Bol’shakovskii gabbro massif as a fragment of the Southern Urals zone of Early Carboniferous rift

In this paper new data on the absolute age and geochemistry of rocks of the Bol’shakovskii massif, situated in the central part of the Aramil-Sukhteli zone of the Southern Urals, are given. The obtained values are evidence for its Visean age. By the geological-petrographic and petro- and geochemical features, the rocks of the Bol’shakovskii complex differ sharply from ophiolite-type gabbroids, although they reveal a substantial similarity with the gabbro-granite formation of the Magnitogorsk megazone. The Bol’shakovskii massif is situated in the northern branch of the South Urals zone of Early Carboniferous riftogenesis; and its formation is most probably associated with magmatism events during the rift regime in the died_out island arc.     

Zircon geochronology of the Klyuchevskoi gabbro-ultramafic massif and the problem of the age of the Mohorovicic paleoboundary in the Central Urals

The Klyuveskoi gabbro-ultramafic massif is the most representative ophiolite complex on the eastern portion of the Uralian paleoisland arc part. The massif is composed of dunite-harzburgite (tectonized mantle peridotites) and dunite-wehrlite-clinopyroxenite-gabbro (layered part of the ophiolite section) rock associations. The U-Pb age was obtained for the accessory zircons from the latter association using a SHRIMP-II ion microprobe at the Center for Isotopic Research at the Karpinskii Russian Geological Research Institute. The euhedral zircon crystals with thin rhythmic zoning from dunites are 441.4 ± 5.0 Ma in age. Zircons from olivine clinopyroxenite show three age clusters with sharply prevalent grains 449.0 ± 6.8 Ma in age. Two points give 1.7 Ga, which is probably related to the age of the mantle generating the layered complex. One value corresponds to 280 Ma, which possibly reflects exhumation of ultramafic rocks in the upper crust during the collision of the Uralian foldbelt. Thus, dunites and olivine pyroxenites from the Klyuchevskoi massif are similar in age at 441–449 Ma. The bottom of the layered part of the ophiolite section corresponds to the M paleoboundary and, consequently, the age of the Mohorovicic discontinuity conforms with the Ordovician-Silurian boundary in this part of the Urals.     

Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (⁸⁷Sr/8⁶Sr)₀ = 0.70428 and ?_Nd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.     

U-Pb systematics of detrital zircons from the Serebryanka Group of the Central Urals

Based on the LA-ICP-MS data, detrital zircons from the tillite-type conglomerates of the Tanin Formation (Serebryanka Group) on the western slope of the Central Urals include approximately equal proportions of crystals with Neoarchean and Paleoproterozoic U-Pb ages. Therefore, we can assume that crystalline rocks of the basement beneath the eastern part of the East European Craton served as a provenance for aluminosilicate clastics in the initial Serebryanka period. Detrital zircons from sandstones of the Kernos Formation have the Meso-Neoarchean (∼15%), Paleoproterozoic (∼60%), and Mesoproterozoic (∼26%) age. Comparison of the obtained data with the results of the study of detrital zircons from Riphean and Vendian sandstones of the Southern Urals shows that the Riphean and Lower Vendian rocks are mainly represented by erosional products of Middle and Upper Paleoproterozoic crystalline rocks that constitute the basement of the East European Craton. In addition, a notable role belonged to older (Lower Proterozoic, Neoarchean and Mesoarchean) rock associations during the formation of the Serebryanka Group. The terminal Serebryanka time (Kernos Age) differed from its initial stage (Tanin Age) by the appearance of Mesoproterozoic complexes in provenances. According to available data, these complexes played an insignificant role in the formation of Riphean-Vendian rocks in the neighboring South Uralian segment. This implies a spatiotemporal diversity of clastic material sources for Upper Precambrian rocks in the western megazone of the Southern and Central Urals.     

