Variscan structures and their control on latest to post-Variscan basin architecture; insights from 1 the westernmost Bohemian Massif and SE Germany

The Bohemian Massif exposes structures and metamorphic rocks remnant from the Variscan Orogeny 10 in Central Europe and is bordered by the Franconian Fault System (FFS) to the west. Across the FFS, 11 possible presence of Variscan units and structures are buried by Permo-Mesozoic sedimentary rocks. 12 We integrate existing DEKORP 2D seismic reflection, well and surface geological data with the newly 13 acquired FRANKEN 2D seismic survey to investigate the possible westward continuation of Variscan 14 tectonostratigraphic units and structures, and their influence on latest to post-Variscan basin 15 development. Subsurface Permo-Mesozoic stratigraphy is obtained from available wells and are tied 16 to seismic reflection profiles using a synthetic seismogram calculated from density and velocity logs. 17 Below the sedimentary cover, three main basement units are identified using seismic facies 18 descriptions that are compared with seismic reflection characteristics of exposed Variscan units east 19 of the FFS. Our results show that Upper Paleozoic low-grade metasedimentary rocks and possible 20 Variscan nappes are bounded and transported by Variscan shear zones to ca. 65 km west of the FFS. 21 Basement seismic facies in the footwall of the Variscan shear zones are interpreted as Saxothuringian 22 basement. We show that the location of normal fault-bounded latest to post-Variscan Upper 23 Carboniferous-Permian basins are controlled by the geometry of underlying Variscan shear zones. 24 Some of these Upper Carboniferous-Permian normal faults reactivated as steep reverse faults during 25 the regional Upper Cretaceous inversion. Our results also highlight that reverse reactivation of normal 26 faults gradually decreases west of the FFS. tipping out in the Keuper units. In addition, at the location of 496 these antiformal parts of the shear zone a generally higher amount of upper crustal brittle 497 deformation (normal and reverse faults) occurs, reflecting rather local fault concentration above the 498 antiformal parts of the underlying shear zone. It should be noted that towards the E, at the margin of 499 the Franconian Basin, FFS as the major basin bounding fault system displaces the basal


Introduction 28
Variscan orogenic units and structures in central and western Europe are extensively studied from 29 disconnected exposed terranes in the Bohemian Massif, the Rheno-Hercynian Massif, the Black forest 30 and Vosges, the Armorican Massif and the Central Iberian Zone ). Between exposed 31 Variscan units, sedimentary rocks obscure direct observation of possible lateral extension and 32 architecture of Variscan tectonostratigraphy and structures. In southern Germany, for instance, 33 Variscan units of the Bohemian Massif are correlated with exposed Variscan units in the Black forest 34 and Vosges, ca. 300 km apart from each other, causing uncertainties in the lateral continuation and 35 architecture of the Variscan tectonometamorphic Saxothuringian and Moldanubian zones, originally 36 defined by (Kossmat, 1927). Although few wells provide local but valuable information about 37 basement rock types, only few regional 2D seismic profiles (DEKORP 84-2s and 90-3B/MVE and KTB84) 38 image the Variscan units and structures below the sedimentary cover between the Bohemian Massif 39 and Black Forest exposures (Franke et al., 2017;Behr and Heinrichs, 1987;Wever et al., 1990;Edel 40 and Weber, 1995; Meissner et al., 1987;Lüschen et al., 1987). 