This contribution analyses the role played by the mechanical properties of a decollément shale layer in the evolution of the Mexican Fold and Thrust Belt (MFTB). The mobility of overpressured shales can accommodate large strains by grain-scale plastic mechanisms, and affect the folding and thrusting styles of the overburden. Research on shale deformation mechanisms is necessary to improve the knowledge of these processes and their influence on the structural style of fold and thrust belts. The ductile behavior of rocks involving grain-scale plasticity was documented in the Jurassic Santiago shale sequence using geological mapping, microstructural observations on thin-oriented sections, and scanning electron microscopy (SEM) imaging. Structural styles such as detachment folding, fault-bend folding, and shale-cored fold-thrusts were observed at the regional scale. At the outcrop scale, the shale developed strong foliation and pencil cleavage, with immersed packstone boudins. Observed structures include thrusting, soft and open folds, and buckle folding. In thin section, the ductile textures include a strong penetrative foliation with lenticular and wavy-parallel laminae composed of carbonates, ribbons of reoriented clays and organic matter (clay+OM), s-c structures, porphyroblasts microtextures, development of oblique cleavage concerning folded foliation (crenulation cleavage), and carbonates dissolution. The Santiago shale shows also evidence of brittle deformation including calcite-filled fractures and cataclastic gouges. X-ray diffraction (XRD) analysis of the clay size fraction suggests that the authigenic calcareous shale was deformed in conditions of the deep diagenetic zone (between 100 and 200°C) and fluid overpressure (>70 MPa). The results help to improve the understanding of ductile microstructure and its role in shale deformation cretaceous cover, promoting the formation of localized fault propagation folds in the overburden. This study aims to open new perspectives in the kinematics and rheology interpretations for this sector of the MFTB, highlighting the role of the décollement layers during the progression of the orogen.

Shale ductile décollements can influence the structural style and the segmentation of fold and thrust belts, promoting the slip of large overburden wedge blocks above them and playing a fundamental role in the geometry of thrusting [1-7]. Overpressure shales can also accommodate large strains by grain-scale plastic mechanisms, a process known as shale mobility [3, 8]. The thickness of the shale layer can influence the diffusion of deformation. With the presence of a thin shale, the deformation is localized and discrete [9]; while a thick layer distributes deformation. In some cases, structures from the basal unit (pre-shales) will cross the décollement and affect the upper units [9-11]. The mechanisms for shale mobilization vary depending on the perspective. There is a consensus on pore pressure increase as a triggering factor for mobilization [1, 12]. Some works point to the role of fluids during mobilization as a dominant factor [6, 13, 14], while for others the composition and fabric of clay minerals are influential factors [15]. However, there is a consensus that the forces involved are essential to understanding the initiation and evolution of shale mobilization, with two main mechanisms; gravitational processes (gravity sliding and differential loading) and those where lateral tectonic stresses dominate (burial and overpressure) [12, 16].

Shale décollements can extend from tens to thousands of kilometers [16]. Examples of décollements associated with shortened sedimentary basins in which shale mobilization is an essential feature for their formation and evolution include the Niger Delta [3, 17-20], the Moffat Shale Group in the Ordovician Northern Belt of the Southern Uplands of Scotland [2], the overthrust in the Jura Mountains in Switzerland [21], and the western Xuenfeng Mountains in China [5]. Fundamental research on shale deformation mechanisms increased recently due to the relevance of shales for hydrocarbon prospection [6, 10, 11, 22-30].

In Mexico, early works on décollements have focused on the presence of low-strength evaporites and salt layers in northeastern Mexico (e.g., [31-39]).

While salt sequences are absent in the central part of eastern Mexico, thick shale layers locally interbedded with gypsum or limestones are common in the Jurassic [40] and Cretaceous sequences [6, 41]. The role of these shale sequences acting as ductile décollement layers during the evolution of the Mexican Fold and Thrust Belt (MFTB) has received limited attention, and in particular, microstructural studies on tectonically deformed shales are scarce. The presence of highly sheared, low-strength carbonate sequences at the front of the Mexican orogen has been recognized only recently [6, 41]. For instance, Deville et al. [6] demonstrated the relevance of a shale décollement for deformation and hydrocarbon prospection.

This study aims to infer the evolution of the shale deformation and the kinematics from structural data (bedding, foliation, lineation, axial plane cleavages, reverse faulting, and folding data), coupled with microstructural analyses and description of core samples to document for the first time the ductile deformation of the Santiago Shale (Callovian-Oxfordian). Evidence of the deformation collected in the hinterland exposures was integrated with the description of core samples in the foreland. The data allows the analysis of the deformation of a relatively laterally continuous shale layer with a mechanical response contrasting with the deformation of the overburden and basal rocks. A regional correlation of the deformation mechanisms along the décollement is possible considering the foreland’s well data availability. Integration of the regional data provides a comprehensive framework to understand the role of the ductile deformation of the Santiago shale during shortening.