乌拉尔南部和印度南部地区隐晶质菱镁矿的地质与地球化学对比研究

印度南部和乌拉尔南部都有隐晶质菱镁矿产出，这两处矿床的产出地质环境相似，在矿物学和地球化学上具有广泛的相似性。印度南部的菱镁矿矿化主要与超镁铁质侵入杂岩体有关，并形成了部分已受变质的火山沉积地层。超镁铁质侵入杂岩体由纯橄岩，橄榄岩，辉石岩，辉长岩及它们的变质产物组成。在乌拉尔地区，菱镁矿床位于一个蛇绿岩带上的超镁铁岩地体中。隐晶质菱镁矿就以网脉状产出于超镁铁质岩地体上部的风化带中。印度和乌拉尔两个地区的矿床中的矿物组合都有菱镁矿，石英，方解石和白云石，但在印度南部的矿区中还含有滑石和菱铁矿。两个地区的菱镁矿矿石的质量都很好，所有的样品的主要成分都为菱镁矿(73～96％)，而方解石(1～3％)，白云石(0～7％)，菱铁矿(0～2％)，石英(0～5％)和滑石(O～2％)都只是次要矿物。次生的白云石和菱铁矿使一些矿石含有较高的CaO(最高达2．6％)和FeO(最高达1．6％)，石英和滑石等矿物则使矿石中的SiO2较高(5—8％)。滑石指示了低温成因，它的出现说明两个矿区的菱镁矿可能都是内生或外生的成矿流体在上升或下降的过程中在开放裂隙中沉淀而成的。本文研究表明，全球性的超镁铁岩中菱镁矿成矿事件与蛇绿岩带有关，这对菱镁矿的勘探有指导意义。     

Geochemistry of igneous rocks associated with ultramafic–mafic-hosted Cu (Co, Ni, Au) VMS deposits from the Main Uralian Fault (Southern Urals, Russia)

Ultramafic–mafic- and ultramafic-hosted Cu (Co, Ni, Au) volcanogenic massive sulfide (VMS) deposits from ophiolite complexes of the Main Uralian Fault, Southern Urals, are associated with island arc-type igneous rocks. Trace element analyses show that these rocks are geochemically analogous to Early Devonian boninitic and island arc tholeiitic rocks found at the base of the adjacent Magnitogorsk volcanic arc system, while they are distinguished both from earlier, pre-subduction volcanic rocks and from later volcanic products that were erupted in progressively more internal arc settings. The correlation between the sulfide host-rocks and the earliest volcanic units of the Magnitogorsk arc suggests a connection between VMS formation and infant subduction-driven intraoceanic magmatism.     

Main Uralian Thrust and Main Uralian Normal Fault: non-extensional Palaeozoic high-P rock exhumation, oblique collision, and normal faulting in the Southern Urals

The crustal architecture of the Southern Urals is dominated by an orogenic wedge thrusted westward upon the subducted East European continental margin. The N–S trending wedge constitutes an antiformal stack composed mainly of the high-P Maksyutov Complex, the overlying Suvanyak Complex and the allochthonous synformal Zilair flysch further west. These tectono-metamorphic units are separated by tectonic contacts and record discontinously decreasing metamorphic conditions from bottom to top. In the east, the E-dipping Main Uralian Normal Fault cross-cuts the metamorphic footwall and juxtaposes the non metamorphic Magnitogorsk island arc. This syncollisional normal fault compensated crustal thickening and exhumation of the high-P rocks. Orogenic shortening was accommodated by the Main Uralian Thrust, a W-vergent crustal-scale shear zone at the base of the wedge. Geological investigations and reflection seismics (URSEIS '95) argue in favour of a geodynamic evolution integrating subduction and basal accretion of high-P rocks during sinistral oblique thrusting along the Main Uralian Thrust and coeval normal-faulting along the Main Uralian Normal Fault.     

