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Novelty in the biological world is the culmination of genetic changes often triggered by the physical environment. The most radical phase of biological evolution took place during the Cambrian Evolutionary Radiation (CER). Prior to the CER, bacterial matgrounds and associated communities of Ediacaran organisms dominated the shallow seafloor. Near the end of the Ediacaran Period, ~ 550 million years ago, many soft-bodied biota went extinct. In the Early Cambrian, animals with the ability to burrow vertically altered the ecology of the seafloor and biomineralization became commonplace. Here we link the terminal Ediacaran extinction, the Cambrian substrate revolution and the diversification of biomineralizing organisms to changes associated with the reversal frequency of the Earth's magnetic field. Beginning around 550 Ma and continuing through much of the Cambrian, the Earth's magnetic field was rapidly reversing. Models, and limited paleointensity studies, indicate that rapid reversals are a feature of an overall weaker dipole. A weakened dipole reduces the dimensions of the magnetosphere that provides a barrier to incoming cosmic radiation. Here we show that the environmental effects of that collapse include increased dosing of UVB radiation into the shallow marine environment. Increased UVB radiation in the shallow marine environment provided selective pressure favoring organisms that could detect and avoid UVB damage by burrowing vertically, moving up or down in the water column, growing protective shells and other ‘flight from light’ mechanisms. These changes took place in advance of the CER, but effectively cleared the ecological space for the subsequent changes in the later Cambrian. [ABSTRACT FROM AUTHOR]

The progress in understanding the evolution of the Earth during the Ediacaran–Cambrian is greatly hindered by the scarcity and inconsistency of paleomagnetic data for this time interval. In order to acquire new data and clarify the confusing situation, Upper Ediacaran clastic rocks of the Basu Formation were sampled at several localities in the westernmost parts of the South Urals that is the deformed margin of Baltica at least since the beginning of the Neoproterozoic. With the aid of stepwise thermal demagnetization, a dual-polarity high-temperature component (HTC) was reliably isolated from gray and maroon sandstones and siltstones at 34/49 sites. The HTC mean direction D° = 55, inclination I° = − 35 (k = 31, α 95 ° = 4.5) corresponds to a paleolatitude of 19° ± 3°. The reversal and fold tests are positive for the HTC. The slump test on two meter-sized slumps shows that the HTC predates slumping in one case and is coeval with it, in the other, thus convincingly indicating the primary origin of the HTC. Also, we demonstrate that inclination shallowing is either absent altogether, or, at worst, less than 10°, in these rocks; hence the position of Baltica can be reliably reconstructed for time 560–575 Ma. We reviewed paleomagnetic data with ages from 615 to 530 Ma for Baltica and Laurentia and come to the conclusion that there is still no uncontestable scenario for the opening of the Iapetus Ocean that is based on non-controversial geologic and paleomagnetic data. [ABSTRACT FROM AUTHOR]

Abstract: It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~2000km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north–central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle–Late Ordovician paleolatitudes and came to the conclusion that a nearly E–W trending active margin of the Kazakhstania landmass had existed at low (~10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet–Akbastau, and with the Aktau–Junggar microcontinent by the Ordovician–Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~45°. This changed the orientation of the Kazakhstania to NW–SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt. [Copyright &y& Elsevier]

Abstract: The Cambrian explosion, c. 530–515Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635–542Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~750–635Ma Neoproterozoic glacial episodes by 20–30million years. In this paper, we present evidence for the oldest discovery of the ‘Ediacara’ discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. “Marinoan”) glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata. [Copyright &y& Elsevier]

Abstract: Existing views on the tectonic evolution of the Central Asian orogenic belt (CAOB) are highly controversial and the Neoproterozoic to Cambrian stages of this evolution remain the most enigmatic. However, the views on the Paleozoic evolution of the CAOB crucially depend on these early stages, as different choices of the starting point lead to very dissimilar Paleozoic reconstructions. In this context numerous microcontinents with the Precambrian basement that are included in the mosaic structure of Kazakhstan, Tien Shan, Altai and Mongolia are of particular interest. We undertook a paleomagnetic, geochemical and geochronological study of the Neoproterozoic volcanics from one of these units — the Baydaric microcontinent in Central Mongolia. According to U–Pb (laser ablation) dating the age of the studied Dzabkhan Volcanics is about 770–805Ma. Thermal demagnetization revealed that most of the studied samples retained a pre-tilting component, whose primary origin is supported by a conglomerate test. These new data, together with available geological information allow us to conclude that about 770–800Ma ago the Baydaric domain was located at a latitude of 47−12 +16° N and belonged to one of the following plates: India, South China, Tarim or Australia. [Copyright &y& Elsevier]

