The role of impacts on Archaean tectonics
The role of impacts on Archaean tectonics
复制标题
DOI：
10.1130/g46533.1
复制
发表时间：
2020-02
期刊：
影响因子：
5.8
通讯作者：
C. O'Neill;S. Marchi;W. Bottke;R. Fu
中科院分区：
文献类型：
--
作者：
C. O'Neill;S. Marchi;W. Bottke;R. Fu
来自皮尔巴拉克拉通（澳大利亚）和卡普瓦尔克拉通（南非）的现场证据表明，现代构造过程可能已经在大约3.2 Ga，这个时间也与保存的太古宙撞击指标的高密度有关。最近的研究表明，巨大的影响与冥古宙的构造过程之间存在因果关系。然而，撞击通量估计和球体床特征表明，3.7 Ga 后地球上的撞击器直径为 150 公里（例如，Bottke 和 Norman，2017 年）。由于太古宙地壳的保存有限，这些事件的陆地记录并不完整。在南非卡普瓦尔克拉通的巴伯顿绿岩带（Lowe 和 Byerly，1986；Lowe 等，2014）和澳大利亚皮尔巴拉克拉通（例如 Glikson 等，2016）中已经发现了与撞击相关的球粒床。这些层随着蒸发的撞击物而形成，岩体凝结形成小球体，对于希克苏鲁伯（墨西哥）等大型撞击，预计这些小球体将作为全球连续的沉积物掉落，并保存在有利的沉积环境中。 Kaapvaal craton 和 Pilbara craton 球体层表明在 3.5–3.2 Ga 期间至少发生了九次重大影响（Lowe 等人，2014 年；见表 1），其中许多与铱和铬同位素异常有关。小球层厚度和小球尺寸分布的建模表明，它们的射弹尺寸范围从 ∼30 到 70 km，撞击速度在 18 到 22 km/s 之间（Johnson 和 Melosh，2012）。这些小球床的年代测定表明，许多大型撞击集中在大约。 3.46–3.47 Ga (Glikson et al., 2016) 和 3.2 Ga，发生了三起重大事件，包括最大的估计撞击事件（直径 41–70 km），发生在 17 m.y 内。彼此的。巴伯顿绿岩带拥有大多数公认的 3.0 Ga 之前的球体层，其中一个（S1 层）横跨皮尔巴拉克拉通（Byerly 等人，2002 年；另见 Glikson 和 Vickers，2006 年）。此外，皮尔巴拉克拉通的大理石坝燧石具有两个不同的球体层位（Glikson 等，2016）。最近关于 ca 的工作。 3 Ga Maniitsoq 结构，西格陵兰岛（Garde 等，2012）表明了撞击起源。尽管该结构在形成时埋藏在20-25公里处，但根据与航磁异常相关的区域圆形变形、修正的平面变形特征、广泛的断裂、角砾化和微观结构变形特征，争论了撞击起源。如果属实，这表明周期 3.41–3.47 和大约。 3.2 Ga 在吸积的衰弱阶段保留了强烈撞击的显着记录。洛等人。 (2003, 2014) 指出，巴伯顿绿岩带中球粒床的形成时间约为 10 年。 3.2 Ga标志着构造样式的转变。下方的 Onverwacht 群代表了典型的古太古代非造山火山体系，以科马提岩和玄武质火山作用以及化学生物沉积为主。 3.2 Ga代表无花果树群中隆起、变形和陆源碎屑沉积的开始，代表第一次主要造山运动。洛等人。 （2014）提出了造山运动与保存下来的撞击事件之间的因果关系。 3.2 Ga还标志着皮尔巴拉克拉通主要横向构造运动的开始（Van Kranendonk等，2007），包括卡拉萨和库拉纳地体的裂谷以及第一威尔逊旋回的可能开始。范·克拉南东克等人。 （2007）认为这可能标志着板块构造过程的开始。最近对板块构造起始的许多估计与 3.2 Ga 皮尔巴拉克拉通记录一致，尽管不确定性范围约为700 Ma（Stern 等人，2016）至 >4.4 Ga（Harrison 等人，2005）。约发生构造转变。 3.0 Ga 是根据镁铁质中 MgO 随时间变化的地球化学模型推断出来的（Tang 等，2016）；根据统计地球化学，镁铁质岩性中 MgO 和 Ni 的变化以及表观熔融百分比变化（Keller 和 Schoene，2012 年）；新生的 Rb/Sr 从主要是镁铁质、薄（约 20 km）、前 3 Ga 地壳转变为从较厚的地壳到较高的 Rb/Sr 比率（Dhuime 等，2012）；根据 Hf 系统学，太古代黑色页岩的源材料在 3.0 Ga 处从幼年材料转变为分化材料（Nebel-Jacobsen 等，2018）；页岩中 Ti 同位素的长英质火山活动增加至 3.5 Ga（Greber 等，2017）。 Shirey 和 Richardson（2011）指出，钻石中的榴辉岩包裹体出现在 Kaapvaal 金伯利岩中的 3 Ga 处，并建议由 Ludwig-Maximilians-University 用户于 23 日从 https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46533.1/4880247/g46533.pdf 下载2019 年 11 月 2 www.gsapubs.org |第 XX 卷 | XX 号 |地质学 |美国地质学会提出俯冲起源，从这个时期推断板块构造。斯马特等人。 (2016) 认为太古宙钻石中的氮丰度意味着它们是由氧化流体形成的，并推断其在大约 10 年前通过俯冲作用引入地幔。 3.2 Ga. 太古宙地球动力学模拟在很大程度上表明了早期地球的炎热系统倾向于进入炎热、停滞的火山状态，只有当系统冷却时才会转变为板块构造（参见 O’Neill 等人，2015 年、2018 年以及其中的参考文献）。在这些模型中，从板块前构造到板块构造的转变是非常非线性的，并且可能会表现出许多错误的开始，这与地质观测结果一致（O’Neill et al., 2018），在此期间系统可能对包括影响在内的外部因素敏感。先前对冥宙大撞击地球动力学效应的建模（O’Neill et al., 2017）表明，由于撞击加热地幔的热浮力，极大的撞击火流星体（>~700 km直径）直接引发活动构造和俯冲。它还表明，如果发生在已经准备好俯冲的岩石圈上，更小的撞击可能会引发俯冲。然而，所提出的 3.2 Ga 板块构造起始点将发生在一颗与冥古宙热状态截然不同的行星上。目前尚不清楚（1）中太古代撞击的拟议规模是否可能在此时引发俯冲，或者（2）俯冲可能是自我延续的，并且实际上开始了持续不断的板块构造过程。我们研究的目的是评估中太古代撞击体的大小和通量是否可能引发俯冲事件，评估有利于构造的地球动力学因素，并确定此类事件是否可能发展为自我延续的全球板块构造，或者它们是否失败（Moyen和van Hunen，2012；O’Neill等，2018）。