Age of nephrite-bearing dikes of the Uzunkyr Belt (South Urals): Local U-Pb isotope analysis of zircon and Sr-Nd isotope data of rock-forming minerals

Through local U-Pb isotope analysis of zircon and Sir-Need data on rock-forming minerals, the age of nephrite-bearing monzonite-diorite dikes of the Uzunkyr Belt has been determined. The derived datings coincide with known geological events that took place in the Phanerozoic on the territory of the South Urals. Xenogenic zircons prove the participation of the Upper Ordovician units in the tectonic structure of the studied area. Devonian zircons are associated with assimilation of subvolcanic rocks which are middle and basic in composition and whose formation time correlates with the appearance of the subduction zone with the Magnitogorsk island arc above it. Early Carboniferous datings indicate the relationship between dike formation and formation of the continental arc-shaped structure to which the Syrostan massif (monzodiorite-granite formation) belongs. The age range of the Uzunkyr nephrite-bearing dikes coincides with that of intrusives (350–336 Ma) of the Magnitogorsk Belt, where formation of gabbro series was also changed by formation of subalkali and alkali igneous rocks. According to the analogous data on zircon datings from metamorphic rocks of the Il’menogorskii Complex, the given territory later evolved as a whole.     

The geological setting of lode-gold deposits in the central southern Urals: a review

The late-Paleozoic Uralides represent one of the largest lode-gold metallogenic provinces in the world. In the southern Urals, gold distribution is heterogeneous and is confined mainly to two tectonostratigraphic zones, namely the Main Uralian fault and the East Uralian zone. The important lode-gold districts within and in the immediate hangingwall of the first-order crustal suture of the Main Uralian fault are characterized by a complex tectonic history of earlier compressional tectonics involving thrusting, folding and reverse faulting and later transcurrent shearing. Gold mineralization is hosted by second- and third-order brittle to brittle–ductile strike-slip faults that developed late during the kinematic history of the Main Uralian fault. Strike-slip reactivation of earlier compressional structures was related to the late-stage docking of the passive margin of the East European platform with island-arc complexes of the southern Urals, an event that is tentatively related to changes in plate motion during the final stages of terrane accretion during the upper Permian and lower Triassic. Gold mineralization was controlled by the permeability characteristics of the hydrothermal conduits, as well as by competence contrasts and geochemistry of the mainly volcanic host rocks. Mineralization occurred at relatively shallow crustal levels (2–6 km) and largely post dates peak-metamorphism of the host rocks. The large and very large (up to 300 to Au) gold deposits of the East Uralian zone are hosted by upper-Paleozoic granitoid massifs. Gold mineralization is temporally associated with the main phase of regional-scale compressional tectonics and granite plutonism during the upper Carboniferous and lower Permian. Controlling structures have a dominantly east–west strike and occur as hybrid shear-tensional vein systems in competent granitoids subjected to east/west-directed regional shortening. Deformation textures and alteration mineral assemblages indicate lower-amphibolite-facies conditions of mineralization close to peak metamorphic conditions that are associated with the mid-Permian regional metamorphism and tectonism. Gold deposits in the southern Urals are, therefore, polygenetic and are temporally and genetically distinct in each of the two major mineralized tectonostratigraphic zones of this well-preserved collisional orogenic belt. The different timing of ore fluid generation and fluid discharge is interpreted to be the result of the different tectonic, metamorphic and magmatic evolution of terranes in the southern Urals.     

Peculiarities of the kinematics of the South Uralian suture zones as a cause of the formation of the convergent structure of the East Uralian megazone

The structure of the suture zones surrounding the East Uralian megazone in the South Urals is reviewed. The analysis of mesoscale structures allowed us to prove their strike-slip nature. Their kinematics changed from left-lateral in the Early Carboniferous into right-lateral in the Mesozoic. The age of these dislocations was determined after the age of synkinematic intrusions. The suture zones have divergent structures in contrast to the convergent East Uralian megazone that is located between them.     

