Far from boring: a new Grenvillian perspective on Mesoproterozoic tectonics

As determining when plate tectonics began on Earth is a highly debated subject, it is crucial to understand the “boring billion” (1.8 to 0.8 billion years ago), a period of tectonic quiescence inferred from proxies, such as the average chemical composition of the mineral zircon on Earth and the isotopic composition of seawater derived from marine rocks. Yet this period saw the construction of what may have been the biggest mountain belt that ever existed, the remnants of which are found in the Grenville Orogen of eastern North-America. This contribution �rst exposes a compilation of multidisciplinary geological datasets and new geochemical data from igneous suites emplaced during the Grenvillian Orogeny that are incompatible with the current tectonic paradigm. We then present a completely revised model for Grenvillian tectonics. In contrast with the actual Laurentian-centred paradigm, our model involves the construction of a newly revealed continent by amalgamation of volcanic arcs far away from Laurentia (the craton forming the core of actual North-America) and their collision 60 millions year later than the currently accepted timing. This new model resolves the longstanding contradiction between tectonic proxies and geological record and invalidates the view considering the Mesoproterozoic as a tectonically quiet Era.


Main Text
The Mesoproterozoic Era, more speci cally 1.8-0.8 billion years (Gyr) ago is one of the most enigmatic period of time on Earth that has been referred to as "the boring billion" 1 .It was related to a prolonged phase of tectonic quiescence [2][3][4][5] from several proxies of tectonic activities, notably: 1) the paucity of passive margins in the geological record 6 ; 2) a systematic decrease in normalized seawater 87 Sr/ 86 Sr ratio 7 , which is a hallmark of juvenile (igneous rocks produced directly by mantle melting) rock erosion.
Detrital zircon (of sedimentary origin) of this Era collected on all continents on Earth further show: 3) a systematic decrease in Eu anomaly inferred to indicate a decrease of the average crustal thickness 2 ; 4) a stable high value in εHf indicating a high average mantle input in igneous activity 8 ; as well as 5) a peak in abundance 9,10 and 6) a peak in δO 18 1.1 Gyr-ago 11 .Although, these last two proxies have been related to collisional orogenesis 10 , there is compelling evidence that zircon abundance correlates with subduction ux 12 , and high δO 18 in zircon is usually interpreted as evidence for sediments incorporated into subduction zones 13 .Accepting the view that plate tectonics was active during the Mesoproterozoic [14][15][16] , these characteristics suggest that Earth was dominated by volcanic arcs formed above subducting oceanic crust.This setting derived from proxies of tectonic activity contrast greatly with the geological record.Indeed, the Mesoproterozoic saw the formation of a mountain belt, now exposed as the Grenville Province (Fig. 1), covering more than 25% of the globe with and extensive paleo-uvial system shedding sediments throughout Laurentia 17 .The current paradigm postulates that it formed through a complex evolution, rst involving the growth of an extensive continental arc, very similar to the Andes, by NW-directed (present coordinates) subduction underneath Laurentia between 1.8 and 1.3 Gyr-ago, followed by episodes of continental back-arc opening and closing with localized arc accretions 18,19 .This evolution would have culminated in a collision between Laurentia and another continent, inferred to have been Amazonia (e.g. 20), during the Grenvillian Orogeny, that would have started at 1.1 Gyr-ago.This extensive mountain belt, referred to as the orogenic climax of Earth by some authors 21 , should have strongly in uenced the above-mentioned proxies.The crust would have been thick for most of this period, which should have resulted in a high Eu anomaly, its weathering should have increase 87 Sr/ 86 Sr 7 ratio and decrease the εHf in zircon.Finally, subduction ux would have ceased at 1.1 Gyr-ago, not peaked as suggested by zircon abundance and δO 18 .All these predictions are the opposite of the measured values.
Adding complexity to this paradox is the tectonic setting within the interior of Laurentia.Located in the footwall of the current Grenville Front, the Midcontinent rift (MCR on Fig. 1) and the Southwestern Large Igneous Province 23 were major ma c magmatic events that required signi cant extension of the crust from 1.15 to 1.08 Gyr-ago 24 .Synchronously, major sedimentary basins were created by rifting 25 more than 3000 km towards the hinterland (Fig. 1).Furthermore, recent eld-based and geochemical studies of sedimentary rocks in the MCR documented wave-in uenced structures and marine sedimentation 26,27 .
These results have major implications for Grenvillian tectonism because it suggests that the MCR formed an embayment open to the ocean and that the entire Laurentian continent was under extensional stress at the same time as the Grenvillian continental collision was supposedly at its climax.
In light of all these contradictions, we consider that a critical evaluation of the geological record is required to investigate whether the data could be compatible with another tectonic model than the currently accepted Grenvilian paradigm.

