Orogens are major sites for crustal growth on Earth. They record a unique formation and evolutionary history of the continental crust that distinguishes Earth from other planets. Therefore, orogenesis and crustal growth, and particularly their relationships, are fundamental issues in the Earth sciences.
Orogens are characterized and categorized using a range of features, leading to the identification of convergent and extensional or small–cold and large–hot orogens 1,2, the external and internal orogenic systems3, and Alpine-, Himalayan-, and Turkic-type collisional orogenic belts4. They are commonly grouped into collisional, accretionary, or intracratonic5–9.
However, orogens are mostly composite and experience multiple stages of the orogenic processes, such as the accretion of relatively small terranes (or soft collision) and continental collision4,8,10. Thus, orogens vary in their nature and style, defining a broad spectrum of types that encompass the Wilson Cycle11. So, it is difficult to quantitatively characterize, define, and/or classify the various types of orogens, although some previous studies have sought to compare accretionary orogens based on terrane longevity, accretion rates, and juvenile crust production4,5,6,12. Besides, every orogen contains a distinct complex history, which makes the comparison with other orogens very difficult. Therefore, seeking a way or method to objectively and quantitatively characterize and/or classify orogens based on their final compositional architecture and crustal growth patterns (or stages) is crucial.
In this paper, we applied Nd isotope mapping as a “big data” approach to evaluating the compositional architecture and crustal growth patterns of eight well-studied Phanerozoic orogens to provide a semi-quantitative basis for comparison. Specifically, the areal proportion of primitive/juvenile to evolved/reworked crust has been used as a semi-quantitative criterion to characterize the crustal compositional architecture and evolution of orogens. This study provides insights into the relationships between orogenesis and crustal growth.
1. Typical orogens
Two end-members of orogens have been commonly proposed: accretionary and collisional5,6,8,9. Accretionary orogens form at active oceanic subduction zones whereas collisional orogens form as continental plates are progressively subducted 13. Most accretionary orogens now occur as “fossil” orogenic belts within and/or at the margins of consolidated continents14. Phanerozoic accretionary examples include the four typical orogens (Appendix A).
The Altaids, or Altaid tectonic collage4,15, or most of the Central Asian Orogenic Belt (CAOB) 16, 17, is located between the Siberian, Baltic, and Tarim–North China cratons. It records protracted accretion of the Paleo-Asian Ocean and subsequent collision of ancient terranes from 1000 (mainly 750) to 220 Ma12,17,18, followed by accretion and closure of the Mongol-Okhotsk Ocean from ca. 300 to 150 Ma4,12,15,17–19. The Altaids is generally considered to be the world’s largest accretionary orogen containing considerable Phanerozoic continental growth12,16–18. The Appalachian Orogen in North America was formed by accretion and/or soft collision of several juvenile terranes and ancient crustal blocks and the terminal continent-continent amalgamation of Laurentia and Gondwana in the Permian10,20. One of the best-exposed cross-sections through the orogen is exposed in Newfoundland10. The North American Mesozoic Cordilleran Orogen is considered to be an archetypal accretionary orogen5,7,21. The major accretionary processes occurred during the Mesozoic, although evidence exists that subduction began ~420 Myr ago21,22. The Lachlan Orogen is part of the Paleozoic Tasman Orogenic system of eastern Australia and Gondwana23. The orogen formed during the stepwise closure (500–340 Ma) of sequential oceanic back-arc basins formed behind a long-lived, outboard migrating subduction zone that is preserved in the New England Fold Belt23.
Four typical Phanerozoic collisional orogens are considered (Appendix A). The Caledonian Orogen or Caledonides formed during the Silurian collision between Laurentia and Baltica24. The Variscan Orogen or Variscides was the result of the late Paleozoic collision between Gondwana and Laurussia25. The Qinling–Dabie Orogen, one of the most important collisional orogens in Asia, initially formed during the closure of the Proto-Tethyan Ocean and finally due to the closure of Paleo-Tethyan oceans and the early Mesozoic collision between the North China Craton (NCC) and South China Craton (SCC)26. The Tethyan Tibetan Orogen (i.e., the Tibetan Plateau) experienced Mesozoic accretion–collision of terranes following the closure of the Proto- and Paleo-Tethyan Oceans, and the final Cenozoic (ca. 60–50 Ma) India–Eurasia collision during the closure of the Neo-Tethys Ocean27,28.
2. Isotopic mapping and crustal compositional architecture
We used felsic and intermediate igneous rocks to undertake Nd isotope mapping and then to investigate the compositional architecture and crustal growth patterns, i.e., the final result of preservation of long-term crustal growth and orogenic evolution. These rocks were generated by crustal melting during orogenesis. We selected this range of crustal rocks to ensure equivalent rock types in all cases to compare their source. Besides, these rocks are more widespread and have more available isotopic data. These data are of good quality (Appendix B; Fig. S3).
