Improvements to the Shaw-Type Absolute Palaeointensity Method

Palaeointensity information enables us to define the strength of Earth’s magnetic field over geological time, providing a window into Earth’s deep interior. The difficulties in acquiring reliable measurements are substantial, particularly from older rocks. Two of the most significant causes of experimental failure are laboratory induced alteration of the magnetic remanence carriers and effects relating to multidomain magnetic carriers. One method that has been claimed to overcome both of these problems is the Shaw method. Here we detail and evaluate the method, comparing various selection criteria in a controlled experiment performed on a large, non-ideal dataset of mainly Precambrian rocks. Monte Carlo analyses are used to determine an optimal set of selection criteria; the end result is a new, improved experimental protocol that lends itself very well to the automated Rapid 2G magnetometer system enabling experiments to be carried out expeditiously and with greater accuracy.

Contractional inheritance and rheology controls in a FTB: the Argentinian Precordillera, Central Andes (30°S)

<p>The Central Andes (12°S-36°S) stretches over more than 2400km. They are characterized by strong longitudinal and latitudinal segmentation (Sierra Pampeanas, Precordillera, Cordillera Frontal, Cordillera Principal from east to west), each domain having distinctive basement involvement and showing different structural styles. The Argentinian Precordillera, located at 30°S, has long been interpreted as a thin-skinned wedge detached below into the lower part of Paleozoic succession. It makes up a typical Coulomb foreland thrust belt system. However, the impact of the Paleozoic inheritance derived from the various orogenic stages on the current structural style has been overlooked. The Chanic structures that developed in Silurian / Devonian times have been reactivated by the Andean deformation that took place from Oligocene to Plio-Pleistocene times. The current structure of the Precordillera has been the subject of numerous studies in the last decades. Thanks to compilation of this literature and fieldwork, we present a new cross-section considering these 2 superimposed events. This cross-section can be divided into 2 different zones depending on the dominant structures. The western Precordillera involves an Ordovician succession characterized by Chanic superimposed folding phases with cleavage development. On the contrary, in the eastern part, most of the observed structures were developed during Andean orogeny. The structural style is characterized by thrusts faults and penetrative deformation is absent. The Sierras Pampeanas in the East are a Miocene thick-skinned system that makes up a typical broken foreland system. The association of both systems of Precordillera and Sierras Pampeanas delineate an inheritance-controlled original orogenic thin-skinned system that turns to the east into a broad thick-skinned system involving up to Precambrian rocks.</p>

Geology and petrography of precambrian rocks of Katsina urban area, northwestern Nigeria

This present study, the mapped area is located between latitude E07°35'00'' to E07°39'0011" and longitude N13°00'00'' to N12°57'00" of sheet 34NW, Katsina State, Northern Nigeria. The area is underlain by Basement Complex rocks and syeno-granitic rocks which dominate in the area. The two sets of granitic rocks were observed and distinguished by texture. The northwest of the mapped area consists of very coarse to coarse grained syeno-granitic rocks, while in the southeast, the rocks are medium to coarse grained. These outcrops exhibit various systems of joints. The mineralogical compositions determine by thin section results displays: quartz, biotite, muscovite, plagio-clase feldspar, apatite, sericite, hornblende, augite and microcline. There are two generalizations of joints with primary WSW to ENW direction and NE-SW, MMW-SSE and WWW-ESE secondary direction of NE-SW, MME-SSE and WWW-ESE.

The pre‐Vendian (640–610 ma) granite magmatism in the Central Taimyr fold belt: the final stage of the Neoproterozoic evolution of the Siberian paleocontinent active margin

Eastern part of the Central Taimyr belt is composed of Precambrian rocks penetrated by granites of the Snezhnaya complex (845–825 million years) and later overlain by mid‐Neoproterozoic sin‐ postorogenic sedimentary deposits of the Stanovaya‐Kolosova Group. Two competing concepts on the Precambrian history of the belt are dis‐ cussed. The first suggests that by the middle of the Neoproterozoic amalgamation of various terrains formed the Cen‐ tral Taimyr microcontinent, which afterwards collided with Siberia in Vendian. 2) According to the second point of view, which is shared by the authors of this article, the belt was part of the Siberian craton from at least the Mesopro‐ terozoic, and there is no suture that would separate it from the South Taimyr belt. To our surprise, during the field work in the South‐Eastern part of the Central Taimyr belt near the proposed “Vendian sutura”, assumed by the first concept, we found a granite pluton (Pregradnaya massif) intruding clastic rocks of Stanovaya‐Kolosova Group. Such setting is quite uncommon for the belt and contradicted to publications, describing the mentioned clastic rocks to overlay the granites and contain their debris. Dating of the pluton confirmed the field observations – its SRIMP zircon age has proved to be 609±2 Ma, an unusually young for this region. The pluton is located in a wide deformation zone separating the Precambrian rocks (to the northwest) and the Paleozoic deposits (to the southeast). Two minor bodies of similar porphyritic granite were found in the same zone further to the southwest, and it seemed logical to assume that a chain of Vendian granites marks boundary deformation zone. However, their dating (843±6 и 840±5 Ma) showed that they belong to Snezhnaya complex. In this paper, we discuss two Neoproterozoic magmatic ‘flare‐ups’ in the Central Taimyr Belt, which are dated at 845–825 and 640–610 Ma. Both ‘flare‐ups’ are evidenced by K‐rich per‐ aluminous granite batholiths intruded the upper crust. It is most probable that each flare‐up was related to a collision event completing an independent cycle in the evolution of the active margin of the Siberian paleocontinent.