Australia: The Land Where Time Began
Plate Motion Velocities – Palaeomagnetic Evidence for Modern-Like Velocities at 3.2 Ga
In the Archaean, 4.0 to 3.2 billion years ago (Ga), the modes and rates of tectonic processes and lithospheric growth are subjects of considerable debate. The discussion may be contributed to by quantifying plate velocities from the past. In this paper Brenner et al. report a palaeomagnetic pole for the ~3.180 million years ago (Ga) Honeyeater Basalt of the eastern Pilbara Craton, Western Australia, which is supported by a positive fold test and micromagnetic imaging. Comparison of the palaeolatitude of the Honeyeater Basalt, 44o ± 15o, with palaeolatitudes that have been reported previously requires that the average latitudinal drift rate of the East Pilbara was ≥2.5 cm/year during the about 170 Myr that preceded 3.180 Ga, a velocity that is comparable with plates of the present. According to Brenner et al. this result is the earliest unambiguous evidence that has been uncovered so far for long-range lithospheric motion. If it is assumed that this motion is primarily due to plate motion instead of true polar wander, the result is consistent with uniformitarian or episodic tectonic processes in place by 3.2 Ga.
Throughout the recent geological history of the Earth plate tectonics has been the dominant surface geodynamical regime. In modern plate tectonics a defining feature has been the differential horizontal motion or rigid lithospheric plates. Evidence of plate tectonic or “mobile-lid” processes that include subduction, collisional orogeny, rifting and spreading of oceans, is preserved in the physiography and composition of the modern crust of the Earth. The case for the Earth during the Archaean, from 4.0 to 2.5 Ga, is not clear. The crust from the Archaean that has survived to the present consists of ~35 cratons (Bleeker, 2003), most of which have a characteristic architecture of rounded granitoid intrusive domes that are rimmed by greenstone keels that dip steeply (Thurston, 2003). Composition of extant Archaean crust is substantially more mafic than the composition of modern oceanic crust, having a high fraction of ultramafic rocks such as komatiites (Thurston, 2003; Tang, Chen & Rudnick, 2016; Greeber et al., 2017). A number of proposals that Archaean crust was constructed by exotic processes, which include plume tectonics (Van Kranendonk, 2010; Fischer & Gerya, 2016), sagduction/drip tectonics (Fischer & Gerya, 2016; Sizova et al., 2015), and lithosphere that is vertically overturning (Chardon, Choukroune & Jayananda, 1996; Wiemer et al., 2018), have resulted from these structural and compositional differences. Alternative geodynamical regimes have been proposed for the Earth during the Archaean, as some of these processes are difficult to reconcile with plate mobility, including stagnant-lid and sluggish-lid modes (Lenardic, 2018; Bédard, 2018) in which the lithosphere was rendered immobile, or at least slowed, as a result of decoupling from the asthenosphere under elevated geothermal gradients (Korenaga, 2006). It has been argued for an uniformitarian model of the Earth during the Archaean by some other studies, according to which some variant of plate tectonics of the present was in operation, at least locally, throughout the history of the Earth. Complete understanding of the lithosphere, hydrosphere, atmosphere and biosphere is predicated on being able to distinguish between these proposed models of the geodynamics during the Archaean. In order to gain insights into these components of the inner early Earth it is necessary to gain knowledge of the inner workings of the terrestrial planets generally and which surface conditions and environments hosted the development of first life.
Inferences of a regime transition towards plate tectonics of a modern style are the basis for arguments for alternative geodynamical regimes in the Archaean. Estimates that have been made of the age for such a transition range from the Neoproterozoic to the Hadean, invoking a range of observations that include local and global geochemical records (Dhuime et al., 2012; Korenaga, 2018; Shirey & Richardson, 2011), field observations of possible syn-tectonic rocks (Stern, 2005; Van Kranendonk et al., 2010; Turner et al., 2014), and comparisons of palaeomagnetic poles (Evans & Pisarevsky, 2008; Cawood, Kroner & Pisarevsky, 2006; Cawood et al., 2018; O’Neill, Turner & Rushmer, 2018). The rate of horizontal motion of plates over the surface of the Earth is a key discriminant between stagnant and mobile-lid regimes. Over the last 400 Myr (Matthews, 2016) absolute velocities have typically been ~2 – 10 cm/year, with extremes (0-25 cm/year), while velocities that heave been hypothesised for stagnant and sluggish-lid models are typically less than 2 cm/year (Fuentes et al., 2019). The velocity of crustal blocks in deep geological time may be constrained by palaeomagnetic methods by measuring their history of apparent polar wander. Robust palaeomagnetic evidence for latitudinal motion has, however, been lacking, thus far for times earlier than 2.8 Ga (Evans & Pisarevsky, 2008; Cawood, Kroner & Pisarevsky, 2006; Cawood et al., 2018; O’Neill, Turner & Rushmer, 2018). In this study Brenner et al. have produced a new palaeomagnetic pole from ~3.180 Ga volcanics in the East Pilbara Craton of Western Australia and use this result to assess the presence of plate tectonic-like processes on Earth prior to that time.
