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Volume 31 Issue 3
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Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun, Haonan Li. Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations in the Northwestern Region of Spain's Iberian Peninsula. Journal of Earth Science, 2020, 31(3): 611-620. doi: 10.1007/s12583-019-1268-z
Citation: Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun, Haonan Li. Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations in the Northwestern Region of Spain's Iberian Peninsula. Journal of Earth Science, 2020, 31(3): 611-620. doi: 10.1007/s12583-019-1268-z

Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations in the Northwestern Region of Spain's Iberian Peninsula

doi: 10.1007/s12583-019-1268-z
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Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations in the Northwestern Region of Spain's Iberian Peninsula

doi: 10.1007/s12583-019-1268-z

Abstract: In the present study, Paleozoic Variscan orogenesis was a model of the oroclinal flexion accompanied by extensive magmatism, which could be divided into the following two types:post-tectonic and syn-tectonic tonalite granite, and leuco-granite which were controlled by the tectonic characteristics of the intrusions. It was observed that a very high majority of the samples had displayed discontinuities in their structures, that were later utilized to define the granitoid morphology and development characteristics of the rock during the intrusion phases. Furthermore, it was determined that the tectonics associated with the Alpine orogeny results in the new generation of faults and fractures during the Paleogene Period had produced the development of the Sierras. Due to different weathering processes, the depressions which had resulted in the present granitoid reliefs were found to be exclusively related to the structural development processes during the geological history (either tectonic or magmatic) of the granite, and not as normally interpreted.

Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun, Haonan Li. Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations in the Northwestern Region of Spain's Iberian Peninsula. Journal of Earth Science, 2020, 31(3): 611-620. doi: 10.1007/s12583-019-1268-z
Citation: Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun, Haonan Li. Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations in the Northwestern Region of Spain's Iberian Peninsula. Journal of Earth Science, 2020, 31(3): 611-620. doi: 10.1007/s12583-019-1268-z
  • Oroclines are considered to be among the largest geological structures on Earth, with formations having developed from the Archean Period to the present time. The Cantabrian Orocline located in northern Spain is one of the first orogens reported in researches, and was generally referred to as the "Asturian Knee" by Eduard Suess (Suess, 1883) in the late nineteenth century. The structure has been attributed to Early Carboniferous collisions between Laurussia and Gondwana during the Pangea amalga-mation (Fig. 1), and is defined by a significant bend in the northern Iberian Peninsula.

    Figure 1.  Collision between Gondwana and Laurussia in the Late Devonian between 380 and 370 Ma. The approximate situation of the Iberian Peninsula (Spain and Portugal) was located in the rectangle marked. Modified from Fuenlabrada Pérez (2018).

    The Cantabrian Orocline tends to weave its way through western Europe, and is located at the apex of the Ibero-Armorican arc. The Carboniferous Variscan orogeny is accompanied by extensive magmatism in response to active buckling. It is known that the synorogenic Variscan granitoid magmatism was active from 345 to 315 Ma and from subsequent post-orogenic magmatism emplaced from 310 to 285 Ma (Weil et al., 2013; Auréac et al., 2004). The post-orogenic magmatic included significant foreland magmatism in the core of the Cantabrian Orocline (Fig. 2).

    Figure 2.  Geological map of Galicia with the location of the main granite bodies (modified from Weil et al., 2013).

    The present relief of Galicia has developed on the aforementioned surface. Its age has been determined to be approximately 200 Ma (Mesozoic Period) (Vidal Romaní et al., 2014, 1998), and presents significantly more modern structures than the rock masses (predominantly granite, slate, and schist) on which it developed, which date back to the Paleozoic Period (Liu et al., 2018; Meng et al., 2018; Li et al., 2017; Wang et al., 2016; Han et al., 2014). During the Mesozoic Period the mega-continent of Pangea was divided into many portions, one of which corresponded to the basement rock formations underlying the region of Galicia (Fuenlabrada Pérez, 2018). Since the Galician territories were exposed on the surface, they had been mainly affected by fluvial erosive processes (Vidal Romaní et al., 2014, 1998), and less intensely affected by marine and glacial processes. During the Mesozoic Period, the Galician rivers began to flow towards the present-day Atlantic Ocean coastal regions, digging deeply into the valleys which had been submerged by the sea during their final stretch in the Cainozoic marine transgressions, becoming the area currently known as "Rias" (Vidal Romaní et al., 1998). The most important milestone in the landscape evolution of Galicia dates back to the Paleogene Period (65–34 Ma), when the Iberian Peninsula (a small plate between the Eurasian Plate and the African Plate) was affected by tectonic activities which gave rise to the different current Galician mountain ranges, largely sustained by granitoids (Vidal Romaní et al., 2014). Among the aforementioned mountain ranges, over 1 000 m had been developed during the glacial processes which occurred during the Quaternary Period (2.58–15 ka) in the far areas of western Europe (Vidal Romaní et al., 2014).

