The role of impacts on Archaean tectonics

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).