Field evidence from the Pilbara craton (Australia) and Kaapvaal craton (South Africa) indicate that modern tectonic processes may have been operating at ca. 3.2 Ga, a time also associated with a high density of preserved Archaean impact indicators. Recent work has suggested a causative association between large impacts and tectonic processes for the Hadean. However, impact flux estimates and spherule bed characteristics suggest impactor diameters of 150 km on Earth after 3.7 Ga (e.g., Bottke and Norman, 2017). The terrestrial record of these events is incomplete due to limited preservation of Archaean crust. Impact-related spherule beds have been identified in the Barberton greenstone belt in the Kaapvaal craton, South Africa (Lowe and Byerly, 1986; Lowe et al., 2014), and the Pilbara craton, Australia (e.g., Glikson et al., 2016). Such layers form as vaporized impactor and rock mass condense to form small spherules, which, for large impacts such as Chicxulub (Mexico), are expected to fall out as globally contiguous deposits, which are preserved in favorable sedimentary environments. The Kaapvaal craton and Pilbara craton spherule layers suggest at least nine major impacts in the period 3.5–3.2 Ga (Lowe et al., 2014; see Table 1), many associated with iridium and chromium isotope anomalies. Modeling of the spherule layer thickness and spherule size distribution suggests that they ranged in projectile size from ∼30 up to 70 km, with impact velocities between 18 and 22 km/s (Johnson and Melosh, 2012). Dating of these spherule beds suggests that many large impacts cluster at ca. 3.46–3.47 Ga (Glikson et al., 2016) and 3.2 Ga, with three major events, including the largest estimated impactor (41–70 km in diameter), occurring within 17 m.y. of each other. The Barberton greenstone belt hosts most of the recognized pre–3.0 Ga spherule beds, with one (layer S1) correlated across the Pilbara craton (Byerly et al., 2002; see also Glikson and Vickers, 2006). Additionally, the Marble Bar chert in the Pilbara craton hosts two distinct spherule horizons (Glikson et al., 2016). Recent work on the ca. 3 Ga Maniitsoq structure, West Greenland (Garde et al., 2012), has suggested an impact origin. Despite the features of this structure being buried at 20–25 km at the time of formation, an impact origin is argued based on regional circular deformation associated with an aeromagnetic anomaly, modified planar deformation features, widespread fracturing, brecciation, and microstructural deformation features. If true, this suggests that the periods 3.41–3.47 and ca. 3.2 Ga preserve a remarkable record of intense impacting during the waning stages of accretion. Lowe et al. (2003, 2014) noted that the formation of the spherule beds in the Barberton greenstone belt at ca. 3.2 Ga marked a transition in tectonic style. The underlying Onverwacht Group represents a typical Paleoarchean anorogenic volcanic regime dominated by komatiitic and basaltic volcanism and chemo-biological sedimentation. 3.2 Ga represents the onset of uplift, deformation, and terrigenous clastic sedimentation in the Fig Tree Group, representing the first major orogeny. Lowe et al. (2014) suggested a causative link between this orogenesis and the preserved impact events. 