  • For more than two centuries, the formation and landscape development of granite and granitoids have attracted extensive attention from geologists, geographers, and geomorphologists (Song et al., 2019). In addition, almost without exception, each of the previous studies had assumed that granite rocky landscapes had been formed by weathering and rock erosion, thereby highlighting the importance of climate in the end results. This had become the case for the monographies of granite formations, with emphasis placed on the influences of climate on their formations (Büdel, 1977; Godard, 1977; Ollier, 1969; Wilhelmy, 1958). This was then highlighted by delightful miniatures of block-diagrams, representing different types of granite climatic landscapes. Then, approximately 37 years ago, Twidale (1982) published the first specialized treatise on granite morphology, which was subsequently updated, revised, and improved (Migon, 2006; Twidale et al., 2005; Vidal Romaní et al., 1998). As a result, one might think that after all the significant advances mentioned above, the interpretations of the genesis of the granitoid formations have already been resolved. However, this is not the case, as there are still many unanswered questions regarding the origins of tafone, polygonal cracking, pseudo-bedding, and so on. Therefore, any observer contemplating granitoid rocky landscapes tended to appreciate specific common features which potentially insured the accuracy of specific granitoid geomorphology cases. Furthermore, what is more surprising, Twidale et al. (2005) noted that there were azonal characteristics in the formations. That is to say, associations of forms which were common to all the granitoid rocky landscapes were observed, which were found to be independent of the climate or external geodynamic agents which exist today, or of those which acted in the past (Twidale et al., 2005).

    Our climate has varied greatly throughout the geological history of the Earth, either for astronomical reasons or because of the migration of lithospheric plates (Nonn, 1966). Therefore, an inselberg or a bornhardt, according to classical geomorphology (Bremer and Jennings, 1978) which originally formed in desert zones and were now under tropical, cold, temperate, or humid climatic conditions could be understood as cases of inheritance. For example, this would include climatic formations which do not correspond the present climate conditions in their current locations.

    The almost unanimous assertion is that all of the formations which had developed on granite or granitoid rock masses have exogenous origins, having been formed by the actions of exogenous processes, and were considered to be practically eternal if permanent contact with water had been avoided.

  • However, it should be noted that there is another option to explain the morphological similarities of granite and granitoid rocky landscapes. It has previously been confirmed that magmatic rock tends to form in the interior of the lithosphere from which it is mobilized and then moves toward the superficial levels. These rock formations can reach the Earth's surface as extrusive or volcanic rock, thereby developing the very specific and unequivocal geomorphological characteristics widely described in previously research results (Ahmed et al., 2018; He et al., 2018; Wang et al., 2018). In other cases, the magmatic rocks which become consolidated in the interior layers of the Earth tend to give rise to different types of intrusive rock formations (granite) according to their mineralogy, textures, and structures. In addition, near the end of the intrusive stage (Vidal Romaní, 2008; Auréjac et al., 2004), a very specific structural fabric (discontinuities) will be generated. Once the rock has been exposed on the Earth's surface, the structural fabric (discontinuities) will define the rocky reliefs with a specific morphology for the granitoid reliefs. The novelty of this interpretation lies in the fact that the structural fabric of the plutonic rock does not develop on the surface of the Earth or within the deeper layers, but during the formations' ascent toward the surface of the Earth and always within the lithosphere, as can be observed in different areas of China (Meng et al., 2019; Han et al., 2014; Zhu et al., 2014). In any of the cases near the ends of the aforementioned ascension processes, erosion actions are required. The erosion actions normally respect the original contours of the intrusive magmatic bodies and eliminate the host rock, essentially leaving the granitoid exposed on the surface.