The Sierra Madre Oriental (SMOr) is the orogenic build-up of the Late Cretaceous to Early Paleogene MFTB (Figure 1) [31, 36, 42-47]. Rocks in the central part of the MFTB front and the foreland basin region include a nearly continuous clastic and calcareous sedimentary sequence from the Early Jurassic to the Late Cretaceous (Figures 2 and 3) [37, 47-49]. The stratigraphy of the Jurassic to Cretaceous rock sequences in the foreland is relatively well-known from well-log data [50]. In the Middle and Late Jurassic, continental and transitional deposits filled basins associated with an extensional regime during the opening of the Gulf of Mexico [37, 48, 51-54]. The oldest rocks in the area correspond to the Early Jurassic continental red beds of the Huayacocotla Formation (Sinemurian-Hettangian) (Figure 3), consisting of conglomerates, interbedded sandstone-siltstone in thin layers, and shales at the top with fossil plants [48, 53, 55, 56]. The Huayacocotla red beds are unconformably overlain by the Early-Middle Jurassic Cahuasas formation (Toarcian-Bathonian) composed of sandstones, polymictic conglomerates, and thick massive deposits of siltstone at the top [48, 49, 53]. The redbeds are unconformably overlain by Early Jurassic clay-rich sequences [40] including the calcarenite Tepexic Formation of the Callovian at the base, followed by Callovian-Oxfordian interbedded mudstone-shale and packstones of the Santiago Formation, Kimmeridgian-Tithonian interbedded limestone-shale of the Taman Formation, and thin layers of shale and limestone of the Tithonian Pimienta Fm [40, 51, 52]. The Santiago shale is the thicker clay-rich formation in the study area and was deposited in a marine environment at relatively moderate depths with constrained water circulation and favorable reductive conditions for the preservation of organic matter (OM) [40, 50, 57]. In the study area, the Santiago shale is a dark gray- to black-colored rock composed of mudstone interbedded with packstone and grainstone [27, 40, 51], with apparent layering commonly 30 to 40 centimeters thick, and sparse and deformed marine fossils (radiolaria, ammonite, bivalves, and echinoderms [50, 51, 56]. Calcite veins, fine-sized quartz grains, and authigenic pyrite crystals were observed immersed in the dark-colored groundmass.

For the time-lapse between the early and Late Cretaceous, progressive marine transgression and relatively stable conditions in a passive margin were established [48, 53, 58]. In contrast to the underlying clay-rich sequence, the Early Cretaceous Tamaulipas Formation (Albian-Cenomanian) is a succession of limestones with abundant chert bands [48, 53]. These rocks were covered by a Late Cretaceous transgressive sequence of limestones: Agua Nueva, San Felipe, and Mendez Formations [48, 49, 53, 59].

Shortening associated with the development of the MFTB in the Central part of Mexico spans from the Late Cretaceous to the Eocene. The topic has been extensively studied and summarized by Fitz-Díaz et al. [47]. During the orogenic build-up of the MFTB, the sequence was folded, faulted, and progressively exhumed with an eastward vergence in the Late Cretaceous to Paleogene [31, 45, 47-49, 60, 61]. Progressive and northeast-directed shortening started in the Turonian to early Campanian (ca. 85 Ma) and finished by the Paleocene to Eocene (43 Ma) [60-64]. The shortening affected partially the foreland basin of Chicontepec filled with more than 1.5 km of deep-basin Paleogene clastic sediments derived from erosion processes over the orogen [65, 66]. The Coulomb wedge theory (e.g., [67]) has been invoked for explaining the overall wedge shape tapering toward the east with a dominantly thin-skinned deformation style [60]. The basal detachment of the thin-skinned deformation has been ubicated in the Jurassic layers of evaporites or carbonaceous shales, but few studies detail the role of these rocks on deformation (e.g., [6, 41]). Here, we provide an example of a ductile deformed shale unit acting as a viscous décollement layer during the evolution of the MFTB.

Structural analysis was conducted by field mapping of geological units and structural features in four different field campaigns (2019, 2021, 2022, and 2023). Data includes available topographic maps (scale 1:50,000), geological maps, and the interpretation of satellite imagery. Regional SW-NE cross-sections show inferred relations between minor and major structures and ductile deformation of the Santiago shale. Intense vegetation cover prevented extensive fieldwork and access to several outcrops. This study focused on structural data collection at 18 locations, with key sampling sites (A1, A2, A3, A4, A6) distributed in localities along the San Marcos River (Figure 2, Table 1). The photographic record highlights key features on each outcrop. Structural data were projected in lower hemisphere stereo plots using the Orient 3.1 software. Reverse fault slip data include fault orientation (dip and dip direction), slickening orientation, and shear orientation from kinematic indicators collected on fault surfaces. Beta diagrams [68] were constructed for the analysis of folding axes. The information for structural analysis and cores for geological interpretations were provided by the Comisión Nacional de Hidrocarburos (CNH) and Secretaría de Energía (SENER). Ductile deformation of the Santiago shale was observed in cores of the Pardo well stored at the Litóteca Nacional de Hidrocarburos in 2021.

Given the fine-grained and heterogeneous nature of shales at multiple scales, an integrated multi-scale approach is critical for its characterization [69]. Structures were described at the outcrop (cm), thin section (mm), and scanning electron microscopy (SEM) (μm) scales. Oriented blocks were extracted from outcrops to produce thin polished sections perpendicular to the foliation. Thin sections were examined under cross-polarized light (XPL) and oblique reflected light. Photomicrographs were taken at 10x magnifications as these values were considered representative in terms of covered area and detail for the identification of grain size and shapes, the spatial distribution of the mineralogical phases, fracture orientation, and fracture characteristics. Key properties for describing the microstructure of a shale include mineral content and packing, OM, pores, and microfractures [26, 27]. The microstructural analysis for key samples of the Itzatlan outcrop was refined by SEM analysis on thin polished sections using a Hitachi Table Microscope TM-1000 at 15 kV with a working distance of 8.22 mm. The elemental composition of the Santiago shale at this location was estimated by Jimenez-Camargo et al. [27]. Magnification values were selected according to the relationship between resolution and the covered area of the microfabric features. The following characteristics of the SEM images were evaluated: microfabric, mineral composition, granulometry, crystal habit, microporosity, and microfractures. Ductile features such as grain dislocation, bending on clay platelets, deformation on pyrite framboids, pores at the edges of rigid grains, and the presence of OM were highlighted.

