Classical models of fault rock architecture point to a relatively simple and symmetric architecture of a fault zone, where the fault core is a narrow foliated cataclasite (few centimeters) bounded by a thick damage zone developed cutting through host rocks. Those models are far from the studied fault rocks developed in the Alhama de Murcia Fault (AMF), SE Spain, where fault core is in contact with an almost no deformed hosting rock at one side and to a wide damage zone towards the opposite boundary. The AMF is an active shear zone and the source of destructive recent and historical earthquakes. It has more than 10 km accumulated slip, and it develops a more than 100 meters wide shear zone with fault rocks that have been continuously sampled and analyzed combining a drill core from a 174 m deep vertical borehole, six trenches excavation, and outcrop surfaces cleaning. Hand specimen and microanalyses were used to classify the fault rock in deformation domains in a strongly heterogeneous shear zone according to its lithologies and structural features. It ranges from 10 to 30 meters wide fault core, where steady strain occurs, to an intensely deformed damage zone where strain is concentrated along discrete gouge bands. Trenching also showed a surface rupture that offsets Arabic archaeological remains related to the 1674 catastrophic event occurred in Lorca (Murcia). Steady homogeneous deformation was found in the areas closest to the hanging wall, in the fault core, where Lower Paleozoic Schists are the potolith of ultrafault gouges. As deformation increased, the shear zone involved Permian-Triasic basement rocks and Miocene sedimentary rocks in heterogeneous deformation domains. In the lower domains, strain is located in anastomosing shear bands which are spatially related with a surface seismic rupture of the 1674 destructive earthquake.
During the last decades, the study of fault rocks has increased and, thanks to the deep drilling of active faults, greatly advanced our understanding of fundamental processes of fault rock formation, their evolution, and influence in earthquake physics (e.g., [1–5]). Fault rocks take priority in the study of seismic processes, and their architecture and lithologies are key parameters in order to shed light on how kinematics and mechanics are spatially and temporally distributed in active shear zones (e.g., [6–10]). The fault zone architecture traditionally has been divided into three elements that compose a fault zone: the fault core, the damage zone, and the protolith. Caine et al.  considered that the “fault core is defined as the structural, lithologic and morphologic portion of a fault zone where most of the displacement is accommodated […]. A damage zone is the network of subsidiary structures that bound the fault core […] and [damage zone and fault core are surrounding by] a relatively undeformed protolith, the hosting rock”. This model points to a relatively simple and symmetric architecture of a fault zone developed cutting through predominantly quartzofeldspathic host rocks, where the fault core is a relatively narrow foliated cataclasite (few centimeters) bounded by a thick damage zone (; Chester et al. 1993; [10, 11, 13]). When a fault is developed in a relatively phyllosilicate-rich hosting rock where strain harden may occur, Faulkner et al. , based on Carboneras Fault (SE Spain, and close to the study area Figure 1), realized that several strands of phyllosilicate-rich fault gouges appear (less than 5 m thick and considered to be the fault core) that bound lenses of variably deformed protolith. At these cases, the fault core cannot be considered as a simple, narrow, and prominent slip surface. For these reasons, in this study, and based on ; Chester et al., 1993; Bruhn et al., 1994; ; Clausen, 2002; Clausen et al., 2003, , we consider that (1) the fault core represents the part of the fault zone where most of the displacement is accommodated. It is in a steady step of the deformation (flow pattern and flow parameter remain constant during deformation) and consists of slip surfaces, fault gouges, some breccias, cataclasites, and geochemically altered rock bodies; (2) the damage zone is a deformed rock volume that limits with the fault core; it is in a nonsteady phase of deformation and is composed of subsidiary faults, fault rocks, and breccia; and (3) the hosting rock is a relatively undeformed protolith. Also, in this study, we have introduced the term Subsidiary Slip Zone (SSZ) in order to name gouge strands with more accommodated slip compared to the surrounding fault rocks, but that are not necessary part of the fault core. This term should not be confused with the term Principal Slip Zones that is used to describe one or more gouge or cataclasite strands within a fault core lithologically and structurally heterogeneous that accommodates most of the displacement along them (e.g., [14, 15]).
The Alhama de Murcia Fault (AMF) is an oblique left lateral strike-slip fault with reverse component located in the Eastern Betics (southeastern Spain, Figure 1), one of the areas with the greatest seismic and tectonic activity in the Iberian Peninsula (e.g., [16–19], Martínez-Diaz et al. 2012). In the central section of the AMF a >100 m thick fault (shear) zone that dips 55-70° to the NW appears showing a stacking of different domains composed of rocks from Lower Paleozoic to Quaternary . The quality and quantity of its outcrops offer a good opportunity to get a complete record of the tectonic activity of this fault and understand how an inclined shear zone grows and distributes the strain. The AMF fault rock core has been studied for decades; mechanical or kinematic models have been proposed by, for example, Hall , Rutter et al. [20, 22], Niejemejer and Vissers , Rodriguez-Escudero et al. , or Alonso-Henar et al. . Besides, there is an exceptional track of neotectonic and paleoseismological studies of the most recent strain increments in the Alhama de Murcia Fault that assign its seismic activity in several segments that helps to understand how strain is distributed in this fault zone (e.g., [18, 26–30], among others).
The aim of this study is to shed light on the influence that the fault zone lithologies and their distribution and architecture may have in the seismic behavior and the evolution at the medium and long terms of a fault zone. We have done a broad scale approach, from scanning electron microscope to cartographic scale, to the fault zone architecture and the analysis of the lithologies of more than 100 m wide fault rock. Combining 3D observations from trenches, drilling, and microanalyses, we have classified the fault rock in deformation domains. The existing architecture models do not satisfy the observations, and we propose an asymmetric strain model for this inclined shear zone, where the more intense and steady deformation is located in a boundary (fault core) and decreases gradually towards the opposite border (damage zone, where surface rupture of historical earthquakes occur).
