Structural Analysis and Micro-Deformations Syn-to Post-Folding of the Visean limestones

Abstract


Introduction
The Central Hercynian Moroccan Massif (CHMM) has been the subject of many geological studies and continues to arouse the interest of geologists, offering the diversity of ages and outcrops and different landscapes from the borders of the Alpine Atlas chains to the Atlantic coast (Michard, 1976).This massif is bordered to the North by the Mio-Pliocene South Rifian basin, to the south and southwest by the erosion limit of the secondary and tertiary outcrops of the phosphate plateau, and to the east and southeast by the Trias-Lias plateau of the Middle Atlas (Fig. 1A).
Since the work of Termier (1936), the CHMM is defined as a large area of the Paleozoic chain structured during the major phase of the Hercynian orogeny known as the Namuro-Westphalian phase; and then leveled by a cycle of erosion, resulting in pre-Triassic peneplanation, before being partially covered, discordantly, by Secondary, Tertiary and Quaternary sedimentsary cover.(Michard, 1976 andFadli, et al. 1991) with location of the study area (Red rectangle).
Moreover, the eastern part of this massif, in which we suggest this paper, is a segment of the Hercynian chain with Paleozoic series ranging from the Ordovician to the Namurian (Fig. 1B) (Bouabdelli, 1989).The Lower Paleozoic series are gravity-nappes sequences with westward vergence and form thrusts of the Kasbat Tadla-Azrou anticline zone (Bouabadelli, 1989;Piqué et al., 1993).The front of these thrust faults is constituted by the Devonian-Carboniferous carbonate formations taking place in a context of a compressional foreland basin during the Upper Namurian-Westphalian times.These formations are NNE-SSW aligned direction and appear overlapping the Visean flysch formations (Piqué, 1979;Lagarde, 1985;Hoepffner et al., 2005).
However, many works have interested the CHMM, those concerning folding-fracturing relationships in the carbonate series have not been approached.Our objective is to study fracturing in the Visean limestone series, at the front of the gravity nappes, between Mrirt and Adarouch, constituting the thrust nappe of the Jebel Bou Khemis and its prolongation towards NE in the region of Amghass.
The present work consists of the study of the folding-fracturing relationships in the limestone facies as well as the sandstone beds of the Namurian in the Jbel Bou Khemis.In this context that we have conducted structural mapping and detailed fracturing analysis in several microtectonic sites.
To analyze fracturing, we spotted several microtectonic sites in both limestone facies and sandstone beds, despite unfavorable outcropping conditions.Studied sites are selected in different parts of the same fold (limbs and hinges).Fractures are systematically measured in areas that are tens of meters in surface, when the outcrop conditions permit, but sometimes fractures are measured in 1 m2 stations (unit surface).
Similarly, the limestone and sandstone facies were sampled, of which some 50 oriented thin sections were made for both facies study and strain analysis from deformed geological objects corresponding mainly to oolites, sparry calcite crystals and fluid inclusions contained in these crystals.Thin sections were made at the Faculty of Science in Meknes (Morocco) and the microtectonic study under microscope was carried out at the Faculty of Sciences in the University of Porto (Portugal).
Following the analysis of macroscopic fracturing (measured at outcrop in different microtectonic sites) and the determination of the relative chronology of fractures from their field geometric relationships, we undertook a study of micro-deformations under microscope from oriented thin sections.For each sample, we made 3 orthogonal thin sections (one section parallel to the stratification (S0) and two others perpendicular to S0).This allowed us to characterize the studied microstructures in the three directions of space and to propose a 3D model of these deformations.
Post-Paleozoic cover begins with Triassic formations represented by conglomerates, red sandstones and clays, and doleritic basalts (Hoepffner et al., 2005).Locally discordant marls and limestones in the Triassic and Paleozoic formations represent the Upper Cretaceous deposits, developed in the south (Fig. 3).

