engleski

Paleomagnetic dating of incipient folding SW of the Sava–Vardar subduction zone (Karst Dinarides, Croatia)

1. INTRODUCTION In this paper we focus on remagnetization phenomena of the haematite-bearing Permian redbeds and applicability of this secondary component of a natural remanent magnetization (NRM) for dating of tectonic deformation of the Karst Dinarides (Croatia), related to the collision of Adria microplate with northerly situated crustal units. Northeasterly progression of the Adria microplate during the Late Mesozoic–Cenozoic time was associated with the Sava–Vardar subduction zone, which was active from the Late Jurassic to Palaeogene. The resulted suture is the Sava–Vardar zone, localized just south of Zagreb. However, the initiation of subduction happened earlier, in the Late Jurassic (Kimmeridgian in the Dinarides, in the Northern Calcareous Alps it happened even a bit earlier, during the Oxfordian, both evidenced by obduction of the ophiolites), and in the area of the Dinarides it was marked by significant facies variability within formerly very monotonous platform. Therefore, the Middle Jurassic represented transitional phase of tectonic peace between the extensional tectonics which was active from Permian to the end of the Early Jurassic and compressional tectonics which started in the Late Jurassic and culminated by the uplift of the Dinarides (Vlahović et al., 2005). Late Mesozoic and Cenozoic sedimentary basins, developed on the pre-Late Jurassic basement, were influenced by subduction to the NE in today’s geographic coordinates. They formed several elongated parallel younger flysch basins, ranging in age from oldest in the NE, of the latest Jurassic age, to youngest in the SW of Oligocene/Miocene age as well as the Promina Basin filled with clastic-carbonate rocks during the late Palaeogene. 2. GEOLOGICAL SETTING Our goal were Permian deposits in the Central Velebit Mt. (Croatia), which are composed of two informal lithostratigraphic units: Lower Permian clastic deposits (topic of this paper) and Middle to Upper Permian carbonates. Lower Permian clastic deposits consist mainly of pyritic sandstones, quartz conglomerates and petromictic conglomerates in the lower part and reddish-brownish sandstones and siltstones in the upper part. Uppermost and the thickest unit of the Lower Permian clastic succession in the Central Velebit area is composed of reddish-brown siltstones and fine-grained, middle-grained to coarse-grained sandstones, in places even microbreccia greywackes. Depositional environments were probably continental and deltaic/coastal. They are gradually passing to carbonates and clastics of the Middle to Upper Permian and Triassic age. Karst Dinarides basins were formed under extensional regime until the Late Jurassic, and after that only partially influenced by ongoing subduction in the northern area, resulting mostly in reduction of sedimentary basins causing the final uplift in Cenozoic (only some temporary small basins were formed during the Late Cretaceous). On the basis of clay minerals (Lewandowski et al. 2012) and CAI analyses (see Fio et al. 2010) the Lower Permian rocks studied in this paper should not have been exposed to the temperatures above 200–250°C. 3. METHODOLOGY Palaeomagnetism is a well-recognized tool for reconstruction of the crustal block movements. Secondary magnetization may be applied to dating of fold structures, by a reference of identified characteristic magnetization to known palaeomagnetic poles/directions. This potential will be utilized in this study to show how NRM may be used to date incipient stage of folding at the early stages of collision between the Adria microplate and the European plate. 3. PALAEOMAGNETIC SAMPLING AND METHODS Paleomagnetic sampling and methods, results, their discussion in terms of acquisition of NRM components, and their tectonic interpretations are described in detail in Werner et al. (2015). In brief we have collected ca. 160 oriented cores from two sections of Permian red siltstones and sandstones at Košna and Crne Grede (Velebit Mt., see Figs 1, 2). Both sections consist of redbed deposits, showing variety of microfacies, differing in grain sizes and mineral assemblage, although unsorted quartz grains are predominant. Clasts of the underlying Košna conglomerate were also investigated for conglomerate test. Since haematite was a main magnetic carrier, the cores were subjected to thermal demagnetization with MMTD1 thermal demagnetizer (Magnetic Measurements Ltd., UK) up to 700°C. Their NRM components were determined in the magnetic field-free space with the Model 755 Superconducting Rock Magnetometer (2G Enterprises, USA). Two typical components of ChRM, differed by unblocking temperatures, can be fitted for thermal demagnetization paths of at least 50% of samples from each segment of the section: low-temperature component (LT – at 0–250°C) and higher-temperature component (HT – ranging from 250–350°C to 500–625°C). For a few samples HT component can be anchored at coordinate system’s origin, indicative for a single-component NRM. Conglomerate test proved negative, implying acquisition of a secondary component of NRM. 5. DISCUSSION Characteristic remanent magnetization (ChRM – HF/HT component) in situ is apparently similar to the Permian direction for the African plate, as expected for the Velebit Mt. coordinates from APWP for Africa (Besse & Courtillot 2002), recalculated to the present-day coordinates of the Velebit Mt. Paradoxically, this orientation is observed within the almost vertically dipping beds, i.e. before any correction for the tilted/folded structures. Consequently, ChRM must be considered secondary (as also confirmed by the negative conglomerate test) and its directional agreement with the Permian expected direction for the Velebit locality is purely coincidental (Fig. 3). The orientation of HF/HT component after 100% unfolding with plunging fold axis restored to the horizontal has no reasonable explanations for both Košna and Crne Grede sections, since their inclinations imply too high latitudes, compared to inferred equatorial position of Adria during the Permian time (e.g. Channell 1996 ; Muttoni et al. 2001 ; Lewandowski 2003 ; Cocks & Torsvik 2006). Partial unfolding of HT/HF mean ChRM direction for the Košna section was performed as clockwise rotation around 290/15 fold axis (0–100°). For angle of rotation of 65–70° it roughly coincides with Early Cretaceous–Palaeogene segment of APWP for Africa (Fig. 3). Consequently, we explain this coincidence in terms of syn-folding, Cretaceous remagnetization of the rocks at their subhorizontal position (tilted by at most 30°S). The incipient stage of folding, which we recognized and dated for the Late Cretaceous using palaeomagnetic methods, can be considered as the introduction into the final disintegration of the Adriatic Carbonate Platform, and a herald of the final uplift, which had its maximum later, in the Late Eocene/Oligocene/?Early Miocene time. Subsequent tilting, understood as an anticlockwise rotation of the beds around the axis gently plunging to the West, resulted in an almost vertical, present-day position of strata. A final geometry of the rocks under study was attained probably at the wane of the main uplift phase during the Oligocene/Early Miocene. A soft LF/LT component was acquired after the final emergence of the Velebit block. Only small (up to 10° anticlockwise) vertical axis rotation of the Velebit unit may be inferred from our data.

Palaeomagnetism; Folding; Dinarides

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