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板片碰撞:汤加-瓦努阿图地区的深源地震(《Geology》最新文章)

已有 4309 次阅读 2011-8-2 13:52 |个人分类:最新论文介绍|系统分类:论文交流| 深源地震, 汤加-瓦努阿图, 板片碰撞, 拆离

板片碰撞:汤加-瓦努阿图地区的深源地震

Simon Richards, Robert Holm, & Grace Barber

(周春银 编译)

 

摘要:一组数量超过100的异常地震位于西南太平洋北斐济盆地之下~600 km深度。这些地震被认为是俯冲澳大利亚板块拆离断块内的地震活动。这些拆离板片断块其中之一已经并正在和俯冲带太平洋板块在~500 km深度发生碰撞。这两个板片断块相遇的地幔区域在此深度也以大量的大(Mw > 7.0)地震为特点。斐济下面太平洋板片的褶皱以及这些地震,被认为是这两个板片碰撞并停留在660 km不连续面处所产生的变形以及由变形所增强的相变而形成的。板片断块的拆离被认为发生在大约5 Ma,但是东断块与西倾的太平洋板片之间的碰撞被认为发生在大约4 Ma,这与上覆劳盆地(Lau Basin)的开启时间是一致的。

 

原文链接:http://geology.geoscienceworld.org/cgi/content/abstract/39/8/787

 PDF文档下载:2011-Geology-When slabs collide-A tectonic assessment of deep earthquakes in the.pdf

 

INTRODUCTION(简介)

 

汤加俯冲带下400-600 km深度间的大地震比世界上任何其他地方的都多(Richter,1979; Tibi et al.,2003; Isacks et al.,1969)。大多数深源地震被认为是俯冲板片内的变形作用(Isacks and Barazangi,1977;Frohlich,1989; Gurnis et al.,2000)、脱水反应(Hacker et al.,2003; Omori et al.,2004)或相变(Kirby et al.,1996; Okal and Kirby,1995)引起的。Tibi等(2003)、GiardiniWoodhouse1984)以及Jiao等(2000)研究者认为俯冲太平洋板片中的大地震可能是由相关构造的变形和活化引起的,而这还可能受到来自太平洋超级地幔柱对俯冲的阻力作用的加剧,正如Gurnis等(2000)所提出的。

 

第二个异常地震活动区正好在俯冲太平洋板片的外围(Hamburger and Isacks,1987; McGuire et al.,1997; Okal and Kirby,1998)。有超过150个地震发生在北斐济盆地下面550-650 km深度范围内一个近板状的水平区域内(Fig.1)。这一地震活动异常区看上去与西倾的太平洋板片的和达清夫-贝尼奥夫带(Wadati-Benioff zone)是相关连的(Brudzinski and Chen,2003),OkalKirby1998)认为这些地震活动位于太平洋板片的拆离断块内。相反,HamburgerIsacks1987)则认为这些地震活动发生在现已消亡的南倾的维蒂亚兹(Vitiaz)俯冲带的参与碎块中。各个研究者之间现已达成共识,这些深源地震是由拆离板片停滞部分内的相变引起的,但是这些拆离断块的起源仍不清楚。本研究主要探讨解释汤加地区下面大量大地震和北斐济盆地下面异常地震活动之间的动力学关系。

 

 

Figure 1. Topography, bathymetry, and major tectonic element map of the study area. The Tonga and Vanuatu subduction systems are shown together with the locations of earthquake epicenters discussed herein. Earthquakes between 0 and 70 km depth have been removed for clarity. Remaining earthquakes are color-coded according to depth. Earthquakes located at 500–650 km depth beneath the North Fiji Basin are also shown. Plate motions for Vanuatu are from the U.S. Geological Survey, and for Tonga from Beavan et al. (2002) (see text for details). Dashed line indicates location of cross section shown in Figure 3. NFB—North Fiji Basin; HFZ—Hunter Fracture Zone.

