海洋核杂岩等概念

海洋核杂岩

海洋核杂岩常出现在慢速扩张脊的附近,如大西洋,其形成与洋中脊的区域性扩张过程有关,有些海洋核杂岩的形成可能受到热点或地幔柱的影响。海洋核杂岩的成因机制复杂,除了受拆离断层的控制外,还受正断层附近下盘的抬升作用和先存洋壳结构的制约。拆离作用持续时限还可能受洋中脊岩浆供应模式的制约,从而决定海洋核杂岩的寿命与演化。[1]海洋核杂岩是一种海底的地质特征[2]，是大位移，低角度正断层（拆离断层）抬升的下盘（图1）[3]，形成了垂直于洋中脊的长脊。它包括平滑的圆顶，其上分布有横的山脊，像条纹状的屋顶。其大小不等，范围为长10-150km,宽5-15km, 高500-1500m,如图1，2。[2,3,4]。通常发育在慢速扩展的洋壳边界，岩浆供应受限。这个区域具有低的上地幔温度，且长的转换断层发育，下地壳和地幔岩出露于海底[5]。

洋底深地震剖面上海洋核杂岩结构表现为3部分,第1部分为层1和层2,以脆性变形为特征；第2部分为脆-韧性过渡层；第三部分，核部以塑性变形为特征。第1部分的层1为透镜-断块状构造,主要由火山成因的沉积和在裂谷作用下依次旋回期间溢出海底的熔岩流组成。它以脆性断裂变形为特征。层2为断块状岩石,岩体主要为辉绿岩墙群,是产生于裂谷带的深源侵入体,具有侵入物质冷却时产生的垂直裂缝和垂向脆性断裂，层2与下部洋幔之间为第2部分,相当于拆离断层带。第3部分为海洋核杂岩的核部,属于蛇纹石化的洋幔。[1]

图 1.海洋核杂岩的产生过程.左侧：解释观测到海底的地质现象，红色区域代表新火山活动区域，蓝色代表拆离断层的出现，黑色线代表其他的一些正断层。中间：板块构造的演化过程。右侧：左侧图表的垂直剖面。[3](a) time=0. Initial regime with straight ridge axis and symmetrical magmatic accretion. Bold red line in centre panel is the plate boundary. Black, blue and green lines show successively older isochrons. (b) Shortly after time=0. At the site of the future OCC, magmatic accretion wanes

and main locus of plate separation (i.e., plate boundary) jumps west to a major valley-wall fault (solid blue line with black ticks), here taken arbitrarily to be at the black isochron, transferring a small piece of (grey) seafloor from the western to the eastern plate. To the N and S, symmetrical magmatic accretion continues. Accommodation zones (similar to non-transform offsets) link the offset plate boundary (blue dotted lines). (c) time=0.5 (where 1 is the duration of the coloured band in (a)). Symmetrical magmatic accretion continues to the N and S. At the site of the OCC, plate separation is focused onto the detachment by strain weakening. For illustrative purposes in this cartoon TOCC=1; if 0.5

图2 海洋核杂岩的地貌形态[4]

参考文献:

[1]李三忠,吕海青,侯方辉,等.海洋核杂岩.海洋地质与第四纪地质.2006,26（1）：47-52.

[2]维基百科

[3] C.J.MacLeod, R.C.Searle ,B.J.Murton,etc.Life cycle of oceanic core complexes. Earth and Planetary Science Letters. 2009,287:333–344.

[4] A.Morris, J.S.Gee, N.Pressling,ect. Footwall rotation in an oceanic core complex quanti?ed using reoriented Integrated Ocean Drilling Program core samples. Earth and Planetary Science Letters. 2009,287:217–228.

[5] Alain Tremblay ，Avni Meshi，Jean H. Bédard.Oceanic core complexes and ancient oceanic