2. 自然资源部第二海洋研究所, 自然资源部海底科学重点实验室, 杭州 310012;
3. National Institute of Oceanography, Karachi, Pakistan
2. Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China;
3. National Institute of Oceanography, Karachi, Pakistan
大陆裂解和洋壳生成是板块构造动力学的基本问题,洋中脊是板块分离与洋壳增生的发源地,也是地球上最活跃的板块边界类型之一,全球每年都有大量的新洋壳沿着长约65000 km的洋中脊体系产生(Morgan,1972;Macdonald,1982;Dick et al., 2003;李江海等,2019).洋壳的结构及其增生过程是认识深部岩石圈形成演化的窗口,不同时期不同条件下洋中脊处形成的洋壳结构,反映着洋中脊向两侧扩张的大洋岩石圈动力学演化过程(Morgan et al., 1987;Sempere and Macdonald, 1987;Lin and Morgan, 1992;Morgan and Chen, 1993;李三忠等,2015).
前人大量研究表明,洋中脊的扩张速率和岩浆供给率直接控制洋中脊形态,尤其是中央隆起区的发展(Macdonald,1982;Chen and Morgan, 1990;Morgan and Chen, 1993),但洋中脊的扩张速率与洋壳结构(主要由厚度和地震波速度表达)的关系尚未有统一认识(Goslin et al., 1972;Woollard,1975;Reid and Jackson, 1981;Chen,1992;Niu and Hékinian,1997).Reid和Jackson(1981)通过汇编不同扩张速率洋中脊的洋壳厚度,提出洋中脊扩张速率越慢,其洋壳厚度越薄.Chen(1992)基于1970—1990年间准确的洋壳厚度数据分析,认为洋壳厚度和洋中脊扩张速率并没有直接的关系,但慢速扩张洋中脊比快速扩张洋中脊有着更大的洋壳厚度变化范围.大量海洋深地震探测试验表明,洋中脊具有分层的地壳结构,由较高速度梯度的洋壳层2(简称层2)和较低速度梯度的洋壳层3(简称层3)组成(Spudich and Orcutt, 1980;Smewing,1981),但不同扩张速率下洋中脊分层结构有何异同尚不明确.此外,通过分析洋壳厚度与洋壳年龄的关系,发现洋壳厚度随年龄增长而变大,但增大的幅值及机制尚未有定论(McClain and Atallah, 1986;White et al., 1992).
本文筛选并统计了全球海域最新的正常洋壳1-D地震波速度结构剖面(图 1),通过分类汇编较新较全的海底地震广角折射剖面数据,探索洋壳厚度与波速结构及年龄等之间的关系,以期对上述问题有新的认识和发现.
在前人完成的大量海洋折射地震探测剖面的基础上,统计并筛选出全球海域范围的152处广角折射地震剖面的1-D速度结构(表 1,2),筛选的方式主要依据以下5个原则:
(1) 选取1975年之后使用多层模型射线追踪方法获取的速度结构,该方法比在此之前使用的斜率-截距法更为准确;
(2) 选取提供了1-D速度曲线的结果,以便构制准确的数字化速度结构;
(3) 只选取未明显受大型断裂构造带和热点影响的速度结构;
(4) 对同一地震剖面选取有差异的代表性1-D速度结构;
(5) 对于扩张中心,选取洋中脊轴部或靠近轴部区域的1-D速度结构.
对搜集并筛选出的所需数据进行了分类汇编.首先按地震测线所在区域类型将所有1-D速度结构分为洋中脊和非洋中脊两大类(表 1, 2),再对位于洋中脊处的地震测线按洋中脊的扩张速率大小:快速(全扩张速率> 90 mm·a-1)、慢速(全扩张速率20~50 mm·a-1)和超慢速(全扩张速率 < 20 mm·a-1)(Macdonald,1982;Dick et al., 2003;表 1)进行分类汇总.然后,将分类汇编的1-D速度结构进行数字化投影,将同类速度结构按照0.1 km厚度间距进行插值处理并求取平均值,得到不同类别洋壳的平均1-D速度结构.最后,通过对比不同扩张速率洋中脊洋壳的平均1-D速度结构,找出其中的异同和规律,再对比洋中脊和非洋中脊的平均1-D速度结构,探讨它们之间的差异性.
