Deep External Fluid Source Along the Gubbio Normal Fault (Italy): Implications for Slip Along the Altotiberina Active Low-Angle Normal Fault System

Fluids play a critical role in fault slip, fault and damage zone development, and the distribution of seismicity in regional fault systems; however, determining the source of fluids within fault damage zones is complicated by superposition of fluid-related fabrics. Clumped isotopes applied to tectonic studies offer an opportunity to distinguish between fluid sources based on temperature and stable isotopic composition. Here we use the clumped isotope geothermometer and field observations and compare them with microstructural analysis to investigate the Gubbio normal-fault (GuF) core, a major SW-dipping epidetachment fault conjugate to the active Altotiberina low-angle normal fault in central Italy. Several distinct calcite vein sets reveal the incorporation of multiple generations of fluids during development of the damage zone. Clumped isotope results from these various precipitation phases record crystallization temperatures ranging from 87–154°C. These temperatures are broadly consistent with the presence of calcite deformation twin types II and III and are higher than estimates of regional peak burial temperatures. Additionally, stable isotope compositions within vein calcite are distinct from published isotopic values of the Apennine sedimentary succession, which constitutes the local bedrock. We propose that these observations suggest hydrothermal fluids migrated from depths greater than 6 km, which requires hydraulic connectivity along structural pathways between the shallow and deep crust, and fluid overpressures. These fluids reach the GuF via migration along the Altotiberina low-angle normal fault plane and they may be either of diagenetic or of deeper subduction origin. We suggest they possibly originated from the proximal retreating Apennine subduction system, implying that subduction processes exert spatial control on the distribution of fluid-assisted normal faulting and related seismicity which is consistent with the co-migration of closely coupled subduction and hinterland extension in the Apennines from Miocene to Present.