Geological setting
The Grenville Province is subdivided into two belts separated by the Allochthon Boundary Thrust (ABT).In the footwall of the ABT, the Parautochtonous Belt consists of Laurentian basement rocks overlain by a Paleoproterozoic passive margin sedimentary sequence deposited 1.9 Gyr-ago 19 .In the hanging wall of the ABT, the Allochthonous Belt consists of (all interpretations of tectonic settings according to the current paradigm 19 ): i) juvenile arc rocks formed 1.8-1.6Gyr-ago accreted to Laurentia shortly after their formation; ii) extensive granitoids emplaced in a continental-arc setting 1.5 to 1.3 Gyr-ago; iii) sediments and igneous rocks deposited/emplaced in a continental back arc setting 1.5 and 1.3-1.2Gyr-ago; iv) two volcanic arcs accreted to the continental arc; v) anorthosite-mangerite-charnockite-granite massifs mainly emplaced 1.15 Gyr-ago.The Grenvillian Orogeny itself is subdivided into two main orogenic phases, an early Ottawan and a late Rigolet phase 19,20 .The former phase occurred between 1.09 and-1.02Gyr-ago, is only recorded in the Allochthonous Belt (Fig. 2) and is considered as a Himalayan-style continentcontinent collision, characterized by the ow of hot ductile crust below an orogenic plateau 28 .Renewed convergence during the Rigolet phase, from 1.01 to 0.98 Gyr-ago 22 , propagated deformation towards the foreland and both the reworked Laurentian basement rocks and its sedimentary cover underwent partial melting at high pressure 29 .
The period between the continental arc and collisional phases is contentious.There is evidence for highgrade metamorphism and associated deformation at different times and places between 1.3 and 1.1 Gyrago 19 with the most spatially extensive phase occurring during the "Shawinigan Orogeny" between 1.17 and 1.15 Gyr-ago.Models of continental back-arc closure 30 , volcanic arc accretion 31 and beginning of continental collision 18 have been proposed for this period and are contradictory with each others.

1.8-1.2 Gyr-ago: comparison with the Andes
As the current tectonic paradigm for the rst phase of construction of the Grenville Province (1.8-1.2Gyrago) is very similar to that of the Andes 18 , both orogens should share key characteristics.Rivers and Corrigan 18 presented several similarities between the two including protracted magmatism (> 300 Myr for the Grenville) and several phases of back-arc sedimentation associated with bimodal volcanism, but other characteristics diverge.First, basement inliers crop out along the entire length of the Andes 32 , whereas there are no Laurentian basement rocks in the Allochtonous Belt of the Grenville Province.
Secondly, according to the Andean model, there should have been igneous rocks emplaced in the plate overlying the subduction zone, thus in the overlying Laurentian rocks, however there are no such rocks in the Parautochtonous Belt (Fig. 2).
Andean volcanic rocks present ubiquitous evidence for crustal contamination of subduction-derived magma 33,34 .To test whether igneous rocks of the Grenville Province also present evidence for contamination, we extracted geochemical data for similar ma c rocks from the two orogens.On the Pearce diagram (Fig. 3A), Andean rocks de ne a eld above the mantle array towards high Th values indicating crustal contamination, whereas Grenvillian rocks plots in a eld centered on the array indicating igneous rocks extracted directly from the mantle 35 .
Another way to determine the extent of crustal contamination is to examine the difference between Ndmodel ages, indicating the timing of extraction from the depleted mantle, and U-Pb ages for igneous suites, indicating the timing of their emplacement.In the Central Andes, there is a difference of at least 0.8Gyr between Nd-model ages and emplacement (U-Pb) ages of Neogene-Quaternary volcanics 33,34 , whereas it is less than 0.22 Gyr in the Grenville Province (Fig. 3B).This is best explained by extensive contamination of recent ma c magma by crustal rocks 1 Gyr older in the Andes, but minimal contamination of Grenvillian magma by a basement that should have been 1.7 Gyr older (Archean Laurentian basement).
1.2-0.95Gyr-ago: testing the Grenville Orogen paradigm Other geological features associated with the Grenvillian Orogeny do not t with the current tectonic paradigm.The current model involves thrusting of the Allochthonous Belt over the Laurentian basement along the ABT 1.1-1.05Gyr-ago.There should thus be a tectono-metamorphic imprint in the underlying Parautochthonous Belt, but temporal constraints for deformation and metamorphism document deformation at amphibolite to granulite grade only 40-60 Myr later in this belt (Fig. 2).
Tectonic discrimination diagrams based on immobile trace elements composition of ma c and felsic igneous suites emplaced before and during the accepted timing of orogeny also challenge the current model.Ma c suites with U-Pb emplacement ages between 1.15 and 1.09, 1.09-1.03and <1.03 Gyr-ago plot in the subduction-related, Andean and subduction-related elds, respectively (Fig. 3C).Similarly, felsic suites dated between 1.15-1.03and <1.03 Gyr-ago plot in the arc and slab failure to A-type elds, respectively (Fig. 3D).Combined, these datasets indicate a volcanic arc setting with minimal crustal contamination between 1.15 and 1.09 Gyr-ago that evolved into an Andean setting between 1.09 and 1.03 Gyr-ago, at which point the slab broke and lead to the intrusion of magma derived from mantle melting.This geochemically-derived tectonic evolution is, therefore, strikingly different than an Andean setting until continental collision between 1.09 and 0.95 Gyr-ago.