Geological context of Jbel Bou Khemis
The study area corresponds to the NNE-SSW oriented geological structures, bracketed between Mrirt and Adarouch, along the national road linking Meknes and Mrirt cities (Fig. 4).It corresponds mainly to the Jebel Bou Khemis allochthonous limestones, forming the front of the eastern nappes of the CHMM.

Structural Analysis
The Devonian and Carboniferous formations at Jbel Bou Khemis, are folded and determine NE-SW oriented large anticlines and synclines well known at the Amghass site (Fig. 5 and 6).These P1 folds associated to the main cleavage oriented NE-SW, are linked to the main NW-SE shortening phase attributed to the Upper Visean-Namurian stage.
Sometimes, there are many metric folds (depending on the lithology), type long-short limbs (dipping 40° E to 80° W), especially in the calcareous thin beds (Fig. 5).These folds are tighter in the Namurian flysch, where the S01-type plans are observed.They are accompanied by faults especially in the hinge areas of the competent rock beds.
In fact, in the Namurian flysch units, the deformation is marked by disharmonic folding (P2) with subhorizontal axes N145-10NE, corresponding to a structure of tectonic scales thrusted to NW and delineated at the base by sub-horizontal faults (N155-20NE).These structures are considered as a ramp propagation folds (Figs. 5 and 6), associated to the main reverse faults oriented NE-SW, inherited from distensive episodes (Lower Paleozoic) and reactivated during compressional periods (Upper Visean-Namurian).The shear zone in our study zone, of medium direction N40 and dip of 55° to 65° towards the NW, brings into abnormal contact the targeted greso-pelitic series and the Namur flyschoid series (Fig. 7).In detail, this area is composed mainly of a set of faults of medium direction N45 and variable dip (50° to 70° NW).These flaws show an early reverse game.Hooks from the S01 plane and quartz veins plus tourmaline in pull-apart indicate a normal dextrous replay.
Thus, structurally and cartographically, all the major accidents stalling in the district of El Hammam constitute a regional shear mega-corridor of medium direction N40E of about 8 km wide.These dropouts are characterized by a first reverse game and a normal dextrous late-Hercynian replay in a transtensive regime.Such a dynamic has been proposed to the Upper Permian (El Wartiti, 1990, Ait Brahim & Tahiri, 1996, Saidi et al., 2002).

Brittle Deformations
Faulting and fracturing mainly affect limestone and sandstone beds of the Devonian and lower Carboniferous at Jebel Bou Khmis.Many cartographic faults were observed showing different directions and dips, they can be summarized in three families: (1) faulting oriented N20-25, 65° SE dipping sometimes filled with quartz, calcite and calc-schist elements, (2) sub N-S faults (N170, 82° E dipping) filled mainly with calcite.
These two conjugate fault families (dextral and sinistral) are cross-cut by the later N75, 70W reverse faults characterized by quartz and carbonate fillings.
In order to analyze fracturing affecting both limestone and sandstone beds, we selected many micro-tectonic stations along all the studied geological structures bracketed between Mrirt and Amghass.In flysch deposits, analyzed fractures correspond mainly to reverse micro-faults with westward vergence, determining fish-tail structures, affecting the centimetric to decimetric sandstone beds, accompanying the main folding (Fig. 8).In contrast, in the metric sandstone beds and in the massif carbonate units, fracturing is more expressed and shows at least three main families (Fig. 8), two sub-vertical families, oblique to the fold axes and a sub-horizontal axial family parallel to the axes of folds.Many stations were chosen in different parts of the folds affecting especially massif limestones and sandstones in order to establish the relative chronology between the fracture families 9).It was possible to distinguish systematic and non-systematic fracture networks occurring at the surface of stratification.At the beginning of compressional stages, sedimentary collection corresponding particularly to limestones are deformed through layer parallel shortening.This compression is followed by excessive fracturing corresponding to conjugate households oriented NE-SW and NW-SE which can be now subvertical.
Thus, the set of fracture measurements, recorded in the field, and their stereographic projection allowed us to distinguish three fracture families (Fig. 10): N20 to N40, N70 to N100 and N110 to N140.
These guidelines include oblique fractures to the axis of the fold that grows as response to the strain field (σ1 perpendicular to the fold), they are related to the early degree of folding, with layer parallel shortening (lps) earlier than the initiation of the anticlinal hinge.After some level of shortening, a third circle of relatives of fractures parallel to fold is interpreted as the result of accommodating extension affecting the fold hinge (stearns, 1964 ; ahmadi, 2006 ; habibou et al., 2014).
This family corresponds to axial fractures which are now sub-horizontal.This scenario of folding and fracturing militates in favor of a buckle folding mechanism, which could be associated to the latevariscan folding phases

