A reappraisal of the Carboneras Fault (SE Spain) from new structural, geochronological and thermal constraints

Fault zones are complex structural features wherein each recorded episode of deformation contributes to their structural intricacy. In particular, the absence of tight constraints on the time dimension of the accommodated deformation history makes the understanding of the progressive evolution of fault zones particularly challenging and may lead to inaccurate reconstructions of faulting histories.

With the aim to further improve our understanding of mature fault zones, we studied key outcrops from the northeasternmost sector of the Carboneras Fault (CF) in the Betics of Spain. The CF is a NE-SW striking, >100 km-long, crustal-scale, left-lateral transpressive fault forming part of the Iberia-Africa diffuse plate boundary. The CF has been active from the early-middle Miocene to the Present and it is described as accommodating up to c. 40 km offset. We adopted a multitechnique and multiscalar approach that builds upon the examination and characterization of brittle structural facies (BSFs) that are used as archives of the fault evolution in time and space as expressed by a multitude of geological features and characteristics (composition, shape, color, geometry and kinematics, relative crosscutting relationships, petrophysical properties, absolute age, etc.).

Field structural analysis shows that the CF deforms Permo-Triassic basement rocks (e.g., phyllite, schist, quartzite) and Neogene sedimentary and volcanic rocks. The CF overall structural architecture reflects the tight juxtaposition of several BSFs that are genetically associated with (i) an E-W striking, high-angle, pervasive foliation associated with (c. upright) folds, and (ii) NNE-SSW to E-W-striking, low-angle reverse faults, where inclined folds and oblique foliations indicate top-to-the S/SE transport. (iii) These BSFs are truncated by a steeply dipping, and rather localized BSF defined by pervasive foliation and minor strike-slip faults, which strike NE-SW and are oriented like the CF’s regional trend. Samples collected from 17 BSFs and respective fault rocks were investigated by means of X-ray diffraction, K-Ar dating of synkinematic clay minerals and microtextural characterization. Illite Age Analysis (IAA) K-Ar geochronology of eight fault gouges suggests three faulting events during the (i) Chattian (26.39 ± 2.95 Ma) along E-W BSFs, (ii) middle-late Miocene (between 12.18 ± 0.71 and 10.02 ± 0.52 Ma) along NE-SW BSFs and (iii) late Pliocene-Early Pleistocene (between 3.35 ± 1.60 and 1.08 ± 0.81 Ma) along E-W BSFs. The analysis of mixed-layer illite-smectite (I-S) and the transformation sequence smectite-random-ordered mixed layer (R0 I-S)-ordered mixed layer (R1 and R3 I-S)-illite-di-octahedral K-mica (muscovite) was used to constrain the maximum temperature of synkinematic clay minerals. The highest temperature (≥ 275°C) is associated with the oldest gouge found in the E-W-striking BSFs. The other two age clusters are associated with intermediate (110-140°C) and lower (70-90°C) temperatures.

Results suggest that the CF underwent a long-lived polyphase faulting history at progressively shallower/colder conditions. The main phase of NE-SW directed strike-slip faulting occurred during the late Serravallian-early Tortonian. Recent fault movement reactivated instead E-W fabrics inherited from a phase of c. NNW-SSE directed, late Oligocene thrust-related shortening. These findings lead to new insights into the spatio-temporal evolution and mechanisms of growth and exhumation of major strike-slip faults.