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 <100 km at ca. 3.5 Ga, and it is unclear whether such impacts could perturb the global tectonic system. In this work, we develop numerical simulations of global tectonism with impacting effects, and simulate the evolution of these models throughout the Archaean for given impact fluxes. We demonstrate that moderate-size (∼70 km diameter) impactors are capable of initiating short-lived subduction, and that the system response is sensitive to impactor size, proximity to other impacts, and also lithospheric thickness gradients. Large lithospheric thickness gradients may have first appeared at ca. 3.5–3.2 Ga as cratonic roots, and we postulate an association between Earth’s thermal maturation, cratonic root stability, and the onset of widespread sporadic tectonism driven by the impact flux at this time. INTRODUCTION Solar system impact flux models indicate an extended tail to the accretion process, with basin-forming impacts extending into the Archaean (Bottke et al., 2012; Marchi et al., 2014; Nesvorný et al., 2017; Morbidelli et al., 2017). Comparisons with post–late heavy bombardment, late Imbrian lunar impact rates suggest the production of ∼70 craters with diameters, D, >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).

中文翻译：

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影响对太古代构造的作用

来自皮尔巴拉克拉通（澳大利亚）和卡普瓦尔克拉通（南非）的实地证据表明，现代构造过程可能在大约 3.2 Ga，这段时间也与高密度保存的太古代影响指标有关。最近的工作表明，冥界的大撞击和构造过程之间存在因果关系。然而，撞击通量估计和球粒床特征表明撞击器直径 <100 公里。3.5 Ga，尚不清楚这种影响是否会扰乱全球构造系统。在这项工作中，我们开发了具有冲击效应的全球构造运动的数值模拟，并针对给定的冲击通量模拟了这些模型在整个太古代的演变。我们证明了中等大小（直径约 70 公里）的撞击器能够引发短暂的俯冲，并且系统响应对撞击器尺寸、与其他撞击的接近程度以及岩石圈厚度梯度敏感。大岩石圈厚度梯度可能首先出现在大约 3.5-3.2 Ga 作为克拉通根，我们假设地球的热成熟、克拉通根的稳定性以及此时由撞击通量驱动的广泛零星构造活动的开始之间存在关联。引言 太阳系撞击通量模型表明吸积过程的尾部延伸，盆地形成影响延伸到太古代（Bottke 等人，2012 年；Marchi 等人，2014 年；Nesvorný 等人，2017 年；Morbidelli 等人，2012 年）。 , 2017)。与后期重轰击、晚雨海纪月球撞击率的比较表明，地球上在 3.7 Ga 后产生了 70 个直径 D > 150 公里的陨石坑（例如，Bottke 和 Norman，2017 年）。由于太古代地壳的有限保存，这些事件的陆地记录是不完整的。在南非 Kaapvaal 克拉通的 Barberton 绿岩带（Lowe 和 Byerly，1986 年；Lowe 等人，2014 年）和澳大利亚的 Pilbara 克拉通（例如，Glikson 等人，2016 年）中已经确定了与撞击相关的球粒床）。这些层形成为汽化的撞击物和岩体凝结形成小球体，对于像希克苏鲁布（墨西哥）这样的大撞击，这些层预计会作为全球连续沉积物脱落，这些沉积物保存在有利的沉积环境中。Kaapvaal 克拉通和 Pilbara 克拉通球粒层表明在 3.5-3.2 Ga 时期至少有九次主要影响（Lowe 等人，2014 年；见表 1），其中许多与铱和铬同位素异常有关。球粒层厚度和球粒尺寸分布的建模表明，它们的射弹尺寸范围从 30 公里到 70 公里不等，撞击速度在 18 至 22 公里/秒之间（Johnson 和 Melosh，2012 年）。这些球粒床的测年表明，许多大型撞击聚集在大约 3.46–3.47 Ga（Glikson 等人，2016 年）和 3.2 Ga，三个主要事件，包括最大的估计撞击器（直径 41–70 公里），彼此相距 17 米。