超慢速扩张洋中脊NTD的蛇纹石化地幔:海底广角地震探测
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摘要
非转换断层不连续(NTD)在超慢速扩张洋中脊高频度出现,其地壳速度结构与相邻的岩浆扩张中心(NVR)差别明显.结合表层断层多和减薄的地壳,说明NTD在超慢速洋中脊扩张理论、热液活动甚至矿产资源方面有十分重要的意义.本文使用2条海底广角地震测线的数据,使用射线追踪正反演的方法,获得了西南印度洋中脊28~29扩张段之间NTD下方的地壳及上地幔速度结构.结果表明:(1)NTD下方地壳较薄(3.2~4.5 km),洋壳层2厚约2.0 km,洋壳层3较薄(1.0 km)甚至缺失;(2)洋壳层2表层速度横向差异大,表明存在较多断裂;(3)洋壳层3出现低速区,成因可能是受浅部大断裂或地幔蛇纹石化作用影响;(4)上地幔顶部速度存在低速异常(7.2 km/s),结合NTD地壳较薄(<5 km)和表层断裂多的特征,认为是上地幔发生大规模蛇纹石化作用导致低速,为进一步理解和区分莫霍面与蛇纹石化前缘(Serpentinization Front)提供了一个很好的例证.
Non-transform discontinuities(NTDs) are frequently found on ultra-slow spreading ridges. They have crustal structures that differ significantly from those of neighboring neo-volcanic ridges(NVRs). For an ultra-slow spreading ridge with weakened lithosphere and thinned crust, NTDs play an important role in understanding the mechanisms of ridge spreading, hydrothermal venting and mineral resource deposition. In this paper, we present an extensive ocean bottom seismometer(OBS) experiment that was carried out in 2010 at the Southwest Indian Ridge(SWIR) during R/V Dayang Yihao cruise DY115-21. The seismic source was a four-air-gun array with a total volume of or 98.32 L(6000 cubic inches) and a seismic trace spacing of ~250 m. A total of 10832 shots were fired and 52 survey lines(about 2650 km in length) were acquired. Thirty-eight OBSs, each containing a three-component geophone and a hydrophone, successfully recovered information from the crust and upper mantle in NVR and NTD settings. In this paper, we use two wide-angle seismic profiles and a ray-tracing method to constrain the velocity structure of the crust and upper mantle of the NTD between segments 28 and 29 of the SWIR. The processing methods include relocations of shot and OBS positions, time drift corrections and filtering. The widely used ray-theoretical travel-time inversion codes of Zelt and Smith(1992) were used for the modeling. The method includes a damped least square inversion of picked travel times, and we computed the statistical misfit, resolution and uncertainty of model parameters to ensure that the final velocity model had the best fit to our data. The main conclusions are as follows:(1) NTD has a thinner crust(3.2–4.5 km), with an oceanic layer 2 that is 2 km thick and an oceanic layer 3 that is very thin(<1 km) or absent.(2) Oceanic layer 2 has significant lateral velocity variation, indicating the presence of various faults.(3) The uppermost mantle has low velocities(7.2 km/s), indicative of large and deep faults or the presence of serpentinization. We suggest that the low velocity zone in the upmost mantle, together with the thinner crust(<5 km) of the NTD and the presence of faults, result from large-scale serpentinization of the uppermost mantle, providing a good example for investigations of the difference between the Moho discontinuity and a serpentinization front.
Non-transform discontinuities(NTDs) are frequently found on ultra-slow spreading ridges. They have crustal structures that differ significantly from those of neighboring neo-volcanic ridges(NVRs). For an ultra-slow spreading ridge with weakened lithosphere and thinned crust, NTDs play an important role in understanding the mechanisms of ridge spreading, hydrothermal venting and mineral resource deposition. In this paper, we present an extensive ocean bottom seismometer(OBS) experiment that was carried out in 2010 at the Southwest Indian Ridge(SWIR) during R/V Dayang Yihao cruise DY115-21. The seismic source was a four-air-gun array with a total volume of or 98.32 L(6000 cubic inches) and a seismic trace spacing of ~250 m. A total of 10832 shots were fired and 52 survey lines(about 2650 km in length) were acquired. Thirty-eight OBSs, each containing a three-component geophone and a hydrophone, successfully recovered information from the crust and upper mantle in NVR and NTD settings. In this paper, we use two wide-angle seismic profiles and a ray-tracing method to constrain the velocity structure of the crust and upper mantle of the NTD between segments 28 and 29 of the SWIR. The processing methods include relocations of shot and OBS positions, time drift corrections and filtering. The widely used ray-theoretical travel-time inversion codes of Zelt and Smith(1992) were used for the modeling. The method includes a damped least square inversion of picked travel times, and we computed the statistical misfit, resolution and uncertainty of model parameters to ensure that the final velocity model had the best fit to our data. The main conclusions are as follows:(1) NTD has a thinner crust(3.2–4.5 km), with an oceanic layer 2 that is 2 km thick and an oceanic layer 3 that is very thin(<1 km) or absent.(2) Oceanic layer 2 has significant lateral velocity variation, indicating the presence of various faults.(3) The uppermost mantle has low velocities(7.2 km/s), indicative of large and deep faults or the presence of serpentinization. We suggest that the low velocity zone in the upmost mantle, together with the thinner crust(<5 km) of the NTD and the presence of faults, result from large-scale serpentinization of the uppermost mantle, providing a good example for investigations of the difference between the Moho discontinuity and a serpentinization front.