  • Granitoid rocks, such as granite sensu stricto and its close petrological relatives (Streckeisen, 1967), are material expelled from the interior layers of the Earth. Once the continents were formed, the plate tectonic fragments had dispersed the granite outcrops throughout the terrestrial surface. Such dispersing actions are known to have occurred after the disintegration of the last great supercontinent (Pangea) and continued until the present time (Ganne and Feng, 2018). Plutons are classified partially by their sizes and shapes. However, they are mainly categorized according to whether they are predominantly discordant (post-kinematics) or concordant (syn-kinematics) in respect to the structural characteristics of the host rock. The most suitable geodynamic environments for finding extrusive and intrusive magmatic rock formations are the mid-oceanic ridges, oceanic-continental island arches, oceanic-oceanic island arches, continental plate edges, hot spots and magmatism due to meteorite collision (Ulrich et al., 2018). However, over time, the accretion processes formed by the collision between plates had resulted in the increasing integration of the plutonic bodies toward the interior of the continents. Such was the case of the north western section of the Iberian Peninsula.

  • Recently, it has been explicitly recognized that the movements of granitoid rock masses on their way to the Earth's surface are not in a fluid or molten state (Liu et al., 2019), but in crystallized solid (mush) formations with variable proportions of volatile (between 5% and 20%) content (Vidal Romaní et al., 2018; Cashman et al., 2017; Parmigiani et al., 2016). The volatile content results in solid poly-mineral deposits, and already totally crystallized masses, with some peculiar rheological properties which allow movements similar to that of very viscous fluid (Vidal Romaní et al., 2018; Cashman et al., 2017; Parmigiani et al., 2016).

    In the aforementioned processes, the magma migrates following the fissures or channels (chimneys) formed during the movements of the lithospheric plates. The materials which circulate through the channels are mush rather than molten materials, with masses practically in solid states although volatile residual fractions (between 5% and 20%) are conserved. This confers the moving materials with a great capacity to deform when subjected to directed effects (Cashman et al., 2017) (Fig. 3). During the intrusive stage, the mush will circulate in a forced regime through the predefined fissures or conduits. The first consequences of these movements will be the generation of fluid structures of endogenous magmatic origin, which then culminate in the total consolidation of the mush in plutonic rocks (granite). Petrologists have advanced different models (Cashman et al., 2017) to explain the movements of the mush on a large scale, although the process has also been theoretically defined on a smaller scale in a Hele Shaw cell (Figs. 4a, 4b) (Bensimon et al., 1986; Liang, 1986a, b). Therefore, it is possible to theoretically establish how the mush moves during each previous stage in order to reach the final consolidation stage using the aforementioned model. The authors are particularly interested in the assumptions presented by Liang(1986a, b) (see also Bensimon et al., 1986), in which the movements of the "mush" were produced in the form of cylindrical digitations of approximately circular cross-sections, which were propagated according to a mode of telescopic advance.

    Figure 3.  Two modes of granitoid body ascending through the lithosphere in its two modalities: concordant (laccolith) and discordant (dome or neck) (modified from Cashman et al., 2017).

    Figure 4.  Different modes of mush movements through the upper lithosphere (Liang, 1986a). (a) Single Hele Shaw cell; (b) branched Hele Shaw.

    In previous studies, in-situ cases of the above-mentioned stage were examined in some rocky outcrops. It was found that the Northwest Iberian region, where there are known to have been a large number of intrusions of granitoid bodies during the two stages of the Variscan orogeny, would undoubtedly provide an ideal location for the type of observational examinations mentioned above (Fig. 5). Since it was confirmed that this was the case in Pontevedra (Spain) (see Fig. 5), it was possible to observe magnificent examples of this type of intrusive structures. It was found that the contours of the ducts were marked by enveloping surfaces (films) of phenocrystals, which allowed for the definitions of the dimensions of the chimneys where the mush ascended, as well as the reconstruction of the directions of the magma movements in the area. At the present time, the largest observed structures have diameters of approximately 3 m. In addition, other cases with similar characteristics have been described in northern Portugal (Valpaços, Valverde). However, it is also possible to observe these types structures which mark the movements of mush as enormous ellipsoids (Fig. 6) formed by the total crystallization of granite formations. Previous related studies have very frequently described movements of mush in the xenoliths and xenocrystals, as well as the accumulated and enclave (Corretgé et al., 1984) structures of granite outcrops. These magmatic enclaves of discrete dimensions appear as 3-axis ellipsoids with their major axis oriented in the direction (generally vertical) of the movement of the magma. These are normally up to 20 or 30 cm in diameter, although in exceptional cases, they have been observed to reach up to 2 m in diameter. Therefore, it is a possibility that these ellipsoids were formed by the crystallization and nucleation of the mush (Cashman et al., 2017) around previous solid nuclei, either xenoliths or xenocrystals, or were an aggregation of the mineral enclaves. In any case, the chimneys or ducts, as well as the enclaves themselves, had been formed during the magmatic stages (Twidale et al., 1994), and had no decisive influences on the morphology of the final granitoid bodies.