2 结果 2.1 快速扩张洋中脊本文整理分析的快速扩张洋中脊的1-D速度结构主要来自胡安德富卡脊(JDFR)和东太平洋海隆(EPR),考虑构造上的连续性,本文将中速扩张的JDFR和快速扩张的EPR一并讨论,统称为快速扩张洋中脊.由所选取的1-D速度结构曲线(图 2)可知,快速扩张洋中脊的地壳厚度变化范围为约4.9~8.1 km,平均厚度为~6.4 km,洋壳速度从浅部向深部递增,从层2顶部到层3底部变化范围为3.4~7.4 km·s-1.其中,洋壳层2平均厚度为~2.2 km,速度从顶部到底部变化范围为3.4~6.4 km·s-1,速度梯度~1.4/s;洋壳层3平均厚度为~4.2 km,地壳速度从顶部到底部变化范围为6.4~7.4 km·s-1,速度梯度为~0.2/s.
大西洋中脊(MAR)为慢速扩张洋中脊的典型代表,本文分析了MAR的部分正常洋壳的1-D速度结构,以代表慢速扩张洋中脊的地壳速度结构特征.由图 3可知,慢速扩张洋中脊的地壳厚度变化范围约为4.6~8.7 km,平均厚度为~7.2 km,洋壳速度从浅部向深部递增,从层2顶部到层3底部变化范围为3.5~7.3 km·s-1.其中,洋壳层2平均厚度为~2.1 km,速度从顶部到底部变化范围为3.5~6.3 km·s-1,速度梯度~1.3/s;洋壳层3平均厚度为~5.1 km,速度从顶部到底部变化范围为6.3~7.3 km·s-1,速度梯度为~0.2/s.
超慢速扩张洋中脊的速度结构主要选自西南印度洋中脊(SWIR)50°S、57°S和66°S以及莫恩洋中脊(MR)、克尼波维奇洋中脊(KR)和加克洋中脊(GR)的部分区域(图 4),其地壳厚度变化范围为4.2~10.2 km,平均厚度为~5.3 km,洋壳速度从浅部向深部递增,从层2顶部到层3底部变化范围为3.2~6.9 km·s-1.其中,洋壳层2平均厚度为~2.1 km,速度从顶部到底部变化范围为3.2~6.1 km·s-1,速度梯度为~1.8/s;洋壳层3平均厚度为~3.2 km,速度从顶部到底部变化范围为6.1~6.9 km·s-1,速度梯度为~0.2/s.
对比上述不同扩张速率洋中脊的平均1-D速度结构可知(图 2—4),洋中脊洋壳层2厚度基本相似(2.1~2.2 km),洋壳厚度差异主要表现在洋壳层3(3.2~5.1 km),推测洋中脊扩张速率直接影响熔融地幔上涌过程中喷出层之下的剩余黏附岩浆量(Lissenberg and Dick, 2008).此外,洋中脊洋壳层3速度梯度相近(~0.2/s),洋壳速度结构差异主要来自更浅部的洋壳层2(1.3~1.8/s),这与不同扩张速率下洋中脊地幔熔融程度及构造作用有关(Niu and Hékinian,1997).