INTRODUCTION
It has been widely recognized that fluids circulate within fault breccia and associated veins and fractures (e.g., Sibson, 1987;Sibson, 1996). In some cases, fractures networks record textural evidence that fluid passage can be episodic and contemporaneous with fault slip (Streit and Cox, 2001;Uysal et al., 2011). In addition, high pressure fluids are important in both faulting, reactivation, and vein formation (Sibson, 1996). These fluids reduce the effective normal stress and facilitate slip on faults in turn promoting the propagation of breccia and fracture networks, and mineralization within fault breccia and vein systems records multiple episodes of fault reactivation as shown by crack-seal mechanisms (Passchier and Trouw, 2005;Nuriel et al., 2011). In this context, microchemical analyses on fault breccia and veins provide information about deformation related fluids (e.g., Kirschner et al., 1993).
Carbonate clumped isotope (Δ 47 ) geothermometry, due to its ability to determine precise calcite crystallization temperatures (e.g., Eiler, 2011), has emerged as a tool for studies of shallow crustal tectonic environments (e.g., Beaudoin et al., 2020;Curzi et al., 2020) particularly when paired with other methods such as calcite twinning geothermometry, U-Pb geochronology, and fluid inclusion microthermometry (e.g., Beaudoin et al., 2020;Curzi et al., 2020). These measurements provide both formation temperature estimates (from Δ 47 ) and source fluid compositions (δ 18 O, δ 13 C), thereby providing the means to differentiate meteoric versus deep fluids and examine ascending/descending pathways (e.g., Bergman et al., 2013), as well as delineate generations of cementation and deformation along fault zones (e.g., Hodson et al., 2016). Application of Δ 47 geothermometry presents a novel opportunity to investigate fluid dynamics and cement generation in fault complexes and fracture zones associated with fault systems where fluid connectivity may play an important role between shallow and deep crustal processes.
Calcite e-twin width and morphology has been correlated with temperature of deformation in naturally deformed calcite (e.g., Burkhard, 1993). For a given amount of twinning, strain mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate up to 170°C and thick twins dominate above 200°C (Ferrill, 1991;Burkhard, 1993;Ferrill, 1998;Ferrill et al., 2004;Passchier and Trouw, 2005). This relationship between twin thickness and temperature has recently been questioned by authors who pointed out that the increasing duration of stress, and hence strain, may cause widening of twin lamellae even at room temperature (Rybacki et al., 2013;Parlangeau et al., 2019). Nevertheless, the optical distinction between different calcite twin type morphology has been successfully applied to tectonic studies where calcite is particularly abundant, especially when coupled with other thermometric techniques (e.g., Meneghini et al., 2012;Musumeci and Vaselli, 2012;Clemenzi et al., 2014;Clemenzi et al., 2015;Marroni et al., 2015;Storti et al., 2018).
The combination of Δ 47 geothermometry with microstructural analysis on calcite e-twin width and morphology presents an opportunity to investigate fluid dynamics and cement generation in fault complexes and fracture zones associated with fault systems where fluid connectivity may play an important role between shallow and deep crustal processes, such as in the epidetachment faults associated with low angle normal faults. Low-angle (<30°) normal faults (LANFs) are extensional features with a shallow dip that can have horizontal displacements large enough to exhume metamorphic rocks from the middle crust (Davis and Lister, 1988). Normal slip along such low-angle structures is unfavorable according to frictional slip theory (Sibson, 1985), and a number of dynamic models have attempted to reconcile this mechanical paradox geometrically (e.g., rolling hinge; Wernicke and Axen, 1988;Lavier et al., 1999;Mizera et al., 2019). Nonetheless, active slip along low-angle detachments has been observed using seismological and geodetic techniques (e.g., Abers, 1991;Abers et al., 1997;Chiaraluce et al., 2007;Hreinsdóttir and Bennett, 2009) and is feasible when fluid pressures are sufficient to overcome the minimum principal stress (σ 3 ) (e.g., Sibson, 1985;Sibson, 2000) or when frictionally weak minerals are present within the fault core (Hayman et al., 2003;Numelin et al., 2007;Collettini et al., 2009aCollettini et al., , 2009b. Evidence of elevated fluid pressures in the form of mineralized tension gashes and the presence of frictionally weak mineral phases are abundant in exhumed LANFs (e.g., Goodwin, 1999;Manatschal, 1999;Cowan et al., 2003;Isik et al., 2003;Collettini and Holdsworth, 2004) and fluid overpressure is indirectly inferred by modelling (Wawrzyniec et al., 1999;Collettini et al., 2006) and seismological data (Moretti et al., 2009). However, active LANF systems are exceedingly rare, and evidence of high-pressure fluids unambiguously coming from seismogenic depths in the hanging-wall of LANFs far from their breakaway zone is sparse. Epidetachment faults situated within LANF hanging-walls operate in tandem with the detachment and present an opportunity to sample these fluids because they create pathways for overpressured fluids to reach the surface (Reynolds and Lister, 1987;Wawrzyniec et al., 1999).
The Gubbio normal fault (GuF) is a 22 km long SW-dipping normal fault (Barchi, 2002), and the largest of the antithetic epidetachment faults structurally connected to the active Altotiberina low-angle normal fault (ATF) in the northern Apennines, Italy. The ATF is imaged in both the CROP 03 seismic line and commercial seismic profiles (Keller et al., 1994;Barchi et al., 1998b;Mirabella et al., 2004), and is a 70 km long low-angle ENE-dipping extensional detachment active since the early Pliocene (Caricchi et al., 2015). Modern slip along the ATF is observable via microseismicity (Chiaraluce et al., 2007), and geodetic surface velocities (Hreinsdóttir and Bennett, 2009). The GuF intersects the ATF between 4 and 5 km depth and is very well exposed at the surface  and therein references). The GuF damage zone contains abundant secondary calcite, filling opening mode fracture sets and slip surfaces. These damage zone calcite veins were previously investigated using fluid inclusions by Bussolotto et al. (2007), who determined that extensional fabrics formed at depths less than 2.5-3 km, in a confined fluid system not related to meteoric water.
We apply carbonate clumped isotope geothermometry and compare them with qualitative calcite twinning observation and microstructural analysis to the GuF damage zone secondary calcite in order to determine fluid source and discuss implications for connectivity between the ATF and associated epidetachment faults in context of active low-angle normal slip.