2. Tectonic and Geological Setting
The Eastern Betic Shear Zone (EBSZ) forms the continuation towards the Northeast of the Trans Alboran Shear Zone [31, 32]. It is formed by a series of left lateral, oblique faults, with a reverse component, that extend about 250 km from Alicante to Almería: Bajo Segura, Carrascoy, Los Tollos, Alhama de Murcia, Palomares, and Carboneras faults (Sanz de Galdeano, 1990; [33–37]). The EBSZ faults accommodate much of the shortening during the Neogene-Quaternary (Sanz de Galdeano, 1990). The neotectonic transpressive period began at the end of the Neogene due to the Africa-Eurasia convergence, with a N150°E regional direction and rates of 5-6 mm/yr (Serpelloni et al., 2007; [38, 39]). The AMF is one of the largest faults of the Eastern Betics Shear Zone (Montenat, 1973, ), it has more than 10 km accumulated slip , and it is the source of Mw >6.5 historical and prehistorical earthquakes (Martinez-Diaz et al., 2001) and the source of the Lorca 2011 Mw 5.2 destructive earthquake . The horizontal geodetic slip-rate of the AMF is , and studies based on geologically measured indicators yielded a slip-rate of in the central segments , which are considered high slip rates compared to the approximately 5 mm/year convergence between Nubia and Eurasian plates.
Martínez-Díaz et al.  divided the AMF into four seismogenic/structural segments: Goñar-Lorca segment (where this study is focused); Lorca-Totana segment, with an intense registered instrumental seismicity and several branches; Totana-Alhama de Murcia segment, with deformation distributed in several branches; and Alhama de Murcia-Alcantarilla segment, with a continuous trace but with less morphological expression (, Figure 1). Fault rocks associated with the AMF are well exposed in several outcrops along the Goñar-Lorca and Lorca-Totana segments. In the latter, the zone of active deformation consists of two branches, the main, northern branch had extruded the fault gouge, and a southern branch, where most of the studies on active tectonics were focused (e.g., ), and that until recently was the only branch considered active (the 2011 Lorca earthquake and the 1674 historical event had been recently assigned to this branch, e.g., [29, 42]). The present study is focused on the northeastern area of the Goñar-Lorca segment, which has the best record of fault rocks, and the strain is accommodated in a unique branch, where Martinez-Diaz et al.  identified the surface rupture of at least one historical earthquake. The fault rock is >100 m wide and developed across Paleozoic, Mesozoic, and Miocene rocks. Fault rock fabric is consistent with current geodynamics with no evidence of inherited previous fabrics (e.g., [20, 23, 43]).
The AMF is located in the Internal zone of the Betic cordlillera (Figure 1). The basement rocks of the study area are formed (from bottom to top) by the Nevado-Filábride complex, which represent the Variscan basement relicts made by Paleozoic and Triassic rocks , that are not observed in the AMF zone, and the Malaguide and Alpujárride tectonic complexes from the Alboran Domain . The latter are the main protolith source for our fault rock. Basement rocks are overlain by Neogene sediments related to Burdigalian-Tortonian basins and Quaternary Alluvial deposits (; Martínez-Martínez and Azañón 1997; , Figures 1 and 2). The Alpujarride Complex is made of (a) Lower Paleozoic Graphite Mica-Schists that locally could include greywackes [48–52]; (b) Permo-Triasic Fine Grained Blue/Gray Phyllites/Schists [53, 54]; and (c) Triassic Carbonates [55, 56]. The Malaguide Complex in the area is mostly made of Permo-Triassic wine red shales, sandstones, and conglomerates but some Upper Malaguide Units include locally Permo-Triassic Greywackes [57–59]. The sedimentary sequence in the study area is made of Neogene filling of the Lorca Basin: Lower-Middle Miocene (Langhian/Serravallian) Fluviodeltaic Conglomerates, Sandstones, and Limestones that locally overlie Foraminifera rich Marls [60, 61]; Upper Miocene transgressive sequence made of Conglomerates with clasts from Alpujarride Complex [62–64]; and Upper Miocene transgressive sequence is made of conglomerates with clasts from Alpujarride Complex [62–64]; Tortonian gray and yellow marls of Carivete Formation [60, 65] and an upper intercalation of several Messinian sedimentary environments recognizable of evaporitic gypsum and salts. Pliocene and Quaternary sediments are strictly related with the AMF activity: Pliocene Pleistocene Alluvial Fan related with mountain fronts and Quaternary alluvial deposits related with creeks [64, 66]. All these lithologies are involved in the shear zone, and their fabric is partially or totally overprinted by the Neotectonic fault activity (Figure 2).
