In order to enhance our understanding on the morphology of the Study area and complete our geological and structural maps, we applied mathematical operators to the aero-magnetic data. In the following paragraphs, we will present our results and interpretations.
For the geological formations and interpretations, we referred to the geological map of Jbel Saghro-Dades at a scale of 1/200000. This map was the support for the establishment of the geological map of the study area and for the attribution of a possible geological causative bodies of every magnetic anomaly in the reduced to magnetic pole map.
4. 1. Residual magnetic and it’s Reduced to pole maps
Once the aero-magnetic data is gridded, various magnetic anomalies appear, isolated or lapping. These anomalies varied in intensity, size, and shape (Fig. 3A and B). To identify the geological source of each anomaly, we calculate the residual reduced to pole, which removes the presence of two lobes for the same source. That allowed us to associate the magnetic anomalies of high intensity and long wavelengths with highly magnetized structures to the Precambrian basement. On the other hand, negative anomalies are associated with frames of weak magnetization in the cover.
This distribution of magnetic anomalies suggests that the central part of the study area may underline by magnetized rocks with high susceptibility, possibly related to the Precambrian basement. The concentration of positive anomalies in this area may indicate the presence of a magnetic body or a series of bodies with significant dimensions. The smalls positives anomalies pinpointed in the South and the North may correspond to smaller bodies or extensions of the central magnetic sources. On the other hand, the negative anomalies found in the North maybe related to less magnetized rocks or structures with an opposite magnetic polarity, possibly associated with sedimentary or volcanic rocks of the cover. The superposition of these magnetic patterns with geological and structural data can help identify lithologies, structures, and geological boundaries, which may have important implications for mineral exploration and resource assessment. (Table.1)
The central part features the highest concentration of these anomalies follows three principal directions, NE-SW, E-W, and NW-SE (Fig.3B).
The NE-SW direction is the most prominent among these, and it characterizes the anomalies from Isk N'Oufrach to Isk N'Alla, extending towards the sector SW. The sub-circular abnormality located along Isk n Ifsam in the central part of the study area also follows this direction, as does the linear anomaly that stretches more than 50 km west of the study area.
The second direction, E-W, is observed in the North by two minor magnetic anomalies in Oussilkane and Tachil Alitine. It is detected, also in the center and the west at Tadaout n Tourhza and Tifdassine, respectively (Fig. 3B). Finally, the north-east part of the study area reveals a positive triangular anomaly at J. Ougrassennt, with a global NW-SE direction (Fig. 3B).
The northern part of the study area is characterized by negative anomalies, which exhibit a low magnetic response (Fig. 3B). These anomalies are likely associated with the Tertiary and Quaternary coverage or Middle Precambrian deposits. The negative anomalies have variable geometry, with some appearing sub-circular, others triangular, and others elongated.
The negative anomalies mostly reveal two directions, which are E-W and NW-SE. The E-W direction found in the North and the NW of Oukenntar. Meanwhile, the NW-SE direction characterizes the anomalies in the East of Jbel Igourdane and Jbel Bardouz. (Fig. 3B)
Anomaly P1 : The positive anomaly P1 is oriented ENE-WSW and has an intensity ranging from 9 to 108 nT. This middle-aged anomaly occupies the western part of the study area with an elongated shape (Fig. 3B). It is probably associated with the formations of the middle Precambrian era, notably the Alkaline granite of Isk n’Allah, which is the prime intrusion in the Boumalne inlier area and occurred mainly in the Neoproterozoic. It is also possible that the Saghro group composed of rhyolites and ignimbrites, could contribute to this anomaly.
Anomaly P2: On Tifdassine, there is an E-W sub-circular anomaly, with an intensity ranging from 50 to 105 nT (Fig.3B). This anomaly is likely associated with Andesite flows and series breaches and conglomerates in the area.