The mineral composition of the poly-mineral Santiago shale was calculated by X-ray diffraction (XRD) on random bulk powder samples using Miniflex Rigaku equipment with copper radiation Kα 1.5406 Å generated at 30 kV and 30 mA, between 5 and 80 degrees 2θ, with a step of 0.02 degrees and 0.06 s counting time. Six samples, four of shale and two of the silt facies of the underlying Cahuasas sandstone were selected for analyses on the clay size fraction (<2 μm). In these samples, oriented aggregates were systematically prepared for XRD to enhance the clay mineral basal reflections. The use of an ethylene glycol medium allowed us to evaluate the presence of expansive clays. Crystalline phases were identified using the PDF-2 database (International Center for Diffraction Data, 2005, www.icdd.com) and the reference Crystallography Open Database (COD, available at www.crystallography.net/cod/). Before clay fraction orientation, apparent density, and grain size were determined for 6 selected samples. The crystallinity index (CI) was obtained from the full width at half maximum (FWHM) using the illite diffraction peak at a 2θ=8.7 calculated with the software JADE 6.1 incorporated in the measuring device. The Kubler crystallinity index [70] was estimated employing the constrained least-squares refinement method of the IC by comparing the FWHM with standard compound LaB6, obtaining a reasonably good match between calculated and experimental diffraction patterns (Profile fit= 0.146). The average thickness of the illite crystals was estimated using the Scherrer equation [71]:

τ=kλ/βcosθ

where τ is the thickness of the crystal, k =1 is the shape factor, λ is the wavelength of the DRX analysis (1.5406 Å), θ is the Bragg reflection angle in radians, and β is the CI.

4.1. Major shortening structures

Building on previous reconnaissance works [35, 37, 48, 49, 55, 59] this contribution presents geological and structural mapping at the front of the MFTB, in the Puebla state of Mexico (Figure 2). Geomorphologically, the study area is characterized by decreasing slopes to the east. Folds and thrusts show a general NW-SE trend with tectonic transport to the NE and E (Figure 4). Beyond the tectonic front, geomorphological plains indicate the presence of the foreland basin.

The hinterland can be divided into three structural domains. The westernmost domain is a regional uplifted block known as the Huayacocotla Anticlinorium, mainly composed by the early Jurassic Huayacocotla Fm (Figure 4) [48, 49, 55]. The Cretaceous cover is practically absent over the Huayacocotla anticlinorium, and the geometry of the uplifting block controls the regional structural trend. Deformation of the Huayacocotla Formation is characterized by intense and pervasive folding (Figure 5(a) and (b)). At the eastern flank of the anticlinorium, the contact of the Huayacocotla Fm with the Santiago shale is a high-angle (50°) reverse fault.

The second domain of the hinterland is mostly composed of the Jurassic rock sequence. The structural style of the Cretaceous sequence shows kilometer-scale gentle folds with a slight asymmetry that defines eastward vergences (San Pablo syncline and the Tlacuilotepec anticline; Figures 4 and 5(c)). The eastern boundary of this domain is the Tlacuilotepec Thrust, a complex high-angle reverse structure defined by a fault propagation fold (Figure 5(d)). At the lower part of the thrust, the Early Jurassic Cahuasas Formation is placed on top of the Middle Jurassic Santiago Shale along the AT, an eastward vergence, low-angle reverse structure. Since Jurassic layers are duplicated in the stratigraphic sequence across the hanging wall the structures were interpreted as a thrust duplex (Figure 4). The deformation of the Huayacocotla, Santiago, and Taman Formations in the second domain is characterized by open and slightly recumbent fault propagation folds, reverse faults, and shear and axial plane foliations (s1) with vergence to the NW and NNE (Figure 3). Shale rock shows a scaly fabric indicating a strong penetrative deformation. At the mesoscale, the shale shows variations from thin disharmonic to thick harmonic detachment folds with uniform flanks and constant wavelength.

The eastern domain of the hinterland is defined by closely spaced folds built up on the cretaceous sequence. The fold series includes the Tanchitla anticline (TA), the San Antonio syncline (SS), and the Papaloctipan overturned anticline (PA) (Figure 3). The structures observed at the limit between the hinterland and the foreland sedimentary sequence are two east-vergent reverse faults known as Tlaxcalantongo and Papaloctipan Thrusts (T-PT) (Figure 5(e) and (f)).

4.2. Ductile and brittle behavior of the shale sequence

The analysis was focused on three key zones along the San Marcos River: a) near the Pahuatlan village; b) three sites near the Tlacuilotepec town; and c) the Itzatlan locality (Figure 2).