Abstract: Most of Kazakhstan belongs to the central part of the Eurasian Paleozoic mobile belts for which previously proposed tectonic scenarios have been rather disparate. Of particular interest is the origin of strongly curved Middle and Late Paleozoic volcanic belts of island-arc and Andean-arc affinities that dominate the structure of Kazakhstan. We undertook a paleomagnetic study of Carboniferous to Upper Permian volcanics and sediments from several localities in the Ili River basin between the Tien Shan and the Junggar–Alatau ranges in southeast Kazakhstan. Our main goal was to investigate the Permian kinematic evolution of these belts, particularly in terms of rotations about vertical axes, in the hope of deciphering the dynamics that played a role during the latest Paleozoic deformation in this area. This deformation, in turn, can then be related to the amalgamation of this area with Baltica, Siberia, and Tarim in the expanding Eurasian supercontinent. Thermal demagnetization revealed that most Permian rocks retained a pretilting and likely primary component, which is of reversed polarity at three localities and normal at the fourth. In contrast, most Carboniferous rocks are dominated by postfolding reversed overprints of probably “mid-Permian” age, whereas presumably primary components are isolated from a few sites at two localities. Mean inclinations of primary components generally agree with coeval reference values extrapolated from Baltica, whereas declinations from primary as well as secondary components are deflected counterclockwise (ccw) by up to ∼90°. Such ccw rotated directions have previously also been observed in other Tien Shan sampling areas and in the adjacent Tarim Block to the south. However, two other areas in Kazakhstan show clockwise (cw) rotations of Permian magnetization directions. One area is located in the Kendyktas block about 300 km to the west of the Ili River valley, and the other is found in the Chingiz Range, to the north of Lake Balkhash and about 400 km to the north of the Ili River valley. The timing of the ccw as well as cw rotations is clearly later than the disappearance of any marine basins from northern Tarim, the Tien Shan and eastern Kazakhstan, so that the rotations cannot be attributed to island-arc or Andean-margin plate settings — instead we attribute the rotations to large-scale, east–west (present-day coordinates), sinistral wrenching in an intracontinental setting, related to convergence between Siberia and Baltica, as recently proposed by Natal''in and Şengör [Natal''in, B.A., and Şengör, A.M.C., 2005. Late Palaeozoic to Triassic evolution of the Turan and Scythian platforms: the pre-history of the palaeo-Tethyan closure, Tectonophysics, 404, 175–202.]. Our previous work in the Chingiz and North Tien Shan areas on Ordovician and Silurian rocks suggested relative rotations of ∼180°, whereas the Permian declination differences are of the order of 90° between the two areas. Thus, we assume that about 50% of the total post-Ordovician rotations are of pre-Late Permian age, with the other half of Late Permian–earliest Mesozoic age. The pre-Late Permian rotations are likely related to oroclinal bending during plate boundary evolution in a supra-subduction setting, given the calc-alkaline character of nearly all of the pre-Late Permian volcanics in the strongly curved belts. [Copyright &y& Elsevier]

In order to test different hypotheses concerning the Paleozoic evolution of the Ural–Mongol belt (UMB) and the amalgamation of Eurasia, we studied Middle Devonian basalts from two localities (11 sites) and Lower Silurian volcanics, redbeds, and intra-formational conglomerates from three localities (20 sites) in the Chingiz Range of East Kazakhstan. The Devonian rocks prove to be heavily overprinted in the late Paleozoic, and a high-temperature, presumably primary, southerly, and down component is isolated at only four sites from a homoclinal section. Most Silurian redbeds are found to be remagnetized in the late Paleozoic; in contrast, a bipolar near-horizontal remanence, isolated from Silurian volcanics, is most probably primary as indicated by positive tilt and conglomerate tests. Analysis of paleomagnetic data from the Chingiz Range shows that southward-pointing directions in Ordovician, Silurian, and Devonian rocks are of normal polarity and hence indicate large-scale rotations after the Middle Devonian. The Chingiz paleomagnetic directions can be compared with Paleozoic data from the North Tien Shan and with the horseshoe-shaped distribution of subduction-related volcanic complexes in Kazakhstan. Both paleomagnetic and geological data support the idea that today''s strongly curved volcanic belts of Kazakhstan are an orocline, deformed mostly before mid-Permian time. Despite the determination of nearly a dozen new Paleozoic paleopoles in this study and other recent publications by our team, significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late Paleozoic. However, the paleomagnetic results from the Chingiz Range and the North Tien Shan indicate that these areas show generally coherent motions with Siberia and Baltica, respectively. [Copyright &y& Elsevier]

