Horst Zwingmann

Structural and geochronological constraints on the Pan-African tectonic evolution of the northern Damara Belt, Namibia

The Pan-African Orogen formed by convergence of numerous continental blocks during the Neoproterozoic to early Cambrian. This convergence eventually led to amalgamation of Gondwana, a supercontinent crosscut by a network of highly oblique linear orogenic belts that locally intersect each other, as in NW Namibia, where the NNW trending Kaoko Belt joins the NE trending Damara Belt. The northern Damara Belt has preserved well three regional Pan-African tectonic events due to the dominance of weak Neoproterozoic marine sediments (Damara Supergroup) that have been affected by low-grade metamorphism. A newly discovered early N-S horizontal contraction, dated by 40Ar/39Ar at ~590u2009Ma, is tentatively linked to convergence between the Congo and Kalahari cratons. This was superseded by collision between the Congo and Rio de la Plata cratons between 580 and 530u2009Ma that thickened and exhumed the orogenic crust of the Kaoko Belt and produce upper crustal N-S oriented folds of earlier fold trains and associated axial planar schistosities in the northern Damara Belt. A switch from E-W to NW-SE horizontal shortening occurred at ~530u2009Ma as a result of collision with the Kalahari Craton, triggering extensive syn-orogenic magmatism in the entire Damara Belt. During this last event, southward indentation and underthrusting of the Congo Craton promontory below the Neoproterozoic cover sequences produced a deformation front in the northern Damara Belt. Our results show that highly oblique convergent processes competed over a period of ~120u2009Ma to build Gondwana in Namibia during the late Neoproterozoic to early Cambrian.

Correspondence: Reply to ‘Challenges with dating weathering products to unravel ancient landscapes’

As the title of the correspondence by Fossen et al.1 suggests, determining the age of landscape elements of the Earth surface is difficult. We thus welcome the opportunity to clarify our arguments on the contentious themes touched upon by Fredin et al.2 The age of landscapes has been a recurring research topic for the last century. Often, landscape ages can be deduced indirectly through morphostratigraphic correlations leading to relative chronologies. However, when working in geological contexts where a sedimentary cover is not present1, and the traditional geochronological tools are not suitable, not only are absolute dates of etch surface formation essentially impossible to obtain, but even relative chronologies are challenging. In an attempt to circumvent this problem, we have applied an untested methodology to date pockets of weathering products at three different sites in Scandinavia (Ivö southern Sweden, Utsira High offshore Norway, Bømlo west Norway) by K-Ar dating of illite formed authigenically during the weathering of the crystalline host rock2. Our results support weathering in the Late Triassic at all studied localities. We note that Fossen et al.1 do not significantly question our results at two of the investigated localities (Ivö and Utsira High), where there is stratigraphical control on the age of weathering. This selective approach is questionable because the three dated sites are internally consistent with each other, of which two have independent stratigraphical control of the Triassic age of weathering. The utility of the new method should thus be discussed including the whole data set from all dated sites. We start our rebuttal from the concluding remarks by Fossen et al.1, who question the saprolitic origin of the dated outcrop on Bømlo, suggesting that we might have dated a Triassic fracture. We firmly reject this possibility. Mesoscopically, the investigated outcrop lacks any evidence of a ‘structural origin’ of the dated clay-rich material. Comparison with many fractures and brittle deformation zones in the surrounding excludes that the dated illite results from synkinematic authigenic growth during faulting3. More telling, the detailed XRD analysis of clays in three samples from a traverse across the saprolitic outcrop documents clay assemblages that are typical for chemical weathering (Table 2, Fredin et al.2). We already showed that the sample closest to the fresh bedrock exhibits a less mature clay-weathering signature, whereas the sample farthest away from the fresh host rock contains a higher concentration of mature weathering products, such as kaolinite, and lower contents of immature clays such as smectite2. This spatially controlled mineralogical pattern is consistent with rock alteration through chemical weathering and not a faulting, fracturing or hydrothermal origin. Here, we further reinforce this interpretation by comparing the clay mineralogy of a nearby fault (the Goddo Fault, studied in detail by Viola et al.4) with that of the dated saprolite outcrop. The Goddo Fault phyllonitic sample BO_GVI_2 contains illite/mica and interstratified illite-smectite, with only subordinate kaolinite (Fig. 1a). The clayrich sample BO_GVI_1 from the fault core also contains a similar clay mineralogy (Fig. 1a), but with dominant interstratified illitesmectite. In contrast, the saprolitic sample closest to the hosting fresh granodiorite (sample ‘Bømlo 2’ in Fig. 1b) is dominated by smectite with subordinate illite and kaolinite. The central portion of the saprolite outcrop (samples ‘Bømlo 3’ and ‘4’), instead contains predominant kaolinite, an end-member product of weathering. Importantly, samples from the weathering profile still preserve in situ primary mineral textures and grains from the host rock, although the rock is sufficiently altered through chemical alteration to easily disaggregate when manipulated by hand. In addition, the outcrop-bounding bedrock joints exhibit a rounded morphology consistent with the fact that weathering first attacks DOI: 10.1038/s41467-017-01468-6 OPEN