At the southern bank of the San Marcos River near the Pahuatlan village, the Santiago shale has an apparent thickness of 350 m. The Huayacocotla Formation can be observed on the northern bank of the San Marcos River as a redbed sequence with abundant plant fossils. Along the river, the Huayacocotla Formation is placed over the Santiago shale by a reverse fault with WNW orientation and dipping (~20°) to the SW (Figure 4, reverse faults). Foliation, defined by the layering of coarse (micrite) and fine (clay) grain particles, dips between 10° and 40° to the SW (Figure 6(a)); s-c structures were observed in coarser grain domains (Figure 6(b)). Boudins and other ellipsoidal-shaped packstone elements were observed embedded in the fine-grain matrix (Figure 6(c)). Most of those features show sinuous and diffuse borders (Figure 6(d)).

The packstone elements show evidence of necking and detachment, suggesting that they were formed as part of an original continuous layer, and hence boudins can be interpreted to be derived from preexisting coarse grain layers by some deformation mechanism, likely boudinage or microfaulting. Boudins show evidence of deformation, especially slip on basal planes and bending of clay+OM ribbons around rigid elements. The orientation of the long axes of boudins parallel to the foliation orientation suggests a rigid body rotation during shearing (Figure 6(b) and (d)). Grain size reduction by cataclastic processes and pressure dissolution structures such as stylolithic joints were identified.

At the Tlacuilotepec zone, the Santiago shale was observed involved in at least three inverse displacements (Figure 7a). In the uppermost thrust (site A6), the thickness of the Santiago shale reaches 300 m under the Cretaceous sequence. A greater content of OM was observed at this level. The elongated boudins display angular edges and are oblique to the foliation (Figure 7(b)). Penetrative foliation dips 50° to the SW and a pencil cleavage was documented. At the middle level (site A1), the Santiago shale overlies the thick (>200m) Huayacocotla sandstone. Figure 8 shows the typical ductile deformation observed at this level, a compact mudstone layer of 1.5 m in thickness defines undulating, soft, and open folds in the outcrop. A buckle fold exhibits how the underlying shale is injected into the fold hinge. Fold axial planes dip slightly to the SSW and the NE. Imbricated reverse faults dipping 40° to the NW were recognized in the compact mudstone layer. Calcite-filled fractures parallel and perpendicular to the foliation were observed only in the compact mudstone layer. The thin and spaced penetrative foliation dips 30° to the W. In the lower thrust, the Huayacocotla Fm is placed over the Santiago shale. This site is located at the bed of the San Marcos River at 540 masl (Figure 7). The calcareous-carbonaceous shale shows a strong foliation dipping to the SW (~20°), showing immersed packstone boudins up to 1 m in length and parallel to the foliation (Figure 7(c)). Boudins oriented parallel to the foliation also show evidence of necking and detaching, and s-c structures are associated with coarser grain sizes. At the Itzatlan site, the Santiago shale is exposed along the riverbed showing an apparent thickness of about 200 m. The microstructure of the mudstone at this location was documented in detail by Jiménez-Camargo et al. [27]. Strong and pervasive foliation dipping ~40° to the NE and SW and crenulation cleavage were documented. Microfolding of the SW dipping foliation observed in the field was corroborated in the circular distribution of poles observed in the stereographic projection (Figure 9(a)). A parallel alignment of fractures associated with the limbs of the isoclinal microfolds was interpreted as crenulation cleavage (Figure 9(b)). When observed in thin sections, crenulation cleavage is oblique to foliation (Figure 10), with angles ranging between 40° and 70°. Some crenulation planes host calcite veins with similar intersection angles to foliation. At this location, boudins have regular edges and are mostly symmetrical varying in shape from well-rounded to sigmoidal (Figure 9(c)) with a long vs. short axis ratio between 2 and 3. The long axis is commonly parallel to the foliation. The curved shape of the boudin alignment in Figure 9(d) suggests refolding over the foliation.

4.3. Microstructure from Thin Sections and SEM Imaging

Further insights into the microstructural properties associated with the ductile behavior of the shale during shortening deformation were obtained from oriented thin sections. Microscope analysis reveals the authigenic-calcareous nature of the shale with lenticular and wavy-parallel lamination [27]. Mineral phases include carbonates, ribbons of clay and organic matter (Clay+OM), quartz, calcite crystals, and calcite veins (Figure 10). Authigenic pyrite is observed both as cubic massive and as framboidal and cubic crystals (Figure 9(e) and (f)).

Carbonates were present in the micritic matrix or filling fractures (calcite) and pores (Figure 10(a) and (f)). SiO2 was commonly observed as authigenic relatively large quartz crystals (Figure 10(b)) or as cryptocrystalline silica. Under reflected light, Clay+OM ribbons and calcite-filled fractures delimit the rhomboidal-shaped micritic fragments (Figure 10(c) and (e)). Foliation planes are defined by the relative enrichment of Clay+OM on wavy and laterally intermittent bands. Quartz occurs as small grains, fractured and sheared, and along bands elongated in the orientation of foliation. Carbonates have been locally dissolved and precipitated as authigenic calcite into the fractures (Figure 10(b)). Zones of dissolution pressure (dark-colored and fine-grained zones) were identified parallel to foliation. Under reflected light (RL) the foliation is observed in the surrounding areas where crystals are segregated from the carbonates and clay+OM ribbons (Figure 10(e)).

