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新几内亚-所罗门弧俯冲体系位于西南太平洋,处于太平洋板块、印度-澳大利亚板块汇聚地带。北邻加罗林板块,西侧毗邻欧亚构造域,南侧毗邻澳大利亚板块,东侧则毗邻世界上最大的海台—翁通爪哇海台[1-2]。新生代以来,伴随着印度-澳大利亚、太平洋板块及欧亚板块间的汇聚、碰撞、俯冲、拆离、扩张等作用,PN-SL逐渐形成在不足1 000 km空间范围内“四沟夹三盆”复杂的俯冲构造体系[3-4]。该区域是现今地壳运动最为活跃、各类地质作用最为强烈的区域之一,其复杂的构造动力背景致使PN-SL地震频发,有记录以来大于6级的地震超过千次,频繁的大震使得这里的流-固跨圈层物质能量交换活跃,广泛发育海底热液、冷泉活动、多金属硫化物喷发和海底裂隙[5]。其特殊的地理位置及构造、资源、环境效应使其成为一个特殊的俯冲构造动力研究的天然实验室,一致吸引着众多学者的关注[6-9]。
PN-SL俯冲体系作为壳幔物质循环、造山带形成、地震与火山活动等地质作用最为活跃的板块汇聚边界地带之一,特别是横亘在太平洋板块之上高数千米的翁通爪哇海台等凸起构造,随着板块运动,其俯冲构造引发板块构造动力学、壳幔相互作用等诸多科学问题,因此,解析包含海台等“凸起”构造的板块汇聚边界—PN-SL俯冲体系的俯冲起始动力学机制是解析PN-SL俯冲体系多期复杂构造的重要切入点。本文在综合前人研究的基础上,立足已知事实,理清PN-SL俯冲体系俯冲起始的期次、类型以及动力学机制,总结其中仍面临的诸多问题,以期促进对PN-SL俯冲体系动力学过程的合理理解。
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新几内亚-所罗门弧位于印度-澳大利亚板块与太平洋板块汇聚边界及新特提斯构造域东端,这里板块俯冲、弧前弧后扩张、火山喷发、破坏性地震等地球内部动力导致了完整的沟弧盆系[10-11]、不同时代不同类型的俯冲带[12-13]、全球海域唯一见到的低角度(25°~30°)正断层[14]和异常活跃的海底热液与冷泉活动[5](ODP193)等。晚白垩世以来,历经50、45、~25、~8、~5 Ma的关键构造期次[3-4],逐渐演变成目前1 000 km范围内自东向西分布的西美拉尼西亚-北所罗门海沟、俾斯麦海盆、新不列颠-圣克里斯托瓦尔海沟、所罗门海盆、特罗布里恩海沟、伍德拉克海盆、波克灵顿海沟地质构造面貌(图1)。目前,印度-澳大利亚板块仍以90~110 mm/a的速率相对于太平洋板块向E-NE向移动[11]。
图 1 新几内亚-所罗门弧俯冲体系区域构造图
Figure 1. Tectonic map showing the tectonic settings and geological tectonic units within the PN-SL subduction system
西太平洋板块与印度-澳大利亚板块沿PN-SL汇聚俯冲导致了多期次分布复杂的岩浆活动。在新几内亚岛上发现指示板块俯冲启动的蛇绿岩等变质岩带多形成于晚白垩世—古新世,为早期澳大利亚板块与新几内亚岛弧-陆俯冲碰撞的记录[17];在所罗门岛弧的岩浆活动可以明显地划分为6 Ma之前由俯冲的太平洋板片控制和之后由俯冲的印度-澳大利亚板块控制的两期[7, 9];西美拉尼西亚出现的大量斑岩显示海沟俯冲自43 Ma即已开始,伴随着26~20 Ma多次深部动力环境和火山活动的变化[18];而板片窗的出现使得伍德拉克海盆扩张轴俯冲位置附近的岛弧岩浆活动变得尤为复杂[8]。零星发现的地幔捕掳体[19-20],显示了深部地幔物质组分与岛弧岩浆活动耦合的多样性。
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围绕新几内亚-所罗门弧,人们自20世纪70年代起通过海底地形观测、磁异常分析等调查研究,试图揭示其深部结构和构造动力学特征。Taylor[10]通过海底地形、震源机制及海底磁异常数据对俾斯麦海盆的构造特征进行分析,厘定俾斯麦海盆南、北块体之间的边界[21],其中两条大尺度(延伸>200 km)走滑断层的发育更是预示着南、北微板块之间的分离[11, 22],意味着该边界未尝不会类似于皮斯格-峡湾海沟的形成过程最终发育成俯冲带(图2),而海底磁异常显示海盆东侧马努斯盆地形成于3.5 Ma以来的海底非对称扩张作用,在其范围内现今存在着丰富的海底热液系统[5, 23](ODP193)。作为新几内亚-所罗门弧构造带范围内的另外一个扩张海盆,侧扫声纳、单道反射地震、海底磁异常数据分析发现伍德拉克海盆扩张的同时[24],向北传递于新几内亚陆壳范围内[25]。1998年ODP180航段(Site 1108-1118)利用多道反射地震仪在伍德拉克海盆北缘观测到了全球海域唯一见到的低角度(25°~30°)正断层层[14],这一奇特的构造现象也同样得到正断型震源机制解的印证[26],其可能主要是由于塑性下地壳的水平剪切作用所致[14, 26],而上地壳强烈的拉张活动使得地壳减薄的同时,诱使深部低密度(地震低速带)的地幔物质上涌以达到均衡[27]。对于所罗门群岛地区而言,虽有开展地震反射调查工作,但学者们并没有重视伍德拉克新生海盆的俯冲结构特征,而是将注意力集中在岛弧及东侧翁通爪哇海台附近,且受早期海洋探测设备的限制,观测的地震剖面深度不超过30 km[15, 28-29](ODP192航段Site 1183-1187),这对该处印度-澳大利亚板块俯冲深度超过200 km的俯冲带以及对于区域俯冲动力体系有着重要影响,而对深度跨度达上千千米的大火山岩省而言,则显得明显不足。2015年“东方红2号”科考船对西美拉尼西亚海沟偏北一侧的加罗林板块的海底地形进行调查,在西美拉尼西亚海沟向北一侧发现由于断裂切割减压致使海底沉积物上涌堆积形成线状展布的“海山链”[30],但并无涉及西美拉尼西亚海沟等俯冲体系的直接调查。综合上述,明显看出以往的海洋调查工作往往集中于海盆扩张轴、海台等附近,常常关注扩张轴展布、扩张速率、应力状态、周缘地区热液活动等,缺乏针对俯冲带开展高精度、深部地球物理探测的工作,以致俯冲带精细结构特征无法得到有效约束。