GEOLOGIC SETTING
The Northern Apennines are a fold-thrust belt caused by Miocene-Present collision between the Adriatic microplate and European continental margin. This collision is accompanied by an associated retreating syn-convergent subduction system where the coupled compressional foreland and extensional hinterland migrate to the northeast onto the subducting Adria lithosphere (Elter et al., 1975;Reutter et al., 1980;Barchi et al., 1998a). Active extension affects the axial zone of the belt, while the present thrust front is situated off the Adriatic coast of Italy (Boncio and Lavecchia, 2000;De Luca et al., 2009;Chiaraluce et al., 2017). The extensional deformation phase began to affect the western Umbria-Marche Apennines in the early Pliocene (Caricchi et al., 2015), and is parallel with respect to the previous contractional tectonic phase with a minimum principal stress (σ 3 ) oriented NE-SW. Local modern extension rates, measured by satellite geodesy, are~2.7 mm/ yr (Hreinsdóttir and Bennett, 2009) and accompanied by moderate (Mmax>6.5) normal sense focal mechanism Frontiers in Earth Science | www.frontiersin.org February 2022 | Volume 10 | Article 811339 3 earthquakes in the shallow crust (Pondrelli et al., 2004;Chiaraluce et al., 2017).
Extension in the northern Apennines is accommodated, in part, by a set of regional low-angle (20°-30°) detachment faults dipping to ENE, which young to the ENE, the youngest and only active of which is the ATF, a~70 km long structure which borders the upper Tiber Valley (Barchi et al., 1998b;Mirabella et al., 2011;Caricchi et al., 2015). A set of seismogenic antithetic high angle SW-dipping normal faults are situated in the ATF hanging wall (Boncio and Lavecchia, 2000). Among these, the Gubbio normal fault (GuF) crops out 25 km east of the ATF breakaway zone near the town of Gubbio in central Umbria ( Figure 1). The GuF exposes Meso-Cenozoic carbonate strata belonging to the Umbria-Marche stratigraphic succession (Cresta et al., 1989) in its footwall, and bounds the Gubbio Quaternary continental basin which is situated upon the hanging wall. Based on seismic reflection, the subsurface geometry of the GuF is interpreted to be listric, shallowing to~10°-15°dip near the intersection with the ATF between 4 and 5 km depth (Mirabella et al., 2004). The maximum displacement of the GuF is~2.3 km,~1.5 km of which is attributed to Quaternary extensional slip conjugate to motion along the ATF, based on variation in the thickness of Miocene foredeep deposits across the GuF (Mirabella et al., 2004).

Field Locations
We sampled two locations along the Gubbio Fault ( Figure 1) for damage zone vein calcite: the Gubbio locality (GL), and the Mocaiana locality (ML) ( Figure 1A). The GL is situated in the central portion of the fault near the town of Gubbio, where the primary fault surface is well exposed forming a steeply-dipping 100 m high and~170 m wide wall. The GL was the focus of the detailed deformation fabric study of Bussolotto et al. (2007) due to the excellent exposure of the damage zone along a road cut that traverses a contiguous transect and extends from the hanging-wall basin sediments through to the damage zone and into the relatively undeformed footwall. At the GL, the exposed footwall includes a substantial portion of the Umbria-Marche carbonate succession representing the upper early Cretaceous to the Eocene section (Marne a Fucoidi, Scaglia Bianca, and Scaglia Rossa formations). This exposed section allows for comprehensive measurement and sampling of the varying styles of damage within these different lithologies, including calcite vein systems and fracture geometries.
The Mocaiana locality (ML) is located approximately 10 km NW of the GL near the northwesternmost surface termination of the GuF. The ML is in a limestone quarry that affords excellent exposure along an approximately 1.1 km transect of the footwall but is limited to the Late Cretaceous Scaglia Group (Scaglia Bianca and Scaglia Rossa) portion of the Umbria-Marche succession involved in the GuF damage zone. At both locations, exposed fault zone tectonites are characterized by foliated gouge through breccia bearing abundant sigmoidal brittle S-C fabrics, and subsidiary fault planes oriented parallel and conjugate to the primary fault surface ( Figure 2).