Platinum-group elements in alpine-type ultramafic rocks and related chromite ores of the main ophiolite belt of the Urals

On the basis of a representative collection of ultramafic rocks and chromite ores and a series of technological samples from the largest (Central and Western) deposits in the Rai-Iz massif of the Polar Urals and the Almaz-Zhemchuzhina and Poiskovy deposits in the Kempirsai massif of the southern Urals, the distribution and speciation of platinum-group elements (PGE) in various type sections of mafic-ultramafic massifs of the Main ophiolite belt of the Urals have been studied. Spectral-chemical and spectrophotometric analyses were carried out to estimate PGE in 700 samples of ultramafic rocks and chromite ores; 400 analyses of minerals from rocks, ores, and concentrates and 100 analyses of PGE minerals (PGM) in chromite ores and concentrates were performed using an electron microprobe. Near-chondritic and nonchondritic PGE patterns in chromitebearing sections have been identified. PGE mineralization has been established to occur in chromite ore from all parts of the mafic-ultramafic massifs in the Main ophiolite belt of the Urals. The PGE deposits and occurrences discovered therein are attributed to four types (Kraka, Kempirsai, Nurali-Upper Neiva, and Shandasha), which are different in mode of geological occurrence, geochemical specialization, and placer-forming capability. Fluid-bearing minerals of the pargasite-edenite series have been identified for the first time in the matrix of chromite ore of the Kempirsai massif (the Almaz-Zhemchuzhina deposit) and Voikar-Syn’ya massif (the Kershor deposit). The PGE grade in various types of chromite ore ranges from 0.1–0.2 to 1–2 g/t or higher. According to technological sampling, the average PGE grade in the largest deposits of the southeastern ore field of the Kempirsai massif is 0.5–0.7 g/t. Due to the occurrence of most PGE as PGM 10–100 mm in size and the proved feasibility of their recovery into nickel alloys, chromites of the Kempirsai massif can be considered a complex ore with elevated and locally high Os, Ir, and Ru contents. The Nurali-Upper Neiva type of ore is characterized by small-sized primary deposits, which nevertheless are the main source of large Os-Ir placers in the Miass and Nev’yansk districts of the southern and central Urals, respectively.     

New data on the composition and age of granitoids in the northern part of the Tagil structure (Ural Mountains)

The Tagil structure representing a large fragment of the Paleozoic island arc on the eastern slope of the Urals has been sufficiently well studied in its southern part (Middle Urals). In contrast, reliable data on the age and geochemical properties of various, including granitoid, rock complexes available for its northern part are scarce. The first data on the U–Pb LA–ICP–MS age of zircons from quartz diorites of the Man’ya massif of the Petropavlovsk Complex (436 ± 3 Ma, MSWD = 1.3), tonalites of the same complex (439.4 ± 1.3 Ma, MSWD = 1.3), granites of the Yuzhno-Pomur massif of the Severorudnichnyi Complex (422.4 ± 3 Ma, MSWD = 1.5), and titanite of the same massif (423.4 ± 4.4 Ma, MSWD = 0.84) have been obtained. Based on these data combined with the geochemical properties of the host rocks, the conclusion that they were crystallized at the initial stages of the formation of comagmatic volcanic series is supported; by their composition, granitoids correspond to island arc igneous rocks.     

Actual Problems of Prediction of Manganese Deposits in the Urals

Based on recent publications and our long-standing investigations, the most reasonable ways were proposed to obtain the manganiferous raw material for the Uralian metallurgic industry, which lost the traditional raw base (Ukrainian and Georgian deposits) after the USSR breakdown. The Urals is region is perspective for the discovery of two types of manganese deposits. (1) Insignificantly metamorphosed siliceous–carbonate ores with silica modulus M_Si (i.e., MnO : SiO₂) ranging from 1 to 2 and manganese content of 17–20%. The most promising are the areas with different-age, mainly carbonate rocks in the Urals (Northern and Polar) and Pai-Khoi regions. (2) Oxide, mainly pyrolusite–psylomelane ores with Mn content of 30–35% in the Meso–Cenozoic manganese hats developed in the Paleozoic manganiferous (volcanogenic–siliceous and carbonate–siliceous) rocks and noneconomic (small) deposits. The most promising areas are Late Cretaceous and Paleogene peneplains of the Southern Urals (Trans-Uralian and Zilair regions). It is necessary to intensify works on the improvement of concentration technique for manganese ores and to carry out the marketing study of the expediency of replacing imported manganiferous concentrates by those obtained from the Uralian ores.     