3. Drilling, Trenching, and Sampling
This study is focused on La Torrecilla Site, where the AMF shows its clearest features and the fault rock is well restricted by previous studies [7, 20, 23–25, 43]. In 2015, a vertical borehole was drilled at this place (FAM-1 Borehole at Figure 2, ), and 174 m of drill core was sampled where almost a half was part of the fault rock. The high dip of the AMF (70°NW) made that the vertical borehole from the hanging wall crossed obliquely the fault zone and only sampled the upper domains of the fault rock (domain 1), but a continuous record of the transition between the hanging wall rocks and the fault core was obtained. In addition, in order to get a continuous record across the fault rock, six trenches were dug transversal to the fault zone structure, and two outcrop surfaces were cleaned. For an appropriate selection of trench sites, high-resolution aerial views from drone images were obtained, and field observation and detailed mapping of the shear zone were done previously to trench excavation (Figure 2(b)). Four of the trenches were made at “La Torrecilla creek” (To-1 to To-4), and two more trenches and two surface exposures were made 300 meters southwestward in an area named “Blas el Cojo” (trenches named BC-1 and BC-2, and surfaces SL-1 and SL-2). At La Torrecilla Site, Holocene sediments from La Torrecilla Creek cover most parts of the trenches, which was useful in order to assess the most recent increments of the deformation history of the shear zone. The trenches were cleaned and exhaustively measured and sampled; both oriented and nonoriented samples were taken. Trench photographs as logging are in the Supplementary Material; the summary of all the observations are in Figures 2 and 3; all the used outcrop scale pictures used in Figures 4–10 are sections of the trenches exposed in the Supplementary Material. Orientation data are also summarized in equal area lower hemisphere plots in Supplementary Material.
We have made 54 thin sections from the samples taken both in trenches and borehole, and some scanning electron microscope images were used focused on the fault core. The identification of deformation mechanisms and kinematic features and the fault rock classification were done using a polarized light optical microscope. The thin sections of unconsolidated samples were prepared by dry cutting after multicomponent resin primer. The studied sections of some samples were constricted by Toy et al.  method in order to constrain the Vorticity Normal Section ), but some of them are only vertical or horizontal sections of the fault rock. The used criteria to fault rock classification are the ones of Woodcock and Mort  that consider that matrix are all of those components smaller than 0.1 mm. The matrix/clasts proportion was obtained semiquantitatively using comparison charts for estimating percentage composition. The crystallographic preferred orientation (CPO) was qualitatively identified under cross polarized light and wave plate inserted. The shape preferred orientation (SPO) was measured in individual clasts or protoliths obtained from the division of major diameter by orthogonal minor diameter of a best fitting ellipse using Fiji software . SEM observations were performed using a JSM 6335F SEMCamScan microscope operating at 10-20 kV and equipped with a Link System energy dispersive X-ray (EDAX) microanalyzer. One cm3 irregularly fractured samples were dried using the successive procedure for unconsolidated sediments described by Tovey and Wong  and were prepared using epoxy resins and fresh surfaces coated with gold under vacuum.
4. Fault Rock Domains
We have distinguished five fault rock domains grouping lithologies or structural features across the shear zone. Here, we describe compositional and tectonite observations of each domain and the damage zone both at outcrop and microtectonic scale. All the descriptions are presented from the hanging wall to the footwall of the shear zone, being domain 1 the uppermost domain of the shear zone (closest to the hanging wall). The borehole FAM-1 is used mostly in the description of the shear zone boundary and domain 1; the rest of the classification is based on the trenches. All the observations are summarized in Table 1, their main microtectonic and outcrop features are presented in Figures 4–9, and their relative position is presented in Figure 3. The boundaries of the domains in some cases are sharp but some of them are gradual transitions of some properties of the fault rock (relative composition, matrix percentage, or deformation style) that are not a single plane; those boundaries should be taken as illustrative and variable along strike. This is the cases of domains 3, 4, and 5.
Several fault rock classifications are available based on relative amounts of matrix and clasts, the nature of matrix, grain growth, and cohesion among other criteria [72, 73]. Here, we used the nongenetic classification of Woodcock and Mort  for fault rock hand specimen that classifies the fault rocks according to three components: percentage of large clasts (>2 mm), small clasts (0.1 to 2 mm), and matrix or cement (<0.1 mm). The fault breccia are those fault rocks with more than 30% of large clasts, and three main subdivisions are distinguished: chaotic breccia (30-60% of large clasts), mosaic breccia (60 to 75% of large clasts), and crackle breccia (large clasts >75%). When large clasts are less than 30%, we used the prefix proto-, meso-, or ultrafollowed by cataclasite or fault gouge in order to distinguish between fault rocks with a percentage of cement or matrix less than 50, between 50 and 90%, or greater than 90, respectively. If foliation is present, it is indicated in each case. If the fault rocks at present-day outcrop are cohesive, they are considered cataclasites, or fault gouges if incohesive. The protoliths involved in the fault rocks are also mentioned in each case.
4.1. Shear Zone Boundary
The shear zone boundary (SZB) at outcrop scale is sharp and discrete, and it is oriented 70/315. It puts the lower unit of the Alpujarride complex (Lower Paleozoic) in contact with the uppermost fault rock domain of the shear zone, which is a broadly studied black fault gouge here described as domain 1 (e.g., [20, 21, 23, 24]). At outcrop scale, the contact between both is solved in few centimeters and shows cleavage dip differences from 30° to 40°, but in some cases, we have identified thin fault gouges strands (centimeter scale) of domain 1 parallel to the SZB few meters within the hanging wall that were used to define the hanging wall damage zone. At microscope scale, in the transition between the deformed and undeformed rock, we have identified evidence of reactivation of the schist fabric in at least 1 meter. In Figure 4(a), we show the original tectonic fabric of the Alpujarride Schist, with anastomosed cleavage that separates mica cleavage domains and quartz microlithons (at the upper part of microphotograph) and parallel zonal cleavage in the lower part with discrete boundary between them. We have identified thin shear bands parallel to the cleavage that offsets thin calcite veins (<0.01 mm wide) and some brittle fractures that offset cleavage domains (see Figure 4(b) and the detail in Figure 4(d)); in these offsets, some calcite porphyroclasts appear in the little shear zones that allow us to obviate refraction effects during vein injection. The crenulation cleavage is parallel to the SZB (detail in Figure 4(c)), and some quartz microlithons show domino fractures (Figure 4(e)). The dark aspect of some bands within the schist is interpreted as shear bands along the mica domains cleavage of the protolith, and their darkness is attributed to the presence of oxides and a drastic comminution of phyllisilicates during cataclasis processes that reactivated the original fabric. As the deformation increases, close to the SZB, protolith blocks are bounded by anastomosed gouge bands (Figure 4(f)) that in detail show sigma objects and S-C fabric (only visible at microscopic scale). Deformation also appears within the protolith blocks where we have identified shortening evidence reflected as chevron folds with thickened hinges and flexural slip in their limbs (upper-right corner of Figure 4(f)). In some cases, we have identified that the SZB is discrete as is shown in Figure 4(g), where we show a sharp contact between the hanging wall and the shear zone with intense deformation.