MATERIALS AND METHODS
At the GL and ML localities, structural and kinematic data were acquired from multiple outcrops ( Table 1). At the GL, samples were taken from calcite veins and slicken-lines belonging to the main fault surface and from secondary main-fault-related subsidiary fault planes within the damage zone. Samples from the ML were taken mostly from calcite veins and slicken-lines hosted in the Scaglia Rossa Formation along the main fault plane. Samples were at least 1 cm 3 in size with most between 4 and 7 cm in width to allow for enough material to be used for thin section and clumped isotope analyses.

Laboratory Analyses
We analysed 16 thin sections from two well exposed portions of the GuF ( Figure 1A) described in the literature Bussolotto et al., 2007), using cathodoluminescence (CL) for texture. Of these, 7 samples were analyzed for Δ 47 geothermometry ( Table 1). Temperature estimates based on calcite twin morphology were assessed qualitatively following criteria defined by Burkhard (1993) and additional observations by Ferrill et al. (2004), and are discussed in comparison with Δ 47 geothermometry data.

Cathodoluminescence
Cathodoluminescence reveals textures not readily visible in plain or cross polarized light with colors and luminescence that are dependent on the distribution of trace minerals within a crystal grain (Pagel et al., 2000). Calcite luminescence is controlled by the distribution of Fe and Mn, which quench and increase luminescence, respectively (Pierson, 1981). Consequently, variation of luminescence in precipitated calcite vein-fill has been interpreted as the result of variable fluid source composition (e.g., Bussolotto et al., 2007). All CL analyses were conducted on a RELION cathodoluminescence microscopy system attached to an Olympus BX-51 petrographic microscope in the Department of Geological and Environmental Sciences at Appalachian State University (ASU). Image acquisition was conducted under 35 millitorr vacuum pressure, 8 kV acceleration voltage, and 0.5 mA beam current, and were processed using Olympus Streamstart 2.3.3 software.

Clumped Isotopes
The carbonate clumped isotope (Δ 47 ) thermometer is based on the theoretical temperature dependence of the abundance of multiply substituted isotopologues containing both 13 C and 18 O in excess of the stochastic distribution within the solid carbonate phase, which is independent of the isotopic composition of the fluids in which the mineral precipitated. The relative enrichment of clumped isotopologues measured in mass-47 CO 2 (primarily 13 C 18 O 16 O) derived from carbonate is known as the Δ 47 value, and varies with the growth temperature of the sampled carbonate (Ghosh et al., 2006). Thin sections were microsampled and powdered samples were analyzed (δ 18 O, δ 13 C, Δ 47 ) in the Paleo 3 Laboratory at North Carolina State University (NCSU). Samples (500-1,500 µg) were digested with phosphoric acid (specific gravity 1.95 g/ml) at 70°C on a Nu Carb automated carbonate device, and resultant CO 2 was cryogenically separated and passed through a Porapak Q trap at -30°C. CO 2 was automatically transferred via dual inlet to a Nu Perspective IS isotope ratio mass spectrometer configured to measure m/z ratios for masses 44-49, which produced δ 18 O, δ 13 C, and Δ 47 values reported relative to the Vienna Peedee Belemnite (VPDB) and the absolute reference frame/Intercarbcarbon dioxide equilibrium scale (ARF/I-CDES; Dennis et al., 2011;Bernasconi et al., 2021). ETH (ETH-1, ETH-2, ETH-3, ETH-4; Bernasconi et al., 2018) solid standards were run concurrently, and data were processed using Easotope software (John and Bowen, 2016) corrected with 70°C acid fractionation (Petersen et al., 2019) and Brand et al. (2010) 17 O correction parameters. Replicates with Δ 48 measurements exceeding ±2‰ were rejected due to contamination by organics/hydrocarbons, and a Pierce Outlier test removed statistical outliers (e.g., Huntington et al., 2009). Standard error was measured using a 95% confidence  Burkhard, 1993;Ferrill et al., 2004), and clumped isotope temperatures are provided. Calcite twin types are grouped morphologically and annotated as follows: Type I and II twins are grouped together as "I/II", Type III twins are separated and denoted as "III". The asterisk (*) indicates samples that were used for clumped isotope analysis.