Anomaly P3 : with an intensity of -14.1 to 108.1 nT. Straight anomaly traversed the Tiwit district on a broadly NE-trend to the SW (Fig. 3B). It is the elongated anomaly in the area, which is about 200m; This anomaly coincided with the mafic Foum Zguid Dyke extending from the South-West to the North-eastern part of Jbel Saghro. It has slightly sinuous and in echelon form.
Anomaly P4 : This anomaly has an elliptical shape and is oriented NE-SW, with a strength ranging from 1.9 to 70 nT (Fig. 3B). This anomaly is likely associated with the sedimentary formations of the Middle Precambrian era, which included conglomerates and sandstones, as well as the upper rhyolites. These formations are known to have magnetic properties that could contribute to this anomaly Fig. 3B).
Anomaly P5 : The magnetic anomaly in the North of Tadaout n’Tourhza, which ranged in intensity from 10 to 40 nT and has an NW-SE orientation, is likely caused by the sandstone and quartzites of the Paleozoic cover (Fig. 3B). These sedimentary formations were deposited in a silico-clastic passive margin and were affected by eustatic changes and local tectonic movements during the late Paleozoic. The Paleozoic cover in this area mainly influenced by Hercynian orogenesis, which could have contributed to the formation of magnetic minerals in the sandstone and quartzites (Gasquet, et al., 2005; 2008).
Anomaly P6 : this anomaly englobe two Anomalies, ones located in the western part of Rherrhiz Bu Daoud with an orientation of WNW-ESE and an intensity ranging from 40 to 75 nT (Fig. 3B). This anomaly is likely associated with the upper rhyolites, tuffs, and or the breaches and conglomerates of the middle Precambrian era. These formations are known to have magnetic properties that could contribute to the anomaly. The second in the southern region of Akka N’Oulili, an elongated anomaly ranging from 13.1 to 60.8 nT in intensity. The anomaly is oriented NW-SE and is likely associated with the Phonolite complex of Jbel Saghro and or the schist-dolomitic series of the Lower Cambrian, along with the Tiwijiwine diorite, which intersects by the red quartz veins and the basalt-dolerites of the Triassic.
Anomaly P7 : The formations of schist-dolomitic series and phonolitic complex of Jbel Saghro may produce the detected anomaly oriented NW-SE in the North of J. Bourhdad with an intensity of 30nT to 70nT (Fig. 3B).
Anomaly P8 : The Tizi Moudou anomaly locate between the Oussilkane granite and the Cryogenian meta-sedimentary rocks, with an intensity ranging from 66.8 to 70.0 nT and NW-SE orientation (Fig. 3B). Thus, this anomaly can correspond to approximately N-S directional trashy-designates dykes. On the other hand, this anomaly also coincided with rhyolite levels in the West of Tizi Moudou.
Anomaly P9 : The zone in the South of Tafraout is oriented E-W and characterized by a complex elongated shape with an intensity ranging from 49.8 to 74.7 nT (Fig. 3B). This anomaly may coincide with the Igourdane granite, which has known magnetic properties that could contribute to the anomaly observed in magnetic surveys.
Anomaly 10: An intensity ranging from -74.2 to 108.7 nT, for short wavelength in the East of Tifrnine, The irregularity has a medium size and it is less deep (Fig. 3B). Geologically, it may correspond to layers of aphanitic basalts rich in magnesium and barite. The anomaly is oriented NW-SE. It may associated with Andesite and Oulggou lava flow.
SW of Anomaly P10: Tizi N Hbab anomaly has an NW-SE orientation of -51.3 to -30.1 nT and a short wavelength (Fig.3B). In addition, this anomaly may relate to all rhyolite or Andesite dykes.
Anomaly N1: characterized by a decrease in the magnetic field intensity. Regarding its source, it suggested that it could be the alkaline granite of Isk n’Allah or the lower rhyolite complex of Saghro (Fig.3B). The anomaly situated in the North-West of Oukenntar oriented in the East-West direction. Its intensity ranges from -200.2 to -239.0 nT,
Anomaly N2: This E-W blip in the North of Oukenntar has an intensity ranging from -189.0 to -207.3 nT (Fig.3B).