图 2 PN-SL俯冲体系地震及震源机制展布图
Figure 2. Spatial distribution of the local seismicity and mechanisms of great earthquakes within the PN-SL subduction system
作为地应力的释放形式,地震活动清晰地记录了板块几何学、运动学特征(图2,3)。自1900年以来,新几内亚-所罗门弧地区震级>6.0的地震事件超过1 000个(USGS),强烈的地震活动使得依据地震信息厘定俯冲带的空间、结构特征成为可能。据震中、地震层析成像等研究可知,印度-澳大利亚板块和太平洋板块分别沿新不列颠海沟、西美拉尼西亚-北所罗门海沟以超过70°的高角度俯冲,板块俯冲深度超过500 km[13, 31]。对于新几内亚-新不列颠岛地区而言,相较于新不列颠海沟>70°的高角度俯冲[13],震中空间展布显示板片沿新几内亚东侧休恩半岛俯冲至100 km深时逐渐变为水平状态,自150~250 km又变得几乎垂直,这可能与板片沿特罗布里恩海沟俯冲的残余部分、板片沿莫尔兹比海槽NE向俯冲以及岩石圈加厚或者岩石圈下方不稳定造成的产物有关[32]。该区域0~250 km震源机制解析显示,所罗门海块体双向俯冲的形态及与上覆印度-澳大利亚板块间的解耦,则充分解释了该区域俯冲形态异常的来源[12, 33]。在偏南一侧所罗门岛弧周缘俯冲带地区,Wadati-Benioff带和地震层析速度剖面显示由所罗门岛弧南侧的双向俯冲向北逐渐变为印度-澳大利亚板块沿新不列颠海沟高角度东向俯冲[6, 31],俯冲形态剧烈的空间差异致使基于少量的横波分裂信息厘定深部地幔运动状态变得十分困难[34]。据伍德拉克海盆东侧OBS台站记录的震中信息可知,新生的伍德拉克海盆沿圣克里斯托瓦尔海沟俯冲角度仅为30°,且地震带上界面向陆迁移。综上可知新几内亚-所罗门弧俯冲类型、俯冲结构的多样性及空间差异性,纵然基于地震信息可以大致勾勒出俯冲带的几何形态(图3),但并没有获取俯冲带俯冲板片、上覆板片及深部地幔清晰的结构、运动信息,至于新几内亚-所罗门弧构造带俯冲构造动力学的复杂性与各板块俯冲系统启动的关系是什么,以及各俯冲系统启动和演变的动力学特征以什么方式耦合,我们还不得而知。
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西太平洋地区处于特提斯构造域与太平洋构造域交汇地带,是中生代以来太平洋板块与欧亚板块、印澳板块俯冲拼接转换的长期演化的结果。由两大洋三大板块撞击形成延绵数千千米、具有全球规模的陆-陆、洋-陆、洋-洋巨型板块汇聚、拼合、交接的壮观雄伟的构造区,包含极其复杂的俯冲、碰撞、增生以及弧后扩张等纵横深浅的构造演变过程(图4)。印度板块与欧亚板块陆-陆俯冲汇聚及其后的楔入作用形成了世界上最高的高原—“第三极”青藏高原,持续的俯冲汇聚导致高原周缘块体的挤出并影响了太平洋板块的俯冲后撤[35]。与此同时,涵盖弧-陆、洋-洋俯冲、俯冲方向频繁反转、沟-弧-盆体系多次叠加以及现今仍持续扩张海盆的新几内亚-所罗门弧俯冲体系[4],不仅是地球表层板块俯冲起始发育异常突出的构造带、复杂的洋陆沟弧盆和多重洋-洋俯冲、弧前海盆扩张构成的构造地貌单元,而且还记录着中新生代以来地球深层印澳、太平洋两大板块地幔状态及相互作用,清晰地反映出深层地球动力学作用与过程对地球表层系统巨大的控制和影响[31]。
图 4 PN-SL俯冲体系关键构造演化阶段模式图(修改自Schellart等[3])
Figure 4. Tectonic evolution of the PN-SL subduction system during the critical periods
由依据古地磁等资料开展的板块重建研究可知[36-38],对于PN-SL俯冲体系而言,印度-澳大利亚板块的北向、北东向汇聚俯冲体系发育过程,可以追溯到中侏罗纪末期,大印度板块开始从冈瓦纳大陆裂解。白垩纪时期,随着印度洋中脊的扩张,印度-澳大利亚板块从南极洲裂离,快速向北运动[36],进而推动北缘新特提斯洋加速俯冲于欧亚板块之下,直至消亡。此时,澳大利亚板块沿其北侧俯冲于太平洋板块之下,而在其板块东北侧,太平洋板块则沿西美拉尼西亚-北所罗门等海沟俯冲于澳大利亚板块之下(图4)[37-38]。受太平洋板块俯冲起始诱发作用,平行海沟方向印度-澳大利亚板块发生强烈的拉张作用,形成波克灵顿、珊瑚海、新喀里尼西亚等一系列弧后海盆[36, 39]。伴随着澳大利亚板块弧后海盆扩张的停止,中晚始新世时期(~45 Ma),作为新生的弧后扩张盆地—波克灵顿海盆开始向北俯冲于太平洋板块之下,板块俯冲方向第一次反转,而这是否可以归因于板块运动方向变化致使板块挤压应力增强,还需要进一步的探究[3]。晚始新世(40~35 Ma),新几内亚岛弧和所罗门海盆开始形成,至于所罗门海的扩张为单一俯冲体系弧后盆地还是双向俯冲体系的弧后盆地,及其与两侧俯冲体系板片俯冲运动、深部地幔活动的耦合方式,目前仍处于未知状态。渐新世时期,波克灵顿海盆完全俯冲消亡,新几内亚岛弧与澳大利亚北缘被动大陆边缘发生碰撞隆升,所罗门海随之停止扩张[3]。渐新世末—中新世早期,在印澳板块东北侧,翁通爪哇海台与所罗门岛弧开始碰撞汇聚,太平洋板块沿西美拉尼西亚、北所罗门海沟的俯冲运动逐渐减缓直至停止[40]。~8 Ma,所罗门海盆分别沿特罗布里恩、新不列颠、圣克里斯托瓦尔海沟向S、N、NE向俯冲[3]。但就翁通爪哇海台的碰撞如何在弧后环境引发板块俯冲方向第二次反转—大规模年轻板片(<35 Ma)开始被动俯冲于古老板片之下?所罗门海盆为何会形成双向俯冲?特罗布里恩俯冲带是何时何种机制停止的?我们目前仍不得而知。~5 Ma以来,伍德拉克、俾斯麦海盆相继扩张,持续至今[4],但伍德拉克海盆作为弧前海盆又是如何开始扩张、活跃至今的同时,沿圣克里斯托瓦尔海沟NE向起始俯冲于白垩纪年龄的太平洋板片之下呢?学者们没有给出明确的解答。