41 The recently acquired FRANKEN 2D seismic survey is covering the Carboniferous-Permian Kraichgau 42 and Naab basins (Paul and Schröder, 2012;Sitting and Nitsch, 2012) and the overlying late Permian to 43 Triassic Franconian Basin (Freudenberger and Schwerd, 1996) in the western vicinity of Bohemian 44 Massif in SE Germany (Fig. 1). The FRANKEN survey is tied to the DEKORP 3/MVE-90 profile creating a 45 grid of regional seismic reflection profiles imaging exposed and buried Saxothuringian units and 46 structures of the Variscan Orogeny across the Franconian Fault System (FFS, Fig. 1). In this study we 47 investigate 2000; Kroner and Goerz, 2010; Franke, 1989). A third deformation phase (D3) records latest Variscan 78 tectonics at ~320 Ma and is represented by the folding of synorogenic deposits during general NW-SE 79 to NNW-SSE shortening (Hahn et al., 2010). D3 is dominated by a wrench tectonic phase and the 80 collapse of thickened crust, resulting in the development of dextral strike-slip faults initiating fault-81 bounded graben and half-graben basins in Central Europe, including the study area in SE Germany 82 (Schröder, 1987 (Gudden, 1977;Emmert et al., 1985). Muschelkalk 140 units crop out along the FFS and the Eisfeld-Kulmbach fault and also west of well Eltmann (Fig. 1). The 141 Keuper Group consists mainly of sandstones that are 530.2 m thick in well Staffelstein 1, 532 m in well 142 Staffelstein 2, decreasing southeastward to 483 m in well Obernsees (Franz et al., 2014;Gudden, 1977;143 Emmert et al., 1985). Keuper units are broadly exposed in the western and northwestern part of the 144 study area and in the fault block bounded by the Eisfeld-Kulmbach and Asslitz faults (Fig. 1). Jurassic 145 units preserved in the central and eastern parts of the study area, but eroded towards the west and 146 northwest (Fig. 1). Jurassic outcrops to the east are fault bounded and are limited to the footwall of 147 Eisfeld-Kulmbach, Asslitz and Lichtenfels reverse faults (Fig. 1). The Jurassic interval is 102-104 m thick 148 in wells Staffelstein 1 & 2 in the north and 140 m thick in well Obernsees in the SE (Table 1, (Meyer,149 1985; Gudden, 1977). Cretaceous sedimentary rocks are preserved in the central and southeastern 150 parts of the study area ( Fig. 1). 151 The structural architecture of the eastern study area is characterized by ten to hundreds of kilometer 152 long NW-SE striking multi-segmented reverse faults (e.g. Eisfeld-Kulmbach and Asslitz faults), whereas 153 towards the west only normal faults (e.g. Bamberg fault, Kissingen-Haßfurt fault zone) are developed 154 (Fig. 1) Franconian Basin to the west (Fig. 1). The FFS initiated most likely during latest Variscan tectonics and 157 has been reactivated at least during Early Triassic and Cretaceous times (Carlé, 1955;Freyberg, 1969;158 Peterek The FRANKEN 2D seismic survey is comprised of four seismic lines, with a total line length of 230.8 km. 179 The survey area is situated in northern Bavaria, SE Germany covering an area of approximately 90 km 180 x 45 km (Fig. 1) links to the DEKORP-3/MVE-90 profile in the NW and to the OPFZ-9301 profile towards the SE (Fig. 1). 185 FRANKEN-1802 and 1804 strike NE-SW and are perpendicular to the major fault zones. Table 2  186 summarizes acquisition and processing parameters of the FRANKEN seismic survey. 187

Seismic interpretation methods 188
In this study we integrate information from 9 deep wells (1100-1600 m) and surface geology to 189 interpret the newly acquired FRANKEN seismic reflection survey in SE Germany. Available wells are 190 mainly located in the center and the western part of the study area ( Fig. 1 and Table 1). Seismic-well 191 tie and time-depth relationships are established using sonic velocity and density logs of the Mürsbach 192 1 well (Gudden, 1971). The calculated synthetic seismogram is correlated with the real seismic traces 193 at the well location and enabled us to transfer geological, in particular stratigraphic information from 194 the well to the intersected seismic profiles ( and dolostones, that are recorded by two distinct seismic facies in the study area, a semi-continuous 219 and medium amplitude reflection with ca. 50 ms thick on top and continuous and high amplitude 220 reflections at the bottom (Fig. 3C). The sandstone dominated Buntsandstein Group is characterized by 221 semi-continuous and rather medium energy amplitudes that gradually show slightly higher energy and 222 continuity of reflections towards the top (Fig. 3D). A continuous and very high amplitude reflection 223 defines the Permian-Triassic boundary between the Buntsandstein and the underlying Zechstein 224 Group (Fig.3D). The latter shows ca. 25-30 ms of continuous and high amplitude reflections which are 225 correlated to an anhydrite and dolomite bearing interval in the upper part of the Zechstein (Gudden, 226 1977;Schuh, 1985;Gudden and Schmid, 1985). Below the Zechstein high amplitude reflections, semi-227 continuous and medium amplitude reflections of the Rotliegend occur (Fig. 3E). These reflections 228 represent the upper parts of the Rotliegend and gradually become less reflective and discontinuous 229 with depth with some reflections being only locally present and laterally becoming less reflective and 230 partly transparent (Fig. 3E, 4A & B). The boundary between the sedimentary cover and the pre-231 Permian low-to medium grade metasedimentary rocks (hereafter considered as basement) is drilled 232 by wells Wolfersdorf and Mittelberg in the north, well Eltmann to the west and the well Obernsees to 233 the southeast and is not particularly reflective in the seismic survey (Table 1 and Fig. 4A  Similar low amplitude and low frequency reflections of BSF1 are also observed at the NW end of the 262 DEKORP85-4N profile ( Fig. 5A & B). There, these reflections are associated with low-grade Lower 263 Carboniferous Flysch deposits (inner and outer shelf facies) exposed at the surface (DEKORP Research 264 Group, 1994a). Based on seismic facies description and in the lack of well information, differentiation 265 between allochthons, flysch sedimentary rocks, inner and outer shelf facies is ambiguous. BSF1 is 266 therefore interpreted as the W-SW extension of low-grade inner and outer shelf facies, low-grade 267 Lower Carboniferous flysch sedimentary rocks and possible Variscan allochthons (DEKORP Research 268 Group, 1994b). Correlating with exposed basement units E-NE of the FFS, these units are interpreted 269 to represent the W-SW extension of the Ziegenrück-Teuschnitz Syncline of the Saxothuringian zone.

Profile FRANKEN-1801 305
Profile FRANKEN-1801 is 47.9 km long and extends NW-SE from south of Bamberg to the NW of 306 Haßfurt ( Fig. 1). At the surface, mainly Keuper units are exposed (Fig. 1). Thicknesses of remnant 307 Keuper units progressively decrease to the W-NW and at the northwestern edge of profile FRANKEN-308 1801, Muschelkalk units are exposed at the surface in the footwall of a segment of the Kissingen-309 Haßfurt Fault Zone (Fig. 6). This fault zone is mapped over ca. 60 km with ca. 7-10 km width, sub-310 parallel to the NW-SE striking FRANKEN-1801 profile ( Permian rocks and are underlain by a Variscan shear zone (BSF2, Fig. 6). From the SE, the Variscan 322 shear zone shallows to the NW and reaches ca. 700 ms TWT at the center of the profile (Fig. 6). 323

Profile FRANKEN-1802 324
Profile FRANKEN-1802 extends NE-SW with 47.7 km length (Fig. 1). This profile is at a high angle to the 325 prominent NW-SE faults, and therefore provides a good subsurface image of these structures (Fig. 7). 326 Profile FRANKEN-1802 is tied to the well Eltmann and is in the vicinity of wells Mürsbach 6 (630 m to 327 the S), Staffelstein 1 (1235 m, to the SE) and Staffelstein 2 (890 m, to the SE). Profile FRANKEN-1802 328 is used as the reference profile for the seismo-stratigraphic interpretation (Fig. 7). Jurassic rocks are 329 preserved in the footwall of the Mürsbach and Lichtenfels reverse faults drilled with 104 m thickness 330 by well Staffelstein 2 (Table 1; (Gudden, 1977). Keuper strata are exposed in the hanging wall of the 331 Lichtenfels Fault at the NE edge of profile FRANKEN-1802 (Fig. 7). Keuper is drilled with 532 m in 332 thickness by well Staffelstein 2. Towards the SW the Keuper is increasingly eroded and only 178.6 m 333 are preserved at the location of well Eltmann ( Fig. 7 and Table 1, (Gudden, 1977;Trusheim, 1964). 334 Muschelkalk and Buntsandstein sedimentary rocks are tabular and regionally dip to the E-NE (Fig. 7). 