We conducted whole-rock Nd isotope mapping (methods in Appendix C). The datasets (9332 Sm-Nd data with ages) were collected from the literature and databases on websites, including 71 (in Newfoundland) and 169 (in Mongolia and China) new analyses (Table S1). Nd isotope parameters, specifically εNd(t) values and TDM2, are shown as contour maps in Figs. 1 and S1. These maps demonstrate the distribution of isotopic domains and their area extent (Table S2). These domains can be summarized into six isotopic provinces (Table S3).
These isotopic domains reflect the distributions of isotopic signatures of the source rocks in the basement. Effectively, the approach is “basement terrane mapping”. Accordingly, we denote juvenile versus reworked crust and place constraints on the compositional architecture. From these maps, we obtain the relative areal proportions of primitive and evolved isotopic compositions and can determine juvenile and reworked crustal provinces, respectively (Table S2, Figs. 1 and 2). The juvenile areal proportions of the Altaids, North American Cordillera, Newfoundland Appalachians, Lachlan, Tethyan Tibet, Caledonides, Variscides, and the Qinling-Dabie Orogen are ~58%, ~48%, ~40%, ~31%, ~4%, ~1%, ~1%, and 1<% of total areas of each orogen, respectively, quantitatively evaluating the degree of preservation of crustal growth (Fig. 2).
The four accretionary orogens (Altaids, North American Cordillera, Newfoundland Appalachians, and Lachlan) are all located on the left sides of the cumulative frequency curves in Fig. 3, each containing > 30% of the juvenile crust. The Altaids hosts the highest accumulation and/or percentage of juvenile crust, followed by the North American Cordillera, Newfoundland Appalachians, and the Lachlan Orogens, with the Tethyan Tibet, Caledonides, Variscides, and the Qinling-Dabie Orogens having the least amount of juvenile crust. Clearly, the accretionary orogens contain much larger volumes of juvenile crust than the collisional orogens (Figs. 1, 2, and 3), quantitatively confirming that accretionary orogens are primary sites of juvenile continental crust production5,13,29,30.
Our Nd isotope mapping results are consistent with those of zircon Hf isotope mapping and xenocryst studies of some orogens, such as in the Altai–Junggar–Tianshan region of the southern Altaids37–39, Qinling-Dabie40, and the Tethyan Tibet28. The isotopic mapping results are also consistent with geophysical investigations. For example, primitive isotopic domains (province I; Table S3), i.e., juvenile crust, in western Junggar of the Altaids overlap the region of deep-seated Paleozoic paleo-oceanic crust revealed by magnetotelluric imaging41.
3. Characterizing orogens based on compositional architecture
The proportions of juvenile to reworked crust contrast between the orogens. The Altaids contains a large area of juvenile crust accounting for ~6, 315, 326 km2 and ~58% area (Figs. 1 and 1; Tables S2 and S3). Assuming that the deep-seated juvenile crust represents the whole crust (as little significant over thrusting) and considering a 40–50 km crustal thickness in the Altaids42, we estimate the juvenile crust as a volume of 284,189,670 km3. The Altaids hosts both the highest percentage (~69%) and the largest volume of juvenile crust among the eight orogens. Comparably, the North American Cordillera (~48%), Newfoundland Appalachians (~40%), and the Lachlan (~31%) contain less juvenile crust and more evolved crust, similar to the complex accretionary orogens defined by Condie6. These are consistent with their geological characteristics. For instance, the Newfoundland Appalachian was subjected to the accretion of Gondwana-derived terranes with an older pre-orogenic history of crustal formation10. The Lachlan Orogen acquired its transitional character by deposition of Proterozoic sediments into backarc basins and the sediments were buried and melted during orogeny29.
In contrast, the above four collisional orogens, located on the right sides of the curves (Fig. 3), contain much less juvenile (or much larger reworked) crust (Fig. 1). The Qinling–Dabie Orogen contains not only the least (<1%) juvenile crust (or the largest reworked crust) but also the most ancient crust (εNd(t) < –16; Figs. 2 and 3). This is consistent with the orogen’s character that the South China Craton was overridden by the North China Craton (deep continental subduction) as indicated by the continental ultra-high-pressure metamorphism43.
From the above, we propose an approach to divide the orogens into primitive/juvenile and evolved, based on the areal proportions of primitive/juvenile crust (Fig. 4). In theory, an orogen can be placed into six categories, based on its compositions and timing of orogeny (Fig. 4).