A new tectonic model
We propose a radically new model for the tectonic evolution of the Grenville Province that reconciles all the issues raised above.After the passive margin phase 1.9 Gyr-ago, intra-oceanic subduction developed thousands of kilometers away from Laurentia and formed several volcanic arcs accompanied by opening and closing of back-arc basins between 1.7 and 1.2 Gyr-ago (Fig. 4A).The nal amalgamation of all arcs occurred during the Shawinigan Orogeny between 1.17 and 1.15 Gyr-ago (Fig. 4B) to form a microcontinent, here named Shawiniga.Andean-style tectonism then started with southward subduction of oceanic crust beneath Shawiniga.We follow Swanson-Hysell et al. 24 who proposed a model of slab avalanche dragging Laurentia rapidly southward to explain the fast plate motion derived from paleomagnetic data from the MCR for the period between 1.1 and 1.08 Gyr-ago (Fig. 4C).In accordance with numerical model of slab avalanche predicting shortening in the upper plate 38 , we further argue that fast subduction underneath Shawiniga lead to signi cant crustal shortening and thickening (Fig. 4C).Because it is separate from the later continental collision phase, we propose the name Ottawan Orogeny for this slab-avalanche phase, which was previously considered as the initial phase of the Grenvillian Orogeny.
Collision between Laurentia and Shawiniga occurred during what was previously considered as the second phase of the Grenvillian Orogeny (the Rigolet).We estimate the timing of this collision at ca. 1.03 Gyr-ago based on the drastic change of chemical composition for ma c and felsic igneous rocks (Fig. 2, 3C & D), and the timing of metamorphism in both the Parautochtonous and Allochtonous belts suggest it lasted until ca.0.96 Gyr-ago (Fig. 2).

Discussion
The new Grenvillian tectonic model presented herein (Fig. 4) is compatible with more geological data than the current tectonic paradigm.All the geological features of the Allochtonous Belt described in the geological setting and comparison with the Andes sections could have formed in oceanic arcs rather than continental arcs (Fig. 4A).The age pattern of detrital zircons from ca. 1.5-1.3Ga Grenvillian basins in the Allochthonous Belt, including 2.7 and 1.8 Ga peaks, was used to imply a Laurentian a nity 39 , however it was demonstrated that such peaks are common to all continents and therefore are not diagnostic 9 .Amalgamation of several arcs (Fig. 4B) is also compatible with the spatially and temporally variable events of metamorphism and deformation documented between 1.3 and 1.1 Gyr-ago 19 .Most importantly, the new model reconciles the absence of Laurentia in both the rock record and geochemical characteristics in the Allochthonous Belt (Fig. 3A&B), as well as the absence of Mesoproterozoic subduction-related intrusions in the Parautochthonous Belt (Fig. 2).Furthermore, a collision between the newly-named continent Shawiniga (comprising rocks of the Allochthonous Belt) and Laurentia 1.03 Gyrago provides an elegant explanation for the lack metamorphic and deformation imprints in the Parautochtonous Belt 1.1-1.05Gyr-ago, but reworking of Allochthonous metamorphic rocks between 1.03 and 0.95 Gyr-ago (Fig. 2).Finally, tectonic discrimination diagrams for felsic and igneous rocks emplaced between 1.15 and 0.95 Gyr-ago present a pattern that strongly supports the model presented herein (Fig.

