Australia: The Land Where Time Began

A biography of the Australian continent 

Record of 3.8 Billion Years of Sea Floor Spreading, Subduction and Accretion – Recognition of Ocean Plate Stratigraphy in Accretionary Orogens Through the History of the Earth

Ocean Plate Stratigraphy (OPS) is a term that is used to describe the sequence .of sedimentary and volcanic rocks that were deposited on the oceanic crust substratum beginning at the time it forms at a spreading centre, and ends when it is incorporated into an accretionary prism at a convergent margin. In this study, Kusky et al. review the characteristics of relic oceanic crust dating from the Cainozoic to Early Archaean and OPS preserved in Alaska, Japan, California (Franciscan Complex), Central Asia, British Isles, Canada (Slave Province), Australia (Pilbara Craton), and Greenland (Isua and Ivisaartoq belts). In accretionary orogens spanning the duration of the rock record of the Earth an assessment of OPS shows remarkable similarities between OPS of all ages in terms of structural style, major rock components, accretion sequence, and geochemical signatures of trace elements. Volcanic rocks that were preserved in orogenic belts are characterised predominantly by oceanic island arc basalts, island arc picrites, mid-ocean ridge basalts, back arc basalts, oceanic plateau basalts, and boninites, with extremely rare komatiites. This demonstrates that seafloor spreading, lateral movement of oceanic plates as well as accompanying sedimentation over the oceanic substratum, and accretion at convergent margins have been major processes since 3.8 Ga, at least. Some changes have taken place in the rock types in OPS, e.g., changes in carbonates and radiolarian cherts whose sources were in the biota in existence in the Phanerozoic though were not present in the Cambrian, but overall, there have not been many changes in the OPS accretion style over time. Komatiites and banded iron formations occur predominantly in orogenic belts that date to the Archaean, which reflects higher mantle temperatures and a composition of seawater that is less oxic, respectively, prior to 2.5 Ga. According to Kusky et al. this is clear documentation that plate tectonics, which includes the lateral movements of oceanic lithosphere, has been a major mechanism of heat loss on Earth since the Early Precambrian.

Introduction

At sites of subduction of oceanic lithosphere accretionary orogens develop and are characterised by tectonic elements that include accretionary wedges, material scraped from the subducting and overriding plates, assemblages of island arc and back arc, ophiolites and ophiolitic fragments, oceanic plateaus, exotic continental blocks, magmatic and metamorphic rocks that are related to the subduction of ridges, post-accretion granitoid rocks, metamorphic rocks that reach granulite facies and locally UHP and UHT assemblages, and sedimentary basins that are deformed (e.g., Kusky & Polat, 1999; Cawood et al., 2009). Involvement in a phase of collisional orogenesis when the oceans close and the bounding continents collide is the ultimate fate of all accretionary orogens, in which the structure of the original accretionary orogen is modified significantly, leading in some cases the complete removal of many parts of the original orogen by deep subduction, thrusting, crustal shortening, and associated erosion. Then, the challenge is how to recognise the early accretionary orogenic phase of ancient mountain belts, as well as to trace precisely the early tectonic history and crustal growth of these belts. Subduction erosion characterises some convergent margins (e.g., von Heune &
Scholl, 1993; Stern, 2011), whereas others are sites of accretionary material from the descending plate to the plate that is overriding. In this paper Kusky et al. focus on only the accretionary orogens in which a record of subduction is preserved in the accreted material.

A mixture, which is sometimes a chaotic mélange, of material that has been scraped off oceanic lithosphere that is subducting, is one of the hallmark units of accretionary orogens. Included in this material may be various types of completed and incomplete ophiolites, as well as all the magmatic and sedimentary material that had been deposited on top of the oceanic crust as it moved from the oceanic ridge to its incorporation in the accretionary orogen. When these rock units form they are highly variable, depending on the magmatic setting of oceanic lithosphere (whether at MOR, BAB or forearc, arc, etc., and depending on the relative rates of extension and the supply of magma – (see Kusky et al. 2011 for a review), and as it has undergone several diachronous deformation events, typically at different metamorphic grades. According to Kusky et al. the aim of this paper is to define the different types of oceanic plate stratigraphy (OPS), what may happen to this stratigraphy as it is incorporated into accretionary orogens, and how to recognise oceanic plate stratigraphy in accretionary orogens, many of which have undergone later collisional events. Following this Kusky et al. present several examples of oceanic plate stratigraphy that have been well documents from orogens of different ages through the history if the Earth. They then assessed changes in OPS through 3.8 billion years, based on the description OPS from different orogens.