Abstract: A prominent feature of the central part of the Ural–Mongol orogenic belt is a series of concentric horse-shoe shaped volcanic arcs, with the youngest arc on the inside. This structure was long-suspected to be an orocline, but unequivocal evidence for this was lacking, mainly because paleomagnetic results of suitable age from this area remained sparse, but also because their interpretation was not straightforward due to a long history of deformations associated with the protracted late Paleozoic assembly of Asia. Our paleomagnetic study of Middle Devonian basaltic and andesitic flows in southeastern Kazakhstan revealed two main components of magnetization. The primary nature of a high temperature magnetization (tilt corrected Dec=286.5, Inc=46.4, α 95 =7.8, k =29.2, N =13 sites) is supported by the presence of antipodal directions and a baked-contact test. We also isolated a post-folding overprint with an in situ mean direction Dec=134.9, Inc=−43.0 (α 95 =4.9, k =71.6, N =13 sites). The age of this overprint can be estimated as Early Permian with a high degree of confidence. The declination of the overprint is seen to be deflected counter-clockwise by 100±6° relative to the 290-Ma reference direction, indicating that the studied locality, similar to many other localities in the region, was affected by late-orogenic rotations. We use the overprint''s deflection to correct the declination of the primary Devonian magnetization for these late-orogenic block-rotations. Declinations from other Silurian and Devonian paleomagnetic results in the subduction-related Devonian volcanic arc of Kazakhstan have been corrected for such rotations wherever the latter are reasonably well documented. Using corrected declinations as passive markers we restored the trend of the volcanic belt to its Devonian configuration. Our analysis indicates that the presently curved belt was nearly straight and NW–SE trending. This ∼1500 km long volcanic belt characterized the northeastern margin of a landmass in today''s central Kazakhstan where subduction occurred towards the southwest. Oroclinal bending of this arc took place in the interval between the Middle Devonian and the Late Permian. [Copyright &y& Elsevier]

Conflicting paleogeographic reconstructions of the Ural–Mongol fold belt have been developed over the past decade; very few of them, however, have been based on paleomagnetic data, which still remain scarce. The Chingiz Range in northeastern Kazakhstan provides an opportunity to evaluate the tectonic evolution of the central part of this belt through extensive paleomagnetic studies. In this paper, we report the results from Upper Cambrian andesites and Lower Ordovician tuffaceous sandstones within the Chingiz Range. A high-temperature component (HTC), decaying to the origin, is isolated from Upper Cambrian rocks at eight sites with an easterly and low-intermediate upward direction. This component passes the tilt test and is likely primary. A HTC is also isolated from Lower Ordovician rocks at 11 sites. These site means exhibit a shallow-inclination girdle distribution within the southeast quadrant, likely due to net rotations within the complex structure studied. The inclination-only tilt test is positive, and the best grouping of data is observed at 100% untilting. Also included in our paleolatitudinal analyses are new middle and late Paleozoic paleolatitudes from nearby parts of the Chingiz Range as well as published paleomagnetic data from central and east Kazakhstan. We favor normal polarity of the early Paleozoic results of this paper and conclude that the study area moved northward throughout the Paleozoic. The observed paleolatitudes generally fit the Siberian reference data and hence indicate coherent latitudinal motion of the Boshekul–Chingiz tectonic zone and the Siberian plate. [Copyright &y& Elsevier]