4.2. Domain 1: Homogeneous Foliated Ultrafault Gouges Overprinting Lower Paleozoic Schist
This domain is a black phyllosilicate/clay rich cataclastic matrix including lens-shaped fragments of schist/phyllite elongated parallel to the domain boundaries. Foliated ultrafault gouges are dominant, but other rock units as breccias and distorted schists/phyllites are found close to the embedded protolith blocks representing different stages of deformation. This domain has been studied by several authors (p.e. [20, 23–25]). At outcrop scale, this domain is a 10 to 30 meters wide gouges with S-C and S-C’ structures including quartz and schists lenses (Figures 3 and 5). In the first two meters, in the area closer to the SZB, foliated and cemented ultracataclasites appear (less than 30% of clasts higher than 2 mm and more than 90% of fine grained (clay) matrix); within this first 2 meters, the only protolith is the Alpujarride Graphitic mica schist. From meter 2, towards the footwall, some calcite rich phyllites appear (from the lower unit of the Alpujarride Complex) forming lenses within the fault rock that in some cases show their original cleavage but it is mostly overprinted by foliated ultrafault gouges. From meter 7, the black fault gouge is dominant again, and some evidence of discrete fault planes starts to appear. Close to the boundary with domain 2, some 10 to 20 centimeters diameter quartz delta and sigma porphyroclasts appear all parallel to the SZB indicating simple shear sense. The boundary with the next domain is sharp and parallel to the shear zone boundary, with rotated foliation that is also parallel to the SZB which is indicative of intense deformation (Figure 5(a)).
At optical microscopic scale petrographic analysis of samples from domain 1 together with XRD and SEM analysis performed by Rutter et al.  and Rodriguez-Escudero , they confirm that all rock units derived mainly from mechanical fragmentation of schists from the lower unit of the Alpujárride Complex. Mineralogy of wall rocks and derived fault rocks is predominantly composed of quartz and white mica (muscovite, illite, and paragonite), with the gouges/cataclasites being enriched in clay fraction compared to the protolith (Figures 5(c) and 5(d)). Geometric structure of the rock unit is systematic over the range of studied scales, from outcrop to SEM. Gouges/cataclasites occur in a network of C and C’ shear bands enclosing lens-shaped fragments of protolith and quartz (Figures 5(c) and 5(h)). Arrays of distorted schists and breccia are typically located in pressure shadows of the wall rock fragments. Breccia are composed of angular to subangular clasts of schists in a finer-grained matrix which is often cemented. Distorted schists correspond to protolith fragments affected by a dense network of oriented fractures leading to a prominent loss of cohesion, but original structures are clearly recognizable. Precipitation of carbonates and iron oxides is common along fractures in protolith fragments and coarse-grained rock units, leading to reddish to brown discolouring (Figures 5(b) and 5(d)).
The very fine-grained nature of the gouge matrix means that it appears black when viewed under an optical microscope in crossed-polarized light (Figure 5(e)). Gouge/cataclasite shows grains down to about 200 μm. However, these grains are clasts belonging to a matrix-based fabric of even smaller particles (<50 μm) mainly composed of preferentially oriented phyllosilicates/clays and isolated subangular to subrounded quartz grains (or survivor grains, Figures 5(f) and 5(g)). Gouge frequently incorporates asymmetric schist clasts and pulverized quartz clasts (>>2 mm), which are oriented parallel to the foliation. While schist clasts are regularly sheared along gouge-internal shears, there is no obvious preferred orientation of the fractures in the quartz clasts, suggesting that they may result from coseismic pulverization as stated in . On the other hand, in the arrays of distorted schists and breccias located in pressure shadows of sigmoid, there is slightly Crystal Preferred Orientation (CPO), which is not evident in protoliths.
4.3. Domain 2: Homogeneous Foliated Ultra- and Mesofault Gouges Overprinting Blue/Gray Phyllites
Four meters wide domain of ultra/mesofoliated fault gouges overprinting Blue/Gray Phyllites with quartzite layers from upper units of the Alpujarride Complex (Figure 3). Incohesive fault rock with approximately 30% of clasts higher than 2 mm and more than 50% of clay rich matrix form S-C/S-C’ structures with centimeter scale quartz and phyllite porphyroclasts. In the upper part of domain 2, we still observe some porphyroclasts of calcophyllites that disappear downward. Some disperse fragments of reddish/orange silts and sandstones from Langhian/Serravallian Miocene Units appear. In the most northeastern part of our excavations, an exotic thin level of fault rocks appear overprinting Devonic-Carboniferous greywackes (trench 2 and surface 2, Supplementary material and Figure 5). At outcrop scale, it is possible to identify the original Blue/Gray Phyllites tectonic fabric with the overprinting neotectonic S-C and S-C’ fabric.