    Figure 5.  Field photographs showing the cross sections of a single Hele Shaw of granitoid (Pontevedra, Galicia).

    Figure 6.  Flow of granitoid mush in concordant granitoid from Santa Valha (Portugal) showed by a granitoid ellipsoid with concentric envelopes. Elongation axis parallel to the direction of movement.

    However, it remains unclear what occurs when erosion actions eventually place these granite formations on the Earth's surface. Generally speaking, it has been found that they present two types of morphology (Twidale, 1971; Badgley, 1965): either discordant (post-tectonic) domes or concordant (syn-tectonic) (laccoliths) granitoid bodies (Cashman et al., 2017).

  • In the cases of syn-tectonic magmatic bodies, their structural characteristics are only observable after erosion has eliminated the host rock, leaving the granitoid exposed on the surface. From the aspect of morphology, they tend to display reliefs formed by lowered bodies, with their greater flat dimensions parallel to the surface of the land (Sánchez Cela, 2004). In regard to the laccoliths, the final intrusions have taken place in domains close to the Earth's surface. As a result, the effects of the low confining pressure (Cashman et al., 2017) favours the lateral extension of the granite mass that adopts a very characteristic tabular morphology. The dominant internal structure in this type of granitoid bodies is the so-called sheet structure, formed by the injection of plutonic rock (mush) into the previous terrain in superimposed sheets. The injection always implies a friction of the intrusive with the host terrain or with itself in each layer. In transversal section these concordant bodies are shown as large parallel sub-horizontal sheets of variable thickness (between half a meter or less to a few meters), which indicates that it was produced by a successive injection. The contact between each sheet can be net or gradual as indicated by a shear zone with a variable thickness from a few centimeters to several meters in some cases. There the rock is finely divided into sheets or planar scales formed during a tectonic shearing process. They have a "morphological" similarity to a porphyroblastic gneissic structure although here the dimensions are much larger in metric order (Fig. 7). Then, when the later erosion made the sheared granite disappear, the intact granite "eyes" became loose. On occasion, remains of the foliated rock which originally enveloped the granite have been observed. Some studies (Vidal Romaní, 2008) have also associated these zones with other typical shear structures, such as polygonal cracking, which are only obviously conserved on the surfaces of the granite blocks in some cases, or on different faces of the blocks. In such cases, polygonal cracking is associated with the development of tafoni, not only at the base of the granite blocks, but in any or even all of the block faces. This is very characteristic of the association of structures with the following order of generation: shearing, polygonal cracking, and the development of tafoni. This type of desquamation structure or sheet structure formed by scales (leaves) of intact rock and of great radius defines a very characteristic landscape of domes of very low relief (almost flat) which interfere laterally between them by the juxtaposition/imbrication of the sheet structure.

    Figure 7.  Surface morphology of a granitoid body concordant with its largest dimension parallel to the Earthʼs surface from Valverde, Portugal.

  • However, when the intrusion of the mush occurs at a greater depth under a higher confinement pressure conditions, the granitoid bodies will definitely consolidate at greater depths. In such cases, the intrusions define bodies of smaller dimensions, with external contours of the ellipsoidal or cylindrical sections (circumscribed massifs; domes) (Fig. 8) (Cashman et al., 2017). The elongation axis will generally be vertical and may sometimes intersect through some previous laccoliths. Although the cylindrical domed bodies are not as frequent as the laccoliths, they are the best known, perhaps due to the dramatic aspects they present when erosion exposes them on the surface. Therefore, the following tend to be highlighted in the surrounding reliefs of the totemic shapes in granite landscapes: domes, inselbergs, or bornhardts.

    Figure 8.  Granitoid dome as an example of a discordant body. O Pindo, Galicia, Spain.