Microstructural Observations
Observation of calcite twin textures allows qualitative estimation of deformation temperature (Burkhard, 1993;Ferrill et al., 2004). Burkhard (1993) describes four twin morphologies (Types I-IV, Burkhard, 1993) that can be correlated to temperature ranges between <170°C to >250°C (Ferrill et al., 2004).  Groshong (1972). Nonetheless, optically distinguishing between calcite twin type morphology provides a useful qualitative tool to estimate temperatures, particularly when coupled with additional thermometric techniques. Earlier heating during burial of the Umbria-Marche succession in the Apennine foreland likely resulted in calcite twin fabrics. In order to avoid these pre-existing fabrics we focused on deformed crystalline calcite filling veins associated with the damage zone of the GuF described in detail by Bussolotto et al. (2007), which is readily distinguishable from the fossiliferous microcrystalline calcite of the host lithologies.

Field Description
At both sampled locations (GL and ML), the bedding strikes NW, with medium to high density fracturing near areas where distinct brittle-ductile features are observed, such as sigmoids with S-C planes and fault breccia ( Figure 2; Table 1). The predominant fabric is NW-SE trending veins with thicknesses less than 1 cm, comparable to domain Dc-1 in Bussolotto et al. (2007). We also distinguish: 1) E-W trending veins with mostly millimeter scale thickness, and a curvilinear geometry terminating against bedding planes; and 2) NE-SW trending fractures with mm-cm scale thickness which are crosscut by the previously described fracture sets. Stylolites that are predominantly parallel or subparallel to bedding, and likely related to burial, are ubiquitous within and outside of the GuF damage zone and have peak heights up to 5 mm. Stylolites associated with tectonic deformation are less common at the outcrop scale but are present along the edges of the sigmoidal foliation fault gouge structures.

Mineral Assemblage
Calcite is the dominant mineral in the clast, matrix, and vein material within all samples, which is consistent with the near ubiquity of limestone in the Umbria-Marche succession. Accessory minerals consisted of 1-10 μm scale baryte, pyrite, and apatite, with very rare zircon and titanite. Clay minerals, predominantly illite, were undetectable except when concentrated as insoluble material along stylolitic surfaces.

Microstructural Observations
Calcite textures from vein material in the GuF contain a range of twinning types (Types I -III; Burkhard, 1993;Ferrill et al., 2004), with most of the samples showing textures of more than one twin type. Each sample exhibits at least faint Type I fabrics, with seven samples strongly exhibiting textures indicative of temperatures >170°C (Type II or greater), and three samples exhibiting textures indicative of temperatures >200°C (Type III fabrics; Figure 3 and Table 1). Cross-cutting features are also evident: Type I twins are consistently cut by Type II and Type III twins in every sample where these types are present. Type II twinning patterns are cut by different generations of Type II, as well as Type I and Type III twinning patterns. In all samples where Type III calcite twinning patterns are found, Type III twins crosscut Type I and Type II; however, we only rarely observed the opposite relationship where Type III twins are crosscut by either Type I or II twins.

Stable and Clumped Isotopes
Stable and clumped isotope analyses were conducted simultaneously on 7 samples from within the damage zone of the GuF at both the GL and ML localities ( Table 1). Bulk carbon isotope (δ 13 C) values ranged from +2.2 to +2.5‰ (VPDB) and oxygen isotope (δ 18 O) values ranged from 0.4 to 8.1‰ (VPDB) ( Figure 4A). Carbonate clumped isotope (Δ 47 ) values ranged from 0.468 to 0.553‰ (I-CDES), corresponding to temperature estimates of 87-154°C ( Figure 4B). Calculated source fluid oxygen isotope (δ 18 O sf ) values ranged from +5.3 to +20.5‰ (VSMOW). While there is no apparent trend between bulk isotopic composition and texture or temperature ( Figure 4A), clumped isotope temperatures do correlate with textural type (and therefore calcite-twinning temperature ranges) and show a trend of substantial δ 18 O sf enrichment with higher temperatures ( Figure 4B), in general agreement with the presence of elevated temperatures as indicated by calcite twinning textures.