作为区域俯冲体系发育演化的物质记录,变质岩带、岩浆活动清晰地反映区域构造活动期次和深部地幔活动特征等。在PN-SL范围内,目前发现的蛇绿岩等变质岩带(如巴布亚、马鲁姆、伊里安蛇绿岩带等)主要集中于新几内亚岛上,通过古生物、锆石U-Pb、角闪石Ar-Ar等年代学记录显示,上述变质岩带多形成于晚白垩世—古新世,为早期澳大利亚板块与新几内亚岛弧-陆俯冲碰撞的记录[17, 41]。受印度-澳大利亚板块与太平洋板块俯冲作用,新几内亚-所罗门弧新生代以来表现出强烈的火山活动,并诱发形成世界级斑岩铜矿和浅成热液金属矿[42]。对于俾斯麦海盆而言,东侧马努斯盆地海底火山玻璃分析显示,其岩浆组分中的Cl及Re含量与板片俯冲过程诱发的流体有关[43-44]。由新几内亚马拉莫尼岛地区侵入岩U-Pb年龄及Hf等元素分析可知,~12 Ma时期,受沿波克灵顿海沟俯冲澳大利亚陆块的影响,新几内亚地区开始隆升,至9 Ma,俯冲活动诱发强烈的岩浆活动,7 Ma,俯冲板片的拆离致使重稀土元素(HREE)亏损的岩浆活动形成[45]。据新不列颠岛斯木库岩浆杂岩U-Pb年龄及主微量元素分析,西美拉尼西亚海沟俯冲带至少于43 Ma已开始活动,伴随着26 Ma翁通爪哇海台的碰撞,俯冲带活动性减弱,受俯冲深部动力环境的改变,24~20 Ma岛弧火山活动继续的同时,研究区开始形成大量斑岩。20 Ma,西美拉尼西亚海沟俯冲带活动停滞[18]。对于新几内亚-新不列颠岛弧新近活跃的火山活动而言,主微量元素分析对比发现,相比于新不列颠岛弧深部发育的亏损地幔,新几内亚西俾斯麦岛弧地带深部地幔经受源自俯冲板片沉积物混染量逐渐增加[33]。所罗门岛弧地区岛弧岩石样品Pb同位素数据显示,该区岩浆活动可以明显地划分为两个阶段—6 Ma之前由俯冲的太平洋板片控制,之后由俯冲的印度-澳大利亚板块控制[7, 9],而形成于晚上新世以来的加列戈火山岩地化分析显示,其深部地幔源仍会经受东侧俯冲的太平洋板片、翁通爪哇海台的次要影响[46]。与此同时,板片窗的出现使得伍德拉克海盆扩张轴俯冲位置附近的岛弧岩浆活动变得尤为复杂[8]。由此可见,印度-澳大利亚、太平洋等板块的俯冲主导了新几内亚-所罗门弧的岩浆活动规律,那么各俯冲系统的启动、发育如何控制、影响新几内亚-所罗门弧的岩浆活动呢?岛弧岩浆活动期次、空间分布规律又是如何与俯冲系统的多次迁移、反转耦合的呢?虽然有地幔橄榄岩的发现[19-20, 47],但是板片俯冲运动与岛弧岩浆活动之间重要的枢纽—深部地幔运动又是如何与板片的俯冲、岛弧岩浆活动耦合的呢?这些问题亟需回答。
综合上述分析可知,历经新生代时期的构造演化,由多期次板块俯冲起始串联,PN-SL发育成具有异常丰富模式、时序完整的俯冲体系(图3),除包含西美拉尼西亚、北所罗门超过40 Ma成熟的、极可能为“主动”的俯冲体系[3],还涵盖了形成时间不足8 Ma的新不列颠-圣克里斯托瓦尔、特罗布里恩如此年轻板块(<35 Ma)俯冲于古老板块(>100 Ma)之下的“被动”俯冲带[4],且并未经受后期强烈的改造,几乎完整地保留了板块起始俯冲状态。最为特别的是,上新世以来,新生伍德拉克海盆持续扩张的同时E—NE向俯冲于白垩纪的太平洋板块之下,成为全球唯一可见的活跃海脊俯冲实例。而南北俾斯麦海盆微板块边界的形成,更是预示着未来演变成板块俯冲起始的可能[11]。可见,正是由于PN-SL俯冲体系多期次、多类型板块俯冲起始作用奠定了PN-SL俯冲体系纷繁复杂的俯冲发育演化过程。而新生代时期区域岩浆活动的时空规律,则是对于多期次、多类型板块俯冲起始作用在PN-SL俯冲体系发育演化过程中扮演关键角色的直接反映。
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处于太平洋板块西南一侧的翁通爪哇海台,作为世界上最大的海台,其运动过程甚至对于全球板块运动都有着显著的影响[1-2],伴随着印度-澳大利亚、太平洋板块强烈的挤压作用,其西侧毗邻的涵盖俾斯麦、所罗门海等微板块的新几内亚-所罗门弧(PN-SL)俯冲体系成为世界上地震活动最为活跃的俯冲系统之一[48-50]。因此,以世界上最大的海台—翁通爪哇海台及与其相互作用的PN-SL俯冲体系为研究对象,无疑为探究包含海台等“凸起”构造的板块汇聚形变、构造动力过程提供最佳的实验场地。
翁通爪哇海台地处PN-SL俯冲体系东侧,历经太平洋板块下方地幔柱~122 Ma、~90 Ma两期拉斑玄武岩浆活动形成,地形高出洋底约2 000 m,覆盖面积达1.9×106 km2,是世界上最大的海台,也是世界上保存最大的大火山岩省[51-53],甚至有学者提出翁通爪哇海台在~120 Ma形成之初与马尼希基(Manihiki)海台、希古朗基(Hikurangi)海台仍为一体,三者构成更大的“超级海台”[54]。白垩纪晚期,在太平洋板块裂解作用下,随着奥斯本(Osbourn)海槽、埃利斯(Ellice)海盆的快速扩张(扩张速率分别为~90、~84 km/Ma),三者发生分离[52, 55]。翁通爪哇海台由形成之初的24°±4°S位置沿NW向历经缓慢、快速、缓慢等差异性迁移过程[56],并顺时针旋转了40°[54],最终与PN-SL俯冲体系汇聚。