Paleomagnetic results on thick lava series are among the most important sources of information on the characteristics of ancient geomagnetic fields. Most paleo-secular variation data from lavas (PSVL) are of late Cenozoic age. There are far fewer results from lavas older than 5 Ma. The Central Asia Orogenic Belt that occupies several million square kilometers in Asia is probably the world’s largest area of Paleozoic volcanism and is thus an attractive target for PSVL studies. We studied a ca. 1700 m thick lava pile in eastern Kazakhstan of Early Permian age. Magmatic zircons, successfully separated from an acid flow in this predominantly basaltic sequence, yielded an Early Permian age of 286.3 ± 3.5 Ma. Oriented samples were collected from 125 flows, resulting in 88 acceptable quality flow-means ( n ⩾ 4 samples, radius of confidence circle α 95 ⩽ 15°) of the high-temperature magnetization component. The uniformly reversed component is pre-tilting and arguably of a primary origin. The overall mean direction has a declination = 242.0° and an inclination = −56.2° ( k = 71.5, α 95 = 1.8°; N = 88 sites; pole at 44.1°N, 160.6°E, A 95 = 2.2°). Our pole agrees well with the Early Permian reference data for Baltica, in accord with the radiometric age of the lava pile and geological views on evolution of the western part of the Central Asia Orogenic Belt. The new Early Permian result indicates a comparatively low level of secular variation especially when compared to PSVL data from intervals with frequent reversals. Still, the overall scatter of dispersion estimates that are used as proxies for SV magnitudes, elongation values and elongation orientations for PSVL data is high and cannot be fitted into any particular field model with fixed parameters. Both observed values and numerical simulations indicate that the main cause for the scatter of form parameters (elongation values and elongation orientations) is the too small size of collections. Dispersion estimates (concentration parameter and standard angular deviation) are more robust, and their scatter stems from other sources, which may include non-stochastic features of datasets like clusters, loops etc., or non-stationary behavior of secular variation magnitude over time intervals of many million years. [ABSTRACT FROM AUTHOR]

Abstract: The Kazakhstan orocline, a horseshoe-shaped belt with volcanic arcs of Devonian (external) and late Paleozoic (internal) age, is thought to have formed as a result of convergence between the cratons of Siberia, Baltica and Tarim leading to the amalgamation of Eurasia. Paleomagnetic and geologic data indicate that in the Middle Devonian the arc, which is now strongly curved, was nearly straight near the northwest–southeast trending volcanic margin of a Kazakhstania continent. To constrain the age of oroclinal bending we conducted a paleomagnetic study of Late Carboniferous to Late Permian subduction-related volcanics from the middle (NW) and north-eastern (NE) limbs of the orocline. Our new results indicate that the rotation of the middle arm of the orocline was essentially completed by the earliest Permian, while the NE arm probably was still ∼50° short of its final orientation with respect to Baltica. The rotation of, or rotation within, the NE arm was completed by the Late Permian. The paleomagnetic data constraining the timing and rotation patterns lead us to propose the following scenario for the bending of the Kazakhstan orocline. The orogenic deformation scenario began in the Late Devonian when an initial collision with Tarim pinned Kazakhstania''s southern corner, while a dextral shear motion and a considerable clockwise rotation of Siberia dragged its northern end. Relative convergence between Siberia and Tarim caused initial buckling of the Kazakhstania continental element trapped between them, subdividing the belt into three (SW, NW, NE) segments. Continued subduction under the established limbs of the orocline with an estimated outward-directed subduction velocity of well less than 1 cm/yr gradually led to closure of the intervening Junggar–Balkhash oceanic basin and tightening of the orocline. [Copyright &y& Elsevier]

Three new Middle–Late Ordovician and two new Early Carboniferous paleomagnetic poles have been obtained from the North Tien Shan Zone (NTZ) of the Ural–Mongol belt in Kyrgyzstan and Kazakhstan. Paleolatitudes for the Carboniferous are unambiguously northerly and average 15.5°N, whereas the Ordovician paleolatitudes (6°, 9°, and 9°) are inferred to be southerly, given that a very large (∼180°) rotation of the NTZ would be necessary during the middle Paleozoic if the other polarity option was chosen. Thus, the NTZ drifted northward during much of the Paleozoic; east–west drift cannot be determined, as is well known, from paleomagnetic data. In addition, detailed thermal demagnetization analysis reveals two overprints, one of recent age and the other of Permian age, which is a time of strong deformation in the NTZ. The paleolatitude of the combined Permian overprint is 30.5+2°N. The paleolatitudes collectively track those predicted for the area by extrapolation from Baltica very well, but are different from those of Siberia for Ordovician times. This finding is compatible with Sengo¨r and Natal'in's [Sengo¨r, A.M.C., Natal'in, B.A., 1996. Paleotectonics of Asia: fragments of a synthesis. In: Yin A., Harrison, M. (Eds.), The Tectonic Evolution of Asia. Cambridge Univ. Press, Cambridge, pp. 486–640] model of tectonic evolution of the Ural–Mongol belt and disagrees with the models of other researchers. Declinations of the Ordovician and Early Carboniferous results range from northwesterly to northeasterly, and are clearly affected by local relative rotations, which seem characteristic for the entire NTZ, because the Permian overprint declinations also show such a spread. Apparently, the important latest Paleozoic–Triassic deformation involved shear zone-related rotations as well as folding and significant granitic intrusions. [Copyright &y& Elsevier]