The pelagic sedimentation continues as the oceanic basement moves towards the trench until the moment any point on the oceanic plate passes over the outer trench slope, and enters the trench. Depending on the sedimentary fill of the trench, at this point the pelagic sediments will be covered by hemipelagic shale and chert, then a sequence of greywacke and shale that represent erosion from the overriding accretionary wedge and the convergent/accretionary orogen behind the wedge. Included among these deposits may be disrupted layers such as olistostromes.

The OPS becomes severely disrupted by many thrust faults and dewatering structures related to the offscraping, underplating and accreting of this material to the overriding plate, as the material that has been deposited on the oceanic substratum is pulled in to the subduction trench. Only parts of the OPS are offscraped at one point, in some cases, and typically repeated by hundreds of thrusts, though in other cases whole sections of the OPS that include slices of the different levels of the underlying oceanic crust may also be included. Therefore, the simple model of a regular stratigraphy is disrupted, once the OPS is added to the overriding plate, and the OPS becomes structurally complex, and repeated in many thrust imbricates or complex mélanges.

Variation in the oceanic substratum of OPS

In the above simple model of OPS there are many variations. The first of these variations stems from the different types of oceanic substratum that may underlie the cover sequence. It has been revealed by studies of modern ocean floor and obducted ophiolites that there is a much larger variation in the types of oceanic lithosphere that are generated at MOR and other environments that is suggested by the Penrose definition of ophiolites (Anonymous, 1972) (e.g., Dilek & Furnes, 2011; Kusky, 2011), and these variations may be recorded in the basement OPS in accretionary orogens. It has been shown that there are different types of ophiolites and oceanic lithosphere that were generated in different balance settings between the rates of supply of magma, extension, tectonics setting, All of these different types of OPS substratum may be preserved as the basement component to OPS, and it is not necessary to find a complete ophiolite sequence of “Penrose type” in accretionary orogens in order to recognise the sequence as formed in an oceanic setting. When these different types of oceanic substratum are incorporated into the accretionary complex further complications arise based on different levels of detachment and preservation.

Variations in cover sequences of OPS

Different types of cover sequences may be developed by the oceanic substratum, depending on the geological age (i.e., radiolarian cherts were not deposited in the Precambrian prior to the evolution of these life forms), the type of material eroded from uplifted orogens and deposited in the trench (i.e., the presence of carbonates, coal, etc.), can be influenced by climate, and the specific environments, especially as the oceanic substratum approaches the trench can vary greatly. The Peru-Chile trench from South America is an example in which it is intersecting the Chile Rise that is subducting. High rates of erosion from glaciation and the trench is filled with greywacke-shale turbidites, which overlie the oceanic substratum, whereas areas to the north are experiencing a climate that is more arid and the trench is underfilled, which leads to a thin carapace of turbiditc sediments on top of the pelagic sequence.

A simple model was developed by Kusky et al. (1997) for the controls on whether the accreted OPS in accretionary orogens tends to be in the form of a thin belt of chaotic mélange or thick packages of greywacke turbidites that are relatively coherent. In this model, Mélanges like the McHugh Complex of Alaska typically form during the process of subduction, where there is only a thin veneer of pelagic sediments on the downgoing oceanic plate and shear strains that are subduction-related are concentrated in a narrow zone above the plate boundary (Benioff) zone. For examples of OPS that include vast and thick packages of accreted rocks of flysch type, in contrast, and these are believed to form in situations where an arc has collided with a continent, which leads to the uplift of ranges, and the trench becomes thickly sedimented and the shear strains above the plate boundary zone are distributed across trench fill deposits that are a couple of km thick, which results in packages of accreted rocks that are more coherent. Due to climate and other variables, discussed above, variations of the cause of trench that is thickly sedimented are possible.