Fluid Temperature
Temperatures determined from clumped isotopes are in the range of 87-154°C (Table 1). Our calcite twinning observations, indicate temperatures in the same range or slightly higher (Figure 4). We observe fabrics consistent with temperatures >170°C in all but five samples. However, all samples that contain high temperature textures (Types II-III; Burkhard, 1993) also contain lower temperature fabrics; for example, sample GU-3 has clear Type III fabrics, but also contains Types I and II twins (Figure 3). Lower-temperature (<170°C) fabrics are not completely recrystallized but are often crosscut by later vein generations exhibiting higher temperature fabrics (Figure 3). We interpret them as multiple generations of deformation through a thermally evolving or dynamic system. Clumped isotopes, calcite twin textures, and fluid inclusion geothermometric methods all record distinctly different In contrast, the micro-milling procedure for clumped isotopes in this study likely combines calcite from multiple precipitation phases spanning the entire GuF deformation history. Moreover, solid state reordering of calcite vein material over multiple deformation events may have lowered our temperature estimates following calcite precipitation (e.g., Bonifacie et al., 2012). Our optical and CL observations of multiple cross-cutting vein sets containing an array of deformation temperature fabrics (Table 1), and previous interpretations of cross-cutting deformation fabrics (Bussolotto et al., 2007) are consistent with both incorporation of multiple generations of calcite, and post-precipitation solid state isotopic reordering. The result of these processes is that our T (Δ 47 ) temperatures likely reflect some integrated contribution of multiple heating or fluid generations throughout the extensional exhumation of the GuF damage zone. In addition, the micro-milling process may have also introduced some degree of wall rock into our clumped isotope measurements. Previous estimates of maximum burial conditions (and hence temperature) experienced by the Umbria-Marche succession are in the range 80°-100°C as determined by clay mineral assemblage and vitrinite reflectance (Aldega et al., 2007;Corrado et al., 2010). Incorporation of these lower temperature carbonate materials would also reduce our clumped isotope-based temperature estimates; we therefore interpret these T (Δ 47 ) temperatures as minimum estimates of peak fluid temperature.
In contrast, our calcite twin observations suggest temperatures in excess of 170°C in most samples, which is higher than both the fluid inclusion estimates of Bussolotto et al. (2007), and our T (Δ 47 ) estimates. The co-dependence of calcite twin fabrics on strain could possibly result in overestimation of deformation temperatures via textural interpretation, particularly in high strain environments, and previous attempts to quantify this twinning-temperature relationship have relied on calcite deformation fabrics from samples situated away from shear zones (e.g., Ferrill et al., 2004). We therefore consider these texture-based estimates from the GuF damage zone rocks as high-end temperature values, limited by the absence of Type IV twins which have been observed to form at 250°C or above in natural samples (Groshong et al., 1984;Burkhard, 1993;Ferrill and Groshong, 1993). Our combined estimates, nonetheless, point to the abundance of higher temperature fluids, with the highest temperature fluids in the range of~150-250°C, which suggests a likely deep origin of overpressure fluids.