Geologic settings of the Honeyeater Basalt
In the East Pilbara Craton in Western Australia and the Kaapvaal Craton of South Africa the Greenstone Belts are 2 of the only well exposed regions of the Earth that have preserved rocks that are ≥3.0 Ga that have been metamorphosed to greenschist facies conditions or lower (250oC – 500oC, 1 to 8 kbar), which has made them potentially suitable for palaeomagnetic analysis. Archaean palaeopoles that had been reported previously from the East Pilbara fall into the Neoarchean (3.35 – 3.5 Ga) groups and are discussed in detail in the Supplementary Materials (Mesoarchean Palaeogeography”). The history of East Pilbara motion prior to 2.8 Ga can potentially be refined by palaeomagnetic study of a rock that is intermediate in age between existing Palaeoarchaean and Neoarchaean poles. The Soansville Group (Hickman, 2009) is one such candidate that dates to ~3.220 Ga to 3.170 Ga, which is a predominantly mafic volcano-sedimentary succession that outcrops mainly along the western margin of the East Pilbara. The lower Soansville Group (~3.220 – 3.180 Ga) is a siliciclastic succession that is up to 3.500 Ga of sandstone, turbidites, conglomerates, and minor cherts and banded iron formations (Van Kranendonk et al., 2010). The overlying Honeyeater Basalt (HEB: 3.192-3.176 Ga), which was sampled in this study, contains up to 1,050 m of pillowed and massive flows of tholeiitic and komatiitic metabasalt (Van Kranendonk et al., 2010). Mafic-ultramafic dykes and sills of the Dalton Suite, which is contemporaneous and probably comagmatic with the HEB (Van Kranendonk et al., 2010) cuts the lower Soansville Group succession. The U-Pb Sensitive High-Resolution Ion Microprobe (SHRIMP) was used to date magmatic and detrital zircons and baddeleyites from the Dalton Suite in 5 greenstone belts (Van Kranendonk et al., 2010), and derived the date range of 3.192 - 3.176 Ga age range of the HEB.
It has been hypothesised from previous studies that the Soansville Group had a rift origin (Van Kranendonk et al., 2010; Van Kranendonk et al., 2010; Van Kranendonk, 2007; Van Kranendonk 2006), which may be linked to the initiation of a primitive Wilson cycle (Van Kranendonk et al., 2010). Brenner et al. suggest its fining upwards siliciclastic succession and subsequent basalts are consistent with a modern rift setting. Also, extension and basement involved in normal faulting has syndepositionally deformed the lower siliciclastic portion. The Soansville Group would represent structural evidence of Mesoarchaean horizontal tectonics in the Pilbara Craton if this interpretation is true.