  • Without a doubt, during the formation development of granitoid massifs, the most relevant structural characteristic of the morphologically includes the exfoliation (sheet structures) since it is the one which defines the bornhardt dome reliefs, which are considered to be the most notorious form (although not the only exclusive form of granitoid reliefs) (Fig. 9) (Twidale et al., 2005). The bornhardts (Rey et al., 2011; Twidale et al., 2005, 1994) are the last remnants which have survived the long-distance scarp retreat processes. This is due to their massive structures within the innermost areas of the granite bodies, which have not been affected by the discontinuities associated with contacts with the host rock. Both interpretations are partially correct since when the erosion has eliminated the external sections, or essentially the ones most affected by the discontinuity systems, while only the internal and more massive parts of the pluton will remain. These inner sections have resisted weathering effects due to the absence of structures (discontinuities). However, as noted in all of the examined cases, the sheet structures in the bornhardts developed when the rock had become completely crystallized, and could potentially even affect the host rock. Some previous researchers (Tahiri et al., 2007) have referred to the deformative structures developed between the pluton and the host rock contacts as "the strain aureole", considering them even as "magmatic structures developed in solid states" (Petford, 2003; Diot et al., 1987). Therefore, it has been implicitly suggested that the rock was already consolidated when the discontinuities developed (Zulauf G et al., 2011; Zulauf J et al., 2011). This fact has also been corroborated by other results (Vidal Romaní, 2008; Twidale et al., 2005, 1994; Vidal Romaní and Twidale, 1999), in which it was considered that the sheet structures were not the only type of discontinuity, but one more of those which had developed in gradual series of increasing magnitudes due to shear deformations in the contacted granitoid host rock. Most importantly, their development occurred when the rock had become fully crystallized. The shear deformations were limited to the host rock-granitoid contact positions, and gave rise to different types of associated discontinuities, such as lamination, granitoid foliation, pseudo bedding, and in the most extreme cases, a type of deformation referred to as boudinage and polygonal cracking (Zulauf G et al., 2011; Zulauf J et al., 2011; Vidal Romaní, 2008; Vidal Romaní and Twidale, 1999) (Fig. 10). Therefore, all these types of discontinuities were confirmed to not be related to the unloading of the massifs, but are now considered to be shear structures confined to the immediate contact zones between plutonic and host rock. Furthermore, their development is now known to have taken place inside the lithosphere, as evidenced by the fact that in many cases the contacts of the aforementioned discontinuities have been injected by pegmatite, quartz, and leuko-granite bodies, and so on (Tahiri et al., 2007; Twidale, 1982). In addition, the internal domes have characteristic internal structures which are defined by two main types of discontinuities. The first type is developed in the internal zones characterized by having the furthest the contacts with the host rock. In those locations, fluid structures are commonly found, although the rock is totally crystallized. In such cases, the movement implies that the flow is made in a solid state (Rodrigues Waldherr et al., 2018; Vidal Romaní et al., 2018).

    Figure 9.  Sheet structure in granite from Ézaro, O Pindo, Galicia, Spain.

    Figure 10.  Boudinage-polygonal cracking on the surface wall of a granitoid dome as proof of the intrusive origin of the dome from Arouca, North Portugal.

    However, it is more common that the structure developed in contact with the walls of the ducts (sheet structures) (Vidal Romaní, 2004; Vidal Romaní and Twidale, 1999), although variable dips are usually observed in those cases. Some researchers have interpreted the aforementioned as structures of relaxation or erosive unloading. However, others (Vidal Romaní and Yepes Temiño, 2004; Vidal Romaní and Twidale, 1999) have decisively associated them with shearing effects located in the contact zones of the granitoid and host rock which had developed during the intrusive stages of the granitoid. According to the aforementioned research interpretations (Vidal Romaní and Yepes Temiño, 2004; Vidal Romaní and Twidale, 1999), zoning of the sheet structures exist (Figs. 7 and 8) which leads to its gradual disappearance (fading) from its best development points (in contact with the host rock) to its total disappearance inside the granitoid domes.