At microscopic scale, we have identified mineralogy coming from Alpujarride phyllites, white mica, and quartz in the gouge enriched of clay. Calcite is abundant at this point and some opaque mineral appears (possibly Fe and Ti Oxides). Matrix is composed of survivor quartz clasts and clay minerals (illite) from protolith comminution. Calcite is present both in veins with tabular thick twins and as sigmoids clasts with strong CPO in their tails. Quartz is also abundant forming porphyroclasts and/or crystal aggregates. The quartz porphyroclasts show undulose extinction surely inherited from original tectonite. The fine grain matrix is composed of clay minerals but also of tiny quartz and calcite clasts with slightly CPO. The comminution at this point is lower than in domain 1 but still effective with more than 70% of clasts lower than 0.1 mm and in some areas even more. The porphyroclasts have aspect ratio ranging from 2 to 2.5 and showing shape preferred orientation with long axes mainly oriented parallel to foliation (Figure 6(f)) or C shears (Figure 6(g)). Chaotic microfractures are predominantly within protolith lenses being part of cataclasis or comminution during shearing.
4.4. Domains 3 and 4: Heterogeneous Mixture of Paleozoic, Permian-Triassic, and Miocene Protoliths
From this point, the deformation became more spatially heterogeneous, and the domain classification is based on gradual changes in relative compositions of protoliths (boundary between domains 3 and 4) or the deformation intensity (boundary between domains 4 and 5). We have unified the description of domains 3 and 4 in the same section because their only difference is the relative amount of the protoliths, specifically the Blue/Gray Phyllites that became dominant within domain 4. The boundary is gradual, and it could be located in a broad band (few meters wide) across the strike of the shear zone. Even more, these units, together with domain 5, change their width along strike.
Domains 3 and 4 are heterogeneous domains of fault gouges mixing and overprinting yellow marls, silts, and sandstones from Miocene, wine-red shales, and sandstones from Triassic Malaguide Complex and Blue/Gray Phyllites from Permian-Triassic Alpujárride Complex (Figure 3). The upper boundary of domain 3 is a sharp contact between domain 2 and Miocene Marls lenses with low quantity of internal kinematic markers (Figures 7(a), 7(b), and 8(b)). The orientation of the marls lenses has been identified in their boundaries, where phyllites appear (oriented almost parallel to the SZB 60/310). Some lineations are preserved in this unit with chaotic plunges pointing to complexities in deformation style (orientation data are available in Supplementary Material (available here)). At this point (at outcrop scale), the comminution of Blue/Gray Phyllites is still effective developing a cataclastic matrix of more than 50% of the fault gouges. From meter 18, some Triassic sandstone and silt blocks start to appear (meter scale at the top to centimeter scale to the bottom, Figure 3) with sigma and delta shapes indicating simple shear sense (Figure 7(f)). This alternation is constant from meter apx. 18 to meter apx. 31 where Malaguide Sandstone Blocks appear. In the boundaries of the blocks, slip is concentrated, and comminution processes had been more intense forming gouges (Figure 8(e)). The boundary between domains 3 and 4 has been established close to meter 30 from the hanging wall in a Malaguide Complex Block bounded by well-developed gouges that we have defined as Subsidiary Slip Zone (SSZ). From this point, the deformation is characterized by an anastomosed alternation between gouges overprinting centimeter to decimeter scale Blue/Gray Phyllites and marls bands. The sandstone blocks are less abundant, but when appear, they are bounded by penetrative tectonic fabric developing foliated mesofault gouges overprinting the Blue-Gray Phyllites.
At optical microscope scale, the observations in these domains are redundant with the features identified at outcrop scale. Thin sections of lenses or blocks from Miocene deposits (Tortonian yellow marls or Langhian/Serravallian orange silts) show unaltered mineralogy, calcite marls, and foraminifera or quartz-rich fine-grained silt. The studied thin sections of the marls domains do not show clear evidence of internal deformation apart from some isotropic fractures. Some fossil appear (foraminifera) within Marls units, and microfractures are present in both with no remarkable features (Figures 8(b) and 8(c)). Thin sections of the most heterogeneous area (located in the area of Figure 7(f) at around meters 20–30 from the hanging wall) show gouge bands with centimeter to millimeter quartz/phyllites or sandstone lenses porphyroclasts. The clay-rich matrix is mainly composed of white mica and illite with some quartz fragments coming mostly from mechanical comminution of phyllites. Calcite is also abundant as porphyroclasts or vein injection. Some opaque mineral appears overlapping all the deformation (surely alteration minerals). Corroded boundaries and vein injections of apparently devitrified material bounding some sandstone clasts are interpreted as evidence of frictional melting (Figures 8(a) and 8(d)). Shape preferred orientation of clasts and S-C and S-C’ structures are common in the gouge bands. Compared with the other domains, here, the fault rocks show a relative increase in C’ shears. Quartz porphyroclasts have rounded boundaries that indicate mechanic fragmentation, with an aspect ratio around 2 with long axes mostly oriented parallel to C’ shears; the phyllite porphyroclasts have a higher aspect ratio (>2.5) and higher angularity with long axis orientation consistent with the quartz porphyroclasts. Same feature has been identified at the boundaries of blocks where slip is concentrated along gouge bands forming SSZ, where the deformation is more intense than the bounding planes (Figure 8(e)).