3C & D).
In this new model, Laurentia was not involved in Grenvillian orogenesis until its latest stage.Laurentia was thus a passive margin throughout most of the Mesoproterozoic, with access to sea water such as documented in sedimentary rocks of the MCR 26,27 .The extremely fast motion of Laurentia between 1.1 and 1.08 Gyr-ago dragged by slab avalanche, would likely have put the entire continent under extension and generated all the major sedimentary basins and large igneous province such as the MCR that formed during this period 24,25 .Moreover, our model better reconciles proxies of tectonic activities used to infer tectonic quiescence during the so-called "boring billion".First, one of the main arguments for tectonic quiescence, the paucity of passive margins, is invalidated by our model postulating that the entire SE margin of Laurentia would have been a passive margin from 1.9 to 1.03 Gyr-ago (Fig. 4A-C).Most of the sediments deposited during this phase are di cult to recognize because they were deeply buried, metamorphosed and deformed when Laurentia was underthrust by Shawiniga.Second, the consumption of several 1000's of km of ocean crust during the slab-avalanche event proposed in our model is a special event that would likely have resulted in a peak in subduction proxies ca.1.1.Gyr-ago, such as the peak in zircon abundance and their δO 18 values observed in worldwide detrital zircon 9,11,40 .Similarly, by removing the protracted Andean phase and reducing the duration of collisional orogenesis, the proposed Grenvillian model is more compatible with the low crustal thickness inferred from Eu anomaly in zircon 2 .Finally, the recorded decreasing 87 Sr/ 86 Sr 7 ratio in seawater and high εHf in zircon recorded for the Mesoproterozoic 7,8,16 are the expected proxies for our model, dominated by oceanic arcs until 1.1 Gyr-ago (Fig. 4A-C), that would have shed large volume of juvenile sediments in the oceans.Inasmuch as the southward motion of Laurentia 24 suggests that these arcs were at equatorial latitudes, this setting should have led to a cold Earth 41 prior to the assembly of the supercontinent Rodinia.
In conclusion, recasting Grenvillian tectonics into a model involving the formation of a microcontinent comprising several oceanic arcs that would have collided with Laurentia 60 Myr after the currently accepted timing of collision, allows to integrate a larger multidisciplinary geological dataset and tectonic proxies into an internally consistent tectonic model.In contrast with recently proposed views [2][3][4][5] , it demonstrates that the Mesoproterozoic was far from being boring 15,21,42 and tectonically active.We anticipate that this new perspective on Grenvillian tectonism will unlock new research on slab-avalanche in uence on mountain-building processes, paleogeographic reconstruction, Artic basins formation, evolution of plate tectonics through time, as well as quantifying the initial conditions for modeling climate change associated with the transformation of Earth into a Snowball 43 .

Methods
For gure 3A and 3C, geochemical data for basaltic rocks of intrusive suites from the Andes and from the Grenville allochthonous belt were compiled.The Andean arc geochemical data was taken from Georock database (http://georoc.mpch-mainz.gwdg.de).The data from the Grenville province were taken from published data from the literature and unpublished data from Quebec's geoscienti c database SIGEOM.Dykes and intrusions were taken out from this dataset as they are likely post-orogenic late intrusions.Filters were applied to focus on basaltic rocks according to Pearce, 1996: Si <55wt%; Al2O3>20wt%; Sc > 50 ppm and Ni > 200ppm.For gure 3D, granitic rocks data were compiled from published data.A silica lter was applied (Si 55 to 70wt%) for granitic rocks.Diagrams were made using GCDkit software.For gure 3B, depleted mantle model ages (TDM) were compiled (Dickin et al., 2010; Dickin, 2000; Dickin and  McNutt, 2007; Dickin et al., 2016) and overlain on a geological map from the Quebec's geoscienti c database system SIGEOM with QGIS.All data points falling within an igneous suite of known U-Pb age on the time-slice geological maps from https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-grenville_en/ were retained and the difference between U-Pb and TDM ages were used to create the boxplot shown on gure 3B.Orogen-perpendicular 'geochronogram' through the Grenville Province.Ages of granitoid and ma c suites as well as Grenvillian metamorphic ages were compiled from the literature and projected onto a NW-SE plan orthogonal to the average strike of the ABT, the main tectonic discontinuity of the orogen, and positioned according to their orthogonal distance to the ABT.The a nity of granitoid and ma c rocks were determined from the geochemical tectonic-discrimination diagrams presented on gures 3C and 3D (colour code is the same on both gures).Notice the change in geochemical a nities for both types of intrusions 1.03 Gyr-ago.Also notice the absence of metamorphic ages older than 1.01 Gyr-ago in the Parautochtonous Belt (PB), but the presence of metamorphic ages of all ages within the Allochtonous Belt (AB).See text for details and interpretations.

Figures Figure 1
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