Its source is likely associated with the Shale and Sandstone formations, Quartzites, Rhyolites, Tuffs, and Conglomerate lentils. It is also possible that the Basic volcanic series and Microdiorites could contributed to the anomaly.
Anomaly N3: Located in the Eastern part of Jbel Bardouz, this elongated anomaly extended in a Northwest-Southeast direction (Fig.3B). It has an intensity ranging from -187.2 to -208.4 nT and may attributed to the presence of shale, sandstone, quartzites, rhyolite, tuffs, and conglomerate formations, as did basics volcanic series and micro-diorites.
Anomaly N4: In this area, we have detected a calm magnetic field with a blue anomaly that aligned with the Oussilkane granitoids. These granitoids are deficient in ferromagnesian minerals. The blip's source could be the granites, fine-grained dioritic facies, or the Bou Gafer Pyroxene Calco-Alkaline Granite. The anomaly is oriented NW-SE, and its intensity ranges from -175.1 to -208.5 nT. Another possibility is that the anomaly corresponded to veins and layers of aphanitic basalt with high magnesium content that embed in the Ouarzazate series formations.
Table 1. Location, direction, and sources of positive Pn and negative Nn anomalies
Anomaly Label
|
Localisation
|
Direction
|
Corresponding Sources
|
P1
|
Ouest of Isk n’Allah
|
ENE-WSW
|
Alkaline granite of Isk n’Allah and/or Lower rhyolite complex of Saghro
|
P2
|
North of Isk n’Ifsane
|
E-W
|
Upper Rhyolites with Myarol
|
P3
|
From SW to NE of Jbel Saghro
|
NE-SW
|
Foum Zguid Dyke
|
P4
|
Tfdassine
|
NE-SW
|
Myarol upper rhyolite
|
P5
|
North of Tadaout n’Tourhza
|
NW-SE
|
Sandstone and Quartzites
|
P6
|
Ouest of Rherrhiz Bou Daoud
|
NW-SE
|
Lower rhyolite complex of Saghro and/or Tuffs and breaches
|
P7
|
South of Oulili
|
NW-SE
|
Phonolite complex of Jbel Saghro
|
P8
|
Tizi Moudou
|
NW-SE
|
Trachyandetic dyke and/or rhyolites
|
P9
|
South of Tafraout
|
E-W
|
Igourdane Granite
|
P10
|
Est of Tifrnine
|
E-W
|
Oulggou Andesite and Lava flow
|
N1
|
NW of Oukenntar
|
E-W
|
Alkaline granite of Isk n’Allah and/or Lower rhyolite complex of Saghro and/or Tuffs and Breaches
|
N2
|
North of Oukenntar
|
E-W
|
Shale and sandstone, quartzites, rhyolite, tuffs and conglomerate lentils. Basic volcanic series and micro-diorites
|
N3
|
Est of Jbel Bardouz
|
NW-SE
|
Shale and sandstone, quartzites, rhyolite, tuffs and conglomerate. basic volcanic series and micro-diorites
|
N4
|
Est of Igourdane
|
NW-SE
|
Granites and fine-grained dioritic facies and/or Bou Gafer Pyroxene Calco-Alkaline Granite
|
4. 2. Edges detection techniques
4. 2. 1. Total Horizontal Derivative (THD)
Upon examining the map of the total horizontal derivative (THD) (Fig.4), we can see that the THD maxima perfectly align with the edges of the magnetic anomalies, providing a clearer view of the appearance and extent of some anomalies whose limits were not well-defined by the reduction to the pole method. Additionally, the map and the rose diagram of detected lineaments reveals the presence of linear anomalies-oriented NE-SW to E-W and NW-SE, as well as small circular anomalies in the North-East and Northwest magnetic domains (Fig. 4 A and B).
Moreover, magnetic lineaments are visible and appear to intersect with the Saghro massif, indicating a clear difference in lithological contacts along the area (Fig. 4). Structurally, the axes of these lineaments characterize abnormal contacts, faults, and folds that affect the area.