4.5. Domain 5: Fault Gouges Bounding Lenses of Variably Deformed Protoliths
Foliated protofault gouges and crackle breccia overprint Blue/Gray Alpujarride phyllites and red-wine Malaguide shales/phyllites. At this point, the old tectonite fabric is evident, and the overprinting neotectonic fabric and comminution has not been efficient yet (Figures 9(a) and 9(b)). Derived fault rocks show less or circa 50% of matrix and are mostly composed by mica and comminuted clasts from Upper Alpujarride Blue/Gray Phyllites. There are many intact blocks of Alpujarride Phyllites with some alterations and faults and fractures crossing them. Until meter 65 to the hanging wall, there is still some alternation with Miocene Marls lenses. From meter 70, almost all the deformation is located in discrete bands bounding intact blocks, with S1 oriented 70/025; 70/010 (Figures 3 and 9(c), and Supplementary Material for stereoplots showing orientation data). In the discrete shear zones, protofault gouges are developed, with powdered quartz clasts and S-C fabric (Figures 9(f) and 9(i)); towards the footwall of the shear zone, the crackle breccia is more common (mostly overprinting veins), and mosaic breccia overprinting phyllites appears in the protolith blocks (Figures 9(b) and 9(h)), and the gouge strands are less abundant. We found displacement evidence in single and narrow shear bands of fault gouges oriented parallel to axial plane crenullation foliation overprinting the Blue/Gray Phyllites where the original tectonite has not been completely overprinted. In some bands (apx. 1 to 10 cm wide) within domain 5 comminution has been effective enough and almost completely overprinted old fabric. These shear bands are classified as SSZ that seem to reflect the last increment of deformation of the historical record as we discuss below.
At microscale, the identified mineralogy is the same of the protoliths; there is a clay enrichment only in SSZs. The blocks of Red-Wine Phyllites from Malaguide Complex show spaced parallel cleavage, with cleavage domains of white mica and microlithons mostly composed by quartz. At the microlithons, there are some folds with axial plane parallel to cleavage (Figures 9(a), 9(d), and 9(g)). Within the Blue-Gray Phyllites blocks, some brittle fractures are identified, and crackle breccias are overprinting mostly quartz and calcite aggregates. At the boundaries of the quartz veins, there is more intense cleavage (cleavage percentage domains increase, Figures 9(e) and 9(h)). The identified SSZ gouges have same mineralogy as their protoliths, clay enriched matrix with quartz and phyllite porphyroclasts. They show S-C S-C’ structures with sigma and delta porphyroclasts with aspect ratio around 1.5–2 and angular boundaries. Shape preferred orientation is present with the long axes oriented parallel to the boundaries of the SSZ (Figure 9(c), 9(f), and 9(i)).
5.1. On Textures and Slip Distribution (from Core to Surface Rupture of Historical Events)
The textural and petrographic analyses of fault rocks across the AMF fault zone, including FAM-1 borehole data, six trenches excavation, and two cleaned surfaces, reveal a strongly heterogeneous shear zone regarding to its compositional and structural features. It ranges from the fault core rocks in domain 1 (derived from Paleozoic schist), where strain is distributed within several meters wide of foliated ultrafault gouges and ultracataclasites, to domain 5, where strain is concentrated along discrete fault planes (or shear bands) involving Permian-Triassic to Holocene rocks. Even we have identified corroded borders (pointing to frictional melting) and slightly crystal preferred orientation in some units (maybe pointing to intracristaline dislocations), cataclasis, cataclastic flow, and frictional sliding are the dominant deformation mechanisms during shearing. Blenkinsop  showed that the fractal dimension of the Particle Size Distribution depends on several features such as fragmentation process, initial size distribution, number of fracture events, energy input, strain, and confining pressure, and it is increased by last four. Thereby, considering matrix the proportion of clasts lower than 0.1 mm, the percentage of matrix can be used as a qualitative proxy to identify the relative degree of incremental deformation that each domain had accommodated, even more when they have the same protolith, which is the case of the Blue/Gray Phyllites that appears across almost the whole shear zone. Within each domain, it is possible to find more than one kind of fault rock (i.e., within domain 1, we found foliated ultracataclasites and gouges but eventually some fault breccia appears; or in domains 3, 4, and 5 where protofault gouges and breccia are dominant but some shear planes show an increase in their matrix percentage compared with the surrounding rocks). However, the fault rock classification of major features of each domain shows that there is a decreasing trend in the percentage of matrix (clasts lower than 0.1 mm) from the hanging wall to the footwall of the shear zone. This could be pointing to a decrease in the accommodated strain from the upper parts of the shear zone (domains closer to the hanging wall, where foliated ultrafault gouges are dominant) towards the footwall (where protofault gouges and breccia are dominant) (Figure 10).
The uppermost unit (domain 1) is composed mostly of the lower tectono-stratigraphic unit, the Paleozoic Alpujarride Schists, which is considered to have more accommodated strain and here classified as the fault core. This is consistent with its textural features here classified as foliated ultrafault gouges, where the original (old) tectonic fabric of the protolith had been almost completely overprinted. Domain 2, which is composed of upper units of the Alpujarride Complex, is texturally homogeneous, and its structural style has features close to the identified in domain 1 but with a relative decrease in the matrix percentage (here lower but close to 90%) and thereby classified as a foliated (meso) fault gouge. We propose that the textural differences between domains 1 and 2 are due to different stages of the comminution processes, that are in an earlier stage in domain 2 than in domain 1, which is maybe in a steady stage (which is also consistent with the kinematic analyses done in domain 1 in ). The intensity of the deformation in domain 2, its lateral continuity, and homogeneity make us to include it in the fault core.