Fluid Source
We discuss three potential sources for fluids involved in slip and development of damage zone fabrics along the GuF that may be distinguished based on the temperature and isotopic compositions ( Figure 5). In Scenario 1 (pink, Figure 5) meteoric fluid enters the ATF breakaway zone, or percolates down through the hanging-wall blocks, and travels along the ATF fault zone. The ATF fault zone provides a potential pathway as it forms the boundary between metamorphic basement rocks and the sedimentary Umbria-Marche succession, and is likely hosted in a wide permeable damage zone associated with ATF low-angle fault plane. Fluids migrate down dip along the ATF detachment, encountering the GuF at approximately 4-5 km depth as imaged in the CROP-03 seismic line and interpreted by Mirabella et al. (2004), and subsequently travel up the GuF damage zone.
In Scenario 2 (blue, Figure 5) meteoric water percolates through the ATF/GuF hanging-wall, enters the GuF damage zone directly, and ascends along the fault. In both Scenarios 1 and 2, fluid stable isotopic compositions would reflect an open fluid system and a combination of conserved meteoric and host rock isotopic sources. Vein composition provides bulk isotopic (δ 18 O, δ 13 C) and calculated δ 18 O sf values which can be used to distinguish fluid sources; a meteoric fluid source would result in a conserved meteoric composition, or within the range of an FIGURE 3 | Photomicrograph of deformation vein calcite with twinning. (A) plain polarized light (PPL), (B) cathodoluminescence, (C) annotated PPL image. Panel C annotations include Type I and Type II twins in red, and Type III in blue, approximate grain boundaries are delineated in black. Following the convention of Burkhard (1993) Type I and II twins are relatively thin, straight lines seen in yellow, Type III twins are thick, curved, and tapered. Type I and II twins are cross cut by Type III twins, which rarely also contain straight parallel and relatively thick (>5 μm) tabular likely Type II twins.
Frontiers in Earth Science | www.frontiersin.org February 2022 | Volume 10 | Article 811339 intermediate to evolved composition generated via interaction with bedrock (e.g., Hodson et al., 2016;Luetkemeyer et al., 2016). An intermediate to evolved fluid source would be characterized by 1) δ 13 C values between surface water, or shallowly degassed CO 2 (a more depleted fluid source; Chiodini et al., 2004), and a host-rock composition ( Figure 4A), and 2) δ 18 O values evolved via temperature and water-rock interactions in an open system (e.g., Luetkemeyer et al., 2016) from a meteoric composition ( Figure 4B). In Scenario 1, in which meteoric water penetrates to the depth of the GuF and ATF damage zone intersection, the isotopic signature would likely reflect a more evolved composition as isotopic exchange with the host rock would be assisted by an increased temperature at depth. Whereas in Scenario 2, we would expect a maximum temperature corresponding to no more than about 3-4 km (T < 100°C), which is lower than the temperatures measured by clumped isotopes and estimated by calcite twin observations. Scenario 3 (orange, Figure 5), in contrast to Scenarios 1 and 2, includes a closed external deep fluid source rather than a local open meteoric source. In Scenario 3, fluid either originates from down dip along the ATF and migrates up along the detachment fault, or simply from the ATF footwall. In this scenario, precipitated vein calcite from fluids migrating up the GuF would record a hydrothermal/metamorphic stable isotopic composition and likely elevated temperatures in the clumped isotope (T > 100°C) and calcite twinning signatures (Type II and greater). Based on likely meteoric (e.g., IAEA/WMO, 2015) and other surface or bedrock sources for this region (e.g., Bussolotto et al., 2007), measured vein compositions and calculated δ 18 O sf values, as well as interpreted temperatures (Figure 4), do not fit predictions from either Scenario 1 nor Scenario 2. Instead, our results strongly support a deep and externally-sourced fluid (Scenario 3, Figure 5).
Both thermometry techniques yielded fluid temperatures that are consistent with or exceed likely ambient temperature at depth for the GuF (Scenario 1, Figure 5) or for the ATF between the breakaway zone and intersection with the GuF (Scenario 2, Figure 5). Our lowest T (Δ 47 ) temperature estimates are between 87 and 109°C, while our higher estimates range from 128-154°C. As previously discussed, these values are likely underestimated due to the incorporation of lower temperature wall rock and are substantially lower than estimates based on calcite twin fabrics. Using the published geothermal gradient (20°C/km; Mongelli et al., 1989), and assuming no cooling of the fluid during migration, the minimum depth of the fluid source is approximately 6-7 km. Finally, stable isotope ratios are incompatible with any combination of meteoric (IAEA/WMO, 2015) or local bedrock sources, but are consistent with a hydrothermal or metamorphic source (e.g., Della Porta, 2015; Scenario 3, Figure 5). Our results agree with a deeper, nonmeteoric fluid source for GuF vein fill as also previously suggested in the same area by other authors on the basis of CO 2 flux measurements (c.f., Chiodini et al., 2004), and isotopic signature (Bussolotto et al., 2007).
This deeply-sourced external fluid could come from a few different regionally-important mechanisms: 1) from the dehydration of clay minerals which can occurs in the observed range of temperatures in fold and thrust belts (e.g., Meneghini et al., 2012); or 2) from deep fluids, for instance CO 2 , originating from mantle degassing which ascends through the interconnected network of fractures associated with structural deformation around complex tectonic provinces. During their upward FIGURE 4 | Bulk isotopic composition (A) and carbonate clumped isotope temperatures (T [Δ47]) versus calculated fluid composition (δ18Osf) (B) of sampled carbonates and regional sources. Colors indicate corresponding twin type (Type I and Type II; red, Type III; blue). The Scaglias, Maolica, Massiccio, and Burano fields are compositions of local marine carbonate (5. Umbria-Marche carbonates, Figure 1A; e.g., Morettini et al., 2002;Bussolotto et al., 2007) and evaporite (6. Evaporites, Figure 1A; e.g., Lugli, 2001)  migration, these fluids are likely to be entrapped in broad structural barriers like the ATF as previously suggested by other authors (Collettini and Barchi, 2002;Chiodini et al., 2004).