翁通爪哇海台作为目前世界上最大的海台,吸引众多学者通过地质学、地球物理学等手段探究其结构特征[57-62]、深部岩浆活动[63-68]及构造演化过程[4, 18, 52, 69]。但就其形成机制而言,却始终存有较大争议,学者们目前已提出“地幔柱起源”[63, 67-68]、“快速扩张海脊附近地幔被动上涌”[70]、“流星撞击”[71]等模式。其中地幔柱起源模式较为流行,但翁通爪哇海台短期内大规模岩浆活动、>1 500 ℃的地幔热结构致使海台侵位深度、均衡地形等理论值与实际特征(海平面下方岩浆活动、隆升幅度偏小)出现较大偏离[70-73],虽然Neal等[74]、Ito和Clift[75]、Covellone等[61]分别将其归因于海台下地壳出现致密的石榴子石麻粒岩、岩浆底侵形式的地壳长期生长作用或地幔柱头部夹带深地幔榴辉岩所致,但热点海山链的缺乏、至少两期间隔30 Ma的岩浆活动等问题[71, 76],仍亟需进一步的探究。翁通爪哇海台自身起源等方面的诸多争议则无疑为解析其与PN-SL俯冲体系的俯冲构造动力关系增添更多的复杂性。
自20世纪以来,围绕翁通爪哇海台已开展5个大洋钻探航次(如Leg30,Leg130等),获取多个深度近百米甚至超过1 500 m的钻孔资料(图1),枕状构造玄武岩及岩床表层之上覆盖数百米至~1 000 m厚的有孔虫软泥、灰岩等地层结构被充分揭示(http://www-odp.tamu.edu/publications/)。据翁通爪哇海台范围及其西南侧跨北所罗门海沟一带采集的反射地震资料可知(图1,5),海台地壳厚度可达约33~38 km,为正常大洋地壳厚度的3~4倍[16, 77-78],故在海沟处不易俯冲,其强烈的浮力对于板块俯冲过程有着明显的阻滞作用,并促使板块向PN-SL侧发生碰撞、加积,这也得到马莱塔岛出露玄武岩岩石学、地球化学元素分析的印证—马莱塔岛原为翁通爪哇海台一部分,其沿北所罗门海沟仰冲拼贴于所罗门岛弧之上[60-79]。地震反射资料显示,翁通爪哇海台地壳80%的部分可能已俯冲于所罗门岛弧之下,只有最上层的玄武岩层和沉积层部分(~7 km)以增生方式拼贴于上覆板块之上[15, 29]。
近年来,围绕翁通爪哇海台及PN-SL构造体系岩心、出露岩石热年代学和古地磁学的研究对翁通爪哇海台的俯冲阻滞效应提供了越来越多的证据:自早白垩世以来,翁通爪哇海台呈NW向顺时针旋转运动[54, 56],至中新世,翁通爪哇海台、PN-SL俯冲体系之间的俯冲碰撞致使太平洋板块俯冲活动逐渐减弱的同时,板块俯冲方向发生反转,弧后所罗门海盆周缘地带形成新不列颠海沟、特罗布里恩海沟等新的俯冲体系[2, 4, 7, 18, 40]。以岛弧岩浆活动及海山运移特征为约束,学者认为海台-岛弧初始碰撞时间可能为20~25 Ma,软碰撞阶段开启[53, 69];地震反射资料及陆缘地层记录联合分析则表明,经过长时间的过渡阶段,海台与岛弧于约4~5 Ma发生硬碰撞[29, 78]。至于翁通爪哇海台何时俯冲殆尽,亦或是所罗门海盆俯冲消亡后,在海台后方是否会出现新一期的板块起始俯冲过程及海台是否会与所罗门群岛发生拼贴造陆现象,学者们并没有给出明确的回答。历经翁通爪哇海台、PN-SL体系间>20 Ma的俯冲碰撞,最终引发弧后所罗门海盆“反向”、“多向”俯冲的特殊构造过程(图1,3,4),这一构造现象明显区别于以往海台俯冲诱发海沟位置后移、俯冲极性反转二元简单构造模式[80-81]。虽然有学者重视该现象,但往往将研究重点倾向于翁通爪哇海台的深部结构及其与PN-SL体系俯冲碰撞过程方面[16, 59, 82],至于翁通爪哇海台俯冲诱发弧后所罗门海盆的构造形变细节及二者间的耦合关系并不清晰。翁通爪哇海台、所罗门海盆作为上述特殊构造现象中两大主要构造单元,翁通爪哇海台的结构特征、俯冲碰撞细节方面的详实研究意味着,弧后所罗门海盆构造形变的深入探究成为解析上述海台俯冲碰撞构造特殊现象、理清海台俯冲与弧后海盆构造形变间的耦合关系的关键一环。
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板块反向浮力是现代板块的驱动力,而反向浮力的大小往往与板块密度息息相关。当两侧板块密度出现差异,则导致重力不稳定,密度较大一侧板块的反向浮力为板块俯冲提供必要的重力势能[83]。依据俯冲板块、上覆板块密度差异,板块俯冲起始机制模型可以分为主动俯冲[80, 84]和被动俯冲[85-86]两类模式,而俯冲板片负浮力是否超过俯冲阻力是基本特征。然而实际情形要复杂得多,岩石圈物性变化(水、温度等)致使岩石圈强度减弱,被动大陆边缘沉积荷载作用,岩石圈地幔小尺度对流,地幔柱、板块运动方向变化造成的被动大陆边缘或大洋板块破裂,俯冲方向反转或海沟跃迁,扩张洋脊转变为海沟,裂陷过程中岩石圈的拉张解耦等都可能是板块发生俯冲的原因[81, 87],而且板片的主动和被动俯冲可能相互转化和并存[85, 88]。显然,受探测技术、模拟手段和约束资料所限,板块俯冲起始、发育的动力机制是一个跨地球物理、地球化学、构造地质、岩石学等多学科交叉融合的地学难题。
综合以往研究发现,对于海台等“凸起”构造俯冲过程而言,受其低密度结构的影响,往往会引起海沟位置向后迁移或板块俯冲极性的反转(图6)。对于PN-SL俯冲体系而言,古地磁、岩浆活动等研究显示翁通爪哇海台北西向运动与印度-澳大利亚板块碰撞改变早期PN-SL俯冲带形态的同时,海台俯冲阻滞作用甚至重新塑造了PN-SL的俯冲构造格局,而弧后所罗门海盆的复杂构造形变则是对世界上最大的海台的强烈俯冲作用的直接体现[4, 6]。其中翁通爪哇海台的俯冲导致弧后所罗门海盆“反向俯冲”的同时,中新世以来甚至呈现出“NW向、NE向和SW向的多向俯冲过程”(图4,7),这无疑是对以往海台俯冲诱发海沟位置后移、俯冲极性反转二元简单模式提出新的挑战[80-81]。
