，我们认为大多数全新世边缘礁是由于静压海平面下降而暴露出来的，并下降到现代边缘礁的当前位置。这将意味着，静压海平面变化率将在决定现代边缘礁的未来（追赶 vs 跟上 vs 放弃）以及南安达曼和 Swaraj Dweep 的沿海形态方面发挥重要作用，对气候变化情景下的沿海洪水和稳定性产生影响。