Paleomagnetism of late Paleozoic basins of the Moroccan Meseta and the Western High Atlas

Weissbrodt, Vinzenz (2024): Paleomagnetism of late Paleozoic basins of the Moroccan Meseta and the Western High Atlas: the Intra-Pangean Megashear Zone and the paleogeography of Pangea. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

When reconstructing tectonic plates since the Mesozoic, a central cornerstone within the Earth sciences community has always been the agreement on the classical Wegenerian shape, from which Pangea dispersed into the present-day configuration of continents. The configuration of Pangea in the late Paleozoic, however, has been controversial. The small number and low quality of paleomagnetic data for the late Paleozoic resulted in a significant overlap between Gondwana and Laurasia when keeping the conventional Wegenerian (Pangea A) reconstruction. In the alternative (Pangea B) model Gondwana is positioned some 3000 km further to the east overcoming the problem of continental overlap. However, in order to achieve the Pangea dispersion starting in the Jurassic, a dextral (mega-) shear zone located in the Mediterranean is required to transfer from Pangea B to A. In such a shear-zone, vertical axis rotations may be the only quantitative means to clarify its still controversial timing and position and act as supportive evidence of Pangea B in addition to existing paleomagnetic, paleoclimatological, paleontological and geochemical arguments. Proponents of Pangea B have complemented the sparse Permian paleomagnetic data set of Gondwana with data from Adria treating Adria as a tectonically coherent African promontory. On the other hand, it has been argued that the overlap might be an artifact caused by inclination shallowing. Thus, the validity of Pangea A or Pangea B decisively depends on selection and filtering criteria making new high-quality late Carboniferous - early Permian data from Gondwana as well as identification of larger patterns of vertical axis rotations for the same time interval indispensable for a substantial progress in the Pangea debate. In order to contribute to this debate, late Paleozoic continental sediments and well dated intrusive and extrusive rocks of the Moroccan Meseta and Western High Atlas in NW-Africa have been studied paleomagnetically. In total, fifteen late Paleozoic basins and two late Hercynian plutonic massifs were studied and 944 paleomagnetic samples were collected at 152 sites, with age constraints based on absolute and relative dating using geochronological, paleontological and stratigraphical arguments. Rock magnetic studies suggest predominantly magnetite and hematite in various proportions as main carriers of the paleomagnetic signal in the igneous samples whereas hematite is prevalent in the sedimentary samples. Moreover, maghemite is observed within strongly weathered sites. For five basins, no primary high-stability paleomagnetic component could be determined, potentially due to viscous overprinting by the recent geomagnetic field caused by chemical alteration of the magnetic mineralogy or by an IRM imparted by lightning strikes. Difficult drilling conditions at the two late Hercynian (~ 270 – 280 Ma) massifs restricted the acquisition to a small data set only with preliminary but promising results. Moreover, one basin is interpreted to be pervasively remagnetized in the late Permian as revealed by negative bulk and inclination-only fold tests as well as calculated paleolatitudes. At the residual basins, positive inclination-only fold tests support the primary character of magnetization, which is also underpinned by the identification of three reversals recorded at the basins of M’Tal, Chougrane and Souk El Had Bouhsoussene, tentatively related to the late-Artinskian CI2n normal magnetochron (281.24 ± 2.3 Ma). A high-stability component of characteristic remanent magnetization could be identified in 76% of the remaining samples with site-mean declinations frequently distributed along a small circle girdle within respective basins. The magnetization is therefore interpreted as syndepositional and rotational movements are argued to have happened concurrently with sedimentation/intrusion in the early Permian. Resulting VGPs are elongated along a NE-SW trending small circle swath intersecting the late Paleozoic segment of the Gondwanan APWP with a rotation pole located in the sampling area suggesting the presence of vertical axis rotations within the Meseta block which might be related to intra-Pangea shearing. The resulting paleolatitudes yield a position of the Meseta block close to the paleo-equator at ~ 280 Ma irrespective of the underlying lithology (inclination shallowing-resistant volcanics or inclination shallowing-prone sediments) thus widely excluding the influence of inclination shallowing. Comparing data from this study with a combined reference data set of Adria and Gondwana does not support the coherence of Adria and Gondwana. An early Permian Pangea reconstruction based on the data achieved in this study does not result in a significant continental overlap and does not require alternative reconstructions such as Pangea B.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 25. Oktober 2024
• 1. Bericht­erstatter:in: Bachtadse, Valerian