Seismic investigation of an active ocean-continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre
Seismic investigation of an active ocean-continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre
复制标题
活跃洋陆转换边缘的地震调查：天鹅群岛断层带与超慢速扩张的开曼中部扩张中心之间的相互作用
DOI：
10.1093/gji/ggz283
复制
发表时间：
2019
影响因子：
2.8
通讯作者：
Castiello G
中科院分区：
文献类型：
--
作者：
Castiello G
天鹅群岛转换断层（SITF）标志着开曼海槽的南部边界和洪都拉斯近海北美-加勒比板块边界的海洋-大陆过渡。CAYSEIS实验获得了一个180公里长的地震折射和重力剖面，横跨这一转换边缘，距离中开曼扩展中心（MCSC）以西约70公里。这条剖面显示了穿越转换断层系统的地壳结构，该断层系统将中生代大陆地壳与北部1000万年前的超低速扩张洋壳并列在一起。沿剖面部署了海底地震仪，并在重力分析的支持下进行了逆时和正时走时建模，在SITF中，发现了厚度为1.23公里的大陆地壳，该大陆地壳在1.70公里至1.10公里的距离上变薄，与厚度为1.44公里的洋壳并列。这种减薄主要发生在下地壳。由于莫霍面反射没有被广泛观测到，7.0 km s− 1的速度等值线被用来定义莫霍面沿剖面。SITF北部明显缺乏反射，这表明莫霍面更可能是地壳和地幔之间的过渡带。在剖面穿过离轴洋壳的水深高点处，在地壳浅层观察到较高的P波速度。S波到达模型也显示在浅层速度升高，除了邻近SITF的地壳，当在轴上时，该地壳将占据脊转换交叉点的内角高。我们使用1.9的Vp/Vs比值来标记下地壳和上地幔的岩石可能被折返的位置，并定位上地壳到下地壳的过渡，识别残留的洋核杂岩和岩浆形成的地壳区域。Vp/Vs比值的升高不仅表明蛇纹化橄榄岩可能在某些地方出露于海底，而且表明海水能够在20-30 km宽的区域内流入地壳和上地幔深处，这可能解释了为什么没有明显的莫霍面。SITF在较浅深度处的速度比在北部洋壳中观察到的速度高，在海底，它是一个相对较宽的特征。然而，海底速度-深度模型表明，一个断层带的宽度不超过1500 -10公里，与之对应的是一个7500米深的狭窄海底凹陷。重力模拟显示，SITF也位于海底2公里以上的地方，有一个20公里宽、密度大于3000 kg m− 3的区域，可能反映了一个广泛的变质区域。与水深测量相比，整个调查区域的剩余地幔布格异常表明，这种转换也可能有一个左横向跨张位移的组成部分，这可以解释其明显的宽海底外观，我们的研究结果表明，天鹅岛边缘的发展导致了邻近大陆的减薄，这是由于地幔上涌减少和（或）断裂加强了流体流动。
The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. TheCAYSEISexperiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north.Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle.Where the profile traverses bathymetric highs in the off-axis oceanic crust, higherP-wave velocity is observed at shallow crustal depths.S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use aVp/Vsratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevatedVp/Vsratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho.The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at >2 km subseabed, by a ∼20-km-wide region of density >3000 kg m−3that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre.Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.