  • The most decisive properties of granitoid rock masses include their impermeability, low porosity, low solubility, and excellent resistance to weathering when dry (Vidal Romaní et al., 2015). These features explain the importance of the discontinuity systems in this type of rock since when open those systems direct movement toward the interiors of the rock massifs resulting in alterations in the rock. As a consequence of these movements, the distributions of the discontinuities or the fracturing systems tend to control the final morphology of the rocky landscapes developed on granite deposits. However, until recently, the origins of these discontinuities have been misinterpreted (Burton Johnson et al., 2019) due to the lack of understanding, whether implicitly or explicitly, of their formation processes related to the subsequent intrusion-consolidation of the rock. As mentioned above, in magmatic (plutonic) rock, discontinuities are formed on the already consolidated rock formations. However, these are generally not evident on the Earth's surface, but as granite intrusions formed through lithosphere progresses. This explains why the morphological patterns which are defined by the discontinuities of granitoid bodies consistently result in very similar formations due to the intrusions always occurring in the same way. Perhaps that is why the representations of the discontinuity systems which define the plutonic rock bodies are almost always presented (see Brook, 1978) with the same generic scheme. For example, granitoid domes of radial and anticline structures or systems of conjugated discontinuities formed by two or more orthogonal families, almost always perpendicular to the Earth's surface and which (Cloos, 1931, 1923) are invariably related to the flow tray of the batholiths (although this almost never happens in all real cases). The most obvious types of discontinuities are sheet structures, which have been identified as structures of relaxation, decompression, or erosive discharges associated with the dome shapes observed in the contact zones of granite/granitoid deposits and their host rocks. However for some previous related studies (Vidal Romaní, 2008, 2004; Vidal Romaní and Twidale, 1999) have theorized that the formations of sheet structures do not correspond to the effects of unloading, but to the effects of compressive actions in environments dominated by shear stress, as confirmed by the minor structures associated with the contacts between the structural units (leaves) of the sheet structure. These include pseudo bedding, boudinage, polygonal cracking, and even with the formation of tafone (Fig. 10) (Vidal Romaní, 2008, 2004). However, regardless of these factors, there still are many researchers who point out the similarity of the granitoid-salt diapirs in order to explain the intrusions of discordant plutonic rock bodies based on the similarities in the structural and morphology characteristics (Vendeville et al., 1992; Talbot and Jackson, 1987) in granitoid domes. In the aforementioned theories, the related studies have ignored that, although the intrusive processes seem mechanically similar, the characteristics of the material and geodynamic environments (lithostatic pressure, temperature, density) involved in both cases tend to be very different.

  • In regard to granitoid bodies, either concordant or discordant, it has been found that once they arrive at the Earth's surface, they are characterized by a very specific morphology which takes advantage of the preferable systems of discontinuities affecting the rock. These discontinuities are defined during the last intrusive stages of the granitoid and are therefore of endogenous origin. During these endogenous stages, even though total crystallization has occurred, the rock masses have been observed to have very specific rheological behaviors in the proximity of the contact zones of the magmatic bodies and the host rocks. These contacts propitiate deformations by ductile-fragile shearing actions which generate very specific planar structural fabric (Vidal Romaní, 2008; Glazner et al., 2004). For example, having been previously defined for sedimentary or metasedimentary rock masses (Zulauf G et al., 2011; Zulauf J et al., 2011; Ramsay and Huber, 1987) and being unequivocally attributed to deformative processes developed during the folding of the sedimentary masses, these actions are known to be associated with stratification, fracture planes, faults, and so on. Until recently plutonic rock masses which lack stratification planes have been interpreted as sheet structures (Gutiérrez, 2005) of exogenous origin caused by erosive unloading processes. However, the erosion processes are now known to have not eliminated the host rocks (sedimentary or metamorphic) which were less resistant to erosion until millions of years after the granitoid had been intruded and deformed. Therefore, it can be concluded that the granitoid rocky landscapes were not evident until the rock became exposed on the Earth's surface (Gutiérrez, 2005; Twidale et al., 2005, 1994). In addition, this essentially explains why the endogenous features of granitoid rocky landscapes have been misinterpreted as exogenous formations caused by erosion.

    Once the granitoid rock masses reach the surface layers, the meteoric agents gave rise to the only exogenous formations developed on the surface. The most conspicuous cases of the aforementioned are surfaces polished by marine, fluvial, glacial, or wind erosion, as well as the generation of pot-holes of fluvial, marine, or subglacial origin), and grooves and rills caused by the erosion or dissolution of the rock by water.

  • This work was financially supported by the National Natural Science Foundation of China (NSFC) (Nos. 41472155, 41876037), the Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology (No. MGQNLM20 1902), the Scientific and Technological Innovation Project from the China Ocean Mineral Resources R & D Association (No. DY135-N2-1-04). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1268-z.

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