Deciphering the Sardic (Ordovician) and Variscan deformations in the Eastern Pyrenees, SW Europe

Detailed geological mapping of the La Cerdanya area (Canigó unit, Eastern Pyrenees) provides new data characterizing the different structural styles exhibited by Cambrian–Lower Ordovician (Jujols Group) and Upper Ordovician successions. Their unconformable contact, related to the Sardic Phase, ranges from 0° (paraconformity) to 90° (angular discordance). The Jujols Group rocks topped by the unconformity are affected by Sardic foliation-free open folds. The pre-Sardic succession, the Sardic Unconformity and the lower part of the post-Sardic succession (Rabassa Conglomerate and Cava formations) are cut and offset by several Late Ordovician NNE–SSW-trending synsedimentary extensional faults associated with hydrothermal activity, which dramatically affected the thickness of the lower part of the Upper Ordovician succession. We relate (1) the Mid-Ordovician Sardic uplift and erosion, and (2) a Mid- to Late-Ordovician upward propagating extensional fault system bounding the outline of half-grabens, subsequently infilled by alluvial deposits, to a thermal doming event (about 475–450 Ma) that led to the uplift and stretching of the Ordovician lithosphere. Thermal doming may be caused by mafic magma underplating and responsible for the coeval calc-alkaline magmatic activity broadly developed in the Eastern Pyrenees. We discuss the similarities between the Mid-Ordovician Sardic Unconformity and other Early Paleozoic unconformities described in neighbouring areas. Finally, we suggest a geodynamic scenario in which a regional-scale thermal event was related to the opening of the Rheic Ocean that led to the drift of Avalonia from the SW European margin of Gondwana.


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In several areas of the Pyrenees, from Pierrefite to the west (Calvet et al. 1988

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All these pre-Variscan rocks display a polyphase deformation linked to the Variscan 134 deformation and a low-pressure/high-temperature metamorphism (Guitard 1970;Zwart 1979). A 135 pervasive foliation is the main deformational structure in the medium-to high-grade 136 metasediments and the gneissic bodies derived from Ordovician and Cadomian granitoids. This 137 foliation (S1-2) is a composite fabric developed prior to coeval with the Variscan regional 138 metamorphism, being associated with an E-W to NE-SW oriented stretching lineation. S1-2 is 139 folded by later E-W to NW-SE upright folds, forming the mesostructure that depicts the Aston-        Cambrian-Ordovician strata on its northern side. This contact shows a planar to weakly 191 undulated, locally irregular surface capping the Cambrian-Ordovician strata, and can be 192 considered as an unconformity laterally ranging from a paraconformity to an angular 193 discordance. The unconformity exhibits a roughly WNW-ESE trend (194/75) although, locally, 194 the orientation ranges from E-W to NW-SE ( Fig. 5a-b).

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The Upper Ordovician succession shows a NW-SE trend of vertical to subvertical strata. Upper

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Ordovician beds may be either overturned or strongly dipping northwards or dip eastwards in 198 the eastern part of the study area (Fig. 6). In contrast, Cambrian-Ordovician bedding surfaces 199 exhibit a wide range of orientations (see below).

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Orientation of the Cambrian-Ordovician succession presents a marked dispersion and dip 206 values ranging from subvertical to subhorizontal (Fig. 7a). In contrast, the Upper Ordovician 207 bedding poles exhibit a more regular NW-SE trend (Fig. 7b). As we will discuss below, the 208 marked dispersion of the bedding poles in Cambrian-Ordovician strata suggests the 209 superposition of several fold systems. In the description that follows, L1 and L3 refer to different 210 fold axes, and L0-2 and L0-3 to bedding/cleavage intersection lineation.    Cambrian-Ordovician beds still remain vertical and with a similar NE-SW orientation (Fig. 8b).

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This observation suggests that D1 fold axes presented an initial subhorizontal NE-SW 7 7 orientation, with a short limb oriented NE-SW subvertical or strongly dipping, and a long limb 228 subhorizontal or gently dipping towards the SE or NW. However, the effect of superposed D3 229 folds precludes to assess whether D1 folds have a NW or SE vergence. Analysis of the areas 230 where subvertical WNW-ESE and NE-SW orientations coexist allows the estimation of 10 to 100 231 m-scale dimensions of D1 folds (Fig. 9). It should be noted that there is no evidence of an axial 232 planar cleavage development or other mesostructrures related to D1 fold formation.

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Contacts are commonly scoured and grading is both normal and inverse. Breccias and matrix-301 supported conglomerates show either chaotic or amalgamated patterns; they interfinger laterally 302 with packages of clast-supported conglomerates and pebbly sandstones that pinch out or 9 9 change abruptly, both laterally and vertically, into sandstone/shale alternations (Cava 304 Formation). Clast-supported conglomerates show massive to weakly imbricated pebbles and 305 cobbles (Fig. 11f), subrounded, moderately sorted and grade upsection and laterally into 306 litharenites; the latter display centimetre-to decimetre-scale trough and low-angle cross-307 stratified sets alternating with shales ( Fig. 11g), irregularly burrowed. Lithostratigraphycally, the

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Thus, it is assumed that the episode D1 is earlier than the successions deposited above. An

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Taken all together, uplift, denudation, extensional tectonics and magmatic activity may be 444 related to a thermal doming caused by the underplating of hot mafic magmas (Fig. 13). This

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We suggest a regional-scale thermal event ranging in age from Early-Mid to Late Ordovician,

Revision: this chapter has been modified
Line 167: Delete "·". Add "software" after MOVE.

Revision: Done
Lines 194-195: It is confusing talk about cleavage surfaces before the descriptions in the next paragraphs.

Revision: Done
Line 230: I consider that the comment about the S2 deflection is obvious and unnecessary.
Revision: We disagree. No structuralists can find this comment necessary to follow the argument.
Line 256: Where the axe of this fold should be?
Revision: In some part in he middle of the Canigó massif, North of the study area Line 280: "peter out" is informal. Use decrease, or other synonym.

Revision: Done
Line 298: Fig. 6e. Revision: Done 6: What "1%" means in the stereoplots? As is always the same value I recommend add the data to the figure caption. In comparison, too many measures in D5 domain. The map (that is the same that is in fig. 2) is very big in the figure. As the only new information is the domains, I suggest add them to fig. 2