Structure of the Middle Urals,east of the main Uralian Fault

In the Middle Urals, volcanic-arc and back-arc basin rocks of Ordovician to Devonian age occur in the Tagil Synform. These outboard terranes were thrust westwards in the late Carboniferous onto continental margin associations of late Proterozoic and Palaeozoic age, now exposed in the Central Uralian Uplift. The Main Uralian Fault coincides approximately with the suture separating the outboard terranes from the East European Platform margin. New fieldwork in the hinterland of the Middle Urals in the area east of the Tagil Synform has found structural evidence favouring E-directed thrusting of accreted terranes and eugeoclinal allochthons in the late Palaeozoic. The upper tectonic units are composed of ophiolite mélange and volcano-sedimentary rocks of Ordovician to Devonian age; they are thrust onto high-grade gneisses, some of possible microcontinental affinities, extensively intruded by mid-Palaeozoic granitic plutons. The nappes in the hinterland are refolded by major upright antiforms and synforms that fold the entire tectonostratigraphy. After thrust assembly, all tectonic units east of the Main Uralian Fault were intruded by late Carboniferous to early Permian granites. Reflection seismic profiles (recorded to 8 s TWT), recently reprocessed at Cornell University, image the major fold structures and demonstrate that they are restricted to the upper crust, being underlain by an extensive zone of flat-lying middle crustal reflectivity. At 10–15 km depth the latter appears to truncate all structures, including the late- to post-tectonic granitoids and extensional faults, east of the Main Uralian Fault. Previous studies (potential-field, refraction- and wide-angle-reflection seismics) have identified an anomalously deep crust under the Tagil Synform and have concluded that the root zone of the orogen is located beneath this belt. The new evidence presented here supports this interpretation, with back-thrusting of the oceanic rocks eastwards over Palaeozoic accreted terranes. © 1998 John Wiley & Sons, Ltd.     

Structure and formation mechanism of the Nizhny Tagil dunite-clinopyroxenite massif,Central Urals

The results of the structural study of the Nizhny Tagil platiniferous massif in the Central Urals are presented. This is a classic massif of the Ural-Alaskan-type zonal dunite-clinopyroxenite complexes. The massif is characterized by a nearly concentric vertical planar internal structure conformable to petrographic zoning (layering). The primary ultramafic rocks are distinguished by adcumulative protogranular structure with relict euhedral olivine protocrysts and distinct linear orientation of minerals, which was formed as a result of magmatic flow. The deformational linear and planar structures conformable to the early structural elements were formed in the process of subsequent coaxal high-temperature ductile flow. At this stage, dynamometamorphic zoning is formed, expressed in the change of the protogranular microstructure typical of the inner portion of the massif by porphyroclastic and mosaic microstructures in its marginal part. The country rocks underwent conjugate high-temperature metamorphism with the formation of hornfels and kytlymite. The structure of the massif is considered to be a result of dynamic differentiation in the course of magmatic flow followed by high-temperature coaxal flow during intrusive and diapiric ascent of rocks to the crustal level.     

Early Paleozoic Protoliths of Gneisses and Granitic Gneisses in the Eastern South Urals: Results of U–Th–Pb (SIMS) Geochronological Studies

Doklady Earth Sciences - The Early Paleozoic age of the protolith for gneisses in the East Uralian megazone (South Urals) is proved by zircon dating. Two metamorphic complexes have been identified...     