Brenner et al., sampled from 2 of the most extensive exposures of the HEB that is located in the Soansville Syncline (SVS) and the East Strelley greenstone belt (ESGB) (Van Kranendonk et al., 2010). The SVS is an open syncline that plunges to the northeast and outcrops over a region of approximately 100 km2 in the southern Soansville greenstone belt (Van Kranendonk, 2018). A baddeleyite-bearing Dalton Suite sill in the EZGB gave a U-Pb SHRIMP age of 3.182 Ga ± 2 Ga, which agrees with the 3.192 – 3.176 Ga age range of the HEB based on U-Pb geochronology throughout the East Pilbara (Van Kranendonk et al., 2010). As a result of its unique stratigraphic surroundings correlation of the HEB across the ESGB and SVS has been well established. A sequence (bottom to top) of komatiites, felsic volcanics, siliciclastics, and banded iron formations, (the Kunagungarrina, Kangaroo Caves, Corboy, and Paddy Market Formations, respectively), all of which have been intruded by sills and dykes of the Dalton Formation, is just overlain by the HEB (Van Kranendonk et al., 2010). This sequence diagnostically repeats itself in both the ESGB and the SVS (Van Kranendonk et al., 2010). As the Corboy Formation represents the first thick siliciclastic formation in East Pilbara, it is particularly distinctive (Van Kranendonk et al., 2007). In the SVS and the ESGB the age of folding is ~2.930 Ga based on structural comparisons with similar folds of the same age in other parts of the Pilbara (Van Kranendonk et al., 2007). The age corresponds to the widespread emplacement of granitoids of the Sisters Suite (~2.955 to 2.920 Ga) across the western Pilbara Craton, and includes those to the west of the SVS in the Yule Dome (Van Kranendonk et al., 2007). In the region sinistral faulting has been dated to 2.936 to 2.019 Ga and is synkinematic with granite intrusions and folding (Van Kranendonk, 2008; Van Kranendonk & Collins, 1998). In the ESGB as well as the SVS, the HEB is metamorphosed weakly to prehnite, pumpellyite facies (Van Kranendonk, 2000), and possibly represents the oldest unit in the East Pilbara, or on Earth, not metamorphosed to greenschist or higher. Also, a palaeomagnetic fold test is enabled for constraining the age of magnetisation in the HEB by the range of bedding attitudes that is available across structures that date to ~2.930 Ga in both the SVS and the ESGB.
Together with palaeolatitudes from other poles that have been reported previously from the East Pilbara, the new HEBh pole places new lower bounds on the rate of horizontal drift of the East Pilbara between ~3.35 and 2.77 Ga relative to the rotation axis of the Earth, assuming a geocentric axial dipole field geometry at ~3.2 Ga. From the East Pilbara the youngest pre-HEBh pole is that of the 3.350- to 3.335 Ga Euro Basalt (EBm), which provides an estimated palaeolatitude of 8.090 ± 5.3o (Bradley, Weiss & Buick, 2015). It is required by this palaeolatitude, which is distinguishable from HEB at 2σ, that the average latitudinal velocity of the Pilbara exceeded 0.23o ± 10o /Myr or ≥2.50 ± 1.15 cm/year between 3.35 and 3.18 Ga. The Fortescue Group Package 0 (P0) is the oldest post-HEBh, having an age that is most likely within 10 Myr of its minimum age of 2.772 ± 2 Myr (Strik et al., 2003; Evans, Smirnov & Gumsley, 2017). It is implied by its palaeolatitude of 57.3o ± 8o that it has an average velocity of 0.03o ± 0.04o/Myr for the Pilbara between 3.18 and 2.77 Ga or >0.37 ± 0.47 cm/year. These rates are lower bounds of lithospheric motion for several reasons. During the relevant time interval the longitude of the Pilbara is not known, which implies that all above drift rates are lower bounds. In addition, the polarity of the geomagnetic field, which is not known, precludes the rates from accounting for the sign of the palaeolatitude. In addition to latitudinal translations quantified by changes in inclination, declination changes between P0 HEBh, and EBm poles may imply substantial vertical axis rotations. About 124o of clockwise rotation occurred in the ~170 Myr between the EBm and HEBh poles (0.73oCW/Myr) and ~101o of counter clockwise rotation (CCW) in the ~410 Myr between the HEBh and the P0 poles (0,25oCCW/Myr). Brenner et al. suggest these rotations may originate from plate tectonic motion of the East Pilbara Craton, true polar wander (TPW), or post-emplacement block rotations, possibly during the ~2.950 – to 2.930 Gyr deformation that occurred in the Lalla Rookh-Western Shore Structural Corridor (LWSC) that sheared sinistrally the majority of SVS and ESGB (Van Kranendonk, 2008; Van Kranendonk & Collins, 1998). The region in which the EBm pole was measured (Bradley, Weiss & Buick, 2015) would not have rotated substantially during this episode (Van Kranendonk, 2008; Van Kranendonk & Collins, 1998). The SVS and the rotation of its central portion CW (Van Kranendonk, 2008; Van Kranendonk & Collins, 1998) were both apparently produced by local effects related to impingement of the nearby Shaw, Yule and Strelley batholiths, which therefore accounted for some of the EBm-HEBh rotation that has been observed. According to Brenner et al. this would also imply that the rotation that was measured between the HEBh and P0 poles is an underestimation, as CW structural rotation in the LWSC would reduce the CCW rotation that is observed. Brenner et al. say that regardless, they cannot determine whether either of the vertical axial rotations that were observed is due to purely local structural effects, rotation of the entire East Pilbara, or a combination of both processes.