引文
1 Cannat M,Rommevaux-Jestin C,Sauter D,et al.Formation of the axial relief at the very slow spreading Southwest Indian Ridge(49°to69 °E).Geophys Res,1999,104:2825–2843
2 Sauter D,Sloan H,Cannat M,et al.From slow to ultra-slow:How does spreading rate affect seafloor roughness and crustal thickness?Geology,2011,39:911–914
3 Minshull T,Muller M,White R.Crustal structure of the Southwest Indian Ridge at 66°E:Seismic constraints.Geophys J Int,2006,166:135 –147
4 Zhao M H,Qiu X L,Li J B,et al.Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge(49°39′E).Geochem Geophys Geosyst,2013,14:4544–4563
5 Sauter D,Cannat M.The Ultraslow spreading Southwest Indian Ridge Diversity of hydrothermal systems on slow spreading ocean ridges.Geophys Monogr Ser,2010,188:153–173
6 Dick H,Lin J,Schouten H.An ultraslow-spreading class of ocean ridge.Nature,2003,426:405–412
7 Tao C,Lin J,Guo S,et al.First active hydrothermal vents on an ultraslow-spreading center:Southwest Indian Ridge.Geology,2012,40:47 –50
8 Tao C H,Li H M,Jin X B,et al.Seafloor hydrothermal activity and polymetallic sulfide exploration on the southwest Indian ridge.Chin Sci Bull,2014,59:2266–2276[陶春辉,李怀明,金肖兵,等.西南印度洋脊的海底热液活动和硫化物勘探.科学通报,2014,59:1812–1822]
9 Tao C H,Li H M,Huang W,et al.Mineralogical and geochemical features of sulfide chimney from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences.Chin Sci Bull,2011,56:2413–2423[陶春辉,李怀明,黄威,等.西南印度洋脊49°39′E热液区硫化物烟囱体的矿物学和地球化学特征及其地质意义.科学通报,2011,56:2413–2423]
10 Tyler S,Bull J,Parson L,et al.Numerical modelling of non-transform discontinuity geometry:Implications for ridge structure,volcanotectonic fabric development and hydrothermal activity at segment ends.Earth Planet Sci Lett,2007,157:146–159
11 Escartín J,Hirth G,Evans B.Effects of serpentinization on the lithospheric strength and the style of normal faulting at slow-spreading ridges.Earth Planet Sci Lett,1997,151:181–189
12 Minshull T,Muller M,Robinson C,et al.Is the oceanic Moho a serpentinisation front?In:Mills R A,Harrison K,eds.Modern Ocean Floor Processes and the Geological Record.London:Geological Society,London Special Publications,1998.148:71–80
13 Tao C,Lin J,Guo S,et al.Discovery of the first active hydro-thermal vent field at the ultraslow spreading Southwest Indian Ridge:The Chinese DY115-19 Cruise.Ridge Crest News,2007,16:25–26
14 Sauter D,Patriat P,Rommevaux-Jestin C,et al.The Southwest Indian Ridge between 49°15′E and 57°E:Focused accretion and magma redistribution.Earth Planet Sci Lett,2001,192:303–317
15 Li J B,Chen Y J.First Chinese OBS experiment at Southwest Indian Ridge.Inter Ridge News,2010,19:16–28
16 Ruan A G,Li J B,Chen Y J,et al.The experiment of broad band I-4C type OBS in the Southwest Indian Ridge.Chin J Geophys(in Chinese),2010,53:1015–1018[阮爱国,李家彪,陈永顺,等.国产I-4C型OBS在西南印度洋中脊的试验.地球物理学报,2010,53:1015–1018]
17 Ao W,Zhao M H,Qiu X L,et al.The correction of shot and OBS position in the 3D seismic experiment of the SW Indian Ocean Ridge.Chin J Geophys(in Chinese),2010,53:2982–2991[敖威,赵明辉,丘学林,等.西南印度洋中脊三维地震探测实验中炮点与海底地震仪的位置校正.地球物理学报,2010,53:2982–2991]
18 Xue B,Ruan A G,Li X Y,et al.The seismic data corrections of short period auto-floating ocean bottom seismometer.J Marine Sci(in Chinese),2008,26:98–102[薛彬,阮爱国,李湘云,等.SEDISⅣ型短周期自浮式海底地震仪数据校正方法.海洋学研究,2008,26 :98–102]
19 Niu X W,Ruan A G,Wu Z L,et al.Progress on practical skills of Ocean Bottom Seismometer(OBS)experiment.Progr Geophys(in Chinese),2014,29:1418–1425[牛雄伟,阮爱国,吴振利,等.海底地震仪实用技术探讨.地球物理学进展,2014,29:1418–1425]