Seamounts/plateau OPS

This is another variation of “standard OPS”, which is characterised by a regular change of lithologic facies from pelagic through hemipelagic to terrigenous environment, is that of oceanic islands/seamounts/plateau (aka seamounts), when there is a change from shallow water sediments, which are typically carbonates, formed on the top of seamounts, through hemipelagic volcaniclastics and carbonate debris on its slopes or flanks, here called slope facies, and siliceous mudstones and shales of the foothill and finally to pelagic chert that is formed at its base. Oceanic basalts are overlain by the OPS sediments: mid-oceanic ridge basalts (MORB) at the foothill/base, and the basalt (OPB) of the oceanic plateau, and oceanic island basalts (OIB) on the slopes/flanks and top.

Seamount OPS was first suggested for the Akiyoshi accretionary terrane in southwestern Japan, which contains an example of an accreted/collapsed intraplate seamount in the western Pacific (Sano & Kanmera, 1991). The Akiyoshi seamount is composed of a large reef limestone sequence dating to the Carboniferous with underlying OIB type basalts (Kanmera et al., 1990). Many accreted seamounts of the CAOB have been compared to the Akiyoshi terrane so far (Dobretsov et al., 2004; Safonova et al., 2008).

Fragments of oceanic seamounts with OPB- and OIB-type basalts have been found in more than 30 accretionary complexes of Central and East Asia (Safonova, 2009; Safonova and Santosh, 2013). Typically, CAOB seamount localities are characterised by OPS units: carbonate cap-slope facies-foothill and oceanic floor sediments. Included in the carbonate cap may be massive/micritic limestone, typically with fossils. Commonly, the slope facies consist of poorly bedded lime mudstone, calcareous and mudstone conglomerate/breccia. The foothill/floor sediments are interbedded mudstone, siliceous shale, ribbon chert, etc.

The Katun’ accretionary complex, e.g., in the northern part of the Russian Altai, includes volcano-sedimentary rocks, former parts of a palaeoseamount:

1)    Carbonate cap;

2)    Brecciated carbonate-chert-clay-basalt slope faces; and

3)    Foothill basalt-chert-clay assemblages (Safonova et al., 2011).

Identification of seamount OPS units in foldbelts is, however, often problematic as accretionary units are deformed and mixed tectonically with other fragments of oceanic lithosphere. As outcrops of seamounts are typically much smaller than those of island arc units, the result is that they have often been misinterpreted as back arc or island-arc units (Safonova et al., 2008). The following criteria for the occurrence of palaeoseamounts in folded areas were proposed in order to avoid misidentification of seamount OPS:

1)    Basaltic lavas of the main seamount body can be covered by a carbonate cap.

2)    There are signatures of formation in the sediments on seamount/island slopes and further slumping down: syn-sedimentation Z-folding, brecciation, variable thickness.

3)    Within thrust sheets of accretionary prisms that incorporate turbidites, ophiolites and UHP rocks, in which deformation may result in formation of reoriented linear and nappe structures, the fragments of seamounts can be found (Buslov et al., 2001, 2004a,b).

4)    Seamount basalts are typically characterised by:

i)                   medium to high TiO₂ (>1.5 wt.%);

ii)                ii) medium to high LREE La/Sm_n>1.3 and medium to highly differentiated HREE; and

iii)               iii) Nb enriched relative to La and Th with the result that Nb/La_pm>1 and Nb/Th_pm>1 (Safonova et al., 2008; Safonova, 2009).

5)    Seamounts record lavas that are erupted above a single plume over time and, therefore, those that occur within a single fold belt may have different ages (Regelous et al., 2003; Safonova, 2008).

6)    The older basalts among several seamounts of a single chain are typically less enriched in compatible elements compared to ones that are younger (Safonova, 2008, 2009; Safonova et al., 2011).

It is important to recognise seamount OPS in fold belts because:

i)                   The may have been preserved during subduction  as a result of the higher buoyancy of oceanic crust;

ii)                They can “block” the subduction zone if they are big enough and thereby enhance the accretion;

iii)              It is possible to date the basalts by their associated sedimentary rocks; and

iv)              OIBs are indicative of an intraplate geodynamic setting, which is probably related to mantle plumes (Safonova, 2009; Safonova et al., 2011).