3 讨论 3.1 洋中脊扩张速率与洋壳厚度的关系根据统计得到的快速、慢速和超慢速扩张脊的洋壳厚度变化范围分别为4.9~8.1 km、4.6~8.7 km和4.2~10.2 km,也即随着洋中脊扩张速率的减小,洋壳厚度的变化范围逐渐增大,这与前人通过统计1970—1990年间的地震测线数据得到的结果相似(Chen,1992).快速扩张洋中脊的岩石类型以玄武岩为主,只出现少量的辉长岩,这是因为其有充足的岩浆供给,使大量的玄武岩在洋底喷出,覆盖了洋壳层3的辉长岩和地幔橄榄岩.快速扩张洋中脊扩张过程中能够满足岩浆收支平衡,即岩浆上涌形成的新洋壳跟得上板块分离过程,造成其扩张以洋壳增生方式为主,构造拉张量有限,转换断层和拆离断层等构造断裂带较少,因此其洋壳厚度变化范围较小.而慢速扩张洋中脊由于岩浆房小,岩浆供给不足,地幔熔岩流向洋中脊段中心汇聚(Whitehead et al., 1984;Sparks et al., 1993;Rabinowicz et al., 1993;Magde et al., 1997),使得洋中脊段中部的岩石圈厚度更薄、温度更高,而其末端以构造伸展作用为主,形成大量的转换断层及拆离断层等构造活动带(Macdonald et al., 1988, 1991).构造拉张作用使洋壳破裂断开,深部物质直接出露于洋底,且非岩浆扩张段也为地幔物质的出露提供了有利条件(Lissenberg and Dick, 2008;牛雄伟等,2015),因此慢速扩张洋中脊的洋壳厚度变化范围较大.超慢速扩张洋中脊岩浆供应量少,熔融物质趋向于向洋中脊段中心聚集,这种情况比慢速扩张洋中脊更加突出,表现为一个个不连续的规模更大的岩浆增生段,而各岩浆增生段之间的洋中脊段岩浆活动贫乏甚至缺失,以构造作用为主,形成大量非转换断层不连续带(Dick et al., 2003;赵明辉等,2010;Zhao et al., 2013;Li et al., 2015;Niu et al., 2015;牛雄伟等,2015;王伟等,2018),由此造成洋壳厚度范围进一步增大.由于洋中脊形成过程中岩浆与构造活动相互作用,导致靠近断裂带的洋壳厚度明显减薄(White et al., 1992),从而造成洋中脊轴部的洋壳厚度变化范围随洋中脊扩张速率的减小而增大(Chen,1992).
3.2 洋中脊扩张速率与洋壳速度结构的关系不同扩张速率下的洋中脊处的洋壳速度也有明显的差异.由图 5可知,快速扩张洋中脊的洋壳速度明显大于慢速和超慢速扩张洋中脊.洋中脊下的熔岩流控制着洋壳生成的物质来源,所以,可以通过研究洋中脊处的岩石地球化学性质来推测洋中脊的动力学过程(Klein and Langmuir, 1987;Niu and Batiza, 1991;Langmuir et al., 1992;Sinton and Detrick, 1992;Niu and Hékinian,1997;Langmuir and Forsyth, 2007).从洋壳形成演化过程来看,洋中脊两侧板块相互分离,造成深部地幔减压熔融,从地幔岩石中萃取分离的熔融物质向上迁移并冷却结晶形成玄武质洋壳,因此洋中脊玄武岩包含了大量的关于熔体组成、熔体迁移以及地幔不均一性等信息(Langmuir et al., 1992;Langmuir and Forsyth, 2007).通常情况下快速扩张洋中脊下熔融岩浆房中的岩浆分异产生低MgO和低结晶度岩浆,而慢速扩张洋中脊下则表现为岩浆供应不足、高MgO以及结晶度高等特征(Sinton and Detrick, 1992).据此,得到快速扩张洋中脊相对慢速扩张洋中脊处形成的洋壳应该具有更小的密度和速度,这与本文通过汇编洋壳地震波速度结构得到的结果有所差异.玄武质岩浆在迁移过程中的分离结晶作用受控于原始岩浆的组分结构,前人主要通过两种方式获得相对原始的母岩浆,一种是将具有同一构造背景的玄武质岩浆中分异程度最低的玄武岩组成作为其他岩浆组分潜在的母岩浆,另一种方法是通过矿物斑晶内的熔融包裹体成分获得母岩浆的组成(Dungan and Rhodes, 1978;Kress and Ghiorso, 2004).但由于岩浆分异作用的普遍存在,简单地从喷出玄武岩的矿物组分推测得到的原始岩浆未必能严格代表真正处于矿物相平衡的上地幔原始岩浆(Michael and Chase, 1987;Falloon et al., 2007;Herzberg et al., 2007).因此前人通过现有岩石地球化学分析得到的洋中脊洋壳深部结构可能有一定的误差.我们推测本文得到的结果可能与快、慢速扩张脊洋壳生成过程的动力学演化机制不同有关.快速扩张脊下的岩浆房深度浅,岩浆供应持续稳定且熔融程度高(Niu and Hékinian,1997;Carbotte et al., 1998;Dunn and Forsyth, 2003),其扩张机制以岩浆作用为主.快速扩张脊较高熔融度的岩浆处在压力较小的环境下,岩浆黏度小,上涌速度快,容易携带更深部的密度较大的熔融地幔物质,地幔熔融岩浆上涌到洋壳部位后散热较快,且结晶分异形成的重物质可能未及时下沉,随上涌岩浆流上升冷却形成密度大、地震波速快的洋壳.而慢速扩张脊下的岩浆房深度较大,岩浆供应不足导致熔融程度低、黏度大且上涌速度慢,发育大量转换断层和拆离断层(Macdonald et al., 1988, 1991),其扩张以构造作用为主,受海水渗透作用更大,最终上涌到海底形成的洋壳密度会相对较小、速度较慢.