图 6 海台俯冲诱发新俯冲带形成示意图
Figure 6. Models of subduction initiation induced by the subduction of oceanic plateau
图 7 翁通爪哇海台俯冲碰撞诱发PN-SL俯冲体系“反向、多向俯冲”示意图(修改自Holm等[4])
Figure 7. Cross-sections showing the reverse and multiple-direction subduction of the PN-SL subduction system induced by the collision of the Ontong Java Plateau
本文依据已有研究事实,认为PN-SL中新世以来“特殊”俯冲体系的发育在经受翁通爪哇海台俯冲碰撞主导作用下,西侧可能受新几内亚微板块的NE向运动挤压作用,即在东西两侧较厚岩石圈、密度偏低的海台及板块汇聚作用下,密度偏高的所罗门海块体沿所罗门海沟、特罗布里恩海沟发生多向俯冲过程。但如前文所述,翁通爪哇海台俯冲诱发弧后所罗门海盆的构造形变细节及二者间的耦合关系仍不清晰。一方面,虽然学者们自20世纪80年代起通过海底地形、地震观测等方面调查研究,力求揭示PN-SL俯冲体系区域构造背景和动力学特征,然而受构造环境的复杂性及20世纪八九十年代探测手段的局限,前人并未给出所罗门海盆西侧重大构造幕次及形变时序特征。另一方面,所罗门海盆历经新生代早期60~40 Ma或45~30 Ma至渐新世晚期(~25 Ma)扩张期,虽然学者们对于后期翁通爪哇海台在弧后所罗门海盆构造记录的主导性上达成共识,但受海盆周缘构造要素复杂的约束,该时期内海盆构造形变的时空序列始终处于争议之中。截至目前,针对所罗门海盆构造形变过程研究可分为如下3种模式:(1)晚中新世时期(~10 Ma),所罗门海板块南北侧分别形成海沟以致板块双向俯冲的“一阶段”模式[89];(2)中新世早期(约18~15 Ma),所罗门海板块沿特罗布里恩海沟向南俯冲挤压,约6~5 Ma,所罗门海板块北缘地带开启北向汇聚俯冲新一期的构造活动,此为“二阶段-Ⅰ”模式[90-91];(3)~8 Ma,所罗门海板块北缘受挤压应力作用形成新不列颠海沟,约4~5 Ma时期,所罗门海板块南缘特罗布里恩海沟地带俯冲挤压活动起始发育,此为“二阶段-Ⅱ”模式[4, 7, 40]。
由上述分析可以看出,翁通爪哇海台与PN-SL俯冲体系汇聚形变过程并非仅依据板块密度变化来简单解释,需要考虑其复杂的构造环境和诸多的构造要素。
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如上节所述,岩石圈密度差异固然能够引起板块交界处重力不稳定现象的出现,但是板块俯冲起始还存在一个充分前提—岩石圈必须能够产生断裂、弯曲、下沉,而在此过程中岩石圈强度则成为一个关键要素。经研究发现,对于岩石圈强度而言,流体成为一个必须考虑的重要因子。流体会减弱岩石的强度,降低熔点,致使岩石圈强度减弱、软流圈地幔的黏度降低[83]。同时,俯冲带中岩石的部分熔融也与俯冲板块的脱水作用息息相关[92]。近年来,越来越多的证据表明,伴随着大洋俯冲板片向地球深部运移的流体是驱动板块俯冲起始、诱发岛弧岩浆作用和地震分布、影响俯冲体系各层圈物质能量交换的关键因素之一[93-94]。流体以含水矿物相随俯冲板片进入深地幔[95],可以达到地壳以下~200 km甚至更深[96],它的运移使俯冲板片脱水并与地幔楔发生水化交代和橄榄岩蛇纹石化作用[97],流体的变化还影响了俯冲体系壳幔物质组成及其物理化学性质,显著改变了岩石圈流变学性质[98]和地幔熔融温度[99-100],进而可能影响板片形态、俯冲速率、方向以及沟槽迁移等动力活动。同时,热力学守恒使俯冲体系的不同深度处又形成了富水流体、含水熔体和超临界流体,橄榄岩蛇纹石化地幔不仅成为诱发岩浆作用的流体源,而且也是将流体再循环进入更深地幔的有效“岩性载体”。可见,俯冲体系中的流体活动是串联起地球水圈-岩石圈-软流圈物质交换和热动力过程的纽带,也是理解板块俯冲运动的关键切入点(图8)。
图 8 俯冲体系流体分布及迁移示意图(修改自Rüpke等[101])
Figure 8. Schematic diagram showing spatial distribution and migration of the fluid within the subduction zone
俯冲体系中蛇绿岩、火山岩捕虏体橄榄岩以及折返至地壳表层的超高压变质岩的实验岩石学研究为探讨地幔物质组成、壳幔物质循环以及板块俯冲启动时间提供了有效窗口[84-85, 97, 102-103],地震层析成像技术揭示了现今俯冲体系深部的构造样式、大尺度的圈层结构以及地幔各向异性特征[104-105]。近些年联立斯托克斯流体动力学定律、物质守恒定律和能量守恒定律数值模拟俯冲体系动力过程成为再现板块俯冲起始和演化过程的有效手段,数模结果显示地幔羽流对岩石圈的持续弱化减薄[102]、被动大陆边缘的沉积负荷加载[87]、板块边界多孔流体的渗滤弱化作用[106]、地幔对流[107]、洋底高原生长与垮塌[108]、板块作用力在转换断层或边界薄弱带的推挤[85]均可诱发板块的主动或被动俯冲。这些研究告诉我们俯冲带活动比预知的复杂得多,除了与俯冲板片年龄、运移速度和几何属性等有关外,地幔楔流场、流体迁移渗滤等众多因素也影响了俯冲体系热动力学过程[109-111]。
