Lithotectonic Complexes Research Articles

The Archean of the Baltic Shield: Geology, Geochronology, and Geodynamic settings

The Archean provinces and lithotectonic complexes of the Baltic (Fennoscandian) Shield are considered. The supracrustal complexes are classified by age: <3.2, 3.10–2.90, 2.90–2.82, 2.82–2.75, and 2.75–2.65 Ga. The data on Archean granitoid complexes and metamorphic events are mentioned briefly, whereas the recently found fragments of the Archean ophiolitic and eclogite-bearing associations are discussed in more detail. The Paleoarchean rocks and sporadic detrital grains of Paleoarchean zircons have been found in the Baltic Shield; however, the relatively large fragments of the continental crust likely began to form only in the Mesoarchean (3.2–3.1 Ga ago), when the first microcontinents, e.g., Vodlozero and Iisalmi, were created. The main body of the continental crust was formed 2.90–2.65 Ga ago. The available information on the Paleoarchean complexes of the Baltic Shield is thus far too scanty for judgment on their formation conditions. The geologic, petrologic, isotopic, and geochronological data on the Meso-and Neoarchean lithotectonic complexes testify to their formation in the geodynamic settings comparable with those known in Phanerozoic: subduction-related (ensialic and ensimatic), collisional, spreading-related, continental rifting, and the setting related to mantle plumes.

The problem of vortical movements in the solid earth and their role in geotectonics

Crustal structural features having a vortical or spiral shape were discovered in the first third of the 20th century. Since then, such features of various ranks, but similar appearance, have been revealed in different geotectonic settings; however, an adequate tectonic interpretation has not been offered. With allowance for the specific character of vortical movement, the evolution of the structural geometry of the North Atlantic basins and different segments of the global system of mid-ocean ridges is considered in this paper. It is shown that vortical movements do take place in the solid Earth during ocean formation and create scale-invariant rifting and spreading systems, where the spreading axis tends to undergo whirling. The size of these systems differs by more than two orders of magnitude. Many geotectonic phenomena that accompany the formation of oceans, including segmentation of the ocean floor and passive continental margins, folding of the sedimentary cover at these margins, and tectonic delamination of the oceanic lithosphere, may be explained by vortical movements of different ranks. In addition, the vortical structures on continents are variable in size and related to lithotectonic complexes of different ages. The vortical structural units of the Mediterranean Belt are considered as an example. Being driven by the same physical mechanism, the vortical movements depend on the dynamics of different geospheres. These movements are realized only in a nonlinear, nonequilibrium medium. Hence, only nonlinear and nonequilibrium thermodynamics will serve as a theoretical basis for a new concept, which is coming currently to take the place of plate tectonics.

Principal features of the tectonic evolution of sedimentary basins in the Western Chukchi shelf and their petroleum resource potential

The history of the formation of sedimentary basins located predominantly in the western shelf of the Chukchi Sea is considered along with the data on the geology of adjacent areas of the American part of the sea and northern Alaska reported in the available publications on this region. The main lithotectonic complexes that correspond to particular stages of evolution of the region are identified. The tectonic rearrangement of the structural grain in response to the formation of the Canada Basin resulted in the development of the North Chukchi Trough. Intense subsidence of this trough and accumulation of thick sedimentary sequence favored, in turn, the balanced inversion uplift of the Wrangel-Herald Zone spatially related to depocenters of older sediments. Vigorous uplift and folding ceased in the Albian. In the geologic record, these processes are marked by unconformity and overstep of the older structural features. The inversion uplifts continued to rise later. Analysis of the data available provides new insights into the structure and evolution of the region. The sedimentary basins with a great thickness of their fill were favorable for generation and accumulation of hydrocarbons. The large uplifted blocks, extensive wedge-out zones, and stratigraphic unconformities at the walls of troughs and uplift slopes are especially favorable for hydrocarbon accumulation. Permissive beds are correlated with the Neocomian-Aptian sequence in Alaska that hosts oil and gas fields. The Upper Paleozoic and Lower Mesozoic strata, which are the main petroliferous sedimentary rocks in Alaska, may occur at a shallow depth in the Wrangel-Herald Inversion Zone. The conclusions drawn in this paper should be taken into account during reappraisal of the petroleum resources throughout the entire Chukchi shelf. At present, several oil and gas prospects are outlined in the Russian part of the Chukchi Sea.