Constraining the timing of shale detachment faulting: A geochemical approach

K-Ar dating of illite in fault gouges is a useful tool for constraining the timing of brittle fault movement; however, this can be problematic in fault gouges hosted in clay-rich rocks due to the influence of host-rock material. Therefore, this study employs a multianalytical geochemical approach to unravel the influence of host-rock mineralogy, as well as fault zone development, on ages from fault-gouge samples in a shale detachment zone. K-Ar dating of the ≥2 μm fraction of 6 samples from the Sap Bon Formation detachment zone and associated fault zones in the Khao Khwang fold-thrust belt of central Thailand yielded an age range of 262 ± 5.4 to 208 ± 4.6 Ma. Carbon and oxygen stable isotope analysis along with X-ray diffraction mineralogy indicate that the samples with the youngest K-Ar ages are characterized by higher grade clay mineralogy, and hotter, orogenic fluid temperatures. Using these proxies and comparison to existing geochronology of the study area, we correlated K-Ar illite ages to one of three stages of fault zone evolution: detrital, diagenetic (burial), and authigenic (fault movement). The youngest K-Ar dates in the Sap Bon Formation are contemporaneous with recently published zircon province data indicating that faulting and detachment zone formation in the Sap Bon Formation were occurring by the mid-Late Triassic, with deformation continuing as late as the Rhaetian.

TESTING HIGH-VOLTAGE ELECTRICAL DISCHARGES IN DISINTEGRATING CLAYSTONE FOR ISOTOPIC AND MINERALOGICAL STUDIES: AN EXAMPLE USING OPALINUS CLAYSTONE

The radiogenic isotope systematics of clay minerals are complex because of the intimate mixture of minerals from different origins such as detrital and authigenic sources. An important aspect of dating clays is the primary sample preparation and disintegration method. In the present study, a sample of weakly deformed Opalinus claystone from the Mont Terri underground site (Switzerland) was investigated after disintegration by three different methods. The Opalinus Clay was sedimented in the late Toarcian and early Aalenian and reached maximum temperatures of ~85°C during burial in the Cretaceous. The present study reports data from a comprehensive investigation comparing the effects of disintegration by: (1) disc milling; (2) repeated freezing and thawing; and (3) high-voltage discharges. K-Ar age values of the finest clay (<0.1 µm) released by the different disintegration methods are indistinguishable, indicating that the high-voltage liberation method does not influence grains as small as 100 nm. The K-Ar age values of particle-size separates decreased with decreasing particle size. The age values of the 2–6 µm separates correspond to the Carboniferous Period, which reflects the dominance of Paleozoic detritus in that size range. The age values of the smallest separates (<0.1 µm), on average 213 ± 4 Ma, exceed the numerical age of the formation (~177−172 Ma), which show predominance of detrital grains over authigenic grains even in the finest illite. In summary, isotope geochronology data suggest that the high-voltage method can be applied reliably for disintegrating claystones.