Two examples of XRD analysis (Figure 10(g) and (h)) in bulk samples of the Tlacuilotepec site indicate a proportion of carbonates: quartz about 70:30. Jiménez-Camargo et al. [27] reported mineralogical volumetric contents at the Itzatlan site that included carbonates (80-83%Vol), quartz (9-11%Vol), and illite (4–8%Vol.). Pyrite was present with 1–2%Vol. The average OM content is 2% Vol. XRD analysis on the clayey fraction indicates the presence of illite, kaolinite, calcite, quartz, and pyrite. Tables 2 and 3 show the physical and mineralogical properties obtained for the clay fraction of six oriented samples. The Kubler index of crystallinity yielded values between 0.55 and 0.76 for the shales, and 0.58 for the Cahuasas samples. It is important to note that the massive siltstone at the upper part of the Cahuasas Formation (i.e., underlying rocks to the Santiago shale) does not show evidence of intensive shearing or foliation.

The micritic matrix shows a foliated s-c fabric [72] indicating intensive shearing in which undulated bedding-parallel lenses of carbonates form a complex anastomosing network with the Clay+OM ribbons (Figure 10(c) and (d)). The fragmented micrite displays a gradual reduction in grain size when approaching the dissolution pressure zones associated with the pronounced foliation. In the shear bands, reordering of calcite occurs by cataclastic size reduction, rotation, and authigenic growth with irregular edges. Dissolution processes, dislocation, and twinning over carbonates are common (Figure 10(d) and (f)). The foliation microstructure of the Santiago Formation at the Itzatlan location is depicted in the SEM images in Figure 11. The preferred orientation of porosity in clays and the allocation of interparticle porosity in the mechanical interfaces between clays and stiff components make those regions prone to stress concentrations and microfracture growth following the bedding orientation (Figure 11(a) and (b)). Contrasting strengths and deformational character distinguished clays from carbonate domains and stiff components (e.g., silicates, euhedral calcites, or pyrites), with plastic deformation taking place over clay domains where conspicuous inter and intraparticle porosities are present (Figure 11(c) and (d)). Primary deformation mechanisms on clays include bending around stiff components (Figure 11(b) and (d)) and clay dislocations. Figure 11(c) shows dislocation processes on clayey domains where almost no visible microporosity is left due to compaction (zone a). The dislocation processes affect porosity distributions parallel to layering where compacted clay domains have been rotated or segmented (Figure 11(c)).

4.4. Description of Core Samples at the Blind Foreland

Lithological records for nine wells are presented in Figure 12(a), five of these wells reached the Santiago shale at depth. Data allow the interpretation of the lateral continuity of the Santiago shale and its geological structure in the Foreland basin. Core intervals were selected for interpretation along the cross-section (Figure 12(b)), parallel to the main direction of tectonic transport and perpendicular to regional fold and thrust features according to the criteria proposed by [73].

The wells considered for the analysis were designated as T1, E1, E-101, E-102, AF-1, CY-1, Y-1, P-1, and C-2 (Figure 12). Figure 12(a) shows the distribution of the Santiago shale in the wells. The metamorphic basement complex was identified only in 2 wells near the tectonic front: T-1 y E1. Early Jurassic Cahuasas sandstone lying below the Santiago shale was identified in wells T1, E-1, and P-1. The Santiago and Pimienta shales were identified in wells T-1, E-1, E-102, AF-1, and P1. The units related to the early Cretaceous Tamaulipas Inferior and Tamaulipas inferior bentonitic were identified in wells T-1, E-1, E-102, AF-1, and P-1. The Huayacocotla Fm is absent.

Based on the interpretation from well data, regional Tlaxcalaltongo thrust was observed as a repetition of the breccia in the boundary K-Paleogene and the unit KSM in the well T-1, at a depth of 517 m. A second thrust was revealed by close inspection of data from the wells E-101 and E-102, which record a repetition of the Cretaceous sedimentary units Méndez (KSM) and San Felipe (KSSF). The thickness of the upper KSM and KSSF sequences can be estimated at around 80 and 90 m, respectively, with the lower sequence having a thickness of 180 m. Based on the stratigraphic repetition, we infer the location of the Escobal thrust at 620 m below the surface. The Escobal thrust was reported previously as the “Brinco-Escobal” reverse fault based on unpublished seismic data [74, 75].

Core samples of the Pardo well show the Santiago shale with lower strains and an increase in ductility with depth compared to the hinterland samples (Figure 13). For the core sample in Figure 13(a) fragment of packstone immersed in the shale matrix at 3080 m depth is interpreted as a boudin aligned roughly parallel to the foliation, with regular and rounded edges. In Figure 13(b), the core shale sample at 3090 m depth shows evidence of compaction in the clay+OM ribbons and stylolithic joints indicating carbonate dissolution along the bedding surface. At 3100 m depth (Figure 13(c)), imbricate structures and compaction in clay+OM ribbons were observed. Calcite veins and clay+OM ribbons parallel to the bedding were documented producing an incipient foliated structure at 3150 m depth (Figure 13(d)). Note that the stretching of calcite veins produces a segmented boudinage structure. Shearing bands occur in the clay fraction between two laminae in Figure 13(e) at 3200 m depth. In Figure 13(g) and (f), the intense pervasive deformation is similar to that observed in the hinterland. At 3400 m depth (Figure 13(g)), the rock shows undulate fragments of carbonates separated from each other by the clay+OM and calcite veins. At 3660 m, a scaly fabric was developed with rhomboidal fragments of carbonate delimited by calcite veins (Figure 13(f)).