 

METHODS(略)

 

GEOMETRY OF THE PACIFIC SLAB(太平洋板片形态)

 

对太平洋板片的几何解析见Fig.2。在50-250 km深度之间,汤加下面俯冲太平洋板片的结构相对简单,有一个单独的西倾的地震活动带(Fig.2)。随深度增加(350-500 km),板片的结构变得复杂起来。地震密度急剧增加,板片变成了凹形的弯曲状(Fig.1)。超过100个地震也发生在靠近太平洋板块和达清夫-贝尼奥夫带西部及外围的软流圈内(12ºS167ºE以及22ºS179ºW之间)。为了解释其中的部分地震,van der Hilst(1995)将他们解释为俯冲太平洋板片的上下边界(Fig.3)。太平洋板片外围这一异常地震集合也曾被BrudzinskiChen2003)观测到,但是并没有给出他们的成因解释。在3-D中来看,这些地震形成了一个从太平洋板片向西倾的似平板(Fig.23)。根据这一地区有限的CMT数据分析,对这些地震的震源机制尚没有达成一致;但与OkalKirby1998)的研究一直,他们认为拆离板片碎块中的相变引起了这些远程地震。

 

 

Figure 2. Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacific slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

 

GEOMETRY OF THE AUSTRALIAN SLAB(澳大利亚板片形态)

 

西北向东火山岛链,包括新赫布里底斯(New Hebrides)群岛和南所罗门群岛(south Solomon Islands),是由于现已断开的澳大利亚板块和南斐济板块向东倾俯冲形成的。上面的板块是北斐济盆地,它是一个混杂的后弧区,是由活动的以及固定的扩张中心所分开来的一系列亚板块所构成。正在俯冲带澳大利亚板片具有明显的和达清夫-贝尼奥夫地震带,并显示一个最深达300-350 km的相对陡倾的板片。在它的最南端,板片到达的最大深度仅为150 km,止于亨特断裂带(Hunter Fracture Zone)(Fig.12)。

 

在靠近西北部的地区,地震活动带最大深度增加到~250 km。这里,沿板片边缘地震震源地CMT解析显示为一个大致向西北陡然下插的T形,这与在喜马拉雅西部帕米尔地区下面正拆离的特提斯板片(Lister et al.,2008)中所发现的现象相类似。俯冲板块的最北端则与俯冲海沟枢纽和伦内尔(Rennell)海槽俯冲带弯转处(Fig.2)外的地震活动的贫乏相一致。俯冲澳大利亚板片的平均下倾长度只有200 km

 

在北斐济盆地中部正下方有三个明显的地震活动区域。最西部的集合区有40个地震,位于12.8ºS169.4ºE之间,平均深度为~640 kmFig.1)。尽管这是三个地震集合区中最小的一个,但是它却是地震密度最高的。第二个集合区地震密度略低有38个地震,就位于东面并在相似的深度。在这些地震周围区域可以在3-D构建一个简单的由三角形构成的多边形,它呈片(板)状向东扩展到太平洋板片(Fig.23)。进一步向东至太平洋板片,有一些分散的地震位于630-550 km之间深度,形成一个像太平洋板片逐渐变浅的面。地震在横截面上的分布见Fig.3。在东缘,外围地震区垂向俯冲的太平洋板片。在该区下面,太平洋板片停滞在660 km不连续面处(Fig.3)。这些分散集合区中的地震没有超过6.0级的,这与相同深度的太平洋板片中的地震完全不同,这里有不止7个超过7.0级的地震被记录下来。

 

 

Figure 3. Previous interpretation of combined P-wave tomography and seismicity from van der Hilst (1995). Earthquake hypocenters are shown in blue. The previous interpretation of slab structure is contained within the black dashed lines. Solid red lines mark the surface of the Pacific slab (1), the still attached subducting Australian slab (2a), and the detached segment of the Australian plate (2b). UM—upper mantle; TZ—transition zone; LM—lower mantle.

 

EVOLUTION OF THE VANUATU AND TONGA ARC SYSTEM(瓦努阿图-汤加弧系演化)

 

大约15 m.y.以前,太平洋板块的向西运动同时伴随着半连续NW向维蒂亚兹(美拉尼西亚)弧下面太平洋板块WSW向的俯冲(e.g. Hall and Spakman,2002; Falvey,1978)。在大约12-13 Ma时美拉尼西亚高原的碰撞终止了西南向东俯冲,并同时开启了澳大利亚板块沿着瓦努阿图-赫布里底斯俯冲体系向东北的俯冲,真是根据北斐济盆地中最古老的海底年龄12-14 MaPelletier et al.,1993)推测而来的