Discussion
The microtectonic study was performed on thin section samples to understand the penetrative deformations from the three main studied rocks.Before presenting the results of this study, we describe the petrography characteristic of these studied facies.
-Clays: show a clay background rich in phyllites.The latter are arranged in the cleavage plans, well expressed in microlithons by mechanical reorientation or oriented recrystallization.The few quartz grains are well flattened according to shale flows, but the aggregates of chlorite are highly developed, slightly pleochroic and are reoriented according to shale flow.However, these quartz grains remain some clusters emphasizing the initial bedding completely obliterated by the shale cleavage.Detrital white micas are less abundant with a late micro-fracture that is filled with calcite.
-Fine-grained greywackes: The dominant element is quartz.The latter is of angular or rounded shape and of size between 150 and 200 µm.It sometimes has recrystallization tails formed of secondary silica.Large lamellae of muscovite, chlorite and biotite underline stratification and can reach 600 µm in length.Some plagioclase (oligoclases) and elongated sections of tourmaline are also observed.The set is encompassed in a phyllic matrix with a percentage between 20 and 35%.The shale flow, underlined by the brownish streaks of iron oxides, is non-rectilinear, with the average spacing of the planes from 100 to 150 µm.
Medium-grained greywackes: show a percentage of matrix close to 50%, with a percentage of 70% quartz, heterometric with two populations: the grains from 50 to 100 µm of size have a rounded shape against the large clasts, are often cracked and subrounded or angular contours 20% of detrital Micas: muscovite and elongated chlorite parallel to the sedimentary bedding up to 1 mm in length from 5-10% of pelitic rock fragments of 5-10% plagioclase.It is therefore greywackes lithic or feldspathic rich on micas.
The study of thin oriented sections within oriented Visean carbonate samples, reveals the presence of a penetrative deformation in these facies.The microfractures detected in calcite crystals are divided into three main families oriented NE-SW, NW-SE and N-S.On the other hand, in oolitic limestones, microfractures are arranged in two directions: NE-SW and E-W (Figs. 11 and 12).Thus, the work of quantification of this deformation was carried out mainly in the oolitic limestone facies, where the oolites are flattened and sheared.We will present the results of this study on deformed oolites and micro-fractured calcite crystals containing fluid inclusions with preferential orientation (Fig. 13).In addition, the microfractured samples (thin section) are rich in carbonate with calcite-rich breaks in two types, the first of which is rounded and the second is elongated following the direction of the cleavage, with also a richness in fossils and the appearance of oolites, that indicate a restless medium deposit.The cement that appears is of the coaxial syntactic type on plates.Indeed, these samples of the cuts allow for defining several stages, with two types of grains (ends and means).• Stage (4): corresponding to that the flow schistosity is penetrative and affects both sandstone and pelitic beds.All the minerals are arranged parallel to the plans of schistosity, with neoformed micas.There is no trace of the sedimentary fabric.