335 The Zechstein is penetrated by wells Eltmann, Mürsbach 1 and 6, and Staffelstein 1 and is 103-121 m 336 thick (Table 1; (Gudden, 1985). Below the Zechstein units, Rotliegend is drilled by wells Eltmann, 337 Mürsbach 1 and 6 and Staffelstein 1 without reaching the underlying basement. Medium-amplitude 338 and semi-continuous reflections, characteristic of the Rotliegend in the study area, are also locally 339 observed, suggesting the presence of Rotliegend laterally away from wells (Fig. 7). Rotliegend units 340 are wedge shape and are tilted to the E-NE, onlapping to deep sited W-SW dipping normal faults in 341 the footwall of the Mürsbach and Lichtenfels reverse faults (Fig. 7). Interpreted W-SW dipping normal 342 faults appear to be crosscut by oppositely dipping (E-NE) Lichtenfels and Mürsbach reverse faults in 343 Buntsandstein units (Fig. 7). Clockwise E-NE block rotation in the hangingwall of these normal faults 344 created local half-grabens observed exclusively in the Rotliegend section (Fig. 7). In the hanging wall 345 of a normal fault located in the footwall of Lichtenfels Fault, the thickness of the Permian section is > 346 310 ms, TWT (ca. 580 m) thinning W-SW to ca. 85 ms, TWT (ca. 140 m) in the hangingwall of the 347 Mürsbach Fault (Fig. 7). The seismic interpretation of lateral thickness changes in the Permian is in 348 good accordance with 142.3 m minimum thickness of Permian drilled in well Mürsbach 6 (Table 1). 349 The thickness of the Permian section in the hanging wall of Bamberg Fault is > 200 ms, TWT (ca. 360 350 m) decreasing to the W-SW down to 3 m, drilled by well Eltmann (Fig. 7). synform structure (also known as Hollfeld Syncline) where Jurassic rocks are preserved (Fig. 7). The 355 NW-SE striking Lichtenfels Fault is laterally and vertically segmented and is exposed at the surface over 356 ca. 16 km length (Fig. 1). In profile FRANKEN-1802, the Lichtenfels Fault has 135 ms TWT (ca. 230 m) 357 throw, measured at the top of the Buntsandstein (Fig. 7). preserved (Fig. 7). E-NE dipping normal faults interpreted in the SW part of the profile FRANKEN-1802 364 are subparallel to the SE extension of the Kissingen-Haßfurt Fault Zone (Fig. 7). 365 At the well Eltmann location 94 m of ?Devonian metasedimentary rocks are drilled below the 366 sedimentary cover and correlated with BSF1 ( Fig. 7, (Trusheim, 1964 Mürsbach faults and reach to the shallower depth towards the west (Fig. 7). In the center of the profile 370 some high amplitude reflections of BSF2 branch off from the main reflection package and extend into 371 the deeper parts of the crust (Fig. 7). 372

Profile FRANKEN-1803 373
This profile is subparallel to the profile FRANKEN-1801 and strikes NW-SE over 71.8 km length (Fig. 1). 374 Well Obernsees is located 945 m SW of this profile and drilled into the 140 m of Jurassic, the entire 375 Triassic succession and 55.7 m of Upper Permian Zechstein units (Table 1 and  dipping normal fault (Fig. 8). In the hanging wall of this normal fault and to its NW, medium amplitude 383 and semi-continuous reflections below the top Zechstein horizon are interpreted as Rotliegend (Fig.  384 8, (Stettner and Salger, 1985;Schuh, 1985). Permian units are underlain by Paleozoic 385 metasedimentary rocks and Variscan nappes (BSF1 units, Fig. 8). BSF2 reflections are sub-horizontal 386 (between 2000-2500 ms, TWT) along the profile FRANKEN-1803 and gradually get shallower to the 387 NW to reach to ca. 1200 ms TWT. From the SE to the center of the profile, BSF2 reflections become 388 less reflective and appear to be segmented, into a steeper and a sub-horizontal segment (Fig. 8). 389 Farther NW, BSF2 reflections reach to shallower depth and are also imaged by perpendicular 390 FRANKEN-1802 and 1804 profiles. Lateral segmentation and changes in the reflectivity of the BSF2 391 might be related to the 3D geometry of an interpreted detachment/shear zone (Fig. 8). 392

Profile FRANKEN-1804 393