巴伯顿绿岩带拥有大部分公认的 3.0 Ga 之前的球粒床，其中一个（层 S1）与皮尔巴拉克拉通相关（Byerly 等，2002；另见 Glikson 和 Vickers，2006）。此外，皮尔巴拉克拉通中的 Marble Bar 燧石拥有两个不同的球粒层（Glikson 等，2016）。最近在ca上的工作。3 Ga Maniitsoq 结构，West Greenland (Garde et al., 2012) 提出了撞击起源。尽管该结构的特征在形成时被掩埋在 20-25 公里处，但基于与航空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、角砾岩化和微观结构变形特征，人们争论了撞击起源。如果为真，这表明周期 3.41-3.47 和约。3.2 Ga 在吸积减弱阶段保留了强烈撞击的显着记录。洛等人。(2003, 2014) 指出，大约在巴伯顿绿岩带中球粒床的形成。3.2 Ga 标志着构造样式的转变。下伏的 Onverwacht 群代表了典型的古太古宙无造山火山体系，以科马提质和玄武质火山作用和化学生物沉积为主。3.2 Ga 代表无花果树群的隆起、变形和陆源碎屑沉积的开始，代表了第一次主要造山运动。洛等人。(2014) 提出了这种造山作用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的开始（Van Kranendonk 等，2007），包括 Karratha 和 Kurrana 地体的裂谷以及第一次威尔逊旋回的可能开始。Van Kranendonk 等。(2007) 认为这可能标志着板块构造过程的开始。最近对板块构造开始的许多估计与 3.2 Ga Pilbara 克拉通记录一致，尽管不确定性范围从约。700 Ma（Stern 等人，2016 年）到 >4.4 Ga（Harrison 等人，2005 年）。大约在构造转变。3.0 Ga 已经从镁铁质中 MgO 的地球化学模型中推断出随着时间的推移（Tang 等人，2016）；来自统计地球化学的镁铁质岩性中 MgO 和 Ni 的拐点，以及表观熔体变化百分比（Keller 和 Schoene，2012）；幼年 Rb/Sr 从主要为基性、薄（~20 公里）、前 3 Ga 地壳到较厚地壳的更高 Rb/Sr 比率的转变（Dhuime 等，2012）；根据 Hf 系统学，太古宙黑色页岩的源材料在 3.0 Ga 时从幼年转变为分化材料（Nebel-Jacobsen 等人，2018 年）；页岩中 Ti 同位素的 3.5 Ga 长英质火山活动增加（Greber 等人，2017 年）。Shirey 和 Richardson (2011) 注意到 Kaapvaal 金伯利岩中的榴辉岩包裹体出现在 3 Ga，并建议从 https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46533.1/ 下载4880247/g46533.pdf 由 Ludwig-Maximilians-University 用户于 2019 年 11 月 23 日提供 2 www.gsapubs.org | 卷XX | 编号 XX | 地质 | 美国地质学会是俯冲起源，从此时推断板块构造。智能等。(2016) 认为太古代钻石中的氮丰度意味着它们是由氧化流体形成的，并推断它在大约 20 年前通过俯冲引入地幔。3.2 Ga. 太古宙地球动力学模拟在很大程度上显示了热的早期地球系统倾向于进入热的、停滞的火山体系，只有在系统冷却时才过渡到板块构造（见 O'Neill 等人，2006）。、2015 年、2018 年以及其中的参考资料）。这些模型中从前板块构造到板块构造的转变是非常非线性的，并且可能表现出许多错误的开始，与地质观测一致（O'Neill 等，2018），在此期间系统可能对外部因素敏感，包括影响。先前对 Hadean 大撞击地球动力学效应的建模（O'Neill 等人，2017 年）表明，由于撞击加热的热浮力，极大的撞击火流星（直径 >~700 公里）直接引发了活动构造和俯冲。地幔。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，建议在 3 时开始板块构造。2 Ga 会出现在一个与冥王星截然不同的热状态的行星上。目前尚不清楚（1）中太古代影响的拟议规模是否在此时开始了俯冲，或者（2）俯冲可能已经自我延续，并且实际上开始了正在进行和持续的板块构造过程。我们研究的目的是评估中太古代撞击体的拟议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成为自我永存的全球板块构造，或者他们是否失败了（Moyen 和 van Hunen，2012 年；O'Neill 等人，2018 年）。目前尚不清楚（1）中太古代影响的拟议规模是否在此时开始了俯冲，或者（2）俯冲可能已经自我延续，并且实际上开始了正在进行和持续的板块构造过程。我们研究的目的是评估中太古代撞击体的拟议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成为自我永存的全球板块构造，或者他们是否失败了（Moyen 和 van Hunen，2012 年；O'Neill 等人，2018 年）。目前尚不清楚（1）中太古代影响的拟议规模是否在此时开始了俯冲，或者（2）俯冲可能已经自我延续，并且实际上开始了正在进行和持续的板块构造过程。我们研究的目的是评估中太古代撞击体的拟议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成为自我永存的全球板块构造，或者他们是否失败了（Moyen 和 van Hunen，2012 年；O'Neill 等人，2018 年）。