5.1. The Geometry of the Décollement

Even though shale lithologies are widely distributed in Mesozoic basins in central Mexico along the orogen, few works have recognized ductile shale décollements. The inferred evolution of the Santiago shale shares similarities, such as burial depth and estimated overpressure, with the evolution of the Parras shale décollement recently reported in NE Mexico [6]. Both systems can be considered as thin detachment layers with thick covers according to the proposal of Morley et al. [4]; for the Santiago shale, approx. 200 m of ductile rocks were found between the older redbed sequences (Huayacocotla and Cahuasas formations) and the Cretaceous overburden (1100 m).

The uplifting of the Huayacocotla anticlinorium along a steeply inclined reverse fault with eastward vergence favored displacements towards the second domain, driving loads to the ductile shale. The thickening of the shale layer observed at the core of the gentle and symmetric San Pablo syncline and Tlacuilotepec anticline formed on Cretaceous limestones (domain II, Figure 4) was interpreted as indicative of detachment folding. In contrast, the boundary between the domains II and III shows a thrust duplex. The uppermost level of the duplex is represented by a fault-propagation fold (e.g., [76-78] and its associated thrust ramp. At the intermediate level, the trace of a thrust ramp is extended from the surface to the detachment layer. A fault-bend folding structure is interpreted at this point. As expected, the foliation was refolded and thrust in order to travel across the nonplanar fault surface (figures 7 and 8) (e.g., [79]). The cretaceous overburden layers in domain III are less affected by erosional processes and show a higher degree of deformation in the form of tight synclines and anticlines (figures 4 and 5c, and d). The eastern limit of domain III is the Paleocene Chicontepec foreland basin. The presence of a small wedge of the Santiago shale exposed at the footwall of the Tlaxcalaltongo thrust suggests the involvement of the shale décollement in the fault and folding process. For the outermost blind foreland, the well data is interpreted to be consistent with typical detachment-style folding and fault propagation folds in a ductile décollement (e.g., [4, 16]).

5.2. Ductile Meso- and Microstructures of the Santiago Shale

Due to the heterogeneous nature of shale, the assessment of microstructural properties associated with ductile deformation requires multiscale imaging analysis [69].

Shale deformation can influence the structural style of the overburden on a regional scale [1, 3, 4, 7-9, 16]. These features and the resulting structural style have been called shale tectonics in the literature (e.g., [20, 80-83]). Changes in the thickness of the Santiago Shale along the décollement suggest the formation of detachment folding. For instance, thickness duplicates below the Tlacuilotepec anticline hinge (Figure 4) and the shearing is distributed throughout its volume parallel to the reverse faults indicating vergence towards the hinge. Similar detachment structures have been described in other natural systems [6, 7, 84] and analogue models [10], suggesting that shale thickens in a ductile manner below the detachment folds before the episodes of rupture and thrusting.

Depending on the clay content and composition of the clay fraction in the shale, loading and shearing can influence compressibility, mineral dissolution, and precipitation (e.g., [6, 28]). These processes can lead to enhanced plasticity, affecting both brittle and ductile responses. Some features identified at the mesoscale, such as penetrative foliation, scaly clay fabric, s-c structures and cleavage, symmetrical upright folds, and buckle folds of the Santiago shale are indicative of ductility. At the Tlacuilotepec locality (Figure 8) a relatively more competent mudstone layer with calcite-filled fractures forms a stacked thrust sequence with eastward vergence. In the same locality, a buckled anticline formed in compacted mudstone with highly sheared shale at the core showing evidence of diapiric-like synkinematic flow was observed. The cross-cutting relationships observed in this site suggest combined shearing and confined mobilization in the shale throughout the progressive shortening. These structures are similar to those reported in shale décollement in several thrust-fold belts (e.g., [4, 6, 16]), and share similarities with structures generated by disequilibrium compaction that triggers mud mobilization under high confining conditions in regions elsewhere (e.g., [1, 6, 12, 83]). For instance, the hyper-elongated elements of packstone with diffuse edges at the Pahuatlan locality (Figure 6) might have formed by stretching associated with loading under shortening conditions.

At the microscale, the penetrative deformation characterized by foliation, crenulation cleavage, and refolding is noticeable by the complex fabric derived from strength variations between relatively “strong” and “soft” components. Carbonates represent the “strongest” predominant phase showing evidence of brittle deformation, indicated by grain size reduction by cataclasis. Grain reduction and dissolution pressure are localized at the borders of carbonate domains and grains. When fracturing occurs along the micritic matrix, s-c structures develop in the carbonates forming elongated and rotated sigmoidal shapes with associated pressure shadows (Figure 10(d)). Additional ductile microstructures in the carbonates encompass dislocation bands and pressure dissolution borders, especially at contact with the clay+OM ribbons. Ductile deformation also modified the pore distribution forming dislocation fractures aligned with pores in the micritic matrix, creating elongated pores, or interparticle pores between clay particles or at the interface between clay ribbons and the micrite. The presence of rigid grains such as pyrite and authigenic quartz crystals, restrain compaction and shearing along the foliation bands (Figure 14).