This profile strikes NE-SW over 63.3 km length, subparallel to the profile FRANKEN-1802 (Fig. 9). 394 Jurassic units are preserved in the NE and the central part of the profile. To the SW however, Jurassic 395 units are eroded and Keuper sandstones are exposed at the surface (Fig. 9) FRANKEN-1804 (Fig. 9). In the 406 center of the profile, BSF2 reflections are observed at greater depth up to about 3000 ms TWT and 407 are slightly less reflective. Saxothuringian basement and possible lower parts of inner shelf facies 408 (BSF3) characterize the deeper parts of the profile FRANKEN-1804 (Fig. 9). 409 At the NE edge of the profile FRANKEN-1804, the Eisfeld-Kulmbach Fault accumulates ca. 660 ms TWT 410 (ca. 1300 m) of throw, exposing Buntsandstein in its hangingwall (Fig. 9). Across the fault, Jurassic units 411 are preserved in the footwall and thin towards the SW where they are eroded in the hangingwall of 412 the Asslitz Fault (Fig. 9) Muschelkalk. Bamberg Fault detaches into the underlying Variscan shear zone (BSF2) at depth (Fig. 9). 421 Farther north along the profile FRANKEN-1802, Bamberg fault is displaced by the Mürsbach reverse 422 fault (Fig. 7). underlying inner and outer shelf facies ca. 65 km west of FFS (Fig. 10). 447 In the exposed parts of the Saxothuringian zone east of FFS, kinematic indicators show a top-to-the 448 W-SW tectonic transport under NE-SW compression (Schwan, 1974). This deformation phase has been 449 described as "D1" deformation phase before ca. parts of the section are eroded, suggesting that originally even thicker Rotliegend sections (ca. 1000 525 m) were deposited (Herrmann, 1958;Dill, 1988;Paul and Schröder, 2012). About 18 km west of well 526 Wolfersdorf, well Mittelberg drilled only 41 m of Rotliegend before reaching basement rocks (Friedlein 527 and Hahn, 2018). Similar rapid thickness changes of the Rotliegend units were also observed in the 528 Weidenberg, Erbendorf, Weiden and Schmidgaden areas, all originally interpreted as small, isolated 529 fault-bounded basins, but now, interpreted as individual exposures of one coherent depositional area, 530 the NW-SE Naab Basin, where the Rotliegend reaches up to 2800 m thickness (Paul and Schröder,531 2012). The Naab Basin is bordered by normal faults, some of which reactivated as reverse faults or are 532 cross cut by younger reverse faults (Müller, 1994;Peterek et al., 1996b). 533 In addition to exposures along the FFS, several wells in the western parts of the study area (e.g. 534 Staffelstein 1, Mürsbach 1 & 6, and Eltmann) also encountered Rotliegend that relates to the SW-NE 535 Kraichgau Basin (Table 1, Fig. 1) of which the NW-SE Naab Basin is considered as a basin compartment 536 (Paul, 2006  Variscan thrust/shear zones except for the Saar-Nahe Basin (Henk, 1993). Observed variations in post-560 orogenic basin architecture might be related to the differences in the exposed level of the basement. 561 Exposed Variscan shear zones are wedge shaped and thin out towards the W-SW. 617 • Variscan relative autochthons occupy footwall of shear zones. 618 • Shear zones show local syn-and antiformal geometries and reach to the base of Permian-619 Mesozoic sedimentary cover towards the W-SW. 620 • Geometry of shear zones control the location at which major Permian normal faults have 621 developed. 622 • Permian normal faults dip to various orientations, creating Rotliegend graben and half-graben 623 basins. Observed Rotligend half-graben basins in the east are interpreted as the NW 624 continuation of the Naab Basin. Towards the west, observed Rotliegends are associated to the 625 Kraichgau Basin. 626 • Thickness of Triassic sedimentary rocks is fairly constant, highlighting a regional tectonic 627 quiescence in the study area. 628 • Some of the Permian normal faults are cross cut by oppositely dipping reverse faults most 629 likely during the regional Cretaceous inversion event occurred in Central Europe. Reverse 630 faults are interpreted as reactivated preexisting Permian normal faults. 631 • Reactivated normal faults are located to the eastern parts of the study area where preexisting 632 Variscan shear zone show syn and antiformal geometry 633 We        Figure 11