It is therefore indicated by the palaeomagnetic record that the East Pilbara underwent horizontal motion of ≥2.50 ± 1.15 cm/year in the ~170 Myr between 3.350 Ga and 3.180 Ga and ≥0.37 ± 0.47 cm/year in the ~410 Myr between 3.180 and 2.772 Gyr. According to Brenner et al., the former is generally consistent with the hard “stagnant-lid” surface velocities that have been hypothesised and rates of net lithospheric rotation in the Phanerozoic that have been observed (typically <2 cm/year) (Fuentes et al., 2018; Dunlop, 2002). It falls between the 48th and 99th percentiles of the distribution in the Phanerozoic (since 410 Ma) of latitudinal continental plate velocities (Matthews et al., 2016), that have been time averaged over 170 Myr windows, as has been estimated from random point sampling of continental lithosphere. Brenner et al. chose these windows in order to cover the last full Wilson cycle, sampling from the full range of tectonic settings that are encountered on the geodynamically “modern” Earth. Between 3.35 and 3.18 Ga the palaeolatitudinal velocity of the Pilbara is therefore comparable to those that have been observed in modern plate motion, which conforms to either a uniformitarian or an episodic model of the tectonics of the Archaean. A stagnant-lid state with an occasional mobile-lid plate motion (O’Neill et al., 2007) would be combined in an episodic regime, and is possible for both a pre- and post-3.18 Ga periods. In this case, the minimum rates obtained in this study would represent a mix of substantial drift velocities (>2.5 cm/year) and episodes when the motion would be slower. Therefore, episodic transitions between a stagnant-lid – or a sluggish – lid regime and intervals of modern style tectonics prior to 2.8 Ga cannot be rigorously ruled out without further palaeomagnetic studies in which the motions of multiple cratons from the Archaean, over identical, closely spaced time intervals were sampled.
It is not known what the relative contributions to TPW (Tsai & Stevenson, 2007; Creveling et al., 2012) and the differential plate motions to these rates, which represent another opportunity for further work. In order to resolve the TPW and differential components of motion in the interval from 3.35 to 3.18 Ga would require ~3.35 and 3.18 Ga palaeopoles from another craton. An important test of Archaean tectonic style is to distinguish between these contribution to net motion, as motion of a stagnant lid would manifest as a net rotation of the lithosphere (i.e., indistinguishably from TPW in the palaeomagnetic record) though substantial differential motion would be produced by mobile-lid tectonic processes. Measurement of the substantial TPW during the Archaean would be a notable result, which suggests a less stable moment tensor, and possibly more vigorous convection or a hemispherically asymmetrical lithosphere.
It is not known whether long range horizontal motion of lithospheric plates occurred prior to ~2.7 Ga, though geodynamics of the Earth have been characterised by tectonic plates in recent geologic time. Understanding of the formation settings of the settings of the earliest crust of the Earth and nascent biosphere and geodynamics of the terrestrial planets in general would be contributed to fundamentally by resolving this uncertainty. Brenner et al. used palaeomagnetic methods to isolate a high temperature magnetisation in the HEB of the East Pilbara Craton at 3.180 Ga that unblocks beyond the peak metamorphic temperature of the basalt (~250oC). The restored palaeomagnetic directions robustly pass a fold test within the SVS at ~2.930 Ga. Combined with magnetic microscopy and rock magnetic results it is suggested by these observations that there was a primary origin for the magnetisation. A palaeolatitude of 43.7o ± 15.3o for the East Pilbara at ~3.180 Ga and reduces substantially the duration of the longest gap that was not sampled in the available palaeomagnetic record. Brenner et al. show with this record that palaeolatitude records of East Pilbara require an average rate of drift of at least 0.3o/Myr drift of 2.5 cm/year Between 3.350 and 3.180 Ga. The palaeomagnetic record is suggestive of either uniformitarian or periodic operation of plate tectonics in East Pilbara prior to ~3.2 Ga. According to Brenner et al. they cannot yet distinguish between plate motion, TPW, and net rotation explanations for the latitudinal motion that has been observed, and they cannot rule out modern-style or episodic Plate tectonic motion prior to 3.180 Ga. A promising direction for palaeomagnetic studies in the future in the East Pilbara, as well as other cratons, is represented by constraining the relative contributions of each of these components.
Brenner, A. R., et al. (2020). "Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga." Science Advances 6(17): eaaz8670.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|