Mechanically “softer” phases include the clay fraction and theOM shown as clay+OM ribbons. Foliation planes are defined by the alignment of the clay+OM ribbons (Figure 10(c)) and some calcite-filled microfractures oriented parallel to the foliation. Complex microstructures such as reorientation and alignment of the clay platelets around the rigid elements are indicative of intense shearing (Figure 11(d)). The effects of deformation on the OM have been addressed recently by Yang et al. [30]. In their model, the alignment of the clay+OM ribbons is derived from the collapse of OM pores with a potential reactivation of microscopic seepage channels for fluid transport. The seepage of bitumen and other fluids during deformation can drive compaction and enhance the ductile response of the shale (Figure 10(e)).

Multiple sets of calcite-filled microfractures oblique to the foliation evidence the brittle tensional behavior of the shale. These features may be associated with high fluid pressures overcoming the magnitude of the minimum local stress plus the tensional strength of the rock, as suggested by Deville et al. [6]. The crosscutting relations suggest that these episodes of fluid overpressure occurred locally during progressive deformation.

5.3. Conditions of the Ductile Mechanisms

The XRD data for the oriented clay fraction yielded values of the Kubler Crystallinity index between 1 and 0.52, in the deep diagenetic zone, where temperatures between 100 and 200°C can be expected [85-87]. Previous works suggest that the smectite-to-illite transformation starts around 65–75°C and releases water, silica, and cations that can react with calcite and kaolinite to produce ankerite and chlorite (see Soto et al. [11] for a comprehensive review). The same authors suggest that the transformation of smectite to illite is also associated with compaction, reorientation of the clay, and precipitation of authigenic microquartz. The XRD of the clay fraction indicates the absence of smectite and in only one case the presence of muscovite, supporting the classification in the deep diagenetic zone.

Fabric changes in the carbonates, such as grain reduction, redistribution, and authigenic growth (e.g., [88]) and the increase of twinning in carbonates (e.g., [89]) observed in the thin sections also support the increase in temperature. Expected temperatures are in the range reported by Deville et al. [6] between 170 and 220°C, and confining pressures between 120 and 148 MPa measured by fluid inclusion microthermometry and barometry of diagenetic calcite and quartz for the Campanian Parras shale in NE Mexico. The presence of authigenic quartz in these samples provides further evidence of phase transformation during deformation. Authigenic silica was observed as overgrowth quartz grains associated with clay ribbons and as cementing quartz in veins. The experimental evidence on the smectite to illite transformations under high pressures and temperatures [23] suggests that quartz crystals formed at temperatures between 100 and 150°C, while the cementing quartz only formed at temperatures over 150°C. The formation of authigenic quartz cement in shales was also observed by Thyberg et al. [22] at depths around 2500 m (80–85°C) of Upper Cretaceous mudstones in the North Sea.

The depth for the brittle to ductile transition can be likely modified by changes in the amount of clay+OM, temperature, pore pressure, tectonic stresses, and diagenetic changes during burial. At depth, pore fluids that cannot escape during burial or tectonism can lead to overpressure that needs to be dissipated by deformation. The release of structural water during the smectite to illite transformation might also be associated with pore pressure increases. It has also been suggested that overpressure induces dissolution processes on micrite, and the presence of OM provides ion sources for the growth of bedding-parallel fractures that are subsequently filled by bedding-parallel calcite veins [24, 25]. A similar situation is observed in the Pardo well where bedding-parallel calcite veins were observed. Deep burial depths inferred for the Santiago shale are consistent with the ductile character of the deformation.

The Santiago shale is a strongly anisotropic rock with brittle or ductile behavior depending on confining pressure [27]. Employing unconfined uniaxial strength tests in dry conditions, these authors estimated a brittleness index between 0.56 and 0.83, finding that between 0.17 and 0.44% of the total deformation in those tests was accommodated by plastic mechanisms before the rupture. With increasing confining pressure, i.e., depth, the strength of shale rocks increases before reaching the transition from predominantly brittle to ductile behavior [90]. For instance, Yuan et al. [91] estimated a critical confining pressure of 71.2 MPa, or 4470 ± 230 m depth, for the brittle–ductile transition of the tectonically deformed Longmaxi shale in China, using triaxial tests. Depths between 4 and 5 km have been estimated for the brittle–ductile transition by other authors in triaxial tests[15, 92]. Previous works in Mexico reported confining pressures in the order of 70 MPa and an estimated burial depth of about 4-5 km for the brittle-ductile transition in the Parras shale [6], and confining pressure of 77 MPa with a burial depth of 3 km for a décollement in central Mexico [93].

5.4. A Model of Shale Mobility During Shortening

Based on the results, the structural evolution of the décollement-style fold and thrust belt is summarized as follows. We suggest that shale mobilization occurred during the episode of regional shortening deformation, long after significant burial, compaction, and cementation, as inferred from the ductile microstructure and shearing preserved in the Santiago shale. Recent works show that high pressure and temperature conditions plus dewatering through smectite to illite transformations and gas formation can make the shale sequences to behave in a plastic manner [10, 11, 15, 91, 92]. Based on the observation of penetrative deformation characteristics identified in the cores of the Pardo well, such as anastomosed fabric features in carbonates and the formation of the calcite veins, an early mechanical weakening was provoked by lithostatic loads and diagenetic processes during burial. This early stage is represented by the development of stylolithic joints, the segregation of clay+OM ribbons, the occurrence of bedding parallel fractures filled by calcite and/or quartz, scaly clay fabric, and a deformation gradient of the shale layers from more deformed at the bottom to undeformed at the top of the sequence (Figure 15(a)).