4. 2. 2. Tilt Derivative (TA)
The application of this filter allowed us to observe that most of the anomalies have an elongated form, oriented NE-SW to E-W and NW-SE. The intensity is ranging from -1.4 to 1.4 Rad (Fig.5). In addition, an individualization of the boundaries of disturbing geological sources and also the magnetic lineaments represented by the zero-radian contour value of Tilt angle; with a lateral delineation.
We observe a series of anomalies (Fig. 5) that are oriented in E-W direction and are localized within magnetized terrains ranging from Neoproterozoic to Paleozoic. These anomalies exhibit elliptical shapes and are oriented in two main directions: the first direction is NW-SE and the second is NE-SW. Notably, as we move from the SW to the NE, there is an absence of N-S direction, and we observe a strong correlation between the various anomalies and the maximum tilt angle values (Fig. 5). Furthermore, magnetic lineaments are easily identifiable, which may suggest an offset between these three axes, indicating the presence of unmapped field breaks.
4. 2. 3. Analytic Signal (AS)
The AS map provides valuable information about the distribution of magnetic anomalies and geological structures in the study area. It is mentioned earlier, the AS method is particularly useful in identifying the horizontal and vertical location of magnetic sources, making it a powerful tool for exploring subsurface geology. We can identify the edges of magnetic anomalies and better understand the geometry and orientation of geological structures.
The AS map reveals the anomalies localized particularly in the North-Western and the North-Eastern, in addition to few dispersed anomalies in the central parts of the study area (Fig.6). (They are a cause of significant geological structures; and can reliably identified using different magnetic filtering techniques. The well-defined boundaries of geological units (example of this geological units) on the AS map suggest that this method can provide more accurate delineation of geological units and their boundaries than other magnetic filtering techniques.
Furthermore, the AS map highlighted several small magnetic bodies which interpreted as a Quaternary volcanics, mapped or new; And magnetic lineaments in the study area, which may represent hidden mineral deposits or faults. These features can further investigate using complementary geological, geochemical analysis to confirm their significance and potential economic value.
It should also note that the depths of the sources calculated by the AS method are comparable to those deduced from the HD-TDR map. Therefore, the combined AS and HD-TDR maps can provide valuable information about the depth and location of magnetic sources in the study area.
The AS method has a distinct advantage in detecting magnetic sources located at greater depths as it is less susceptible to interference from shallow magnetic sources. The regions where the AS amplitude is low correspond to a decrease in magnetic gradient, indicating the possible presence of magnetic sources at greater depth. (Fig. 6)
4. 2. 4. Euler Deconvolution
The resulting map of Euler solutions represents the sources at their positions, in the horizontal plane, and their depth. Therefore, this filtering helps us at the same time to locate contacts between rocks of different magnetic susceptibilities.
Euler solutions obtained using a structural index value of 0, represented two-dimensional features such as linear faults or high-throw contacts. A window size of 7x7 and a maximum depth tolerance of 9% data points apply to identify possible subsurface features from observed aero-magnetic data. The resulting Euler solutions illustrate the edges of the aero-magnetic sources and provide depth estimates. However, the depths obtained by this method are below the flight level (60 m), The Euler source locations represent using four different symbols based on their estimated depth values, ranging from 1 to 2385m, with 80% being less than 1500 km. Similar contacts previously identified using the horizontal gradient method were found. and other new lineaments were highlighted (Fig.7). The lineaments obtained by this technique are mainly trending NE-SW to E-W and NW-SE, with an abundance of the E-W structures (Fig7 A and B). This remark is confirmed by the geological maps of the southern front of the Saghro massif which show the abundance of the E-W faults.
4. Discussion and tectonic significances of detected lineaments
In the present study, aero-magnetic interpretation helped us to map the subsurface structures in the South-East of Saghro massif. The superposition of all magnetic lineaments obtained from various recent edge detection techniques show a good correlation between these lineaments (Fig 8.). This superposition allowed us to elaborate the structural map of the South-East of Saghro massif (Fig 9.).