20 Zelt C,Smith R.Seismic traveltime inversion for 2D crustal velocity structure.Geophys J Int,1992,99:16–34
21 Kennett J.Marine Geology.New Jersey:Prentice Hall,Englewood Cliffs,1982
22 White R,Mc Kenzie D,O’Nions R.Oceanic crustal thickness from seismic measurements and rare earth element inversions.J Geophys Res,1992,97:19683–19715
23 Muller M,Minshull T,White R.Segmentation and melt supply on the Southwest Indian Ridge.Geology,1999,27:867–870
24 Muller M,Minshull T,White R.Crustal structure of the South West Indian Ridge at the Atlantis II Fracture Zone.J Geophys Res,2000,105 (B11):25809–25828
25 Muller M,Robinson C,Minshull T,et al.Thin crust beneath Ocean Drilling Program borehole 735B at the Southwest Indian Ridge?Earth Planet Sci Lett,1997,148:93–107
26 Zelt C.Modelling strategies and model assessment for wide-angle seismic travel time data.Geophys J Int,1999,139:183–204
27 Horen H,Zamora M,Dubuisson G.Seismic waves velocities and anisotropy in serpentinized peridotites from Xigaze ophiolite:Abundance of serpentine in slow spreading ridge.Geophys Res,1996,23:9–12
28 White R,Minshull T,Bickle M,et al.Melt generation at very slow-spreading oceanic ridges:Constraints from geochemical and geophysical data.J Petrol,2001,42:1171–1196
29 Forsyth D.Finite extension and low-angle normal faulting.Geology,1992,20:27–30
30 Tucholke B,Sibuet J,Klaus A.Proceedings of the Ocean 1000 Drilling Program,Initial Reports.Ocean Drilling Program,College 1001Station,TX,2004,210
31 Bach W,Banerjee N,Dick H,et al.Discovery of ancient and active hydrothermal systems along the ultra-slow spreading Southwest Indian Ridge 10°–16°E.Geochem Geophys Geosyst,2002,3:1044
32 Wessel P,Smith W.New version of generic mapping tools released.Eos Trans AGU,1995,76:329
2 Sauter D,Sloan H,Cannat M,et al.From slow to ultra-slow:How does spreading rate affect seafloor roughness and crustal thickness?Geology,2011,39:911–914
3 Minshull T,Muller M,White R.Crustal structure of the Southwest Indian Ridge at 66°E:Seismic constraints.Geophys J Int,2006,166:135 –147
4 Zhao M H,Qiu X L,Li J B,et al.Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge(49°39′E).Geochem Geophys Geosyst,2013,14:4544–4563
5 Sauter D,Cannat M.The Ultraslow spreading Southwest Indian Ridge Diversity of hydrothermal systems on slow spreading ocean ridges.Geophys Monogr Ser,2010,188:153–173
6 Dick H,Lin J,Schouten H.An ultraslow-spreading class of ocean ridge.Nature,2003,426:405–412
7 Tao C,Lin J,Guo S,et al.First active hydrothermal vents on an ultraslow-spreading center:Southwest Indian Ridge.Geology,2012,40:47 –50
8 Tao C H,Li H M,Jin X B,et al.Seafloor hydrothermal activity and polymetallic sulfide exploration on the southwest Indian ridge.Chin Sci Bull,2014,59:2266–2276[陶春辉,李怀明,金肖兵,等.西南印度洋脊的海底热液活动和硫化物勘探.科学通报,2014,59:1812–1822]
9 Tao C H,Li H M,Huang W,et al.Mineralogical and geochemical features of sulfide chimney from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences.Chin Sci Bull,2011,56:2413–2423[陶春辉,李怀明,黄威,等.西南印度洋脊49°39′E热液区硫化物烟囱体的矿物学和地球化学特征及其地质意义.科学通报,2011,56:2413–2423]
10 Tyler S,Bull J,Parson L,et al.Numerical modelling of non-transform discontinuity geometry:Implications for ridge structure,volcanotectonic fabric development and hydrothermal activity at segment ends.Earth Planet Sci Lett,2007,157:146–159
11 Escartín J,Hirth G,Evans B.Effects of serpentinization on the lithospheric strength and the style of normal faulting at slow-spreading ridges.Earth Planet Sci Lett,1997,151:181–189