3.2 Ga also marks the onset of major lateral tectonics in the Pilbara craton (Van Kranendonk et al., 2007), including the rifting of the Karratha and Kurrana terranes and the possible onset of the first Wilson cycle. Van Kranendonk et al. (2007) suggested that this may mark the onset of plate tectonic processes. Many recent estimates for the initiation of plate tectonics concur with the 3.2 Ga Pilbara craton record, albeit with an uncertainty range from ca. 700 Ma (Stern et al., 2016) to >4.4 Ga (Harrison et al., 2005). A tectonics transition at ca. 3.0 Ga has been inferred from geochemical models of MgO in mafics through time (Tang et al., 2016); inflections in MgO and Ni in mafic lithologies, and apparent percent melt changes, from statistical geochemistry (Keller and Schoene, 2012); a shift in juvenile Rb/Sr from primarily mafic, thin (∼20 km), pre–3 Ga crust, to higher Rb/Sr ratios from thicker crust (Dhuime et al., 2012); a shift at 3.0 Ga in the source material of Archaean black shales from juvenile to differentiated material, from Hf systematics (Nebel-Jacobsen et al., 2018); and increased felsic volcanism from 3.5 Ga from Ti isotopes in shales (Greber et al., 2017). Shirey and Richardson (2011) noted that eclogitic inclusions in diamonds appear at 3 Ga in Kaapvaal kimberlites, and suggested Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46533.1/4880247/g46533.pdf by Ludwig-Maximilians-University user on 23 November 2019 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America a subduction origin, inferring plate tectonics from this time. Smart et al. (2016) argued that the nitrogen abundance in Archaean diamonds imply that they formed from an oxidized fluid and inferred its introduction into the mantle, via subduction, before ca. 3.2 Ga. Archaean geodynamics simulations have largely shown the proclivity of hot early-Earth systems to enter a hot, stagnant volcanic regime, transiting to plate tectonics only as the system cools (see O’Neill et al., 2015, 2018, and references therein). The transition from pre–plate tectonics to plate tectonics in these models is very nonlinear, and may exhibit many false starts, consistent with geological observations (O’Neill et al., 2018), during which the system may be sensitive to external factors, including impacts. Previous modeling of the geodynamic effects of large impacts in the Hadean (O’Neill et al., 2017) showed that extremely large impacting bolides (>∼700 km diameter) directly initiate active tectonics and subduction due to the thermal buoyancy of impact-heated mantle. It also demonstrated that much smaller impacts could act as triggers for subduction if they occurred on lithosphere that was already primed for subduction. However, the proposed initiation of plate tectonics at 3.2 Ga would have occurred on a planet in a vastly different thermal regime to that of the Hadean. It is not clear whether (1) the proposed size of the Mesoarchean impacts could have initiated subduction at this time, or (2) subduction could have been self-perpetuating, and in fact started ongoing and continuing plate tectonic processes. The purpose of our study is to assess whether the proposed size and flux of impacting bodies in the Mesoarchean could have initiated subduction events, assess the geodynamic factors favorable to tectonics, and determine if such events could have developed into self-perpetuating global plate tectonics, or whether they failed (Moyen and van Hunen, 2012; O’Neill et al., 2018).