Structural and magnetic microinhomogeneities in accessory spinel of the system Fe2+(Cr2–xFex3+)O4 from the Kytlym massif (Urals platinum-bearing belt)

The microstructure and magnetic properties of accessory Fe–Cr-spinels from the Kytlym massif of the Urals platinum-bearing belt were studied. Atypical Fe–Cr-spinels in the form of magnetic microareas in grains of primary nonmagnetic Fe–Cr-spinel have been revealed for the first time in the bed dunites of the Kytlym multiphase concentrically zoned massif, North Urals. These spinels are responsible for the magnetic properties of the dunites. It has been established that the microareas are separations in solid solution Fe²⁺(Cr_2–xFe_x³⁺)O₄, which are enriched in Fe^3 + and are probably an intermediate product of the transformation of primary accessory Fe-Cr-spinel during the formation of the dunite massif. These are magnetic microphases with particular chemical composition, cation distribution, and corresponding reversed crystal lattice, which determine the main magnetic properties of the microarea: the magnitude and direction of magnetization vector and Curie temperature. The formation of this earlier unknown type of magnetic Fe–Cr-spinel is probably conjugate with the formation and concentration of PGE mineralization in the bed dunites of the Kytlym platinum-bearing massif. The presence of such magnetization carriers in rocks and ores must be taken into account in geophysical research at the Urals chromite and platinum–chromite deposits.     

Dike swarms and related igneous complexes in the Urals

The dike swarms of the entire Urals are classified for the first time; the related igneous complexes associated with them in space and time are named. The following types and chronological levels of the Uralian dikes are distinguished (proper names are given after type localities). The epicontinental type comprises the Middle Riphean Mashak, Late Riphean Arsha-Serebryanka, Late Cambrian-Early Ordovician Kidryasovo-Lemva, Ordovician-Silurian Ushat, Devoninan Inzer-Timaiz (the most extended of all), Early Carboniferous Magnitogorsk-Mugodzhary, and Triassic Borisovo dike swarms. Many of them are probably related to plume events. The existence of the Early Riphean dike complex remains unclear. Oceanic (spreading or suprasubduction) dike-in-dike type: Ordovician Man’ya oceanic type, Devonian Aktogai backarc and Khabarny suprasubduction types. The igneous complexes associated with dike swarms are rather diverse. In addition to rhyolite dikes, in many cases determining the contrasting character of magmatism, large comagmatic gabbro and gabbro-granite intrusions are noted, as well as minor intrusions of subalkali granitoids, syenites, and, apparently, carbonatites and kimberlites. Flood basalt fields are noted at the periphery of the Urals, implying the occurrence of a feeding dike swarm beneath them.     

Noble metal specialization of Lower and Middle Riphean terrigenous rocks in the South Urals

Results of the study of noble metal specialization of Lower and Middle Riphean terrigenous rocks in the Bashkir Anticlinorium (South Urals) are reported. The study revealed their genetic differences in the relatively unaltered, i.e., “background” terrigenous rocks in type sections of the Burzyan and Yurmatau groups and in sedimentary rocks of the same stratigraphic levels from tectonic zones subjected to local dynamothermal metamorphism of the greenschist facies and intruded by mafic rocks. It has been established that Ru serves as a geochemical marker of the impact of magmatic processes on sedimentary rocks and the redistribution of noble metals during metamorphism and local metasomatism. A generalized model is proposed for the formation of noble metal geochemical specialization of Lower and Middle Riphean terrigenous rocks in the South Urals.     

A possible lower crustal flow channel in the Middle Urals based on reflection seismic data

The Europrobe Seismic Reflection Profiling in the Urals Experiments (ESRU) reflection seismic data from the Middle Urals images c. 10‐km thick band of strong, subhorizontal lower crustal reflectivity and a thinning of the crust that is associated with the East Uralian Zone, a broad strike‐slip fault system containing high‐grade metamorphic rocks and syn‐orogenic to post‐orogenic granitoids. The lower crustal reflectivity consists of discontinuous to continuous, high‐amplitude reflections. Reflections are subparallel to slightly oblique and have a layered to oblate appearance. Geometrical relationships indicate that the reflectivity post‐dates fault activity, suggesting that late‐orogenic processes modified the lower crust. The surface geology indicates that the conditions for lower crustal flow were met in the East Uralian Zone. We suggest that the lower crustal reflectivity imaged by the ESRU data is related to a flow channel that developed at the base of the crust in the interior of the orogen.