12 Minshull T,Muller M,Robinson C,et al.Is the oceanic Moho a serpentinisation front?In:Mills R A,Harrison K,eds.Modern Ocean Floor Processes and the Geological Record.London:Geological Society,London Special Publications,1998.148:71–80
13 Tao C,Lin J,Guo S,et al.Discovery of the first active hydro-thermal vent field at the ultraslow spreading Southwest Indian Ridge:The Chinese DY115-19 Cruise.Ridge Crest News,2007,16:25–26
14 Sauter D,Patriat P,Rommevaux-Jestin C,et al.The Southwest Indian Ridge between 49°15′E and 57°E:Focused accretion and magma redistribution.Earth Planet Sci Lett,2001,192:303–317
15 Li J B,Chen Y J.First Chinese OBS experiment at Southwest Indian Ridge.Inter Ridge News,2010,19:16–28
16 Ruan A G,Li J B,Chen Y J,et al.The experiment of broad band I-4C type OBS in the Southwest Indian Ridge.Chin J Geophys(in Chinese),2010,53:1015–1018[阮爱国,李家彪,陈永顺,等.国产I-4C型OBS在西南印度洋中脊的试验.地球物理学报,2010,53:1015–1018]
17 Ao W,Zhao M H,Qiu X L,et al.The correction of shot and OBS position in the 3D seismic experiment of the SW Indian Ocean Ridge.Chin J Geophys(in Chinese),2010,53:2982–2991[敖威,赵明辉,丘学林,等.西南印度洋中脊三维地震探测实验中炮点与海底地震仪的位置校正.地球物理学报,2010,53:2982–2991]
18 Xue B,Ruan A G,Li X Y,et al.The seismic data corrections of short period auto-floating ocean bottom seismometer.J Marine Sci(in Chinese),2008,26:98–102[薛彬,阮爱国,李湘云,等.SEDISⅣ型短周期自浮式海底地震仪数据校正方法.海洋学研究,2008,26 :98–102]
19 Niu X W,Ruan A G,Wu Z L,et al.Progress on practical skills of Ocean Bottom Seismometer(OBS)experiment.Progr Geophys(in Chinese),2014,29:1418–1425[牛雄伟,阮爱国,吴振利,等.海底地震仪实用技术探讨.地球物理学进展,2014,29:1418–1425]
20 Zelt C,Smith R.Seismic traveltime inversion for 2D crustal velocity structure.Geophys J Int,1992,99:16–34
21 Kennett J.Marine Geology.New Jersey:Prentice Hall,Englewood Cliffs,1982
22 White R,Mc Kenzie D,O’Nions R.Oceanic crustal thickness from seismic measurements and rare earth element inversions.J Geophys Res,1992,97:19683–19715
23 Muller M,Minshull T,White R.Segmentation and melt supply on the Southwest Indian Ridge.Geology,1999,27:867–870
24 Muller M,Minshull T,White R.Crustal structure of the South West Indian Ridge at the Atlantis II Fracture Zone.J Geophys Res,2000,105 (B11):25809–25828
25 Muller M,Robinson C,Minshull T,et al.Thin crust beneath Ocean Drilling Program borehole 735B at the Southwest Indian Ridge?Earth Planet Sci Lett,1997,148:93–107
26 Zelt C.Modelling strategies and model assessment for wide-angle seismic travel time data.Geophys J Int,1999,139:183–204
27 Horen H,Zamora M,Dubuisson G.Seismic waves velocities and anisotropy in serpentinized peridotites from Xigaze ophiolite:Abundance of serpentine in slow spreading ridge.Geophys Res,1996,23:9–12
28 White R,Minshull T,Bickle M,et al.Melt generation at very slow-spreading oceanic ridges:Constraints from geochemical and geophysical data.J Petrol,2001,42:1171–1196
29 Forsyth D.Finite extension and low-angle normal faulting.Geology,1992,20:27–30
30 Tucholke B,Sibuet J,Klaus A.Proceedings of the Ocean 1000 Drilling Program,Initial Reports.Ocean Drilling Program,College 1001Station,TX,2004,210
31 Bach W,Banerjee N,Dick H,et al.Discovery of ancient and active hydrothermal systems along the ultra-slow spreading Southwest Indian Ridge 10°–16°E.Geochem Geophys Geosyst,2002,3:1044
32 Wessel P,Smith W.New version of generic mapping tools released.Eos Trans AGU,1995,76:329
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