Conclusions
The study of fracture networks both in the field and under microscope, affecting the allochthonous limestones and sandstones, allowed us to propose an interpretative fracturing model that summarizes the distribution of fractures in the folds and the relative chronology of the different families of fractures (oblique, transverse and axial) occurring during the late Variscan shortening phases (Fig. 14).The distribution of these fractures along the folds seems very heterogeneous, we can distinguish two high density zones, located on the SE limb and especially along the NW limb.These two zones are separated by a more or less affected part, which occupies the anticlinal hinge of the fold.Indeed, this heterogeneity can be interpreted by the variation in thickness of the carbonate formations and by the integration of the folding process into the reasoning that could provide elements of response, especially as observed fracturing appears to be controlled by its position in the fold.It is strong at the straightened limb and weak at the low-dipping limb (Fig. 12).
Therefore, at the beginning of the compression phases, the sedimentary series corresponding mainly to the limestones are distorted by the layer parallel shortening of the beds.This compression is accompanied by intense fracturing corresponding to NE-SW and NW-SE oriented conjugate families (Tahiri et al. 1987;Tahiri, 1994;Izart et al., 2001), which are now subvertical.These directions contain oblique fractures to the fold axis that grow in response to the stress field (σ1 perpendicular to the fold).They are related to the early folding stage, with a parallel shortening layer (LPS) before the initiation of the anticlinal hinge.After some shortening, a third family of fold-parallel fractures is interpreted as the result of an adaptive extension affecting the fold hinge (Stearns, 1964;Ahmadi, 2006;Habeeboo et al., 2014).
The folds are accompanied by a plan-axial cleavage type with increasing intensity from NE to SW.The fold axes are slightly offset by sub-E-W and N150 shear faults, respectively right and left lateral.These conjugated offsets determine a stress direction that varies from NW-SE to E-W depending on the locations.These faults are responsible for the inflections that affect the axes of the folds.

Fig. 2 .
Fig. 2. Major Hercynian structures of the Moroccan central massif (Extracted from 1m scale of the Moroccan geological map).

Fig. 3 .
Fig. 3. Stratigraphic column of different formations in the study area.

Fig. 4 .
Fig. 4. Geological map of the North-Eastern zone of the Hercynian Central Massif.

Fig. 6 .
Fig. 6.Detailed stratigraphic column of limestone formations and samples studied at the Amghasse anticline.

Fig. 8 .
Fig. 8. (A) Photographs of the main P1 folds affecting Visean limestones at Jebel Bou Khmis area; (B) and (C) Some decametric folds and shears accompanying the main NW thrusting, showing P2 folding

Fig. 10 .
Fig. 10.Examples of fracture networks measured in limestone (A, C), and Conjugated fracture systems in the sandstone beds (B).

Fig. 11 .
Fig. 11.Repartition of the microtectonic measured stations and their rose diagrams.

Fig. 12 .
Fig. 12. Rose diagrams of the microfractures affecting Visean limestones in the Amghas area (NE termination of the studied structure)

Fig. 13 .
Fig. 13.Micro-deformations determined from oriented thin sections in the Visean carbonates.A, B: microfracturing within the cleavage of the calcite, cal: calcareous, QZ: quartz.Thin section.C, D: Fluid inclusions with different directions: inclusion..E,F Thin section microfracturation of different direction (horiz, verti, oblique).G and H: Elongation blades of oolites within the carbonate formations, ol: oolites

•
Stage (1): Starts with a non-planar and non-penetrative flow rate; this is the beginning of the individualization of the shale cleavage.The schistosity is grouped into narrow areas separating large strips where the schistosity is not developed.Detrital minerals (quartz, mica) tend to reorient mechanically in schistosity.• Stage (2): The flow is planar and not penetrative by shale joints that are planar, but the sedimentary bedding remains, sometimes pleated.The quartz grains oriented in the schistosity have a tapered shape with smaller dimensions than those located inside the microlithons.• Stage (3): The flow is planar and penetrative, in the pelitic beds, the planes of schistosity are very close together is less penetrative near the shale joints.The quartz grains are reoriented and undergo dissolution, accompanied by a recrystallization of small micas, oriented parallel to the quartz grains.

Fig. 14 .
Fig. 14.Interpretative structural model showing the folding-fracturing relationships from the Visean studied series.