Oceanic seamounts that consist of OIB-OPB type lavas and associated sedimentary rocks allow the following of the evolution of palaeoceans from the record of their initial opening in riftogenic balastic dikes with OIB-type geochemistry, through their maximal opening, recorded in OPS of seamounts up to their closure that is recorded in accretionary and collisional complexes. Continental growth is contributed to by accretion of oceanic islands, seamounts and plateaus and by the addition of large amounts of rocks that are volcanic to active continental margins in the case of middle to large oceanic plateaus (e.g., Mann & Taira, 2004; Utsunomiya et al., 2018; Safonova et al., 2011; Safonova & Santosh, 2011).

Examples of young OPS from young orogens

Examples from southern Alaska accretionary orogen

OPS of the California Coast Ranges

OPS in current Franciscan units

          OPS as blocks-in-Franciscan mélange

          Coastal Range ophiolite

          OPS as blocks-in-Great Valley Group mélanges

          Concluding statements on OPS in the California Coast Ranges

Japan

          History of recognising OPS

          Ocean plate stratigraphy reconstructed from accretionary complexes in  Japan

          Arrangement of OPS

          Tectonic staking of OPS

          Mélange of OPS

          Disruption of OPS

Scotland OPS

Examples of the Central Asian Orogenic Belt

          Kurai accretionary complex, Late Neoproterozoic OPS

Examples of ancient orogens

          Anglesey

          Ocean plate stratigraphy (OPS)

          Ridge-trench OPS

          Subducted HP, OPS

          Olistostrome-type OPS

          Slave Province

          Pilbara OPS

          The Eoarchaean Isua to Mesoarchaean Ivisaatoq-Ujarassuit greenstone belts,         SW Greenland

          Regional geology and field relationship

          The Eoarchaean Isua greenstone belt (about 3.8 Ga and about 3.7 Ga arcs

          Mesoarchaean (about 3.075 Ga)

          Ivisaatoq-Ujarassuit greenstone composite belt

Discussion and conclusions: changes in OPS with time

It has been found that the description of OPS from accretionary orogens that span the duration of the rock record that has been preserved on Earth demonstrates that there has been no fundamental change in the process of seafloor spreading, oceanic sedimentation, subduction and accretion for 3.8 Gyr. Across the ages, the main rock types are similar, the structural style is also similar showing multiple structional lenses that are fault bounded and thrust duplexes bounded by thin early thrust faults. Where it has been possible to determine ages of accretion, all of the examples show that the age youngs towards the palaeotrench (e.g., see the section on the Franciscan), though the relative ages of rocks in the accreted OPS can vary depending on whether a ridge was approaching the trench, whether it was recently subducted, or if the ridge was moving away from the trench. There has, however, been some changes in the nature of rocks that have been accreted with time in OPS assemblages. Many of these can be related to processes of the surface of the Earth, as has been described in companion papers in this volume and elsewhere (Bradley, 2011; Young, 2012, this issue; Eriksson et al., this issue.

Deep sea oozes containing radiolarians dominate chert dating to the Phanerozoic, whereas deep sea cherts dating to the Precambrian were also deposited, though these rocks were formed by processes of hydrothermal exhalation and replacement. Similarly, calcareous tests of organisms from the Phanerozoic, which did not exist in the Precambrian, dominated carbonates like those deposited on seamounts and at ridges above the CCCD. It appears that most of the carbonates in the Precambrian OPS were also related to CaCO₃ rich fluids that percolated along shear zones, with the result that in Precambrian OPS their distribution is different from that in the younger examples.

The older sequences are dominated by thin layers of pelagic sediments, including hydrothermal cherts, black shales, and mafic sediments, whereas there are typically thicker sections of continentally derived clastic rocks such as greywackes and conglomerates deposited in the trench in younger sequences. There are thick greywacke/shale flysch sequences associated with many of the Archaean examples (such as in Slave Province) though these sequences generally appear to be slightly younger associated with collision of arcs and microcontinents, and true oceanic plate sedimentation are represented by only the earliest stages of them. Kusky et al. suggested that this could relate to different styles of plate configuration, with many arcs and microcontinents, though fewer large continents, dominating the Archaean, and the Phanerozoic being dominated by continental margin arcs, arc-continent collisions, and the supercontinent cycle records. Also, most of the young examples of OPS discussed in this paper are from orogens that have not yet experienced major continental collisions, and much of the accreted OPS may end up being uplifted, eroded or deeply subducted when these collisions happen in the future. Contrasting with this, the ancient orogens that were discussed above have all undergone later phases of continental collision and represent a stage or orogenic evolution that is more mature.