随着俯冲体系中流体活动在诱发地幔楔部分熔融以及板块俯冲启动过程的作用被揭示,近几年一些考虑俯冲板片脱水和地幔楔水化作用的数值模型被研发并应用于俯冲动力学过程的模拟[106, 112]。通过假设初始地壳和地幔水含量的简单线性公式,依据岩石热力学数据库设置平衡相矿物的极限水含量为温度和压力的函数,在模型物理化学条件和热力学平衡假设控制下实现了对俯冲体系中水的释放、迁移和吸收等动力学过程的模拟[113-114]。实验岩石学的流变方程和基于固体基质渗透率的流体-固体耦合模型也被考虑进板片俯冲过程[106, 112]。尽管多数模型对于岩石含水量与流体渗滤过程的假设过于简单,但对俯冲体系中流体-固体相互作用的认识推进了一步。
毋容置疑,综合地质学、岩石学、地球化学和地球物理学多学科观测和研究的俯冲体系地球动力学数值模拟,可以获得俯冲板块随时间演化的动态序列,成为研究板块俯冲起始和重建板块俯冲动力过程的突破口;特定时间-空间尺度流体影响下的俯冲体系各圈层物质-能量交换的地球动力过程再现对比是揭示板块俯冲起始演化机制的钥匙。
对于PN-SL俯冲体系而言,如前文所述,目前学者们主要是通过人工地震探测、自然地震应力解析、岩浆活动分析等方面,就其俯冲体系深部结构、构造形变序列等展开探究,对于PN-SL俯冲体系发育的动力机制分析主要停留在定性分析层面上,鲜有采用动力学数值模拟的定量化研究来厘定其俯冲发育的动力机制。在接下来的研究中,建议采用系统的岩石-热-力学数值模型,研究并计入常常被忽略和简化的流体参数,对PN-SL俯冲体系多期、多类型俯冲起始过程进行定量模拟,厘定密度、流体等因子在板块俯冲过程中所扮演的关键角色,系统揭示板块俯冲起始动力学机制。
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PN-SL俯冲体系位于印度-澳大利亚板块与太平洋板块汇聚边界、新特提斯构造域东端,东侧毗邻世界上最大的海台—翁通爪哇海台,因此,PN-SL俯冲体系是研究板块俯冲起始及俯冲动力演化过程的一个绝佳窗口。综合地质学、地球化学和地球物理学等多学科观测和研究,目前围绕PN-SL俯冲体系的俯冲动力过程已取得丰富的认识,主要体现在以下几个方面:
(1)据板块重建及岩浆活动记录等研究可知,晚白垩世以来,历经50、45、~25、~8、~5 Ma关键构造期内印度-澳大利亚板块和太平洋板块俯冲方向的频繁变化,PN-SL逐渐形成1 000 km范围内“四沟夹三盆”、兼有主动及被动俯冲模式、自孕育至成熟-死亡完整时序的俯冲体系。受多期次、多类型板块俯冲作用的制约,对应于起始时间、俯冲模式的不同,PN-SL俯冲体系深部结构呈现出明显的空间差异性:板块俯冲深度由>500 km减小至不足100 km,而板块俯冲角度则由>70°减小至30°。正是由于多期次、多类型板块俯冲起始活动的主导,最终奠定了PN-SL俯冲体系纷繁复杂的俯冲发育演化过程。
(2)翁通爪哇海台作为世界上最大的海台,其地壳厚度为约33~38 km,近似于正常大洋地壳厚度的3~4倍,其显著的“凸起”构造对于板块俯冲过程有着明显的阻滞作用。岩石热年代学和古地磁学等研究表明,自晚渐新世—早中新世(20~25 Ma)以来,PN-SL俯冲体系中新不列颠海沟、特罗布里恩海沟等特殊构造的发育、演化无不与翁通爪哇海台、PN-SL俯冲体系间的碰撞挤压作用密切相关,即翁通爪哇海台是PN-SL俯冲体系发育的关键动力要素。
(3)板块反向浮力作为板块俯冲起始发育的驱动力,其大小往往与板块的密度结构密切相关。低密度翁通爪哇海台的俯冲作用打破了PN-SL俯冲体系原有的应力平衡状态,重新塑造了PN-SL俯冲体系的构造格局,但不同于低密度结构俯冲诱发海沟位置后移、俯冲极性反转二元经典模式,弧后所罗门海盆发生“反向俯冲”的同时,中新世以来呈现出“NW向、NE向和SW向的多向俯冲过程”。而早—中中新世以来,所罗门海盆所呈现出的南北双向同时俯冲的“一阶段”模式或南北双向差异俯冲的“二阶段”模式等复杂的构造形变时空序列,清晰地说明翁通爪哇海台与PN-SL俯冲体系汇聚形变过程并非仅依据板块密度变化来简单解释,需要考虑其复杂的构造环境和诸多的构造要素。
(4)作为岩石圈强度的重要影响因子,流体致使岩石圈强度减弱、熔点降低的同时,伴随板块俯冲向地球深部运移,促使板片脱水并与地幔楔发生水化交代作用,进而改变壳幔物质组成及流变学性质,诱发地幔楔部分熔融和岛弧岩浆活动。俯冲体系中的流体活动是串联起地球水圈-岩石圈-软流圈物质交换和热动力过程的纽带,也是理解板块俯冲运动的关键切入点。但对于PN-SL俯冲体系而言,鲜有采用动力数值模拟的定量化研究来厘定其俯冲发育的动力机制。在今后研究当中,建议综合PN-SL俯冲体系板块几何结构、应力环境等诸多构造要素,采用系统的岩石-热-力学数值模型,计入俯冲体系密度结构、流体参数,分析二者在PN-SL俯冲体系板块俯冲过程中的关键作用,定量解译PN-SL俯冲体系多向俯冲汇聚形变与翁通爪哇海台俯冲作用的关联,为大洋板块聚合形变研究添砖加瓦,为海台俯冲可能诱发弧后海盆“反向俯冲”、“多向俯冲”这一新的构造模式提供科学依据,进一步丰富海台俯冲碰撞理论体系。
综合上述可以看出,复杂的沟-弧-盆-台俯冲体系、完整的俯冲时序无疑使得PN-SL俯冲体系成为研究板块俯冲动力学特征的最佳实验室,尽管经过近几十年的研究,学者们对PN-SL俯冲体系俯冲动力特征认识取得了相当大的进展,但距离对其深入了解还相差很远,仍有许多问题未得到很好的解决。例如,受研究目标和地区地球物理探测手段局限,缺乏对PN-SL俯冲体系深部结构的认识,无从知晓俯冲体系各层圈结构,进而难以回答俯冲板片俯冲演化与上覆板片、岛弧岩浆活动、软流圈地幔活动是如何耦合的?PN-SL俯冲体系多期次、多类型俯冲过程的主控因素是什么?密度、流体等因素在上述俯冲过程中扮演的角色是什么?翁通爪哇海台作为PN-SL俯冲体系俯冲构造演化的关键动力要素,翁通爪哇海台俯冲作用导致弧后所罗门海盆形成怎样的构造基本场?二者之间的碰撞聚合时空形变耦合关系怎样?翁通爪哇海台作为世界上最大的海台、大火山岩省,其自身起源、构造迁移演化等方面的诸多争议又为解析其与PN-SL俯冲体系的俯冲构造动力关系增添了更多的复杂性。