所有海底地震测线按洋中脊和非洋中脊两类汇总表明(图 6),洋中脊区域平均地壳厚度(~6.3 km)小于非洋中脊区域(~6.6 km),其主要差别在于非洋中脊区域的洋壳层3平均厚度(~4.6 km)要大于洋中脊区域(~4.2 km),而两者的洋壳层2平均厚度则非常接近.洋中脊洋壳层2速度梯度大于非洋中脊区,而洋壳层3速度梯度相近.此外,洋中脊的洋壳厚度变化范围明显小于非洋中脊,这可能与搜集到的非洋中脊区域的洋壳年龄变化范围较大有关.
长期以来有不少学者认为洋壳厚度随年龄增长而变大(McClain and Atallah, 1986;White et al., 1992).即使除去异常区域(如断裂带、火山作用等区域),这种关系仍然存在,只是地壳厚度变大的幅度减小了而已. McClain和Atallah(1986)通过分析太平洋100个地震折射剖面数据,认为太平洋30~100 Ma的洋壳比 < 30 Ma的洋壳增厚~0.34 km. White等(1992)通过汇编大量的大西洋综合地震探测剖面数据,发现大西洋的正常洋壳厚度随年龄的变化关系与太平洋有着相似的规律:大西洋30~100 Ma的洋壳比 < 30 Ma的洋壳增厚~0.6 km.尽管由于地震剖面数量有限导致上述研究的数据不能严格地说明洋壳厚度随年龄的变化关系,但两个洋区的对比结果均符合随着地壳年龄的增长,洋壳每100 Ma增厚0.4~0.5 km这一规律.此外,White等(1992)还得到在同一年龄段内,慢速扩张洋中脊比快速扩张洋中脊在远离断裂带处的正常洋壳要增厚~0.5 km.
本文汇编的不同扩张速率洋中脊处的地壳速度结构整体在10 Ma以内,而非洋中脊区域的地壳年龄均大于10 Ma.由此我们得到全球大于10 Ma的洋壳比10 Ma以内的洋壳厚约0.3 km.这一结果虽然比前人得到的值要小,可能与统计数据中去除异常区域(断裂带、火山作用区)有关,但总体上结果相近.
4 结论本文使用统计分析、求取平均值等方法,对挑选的全球海域49处洋中脊和103处非洋中脊的洋壳1-D地震波速度结构剖面进行对比研究,获得了关于扩张速率与洋中脊洋壳速度结构的认识如下:
(1) 快速、慢速和超慢速扩张洋中脊的正常洋壳平均厚度分别为6.4 km、7.2 km和5.3 km,但其厚度变化范围不一,分别为4.9~8.1 km、4.6~8.7 km和4.2~10.2 km.洋中脊洋壳层2厚度基本相似(2.1~2.2 km),与扩张速率无关,洋壳厚度差异主要源自洋壳层3(3.2~5.1 km),推测洋中脊扩张速率会直接影响熔融地幔上涌过程中喷出层之下的剩余黏附岩浆量.此外,随着洋中脊扩张速率的减小,洋壳厚度的变化范围逐渐增大.
(2) 快速扩张洋中脊的洋壳速度要大于慢速和超慢速洋中脊,可能与快、慢速扩张脊洋壳生成过程岩浆上涌动力学机制不同有关.快速扩张更利于深部高密度岩浆的上涌并均匀地向两侧运移增生.
(3) 洋中脊轴部区域正常地壳平均厚度(6.3 km)小于非轴部区域(6.6 km),其厚度差异主要来自洋壳层3,这种差异性与洋壳年龄具有正相关性.
致谢 成文过程中与谭平川博士、余星博士、张洁博士、于志腾博士以及博士生胡昊、王奥星、王伟等进行了有益讨论,两位匿名审稿人和编辑为本文提出了宝贵的修改意见和建议,在此一并感谢.部分图件使用了GMT绘图软件(Wessel and Smith, 1995).
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