There is another significant difference between the OPS sequences from the Precambrian (especially the Archaean) and the Phanerozoic OPS sequences, the OPS from the Phanerozoic in accretionary orogens are typically associated with ophiolitic fragments, whereas in the Archaean, there are many tectonic slices of basalt, gabbro and ultramafics that are highly dismembered, though few Penrose-type full ophiolite sequences. These fragments of sections that were severely dismembered of oceanic crust “ophirags” as they were termed (Sengör & Natal’ín, 2004) recognising that they originated from tectonically fragmented oceanic lithosphere, though more dismembered and metamorphosed than many younger examples. It was suggested this could relate to Archaean crust having been thicker than equivalents that were more modern (e.g., Sleep & Windly, 1982), with a higher level of detachment than is observed in modern convergent settings (e.g., Hoffman & Ranalli, 1988). The difficulty involved in distinguishing oceanic plateaux/seamounts dating to the Archaean from normal (thicker?) Archaean crust was discussed (Kusky & Kidd, 1992), by use of the Belingwe greenstone belt in Zimbabwe as an example.

A small amount of komatiite is contained in many of these remnants of oceanic crust from the Precambrian, which reflects a temperature that was slightly higher in the Precambrian (especially in the Archaean), and standard “Penrose” type of ophiolite pseudostratigraphy is contained in only a few of them, formed at ridges that were slow spreading where there is a balance between the rate of extension and the influx rate of magma. Instead, most of the examples from the Archaean are fragments of the oceanic sub-stratum that had been subducted, preserving a thick volcanic (typically pillow basalts) section, grading down into sill and gabbro complexes. The higher partial melting conditions in the Archaean could be reflected by this, with melt flux greatly exceeding the rate of extension, and therefore not forming sheeted dike complexes (e.g., Robinson et al., 2008). In spite of the differences in the abundance of minor rock types (e.g., komatiite, banded iron formation), volcanic rocks in both Archaean and post-Archaean orogenic belts share affinities of basalts from mid-ocean ridges, oceanic island arc basalts, ocean island basalts, supra-subduction boninites and picrites, and basalts from oceanic plateaux.

The length scales of accretionary orogens with accreted OPS through time are difficult to assess, as the older orogens have been disrupted by more rifting and collisional events that are later than the young orogens such as Alaska, California or Japan. Some large cratons, such as the Superior in Canada, however, preserve accretionary orogens extending along strike for kilometres, which is similar to the case of young orogens. It is becoming possible to assess the length scales of Proterozoic and even Archaean orogens, as more progress is made in the reconstruction of supercontinents that are successively older. There is no reason at present to suspect that the length scales were significantly different in the Precambrian than the Phanerozoic, though in order to assess this judgement more detailed work is required. The similar thicknesses of OPS in many Precambrian orogens suggest, likewise, that the time scales of the subduction/accretion events in the Precambrian should not have been much different than in younger times.

In accretionary orogens of all ages the data from them demonstrates that the spreading of the seafloor has operated since at least 3.8 Ga, and sediments were deposited on the oceanic substratum and accreted at convergent margins. However, the data from these orogens are not yet adequate to resolve the debate (e.g., Condie et al., 2006) about whether or not the subduction systems were shallow and short-lived in the Precambrian, of if there were long-lived subduction systems where slabs penetrated to great depths in the mantle (past 670 km or to “D”) in early times (e.g., Stern, 2007).

Sources & Further reading

Kusky, T. M., et al. (2013). "Recognition of ocean plate stratigraphy in accretionary orogens through Earth history: A record of 3.8 billion years of sea floor spreading, subduction, and accretion." Gondwana Research 24(2): 501-547.