致谢:衷心感谢刘永江老师对本文提出的宝贵建议。
Subduction dynamics of the New-Guinea-Solomon arc system: Constraints from the subduction initiation of the plate
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摘要: 新几内亚-所罗门弧(PN-SL)位于印度-澳大利亚板块与太平洋板块汇聚边界、新特提斯构造域东端。晚白垩世以来,逐渐演化形成复杂的沟-弧-盆-台、俯冲时序完整的俯冲构造体系。受多期次、多类型板块俯冲起始作用的制约,PN-SL俯冲体系深部结构呈现出明显的空间差异性:板块俯冲深度由>500 km减小至不足100 km,板块俯冲角度则由>70°减小至30°。俯冲体系东侧毗邻的翁通爪哇海台作为世界上最大的海台,其显著的“凸起”构造以及低密度结构,重新塑造了PN-SL俯冲体系的构造格局,但不同于低密度结构俯冲诱发海沟位置后移、俯冲极性反转二元经典模式,弧后所罗门海盆发生反向俯冲的同时,中新世以来呈现出NW向、NE向和SW向的多向俯冲过程。这意味着翁通爪哇海台与PN-SL俯冲体系汇聚形变过程并非仅依据板块密度变化来简单解释,需要考虑其复杂的构造环境和诸多的构造要素。特别是作为岩石圈强度的重要影响因子—俯冲体系流体活动,导致岩石圈强度减弱、熔点降低的同时,伴随板块俯冲向地球深部运移,促使板片脱水并与地幔楔发生水化交代作用,进而改变壳幔物质组成及流变学性质,诱发地幔楔部分熔融和岛弧岩浆活动,是理解板块俯冲构造动力的关键切入点。Abstract: A complicated subduction system, the New-Guinea-Solomon arc (PN-SL), exists in the convergent boundary between the Indo-Australian and Pacific plates at the eastern end of the Neo-Tethyan tectonic domain. Since late Cretaceous, the PN-SL system has gradually become a complex trench-arc-basin-oceanic plateau system suffered various stages of subduction. Constrained by the multi-stages and multi-types of plate subduction initiation, the deep structure of the PN-SL subduction system varies dramatically in space. Among the subduction zones within the PN-SL subduction system, the extension depth of the subducting plate changes from over 500 km to nearly 100 km and the dip angle of the plate decreases from over 70° to 30°. The Ontong Java Plateau, the largest oceanic plateau in the world, is located in the east of the PN-SL subduction system. Owing to the large crustal bulge and associated low-density structure, the tectonic framework of the PN-SL subduction system is reconstructed. Driven by the subduction of the Ontong Java Plateau, the Solomon Sea back-arc basin has subducted beneath the Pacific ocean towards northwest, northeast and southwest directions since Miocene, sharply contrasted with the classical binary model of the subduction polarity reversal and transference or trench jump induced by the subduction of the buoyant lithosphere. This indicates that the convergent deformation process between the Ontong Java Plateau and the PN-SL subduction system cannot just be interpreted as the change in