With increasing shortening, incremental burial, and push from the rear of the orogen, the system reached a critical state for plastic deformation triggering shale mobilization. Relatively rigid layers of packstone fragment by fluid overpressure exceeding the tensional strength of the rock, leading to the formation of fractures perpendicular and oblique to the lamination. Progressive stretching and fragmentation of the packstone layers leads to boudinage, and the clay layers develop foliation around the boudins. This stage favored the development of detachment folds (Figure 15(b)) by ductile mobilization of the shale into the cores of anticlines [94]. At this stage, tectonic shearing has a major influence on the shale microstructure and the detachment folds in the overburdened rocks can either develop an asymmetric geometry or thrust faults.

Progressive vertical load is increased due to thrust stacking in the hinterland, forming fault propagation folds and thrusts that nucleate at the core, or in the limbs of the anticlines in the overburdened rocks (Figure 15(c)). Fault propagation folding was observed at the Tlacuilotepec anticline and was recognized at the Tlaxcalaltongo and Escobal thrust in the Foreland region. The localization of the deformation was interpreted as a process controlled by the reactivation of basal faults at this stage, as suggested by at least two wedges of the Cahuasas Formation overlying the shale in the Tlacuilotepec region. The stacking of the overburden at this stage can be also associated with shale mobilization to outward regions. Complex microstructures associated with this stage might include refolding over the primary foliation (Figure 9(a) and (b)) as shales become involved in thrusting.

The last stage of deformation (Figure 15(d)) is characterized by fault-bend folding. Eventually, progressive shortening reactivates basal faults and leads to the formation of structural highs (e.g., the Huayacocotla anticlinorium).

The aim of this work is not related to the prospecting of hydrocarbons. However, the mobilization of shales generated by the increase in overpressure and temperature due to burial caused by the thrusting of the shale layers has a direct relationship with the generation of hydrocarbons [95]. The highest point of overpressure in the shale lies between 4 and 5 km depth, coinciding with the most profound peak of oil generation and the beginning of the gas-generating window [96, 97]. The increase of the pore pressure is also associated with water released during the illite-smectite transformation, which activates the mobilization of the shales and thrusting. During this process, hydrocarbons (oil) are also released, and these migrate along with the flow of the shale mass [95]. These characteristics make this shale sequence a viable unconventional reservoir similar to other deformed shale sequences reported recently in Mexico [6, 98].

This study provides an example of the role of shale deformation in the contractional setting of the MFTB, improving our understanding of the microstructure related to a décollement layer with shale mobility. The ductile and brittle deformation of the Santiago shale was documented in two adjacent regions of the orogen (hinterland and foreland) using surface geology data, oriented thin sections, and oil-well information. Lateral continuity of the deformed shale layer along the system (hinterland and foreland) was confirmed with the field work and oil-well data. Strong and penetrative foliation of the shale is related to regional shortening and influenced the structural style of the Cretaceous overburden on a regional scale. The presence of the decollement in the hinterland produced structural styles such as detachment folding, fault-bend folding, and shale-cored fold-thrusts. At the outcrop scale, the shale developed strong foliation and pencil cleavage, with immersed packstone boudins. Observed structures include thrusting, soft and open folds, and buckle folding. Specimens observed in the P-1 will confirm the presence of mobile shale in the deformed foreland.

The results help to improve understanding of the development of ductile microstructure and its role in shale deformation. Microstructural features of the shale include dislocation and foliation bands, secondary folding over foliation, s-c structures, grain size reduction by cataclasis, pressure dissolution borders, and alignment of the clay+OM ribbons. Brittle deformation of the shales includes calcite-filled veins and multiscale fracturing and faulting. The shale microstructures confirm that redistribution of shales occurred through combined ductile and brittle deformations.

The absence of smectite and the presence of illite, chlorite, kaolinite, and muscovite suggest conditions for the smectite-illite transition. XRD for the oriented clay fraction yielded values of the Kubler Crystallinity index between 1 and 0.52, classifying the rock in the deep diagenetic zone at temperatures ranging between 100 and 200°C, with fluid overpressures over 70 MPa. Future studies on the tectonic shortening style in Mexico are encouraged to consider the ubiquitous presence of shale lithologies along the orogen.

The data that support the findings of this study are available from the Comisión Nacional de Hidrocarburos, México. Restrictions apply to the availability of these data, which were used under grant number 44533-803-28-III-16 for this study. Derived data supporting the findings of this study are available from the corresponding author on request.

None declared.

MC and LIVB participated in the design of fieldwork, collected data, and designed the layout of the manuscript. MC produced the first draft and most of the discussion. JAJC participated in the analysis of the microstructure in the thin sections and SEM images. CC analyzed the well data and helped in the elaboration of images. All authors revised the first and final drafts and participated in the discussion of the results.

Ricardo Carrizosa and Vania Ferrer assisted during fieldwork. We thank Marina Vega (Laboratorio de Geofluidos, CGEO) and Beatriz Millán (CFATA) for helping with SEM image acquisition for assisting with XRD and Crystallinity index determinations. Hector Ibarra (Laboratorio de Paleomagnetismo, CGEO) is thanked for his help with sample characterization. Juan Tomas (Laboratorio de Laminación, CGEO) produced the thin sections for this study. Gilles Levresse is thanked also for his help on rock characterization.

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