The origin of tectonic lineation in extensional basins: Combined neutron texture and magnetic analyses on “undeformed” clays

Reliability of magnetic fabric of weakly deformed mudrocks as a palaeostress indicator in compressive settings

Mesozoic extension in the Basque–Cantabrian basin (N Spain): Contributions from AMS and brittle mesostructures

Kinematic reconstruction of modern ocean basins shows that since Pangea breakup a vast area in the Neotethyan realm was lost to subduction. Here we develop a first-order methodology to reconstruct the kinematic history of the lost plates of the Neotethys, using records of subducted plates accreted to (former) overriding plates, combined with the kinematic analysis of overriding plate extension and shortening. In Cretaceous-Paleogene times, most of Anatolia formed a separate tectonic plate—here termed “Anadolu Plate”—that floored part of the Neotethyan oceanic realm, separated from Eurasia and Africa by subduction zones. We study the sedimentary and structural history of the Ulukisla basin (Turkey); overlying relics of this plate to reconstruct the tectonic history of the oceanic plate and its surrounding trenches, relative to Africa and Eurasia. Our results show that Upper Cretaceous-Oligocene sediments were deposited on the newly dated suprasubduction zone ophiolites (~92 Ma), which are underlain by melanges, metamorphosed and nonmetamorphosed oceanic and continental rocks derived from the African Plate. The Ulukisla basin underwent latest Cretaceous-Paleocene N-S and E-W extension until ~56 Ma. Following a short period of tectonic quiescence, Eo-Oligocene N-S contraction formed the folded structure of the Bolkar Mountains, as well as subordinate contractional structures within the basin. We conceptually explain the transition from extension, to quiescence, to shortening as slowdown of the Anadolu Plate relative to the northward advancing Africa-Anadolu trench resulting from collision of continental rocks accreted to Anadolu with Eurasia, until the gradual demise of the Anadolu-Eurasia subduction zone.

Kinematics of a former oceanic plate of the Neotethys revealed by deformation in the Ulukışla basin (Turkey)

An integrated structural and paleomagnetic study was carried out on Neogene-Quaternary clastic sedimentary sequences exposed in north Calabria to define the brittle postorogenic evolution of the inner sector of the Calabrian arc. The structural, mineralogical, and magnetic fabric data presented in this paper suggest that the post-orogenic basins of north Calabria originated and developed under an extensional tectonic regime that has been active since the middle Miocene. Results were used to define the spatial and temporal evolution of the extensional fault systems in the framework of extensional processes active in the backarc region of the southeastward-migrating Apennine subduction system. Our data indicate that regional backarc extension operated in the brittle crust, and the data trace a continuous evolution from low- to high-angle extensional faulting. Paleomagnetic data show that this continuous extensional process was accompanied by complex interactions among fault-bounded crustal blocks. In fact, the extensional domain is subdivided into different structural compartments, separated by the adjustment in the fault strike imposed by the regional rotation pattern.

The architecture of brittle postorogenic extension: Results from an integrated structural and paleomagnetic study in north Calabria (southern Italy)

Abstract The low- and high-field magnetic anisotropy (AMS, HFA) of the Rhenohercynian mudstones and greywackes is compared to the theoretical anisotropy calculated from neutron texture goniometry measurements. The magnetic anisotropy is predominantly carried by the paramagnetic phyllosilicates in the form of chlorite/mica stacks and the ferromagnetic contribution is insignificant. The respective principal directions of the theoretical anisotropy and the AMS and HFA are sub-parallel; magnetic foliation reflects the orientation of the maximal concentration of phyllosilicate basal planes, magnetic lineation is subparallel to the intersection axis of those planes. For the purpose of quantitative comparison, the infrequently used standard deviatoric susceptibility as a measure of the HFA degree is employed. A very good linear correlation of the degree of theoretical anisotropy and the measured AMS and HFA is found. The prolate and oblate shapes of the respective fabric ellipsoids are reasonably well correlated. Neutron texture goniometry justifies the use of the conventional magnetic anisotropy technique for the assessment of the mineral fabric of studied rocks. When compared with other works relating the magnetic anisotropy to the mineral preferred orientation (examined by e.g. U-stage or X-ray texture goniometry) neutron texture goniometry seems to be a preferable and very precise method fabric analysis.

Phyllosilicate preferred orientation as a control of magnetic fabric: evidence from neutron texture goniometry and low and high-field magnetic anisotropy (SE Rhenohercynian Zone of Bohemian Massif)

Neutron diffraction is an excellent tool for pole figure measurement of rock samples. Due to high penetration depth of neutrons for most materials neutron diffraction represents an efficient tool to measure complete pole figures with reliable grain statistics even in coarse grained or inequi-granular materials. In the field of structural geology, the measurement of anisotropy of magnetic susceptibility is a standard technique to reveal the tectonic history of deformed rocks. The application of both techniques on still ongoing studies of Precambrian, Carboniferous and Quaternary rocks which are characterised by fundamental different tectonic evolutions and mineralogical compositions shows the wide field of relevance and importance of these methods in understanding tectonic processes in detail.

Anke Hansen

Papers

The origin of tectonic lineation in extensional basins: Combined neutron texture and magnetic analyses on “undeformed” clays

Phyllosilicate preferred orientation as a control of magnetic fabric: evidence from neutron texture goniometry and low and high-field magnetic anisotropy (SE Rhenohercynian Zone of Bohemian Massif)

Neutron pole figures compared with magnetic preferred orientations of different rock types