 

沿瓦努阿图海沟的俯冲几乎伴随着少量澳大利亚板块向北运动时俯冲枢纽的反转。相应地,整个澳大利亚板块的西北角已经俯冲到这里。因此一个相当于北斐济盆地大小的区域必须存在来作为瓦努阿图下面的俯冲物质。

 

自从4-5 Ma以来,汤加海沟也经历了向东的不对称枢纽反转,导致汤加海沟和斐济之间劳盆地的打开(Fig.1)(Taylor et al.,1996)。

 

INTERPRETATION(解析)

 

我们认为上述北斐济盆地下的地震活动位于从瓦努阿图俯冲带拆离的澳大利亚板片断块中;一旦拆离开来,板片向东滑移并与西倾的太平洋板片发生碰撞。HallSpakman2002)的地震层析成像结果也显示在澳大利亚板片和达清夫-贝尼奥夫地震带底部高波速异常中存在一个明显的不连续(break),支持拆离板片断块的解释。此外,这里所讨论的异常深源地震活动极其靠近或完全位于显示高速P波异常的地幔断块中,表明北斐济盆地下面这些异常地震发生在来源于周围俯冲带的板片断块中。将这三个拆离板片断块还原至地表显示他们连结起来的地表区域与现在北斐济盆地几乎是相等的,它也和12 Ma以来澳大利亚板块俯冲区域(Fig.2)是相等的。

 

DISCUSSION AND MODEL(讨论和模型)

 

我们进行简单的还原来说明这两个不断演化的俯冲体系的关系以及来解释北斐济盆地下面深源地震活动的成因(Fig.4A-4E)。

 

12 Ma之前(Fig.4A),沿瓦努阿图海沟的俯冲枢纽反转开始消耗澳大利亚板块(e.g. Meffre and Crawford,2001; Schellart et al.,2002)。在大约5 Ma时(Fig.4B),澳大利亚板片的基部发生拆离,在东部开始分裂并沿着西北向传播。俯冲板块中已有的断裂,如当特尔卡斯托(d’Entrecasteaux)断裂带以及年龄为40-30 m.y.的北Loyalty盆地扩张中心现已俯冲下去的北部,也可能促进了拆离板片分裂为三个独立的断块。快速的板块拆离也得到了BurkettBillen2010)最近研究的支持,他们的研究表明完全的拆离和分裂可以在短至1 m.y.的时间里完成。在4-3 Ma之间(Fig.4D),已完全拆离的板片断块与西倾的太平洋板片产生碰撞,导致太平洋板片的凹陷。劳(Lau)扩张后弧区在此时也开始打开(Taylor et al.,1996),这可能是由初始的板片碰撞引起的。3 Ma至今期间,太平洋板片已经俯冲了~240 km(以目前的俯冲速率),如果拆离板片断块现在停留在~600 km深度,那么初始碰撞可能发生在仅仅300 km的深度。

 

550-600 km深度之间太平洋板片中如此多的地震活动,我们认为,至少在一定程度上,是由于在碰撞处以及当两个断块崩塌在660 km不连续面时(Fig.4E)太平洋板片的褶皱和变形形成的。由于具浮力作用的太平洋超级地幔柱对板片俯冲的阻力作用(Gurnis et al.,2000),变形在此区域可能得到了增强。众所周知的汤加地区下面的深源地震活动,我们认为是各种过程的综合作用的结果,包括板片的变形、相变、以及由变形所促进的相变。

 

总之,我们认为,北斐济盆地下面的异常深源地震活动发生在澳大利亚板片的拆离断块中。板片的拆离发生在大约5 Ma,而拆离断块与西倾太平洋板片之间的碰撞发生在大约4 Ma。两个断块的碰撞导致太平洋板片的变形,以及~500-600 km深度间地震活动的增强。这里提出了一个与北斐济盆地下面大地震聚集的还原和俯冲带动力学研究是一致的解释;提出了发生地震的拆离板片断块的来源;提出了劳盆地开始打开的一个可能的解释;并提出了俯冲太平洋板片中大量深源地震的解释。

 

 

Figure 4. Simplified plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacific slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench; dER—d'Entrecasteaux Ridge.

 

 

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