The analysis and the interpretation of the mains magnetic structures of the newly obtained structural maps accomplished with an extensive geological interpretation of the structural and geological features of the study area allowed us to discuss the major structural features that characterize this area and their extension into region.
The structural map of the South-East of Saghro massif shows that the subsurface geometry of the study area is configured by many faults which trend in different directions and are of varying importance. Comparison of the rose-diagram of aero-magnetic based faults (Fig. 10-B.), with the rose-diagram of faults mapped in Saghro-Dades map, from the outcrops surrounding the study area (Fig. 10-A.); shows a great similarity in the main trends of both subsurface and surface faults. These main trends are: NE-SW, NW-SE and E-W.
i) The NE trending lineaments that are dominant in the study area are well observed in both the tilt derivative and the total horizontal derivative magnetic maps (Figs 4 and 5.). It is the predominant tectonic trend of the faults affecting the investigated area, and it can be interpreted as the subsurface equivalent of the main major surface faults, previously documented and mapped in the Anti-Atlas (JBIAF, TOsF, TWF, SIAF) (Fig.9). The Saghro massif is affected by several ENE to NE-striking tens of kilometres-long faults (Fig. 1). These regional faults cross cut all the Precambrian rocks and locally the folded Paleozoic series or even the Mesozoic-Cenozoic cover, which attests for their repeated reactivation (Soulaimani et al., 2014; Hejja et al., 2020; Aabi et al., 2022). This longitudinal fault system was active during the Late Ediacaran as it controls the thickness of the Ouarzazate Group deposits and the associated magmatic intrusions (Rjimati et al., 1992; Walsh et al., 2012)
They are parallel or nearly parallel to the Foum Zguid dyke (FZD), documented by geological mapping, and could be related to the the Alpin orogeny (Hailwood & Mitchell, 1971; Hollard, 1973; Bouiflane et al., 2017). The filtering allows us to detect the FZD offset.
ii) The NW-SE trending faults are noticeably less present in the majority of area; the NW-striking faults were active during the late Ediacaran in the Saghro massif. This is detected in the Boumalne area where N120E kilometer-long of normal faults control the Jbel Habab graben filled up with Ouarzazate Group formations (El Boukhari et al., 2007, Hejja et al., 2020). The bordering faults of this graben are sealed in the Northwest by Cambrian beds, thus excluding significant post-Precambrian reactivations. Other faults in the same direction are mapped in the area (AOF and TOmF), (Fig. 9)
iii) The E-W trend is well expressed on all previous aero-magnetic treatment. It can be interpreted as the subsurface equivalent of the main major surface faults, previously documented and mapped in the Anti-Atlas, they characterize the Northern boundaries of the study area where the Middle Precambrian and superior formations were deposited along. In addition to the dispersed faults along the map, crossing the Paleozoic cover formations. They include the fault that delimit the sub-basins in the study area, which separate the positive anomalies (NIAF) (Fig.9).
The subsurface structural map that we obtained from interpreting aero-magnetic data and the surface geological map established in the surrounding Precambrian Massifs of Saghro fits well as far as the main Variscan faults are concerned based on geological map. This allowed us to easily tracing the continuity of the main crustal-scale faults zones mapped in the surface of Saghro massif (Fig.9).
The superposition of the main mineralizations of the study area extracted from the geological map with the newly obtained structural map reveals that the mineral indices of Cu, Ag, Pb, Au, Mn, and Ba are concentrated in the Northern to eastern parts of the area, with a performance distribution. In addition, the control of the indices mineralized by lineaments is limited by specific directions. Furthermore, they are generally following the direction trending NE-SW to E-W of major faults (Fig. 9). Lithologically, the mineralization of Cu and Ba is especially concentrated in the Paleozoic cover formations, Feija Intern of the Middle Cambrian and Tata Group of the Lower Cambrian. While the indices of precious metals, Au, Ag, and Pb, are located in the Neoproterozoic Basement (Fig. 9).