Precambrian domains in Lithuania: evidence of terrane tectonics

The West Lithuanian Granulite (WLG) and East Lithuanian domains (ELD) form the Proterozoic basement of Lithuania and can be distinguished on the basis of differing structural patterns, lithologies, and evolutionary histories. They are juxtaposed along the Mid-Lithuanian Suture Zone (MLSZ).In the WLG, the main lithotectonic complexes comprise felsic and intermediate, mostly metasedimentary granulites in the south-west and mafic metaigneous granulites in the north-east. The former are interpreted as marine metapelites, while most of the mafic ones have been derived from island-arc tholeiites. These rock complexes trend NW–SE and are marked by contrasting gravity and magnetic anomalies. NE- and E–W-striking faults and shear zones complicate the potential-field patterns. Sets of NW-trending anomalies also extend from Lithuania across the Baltic Sea to south-central Sweden and indicate that the WLG complexes continue into the Baltic/Fennoscandian Shield. Voluminous anatectic granites alternate with the metapelites, whereas the mafic granulites occur together with enderbites and charnockites.In the ELD, the main structures produce strong, NNE–SSW-oriented gravity and magnetic anomalies which trend parallel to the Belarus-Baltic Granulite Belt (BBG) and other terranes situated still farther east. The ELD is composed of metasedimentary rocks interpreted as one-time graywackes, shales and dolomites accumulated in continental-margin arc and shallow-water basinal environments. Amphibolites and gabbros with MORB and IAT characteristics, and voluminous granitoids are also present. The coexistence of juvenile mafic rocks with continental-margin and shelf sediments suggests an oceanic back-arc setting.The two Lithuanian basement domains display contrasting metamorphic histories that suggest separate developments before the eventual amalgamation. In the WLG, the metapelites indicate peak metamorphism at high temperatures (up to 850–900°C) and moderate pressures (8–10kbar). This was followed by cooling and reheating, and then an uplift event. Repeated magmatic underplating accompanied the metamorphism. In the ELD, in contrast, the rocks have been subjected to comprehensive metamorphism under moderate, amphibolite-facies conditions. That metamorphism, however, was not uniform throughout. The metasediments in the east have recorded pressures similar to those in the neighbouring BBG (7–8kbar) but lower temperatures (650–680°C), while in the central and western parts of the ELD, metamorphism occurred at ca. 480–580°C with pressures increasing from 3–4kbar in the centre, to 6kbar close to the western boundary. Reheating to 700°C due to a ca. 1.5-Ga magmatic event is characteristic.The MLSZ, which separates the two Lithuanian basement domains from each other, is a N–S-oriented, ca. 30–50km wide, westward-plunging crustal discontinuity marked by magnetic and gravity highs, mafic and felsic intrusions, and sheared rocks. Crustal thicknesses change from 42–44km in the west to 50km in the eastern side of the Zone, which also truncates a crustal low-velocity layer characteristic of the WLG. The amalgamation of the WLG and ELD along the MLSZ occurred at ca. 1.71–1.66Ga, after which time both domains were affected by the same post-kinematic, anorogenic magmatism ca. 1.58–1.45Ga ago. That event and related shearing were responsible for some ultimate refragmentation of the Lithuanian basement terranes.

Thermobarometric and fluid expulsion history of subduction zones

Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite → prehnite‐pumpellyite → glaucophane schist → eclogite. Parageneses reflect relatively high‐P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high‐rank blueschists, high‐P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low‐T/high‐P parageneses. Inclusion studies suggest that two‐phase immiscible volatiles (liquid H2O, and gaseous high‐hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher‐grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote‐amphibolite facies assemblages. Alpine‐type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low‐density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high‐P minerals, and lack a low‐pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high‐grade blocks, reflect somewhat higher pressures and lower temperatures. Subducted sections constituting portions of the Franciscan‐type of metamorphic belt evidently moved slowly back up the inclined lithospheric plate junction during continued convergence and sustained refrigeration. Upward motion due to isostatic forces was produced by tectonic imbrication of fault suces, laminar return flow in melange zones, and lateral extension of the underplated accretionary prism. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low‐permeability mudstones tend to maintain fluid values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone‐rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes). Because of substantial updip expulsion of volatiles during prograde recrystallization, only small amounts of H2O and CO2 are available to support hydration and carbonation of the accretionary complex during its return toward the surface; thus limited back reaction takes place and occurs at low Pfluid/Plithostatic ratios, unless an abundance of volatiles is introduced during uplift.

Crustal‐scale thrusting and extension in the Hercynian Schwarzwald and Vosges, central Europe

The Hercynian outcrop areas in the Schwarzwald of SW Germany and Vosges of easternmost France consist from north to south of three major lithotectonic complexes: very low‐ to medium grade metamorphics of the Saxothuringian Belt, the polymetamorphic Central Gneiss Complex, and the tectonic melange of the Southern Complex. Juxtaposition of these three complexes occurred by oblique convergent motions in early Carboniferous time. The Saxothuringian Belt is part of a major SE dipping accretionary wedge that consists of lower Paleozoic meta‐sedimentary and metaigneous rocks and superimposed upper Devonian to lower Carboniferous synorogenic volcanics and elastics. The Central Gneiss Complex consists of polymetamorphic paragneisses, orthogneisses, metabasites, and meta‐ultramafics dipping to the NW. The Southern Complex consists of an imbricated melange of north to NW dipping synorogenic elastics, volcanics, and blocks of ultramafics and gneissic basement. The Central Gneiss Complex (CGC) experienced pervasive Hercynian low‐pressure/high‐temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins. This convergence (340–330 Ma) produced mylonitic‐cataclastic shear zones in the CGC. The most significant shear zones are the SE verging Todtnau fault zone between the CGC and the Southern Complex and the NW verging Lubine fault zone between the Central Gneiss Complex and the rocks of the Saxothuringian Belt. These two fault zones enclose a bivergent “pop‐up” of the polymetamorphic basement. The Todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies. Detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km. Crustal thickening during early Carboniferous convergence led to an extensional phase of crustal reordering (between about 330 and 280 Ma) that was accompanied by the massive rise of “posttectonic” granitic plutons from middle to high crustal levels. Simultaneous creation of a structural “lamination” in the refractory lower crust neglected the main convergent tectonic boundaries; it produced a basin‐and‐range type lower crust and mantle‐crust transition in early Permian time and involved a major advective input of heat.