From domain 3, there is a heterogeneity increase in both textural and petrographic features but a decrease in the accommodated slip. From domain 3 to domain 5, the slip (strain) is mostly accommodated in areas where there is a constant intercalation between protoliths (microphotograph (c) in Figure 10), but with a relative increase in some slip zones bounding bigger blocks. When the size of the different protolith blocks is constant the slip is distributed in wider areas, but when bigger blocks appear (decametric scale), the slip is more concentrated in anastomossed shear zones of meso- and protofault gouges bounding the blocks. Some of the studied blocks show only some microfractures, while in their boundaries, we have identified foliated fault gouges. Domain 4 is in gradual contact with both domains 3 and 5 and is composed of mostly an alternate of Miocene Marls and Upper Paleozoic Blue/Gray Phyllites. Here, the matrix percentage is around a 60% (mesofault gouges), decreasing towards the footwall of the shear zone, until the protolith tectonic fabric is evident, criteria followed to distinguish between domains 4 and 5. Within domain 5, the slip (strain) is preferentially (but not only) accommodated in some anastomosed slip zones (SSZ, classified as meso-fault gouges), bounding Alpujarride and Malaguide Complexes protolith blocks with their original fabric not overprinted yet (Figure 10).
We propose that, from domain 3, the mechanical heterogeneities of the different protoliths may had conditioned the heterogeneity in the slip distribution and the competency contrast favors slip in the block boundaries . This fact is visible both at outcrop scale and microscope scale, where we found that within the protolith blocks, there is almost no cataclasis (apart from some microfractures) while out we find gouges (see Figure 8(a)). The low-lateral continuity of these domains and their internal heterogeneities in both deformation intensity and lithology make us to classify them as a highly intense deformed damage zone. A similar process was describe by Fagereng and Sibson (2010) for shear displacements in subduction channel where the deformation within a melange is concentrated in the contact between incompetent and competent materials, with shear bands localized along lithological contacts or within competent materials while the matrix flow accommodate strain as distributed deformation.
The SSZs within domain 5 or domains laterally equivalent (possibly lateral continuation of domains 3 and 4) could be coincident with the surface ruptures of historical earthquakes. The Quaternary alluvial sediments from La Torrecilla Creek cover the fault rock, but when they appear on the lowest parts of the fault rock (domains 3, 4 and 5), we found faults that offset them (red fault in Figure 2). In T4, Martinez-Díaz et al.  showed a surface rupture that offsets an Arabic archaeological remain at this point (see Supplementary Material (available here)). This rupture was related to the 1674 catastrophic event occurred in Lorca (Murcia). Moreover, domain 1 is covered by undeformed old Quaternary hardened glacis deposits supporting that this sector of the fault zone is not utilized by recent ruptures, at least at shallow levels. This may be pointing that the sense of growing of the shear zone vary from the hanging wall, where steady deformation occurs (domain 1), towards the footwall of the shear, where historical earthquakes ruptures are being recorded (Figure 10).
5.2. On the Architecture of the Fault Zone and Spatial Distribution of Its Lithologies
Summarizing, as we see above, the architecture of the AMF is composed of 10 to 30 meters wide fault core with a narrow damage zone in its upper limit (less than 5 meters wide) and a wide (more than 70 meters wide), highly deformed and heterogenous damage zone towards its lower limit, that is covered by recent sediments that prevent seeing it. All the observations point to an asymmetric slip (strain) distribution across the fault zone; accordingly, this fault zone is far from those conceptual models of symmetric strain distribution of Chester and Logan  or Faulkner et al.  (Figure 11). These models consist of three primary components in a shear zone: the hosting rock, the damage zone, and the fault core (where almost all the strain is accommodated Chester et al., 1993; [13, 76]). Chester and Logan  model consist of a fault core in the middle of the shear zone where almost all the strain is concentrated, decreasing towards both sides equally. The Faulkner et al.’s  model (based on Carboneras Fault, see Figure 1) has a more complex distribution of the strain and anastomosed fault core appear, but strain is still distributed symmetrically (Figure 11). Faulkner et al.  explained the differences into anastomosed versus single straight core of a fault rock based on compositional features; Chester and Logan’s  model could be assumed for quartzofeldesphatic hosting rocks, while Faulkner’s et al.  model is based on philosilicate-rich hosting rocks that bound lenses of variably fractured protolith. The mechanical properties of philosilicate rich fault rocks may explain these features, because strain hardening is a common process in these rocks while weakening is more common in quartzofeldspathic rocks [23, 77, 78]. AMF hosting rocks are very similar to the Carboneras Fault rocks, and the mechanical features of them may explain the anastomosing distribution of the strain, but not its asymmetric distribution.
As explained above from domain 3 Subsidiary Slip Zones appear, whereas in domains 1 and 2 deformation is homogeneously distributed. In domains 3 and 4, SSZs correspond to anastomosed 2 to 10 cm wide shear planes that seem to have accommodated more deformation than the bounding fault rock (that still presents comminution and shearing indicators). However, when the strain decreases and the hosting rock has not been overprinted by the new tectonic fabric, the SSZs still appear. Domain 5 is lithologically almost homogeneous (composed mostly of Blue/Gray phyllites, with some lateral intercalation of Malaguide Complex and Miocene rocks), and the SSZs are anastomosed shear zones bounding blocks with low deformation grade; thereby, we think that the SSZs in this case could be connected with the strain distribution at depth being closer to the Faulkner’s et al.  model.