plate density. Complex tectonic environment and various tectonic elements must be considered in the studies on the subduction and convergent deformation of the oceanic plateau. In particular, as an important influence factor of the strength of the lithosphere, the fluid activity of the subduction system, which may induce the strength weakening and decrease in the melting point of the lithosphere, must be carefully considered. Moreover, the fluid may be transported into the deep part of the Earth together with the subduction of plate and make contributions to the dehydration of plate and the hydro-metasomatism within the mantle wedge, which changes the composition and rheological properties of the crust and mantle and induces partial melting of the mantle wedge and island magmatism. Therefore, it is concluded that fluid plays an important role in the subduction initiation and evolution as a key entry point for understanding the subduction tectonic dynamics of the plate.
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Key words:
- New-Guinea-Solomon arc /
- Ontong Java Plateau /
- subduction tectonic dynamics /
- density /
- fluid
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图 4 PN-SL俯冲体系关键构造演化阶段模式图(修改自Schellart等[3])
WMT.西美拉尼西亚海沟;PB.波克灵顿海盆;SB.所罗门海盆;PT.波克灵顿海沟;NST.北所罗门海沟;NBT.新不列颠海沟;TT.特罗布里恩海沟;WB.伍德拉克海盆;SCT.圣克里斯托瓦尔海沟;BB.俾斯麦海盆
Figure 4. Tectonic evolution of the PN-SL subduction system during the critical periods
WMT. West Melanesian Trench; PB. Pocklington Sea Basin; SB. Solomon Sea Basin; PT. Pocklington Trough; NST. North Solomon Trench; NBT. New Britain Trench; TT. Trobriand Trough; WB. Woodlark Basin; SCT. San Cristobal Trench; BB. Bismarck Sea Basin
图 6 海台俯冲诱发新俯冲带形成示意图
A. 海沟位置向后迁移;B. 俯冲极性反转(修改自Stern等[80];Stern和Gerya[81]);图中S表示缝合线,箭头表示板块俯冲方向
Figure 6. Models of subduction initiation induced by the subduction of oceanic plateau
A. Transference of the trench; B. Polarity reversal. The S denotes the location of the suture line. The arrow illustrates the subduction direction of plate
图 7 翁通爪哇海台俯冲碰撞诱发PN-SL俯冲体系“反向、多向俯冲”示意图(修改自Holm等[4])
Figure 7. Cross-sections showing the reverse and multiple-direction subduction of the PN-SL subduction system induced by the collision of the Ontong Java Plateau
图 8 俯冲体系流体分布及迁移示意图(修改自Rüpke等[101])
Figure 8. Schematic diagram showing spatial distribution and migration of the fluid within the subduction zone
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