It is potentially difficult to distinguish between accretionary and collisional orogens in practice; even the two “terms” are inconsistently understood by different authors. For instance, the Altaids, the typical accretionary orogen12, 17, 18, still hosts collisional features and have been called a Turkic-type collisional orogenic belt4. “The Appalachian orogen is also viewed as a collisional orogen5. The Variscides is generally considered to be a collisional orogen25, but is still considered as accretionary prior to collision (e.g.,5, pp149). Condie5,6 further divided accretionary orogens into simple (chiefly juvenile terranes with lifespans of <100 m.y.) and complex (with both juvenile accreted, components and exotic microcratons, with terrane lifespans of ≥100 m.y.) types.
Our characterization and classification of orogens enable us to objectively and semi-quantitatively contrast between them (Fig. 4). Orogens with juvenile crust between 70–50% correspond to simple (typical) accretionary (e.g., the Altaids, ~58%), and 50–30% to complex accretionary (e.g., North American Cordillera (~48%), Newfoundland Appalachians (~40%), Lachlan Orogen (~31%)). Furthermore, we consider orogens with 30–10% juvenile component as complex collisional, 10–1% as simple collisional (e.g., Tethyan Tibet (~4%), Caledonides (~2%), and Variscides (~1%)). Oceanic arcs containing >70% juvenile crust, and orogens with <1% juvenile crust (e.g., the Qinling-Dabie Orogen), could be regarded as the end members of the entire orogenic array, the former included when they are accreted within an orogenic system. Thus, this study provides a new semi-quantitative approach to characterizing orogen types.
Orogens begin as accretionary and evolve into collisional, culminating in the termination of a Wilson Cycle or the completion of a Supercontinent cycle6, 9, 11. Thus, each orogen could be viewed as having reached a certain stage of its evolution path in the Wilson Cycle. Moreover, active accretionary orogens will continue to evolve; for instance, the accretionary orogenic systems around the margins of the Pacific Ocean at present, such as the North and South American Cordillera, may evolve or be reformed into collisional or even intracratonic orogens if the Pacific Ocean closes in the future (e.g.50). Based on this expected orogenic evolution, we can use the decrease in the juvenile crustal areal proportions to semi-quantitatively trace the orogenic stages from accretion to collision as each orogen progresses through the Wilson Cycle (Fig. 4).
4. Links between orogenesis and crustal growth
Crustal growth is defined as the addition of mantle-derived material to the crust, and its rates are usually based on cumulative frequency plots of global isotopic data31–36. However, there are many unconstrained aspects of the statistical treatment of data (such as data size and their homogeneous distribution), and such global compilations do not reflect the relationship between orogenesis and crustal growth. Here we have applied areal proportions of juvenile crust determined by isotopic mapping to individual orogens to show which ones have contributed significantly to crustal growth and which have not.
The Altaids was regarded as the best example of Neoproterozoic-Phanerozoic continental growth15,16,44. However, this has been questioned recently, where it is argued that no unusual continental growth occurred at this time45. Our study indicates that the Altaids contains both the largest volume and highest proportion of juvenile crust (see above) and much of it formed during the Neoproterozoic-Phanerozoic (Figs 1, 2, and 3). Accordingly, our study helps to untangle the mystery of whether or not significant continental crustal growth occurred during the orogeny, including the Phanerozoic orogeny 16,31,33,35,36.
Untangling the mystery is an interesting and fundamental issue in the Earth sciences. Several mechanisms have been invoked for the generation of juvenile crust during (Phanerozoic) orogeny5,6,33,46,47. Some researchers (e.g.46) suggested that collisional orogens (or collision zones) are primary sites for net continental crustal growth (e.g., the Tethyan Tibet), and the juvenile crust (“syncollisional andesites”) are isotopically dominated by mantle signatures inherited from ocean crust that was derived from the mantle at mid-ocean ridges, or derived from mantle-derived magmas generated during break-off or delamination in a post-collisional setting (e.g.28). However, our Nd isotopic mapping indicates that much less (<5%) juvenile curst occur in typical collisional orogens (Fig. 4). Whereas, accretionary orogens (even Phanerozoic) contain much amount (30-60%) of juvenile crust. For such, there are at least two major reasons and/or mechanisms. Frist, accretion, particular extensional accretion, is a benefit for the generation of juvenile crust5,6,8,9, which is produced by melting of a mantle wedge metasomatized by subducted fluids, or by melting of subducted oceanic crust, or rapid remelting of basaltic underplates at the base of arcs (e.g.47). Second, oroclines and/or soft collision of accretionary complexes may provide good conditions for the preservation of juvenile crust, such as a large number of oroclines in the Altaids4,18,19.
In summary, the crustal pattern or compositional architecture of an orogen reflects both the production and preservation of juvenile crust. Delineation of the compositional architecture and crustal patterns by isotope mapping allows objective and quantitative characterization and classification of orogens, obtaining an indelible imprint that relates to the areal proportion of juvenile crust preserved in the system. Accordingly, we can unveil the relationships between orogenesis, crust-mantle interactions, and crustal growth, providing an alternative approach for constraining the evolution of Earth.