Evolution and properties of young oceanic crust: constraints from Poisson's ratio
Evolution and properties of young oceanic crust: constraints from Poisson's ratio
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年轻洋壳的演化和性质：泊松比的约束
DOI：
10.1093/gji/ggab062
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发表时间：
2021
影响因子：
2.8
通讯作者：
Funnell M
中科院分区：
文献类型：
--
作者：
Funnell M
大洋地壳的地震速度是其物理性质的函数，包括其岩性、蚀变程度和孔隙度。这些性质的变化在年轻的地壳中尤为显著，但也会随着年龄的增长而发生变化，因为它通过热液循环演变，并逐渐被沉积物覆盖。虽然这种变化可以单独通过P波速度来研究，但与S波速度的联合分析允许确定泊松比，这提供了对这些特性变化的本质的更有力的洞察。在这里，我们描述了沿着一条330公里长的∼剖面获得的独立建模的∼-And波地震数据集，该剖面横跨在哥斯达黎加中扩张裂谷形成的新的MYR-古老洋壳。尽管波数据覆盖率几乎比P波数据集低4倍，但这两个速度模型都显示出局部变异性的相关性，以及从S−1到0.8亿公里和0.6亿公里的脊轴上速度随距离的长波增加，从而随年龄的增加。用Vanda-Vs模型计算泊松比(σ)，揭示了年轻洋壳的典型结构，在最上部地壳普遍较高，到1.0-1.5万公里的次基底时降至最小0.24值，然后在整个下地壳再次增加。观察到的上地壳σ的下降很可能是由于裂缝的封闭所致，这得到了通过大洋钻探方案504b孔中的堤坝序列孔隙度随深度的显著下降(从∼15%降至1lt；2%)的观察支持。高泊松比(>；0.31)在CRR轴线北侧的整个地壳中都能观察到，虽然这属于岩性指标的蛇纹岩分类，但普遍存在的地表岩浆作用和有限的构造作用的形态证据表明，原因是以普遍破裂形式存在的孔洞，因此，这是新形成的CRR地壳中地震速度的主要控制因素。在CRR以南，泊松比代表了更典型的洋壳，并且随着与扩张中心距离的增加而减小，这很可能是矿化和裂缝充填增加的结果。这得到了井眼观测和模拟的三维地震各向异性的支持。在半扩张率特别低的时期(<；35 mm yr−1)形成的地壳段相对于背景显示出高的泊松比，这表明它们形成时可能保留了与更大程度的构造作用有关的增加的孔隙度和破裂。在CRR南侧，我们发现地壳上部1公里处的平均泊松比在地壳热封闭前随年龄减少了∼0.0084-−1，这表明，至少到∼-7-Myr，平流热液作用主导了CRR产生的早期洋壳演化，这与热流测量结果一致。
The seismic velocity of the oceanic crust is a function of its physical properties that include its lithology, degree of alteration and porosity. Variations in these properties are particularly significant in young crust, but also occur with age as it evolves through hydrothermal circulation and is progressively covered with sediment. While such variation may be investigated throughP-wave velocity alone, joint analysis withS-wave velocity allows the determination of Poisson's ratio, which provides a more robust insight into the nature of change in these properties. Here we describe the independent modelling ofP- andS-wave seismic data sets, acquired along an ∼330-km-long profile traversing new to ∼8 Myr-old oceanic crust formed at the intermediate-spreading Costa Rica Rift (CRR). DespiteS-wave data coverage being almost four-times lower than that of theP-wave data set, both velocity models demonstrate correlations in local variability and a long-wavelength increase in velocity with distance, and thus age, from the ridge axis of up to 0.8 and 0.6 km s−1, respectively. Using theVpandVsmodels to calculate Poisson's ratio (σ), it reveals a typical structure for young oceanic crust, with generally high values in the uppermost crust that decrease to a minimum of 0.24 by 1.0–1.5 km sub-basement, before increasing again throughout the lower crust. The observed upper crustal decrease inσmost likely results from sealing of fractures, which is supported by observations of a significant decrease in porosity with depth (from ∼15 to <2 per cent) through the dyke sequence in Ocean Drilling Program borehole 504B. High Poisson's ratio (>0.31) is observed throughout the crust of the north flank of the CRR axis and, whilst this falls within the ‘serpentinite’ classification of lithological proxies, morphological evidence of pervasive surface magmatism and limited tectonism suggests, instead, that the cause is porosity in the form of pervasive fracturing and, thus, that this is the dominant control on seismic velocity in the newly formed CRR crust. South of the CRR, the values of Poisson's ratio are representative of more typical oceanic crust, and decrease with increasing distance from the spreading centre, most likely as a result of mineralization and increased fracture infill. This is supported by borehole observations and modelled 3-D seismic anisotropy. Crustal segments formed during periods of particularly low half-spreading rate (<35 mm yr−1) demonstrate high Poisson's ratio relative to the background, indicating the likely retention of increased porosity and fracturing associated with the greater degrees of tectonism at the time of their formation. Across the south flank of the CRR, we find that the average Poisson's ratio in the upper 1 km of the crust decreases with age by ∼0.0084 Myr−1prior to the thermal sealing of the crust, suggesting that, to at least ∼7 Myr, advective hydrothermal processes dominate early CRR-generated oceanic crustal evolution, consistent with heat flow measurements.
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影响因子：
5.3
作者：
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DOI：
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期刊：
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DOI：
--
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DOI：
--
发表时间：
1994
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