For all these reasons, here, we propose a hybrid and asymmetric model based on both Chester and Logan  and Faullkner et al.  models and more specifically on Lin and Yamashita  that showed that there are spatial variations of the fault core location along strike and that the strain distribution could be highly asymmetric even in a strike-slip fault. Here, the strain starts abruptly in one of the sides of the shear zone where the strain is the highest (fault core), and there is almost no damage zone in the transition between the core and the hosting rock. The strain tends to decrease towards the other side and anastomosed shear planes appear, which accommodate more deformation than the hosting rock. Domains 1 and 2 are the fault core; from domain 3, the decreasing trend of the strain starts. From this point, an architecture dominated by an alternation of SSZs and less deformed blocks appear, and the strain differences between SSZs and damage zone are increasing towards the end of the shear zone (Figures 10 and 11).
5.3. On the Asymmetry of the Shear Zone
It is intriguing that, in a shear zone with more than 10 km accumulated slip , the damage zone is located at only one side of the core, which is in contact with an almost no deformed hanging wall to its opposite side. Inclined shear zones display asymmetry of the damage zone (e.g., [79–82]). Numerical models showing mechanical asymmetries in both normal and reverse faults done by Ma (2009) together with several observations in natural damage zones for example Heermace et al. , Yeh et al.  at Chelungpu Fault or Li et al.  at Yingxiu-Beichuan Fault Zone showed that, contrary to the results of this study, the wider damage zones are located in the hanging wall of the fault zones. Seems thereby that the AMF could be an exception where the damage zone is located mostly in the footwall. This strain heterogeneity and the asymmetric distribution of the slip could be explained by several reasons: mechanical heterogeneities related to sedimentary and basement material juxtaposition; mechanical anisotropies related to fault propagation and growth (i.e., splay formation during reverse faulting); intrinsic kinematic features of an inclined shear zone; or a combination of two or more processes.
The mechanical contrasts in shear zones could arise in a heterogeneous strain distribution [86–89]. The basement rocks of Alpujarride and Malaguide Complexes together with sedimentary rocks from the Lorca and Guadalentin basins may arise in a complex distribution of the slip (at least at surface). The phyllosillicate rich protoliths may promote strain hardening and thereby the widening of the shear zone, while the sedimentary rocks are involved in the shear zone introducing heterogeneties. This mixture of textures could incorporate roughness in the lower part of the shear zone, and the asperity removal and comminution place the slip in the boundaries of sedimentary blocks, resulting in the slip heterogeneities found in domains 3 and 4. But this explanation does not completely satisfy the observations in domain 5 which is composed of basement rocks and located at the bottom of the shear zone. Another possibility in order to explain the strong asymmetry in the shear zone could be related to the hanging wall imbrication and splay formation during shearing towards the footwall where recent slip events are recorded. The kinematic evolution of a fold-and-thrust belt implies tilting of the thrusts until reach a critical dip angle enabling the formation of new thrusts towards the foreland (e.g., ). A similar process with different critical dip angles could had promoted the appearance of new faults towards the footwall of the shear zone involving new protolith lenses and promoting the observed asymmetry.
The decreasing trend in strain intensity towards the footwall of the shear zone could be also explained as a part of the deformation mechanism in an inclined transpressive triclinic shear zone. Nabavi et al.  used finite element to model an inclined shear zone similar to the kinematic model of Inclined Transpression with Oblique Extrusion (, that has satisfactorily explained the strain distribution in the AMF, ). With a set-up where two reverse faults (dipping 70°) are bounding a shear zone, Nabavi et al.  revealed that the velocity vectors within the shear zone gradually decrease towards the footwall. This model explains that the accommodated slip close to the hanging wall is the higher, and also, it could explain why steady deformation is located in domain 1, but it does not explain why the surface ruptures of historical earthquakes are located towards the footwall, but this fact, together with splay formation, would explain both the asymmetry and the location of the last increments of strain in the footwall of the fault.
The study of tectonic fabric at both microscale and outcrop-scale of the fault rocks of the Alhama de Murcia Fault reveals that:
Predominant deformation mechanisms are brecciation, cataclasis, frictional sliding, and cataclastic flow, but several optical microscopic observations reveal that frictional melting and intracrystalline deformation could be also deformation mechanisms within this shear zone
The strain distribution in this shear zone is highly asymmetric; it decreases from hanging wall to the footwall. Steady homogeneous deformation was found in domains 1 and maybe 2, areas closest to the hanging wall where comminution made a matrix percentage higher than 90% of the fault rock. As deformation increased, it involved sedimentary rocks of Lorca basin in domains with a highly heterogeneous strain distribution. In the lowermost domain, strain is located in anastomosed shear bands (here classified as Subsidiary Slip Zones) which are spatially related with a surface seismic rupture of the 1674 destructive earthquake
The mechanical heterogeneities of the different protoliths within domain 3 may had conditioned the heterogeneity in the slip distribution and the competency contrast favors slip in the block boundaries. This fact is visible both at outcrop scale and microscope scale, where we found protolith blocks with almost no deformation (apart from some microfractures) while out of them foliated gouges appear
The inclined transpression models together with and splay formation during reverse faulting are the preferred hypothesis to explain the strain asymmetry of the shear zone, but we cannot discard mechanical heterogeneities caused by juxtaposition of sedimentary and basement rocks as an other plausible explanation
Data are available by contacting with corresponding author and in supplementary material.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was supported by the project “QUAKESTEP” (CGL2017–83931-C3-1-P) founded by the Spanish Ministry of Science, Innovation and Universities. We thank Marián Barajas López (Complutense University of Madrid) for thin section preparation. We thank José L. Sanchez Roldán, Yolanda de Pro, and Juan Miguel Insúa Arrévalo (Complutense University of Madrid) for their support during sampling and Alejandra Staller (Polytechnic University of Madrid) for drone image acquisition. We thank Associate Editor Andrea Billi and the reviewer Jack Williams for their constructive comments done on the original version of this article.