Overpressured Fluids
Deeply-sourced ATF/GuF fluids dominate the isotopic signature, and decrepitation of fluid inclusions (Bussolotto et al., 2007) and calcite-filled hydraulic fractures (Bullock et al., 2014) indicate that fluid pressure can exceed local σ 3 . Although hydraulic fracturing in the shallow crust (>3 km) along Andersonian extensional faults (vertical fractures, normal faults dipping between 58°a nd 68°) can be attributed to lower fluid pressures (Sibson, 2000), the sustained presence of overpressured fluids at greater depth in the ATF system is apparent in model results and geologic observations. Collettini and Barchi (2002) provide a mechanical model for localized slip on the ATF which relies upon the pressure condition where pore-fluid pressure, Pf, exceeds σ 3 , attained by entrapment of deep-seated crustal fluids. CO 2 fluid pressure measured in the S. Donato exploration well (Figure 1 and Figure 5) and the Pieve S. Stefano well (50 km NW of S. Donato), which drilled into the ATF footwall, recorded fluid overpressures of~100 and 70 MPa at depths of 5 and 4 km respectively, corresponding to pore fluid pressures of~80% of the lithostatic overburden (Collettini et al., 2006). Craddock et al. (2020) sampled calcite veins present in the GuF footwall in the same GL locality and specifically in the Gubbio fault core (corresponding to our sample GU-15) and determined 230 Th-234 U-238 U ages of 357 ka and 235 ka (Craddock et al., 2020). Their results indicate the presence of recent syn-faulting calcite veins in the Gubbio fault core, suggesting that overpressured fluids have likely been present throughout the Quaternary evolution of the GuF-ATF system.

Tectonic Implications
Deeply sourced fluids play an important role in shallow faulting in the Apennines foreland accretionary prism, and the presence of these fluids are recorded in thrust faults exhumed in the extensional hinterland (e.g., Meneghini et al., 2012). Calcite vein fill within the shear zone of the Monte Rentella thrust fault, part of the Umbria-Tuscan domain of the Apennines fold and thrust belt active during Miocene time (Barsella et al., 2009), records an isotopic signature distinct from the host rock, and consistent with a diagenetic or low-grade metamorphic fluid source from deep structural levels in the accretionary prism (Meneghini et al., 2012). Calcite twin fabrics from within that shear zone display Type II twins, which Meneghini et al. (2012), following Burkhard (1993), interpret as indicative of deformation temperatures between 150 and 200°C. These damage zone temperatures are inconsistent with lower regional peak temperatures, which these authors determine to be between 60 and 110°C based on vitrinite reflectance and clay mineral analysis. Meneghini et al. (2012) interpret the disparity between damage zone fluid temperatures and regional peak temperatures as evidence of hot fluid pumping from deep structural levels within the accretionary prism, and influencing near surface faulting.
Subduction drives burial, diagenesis, and metamorphism in the Northern Apennines, and presents a likely source for fluids, which Meneghini et al. (2012) interpret as migrating along permeable fault zones. The close coupling and proximity of active extensional structures with foreland thrust faulting implies that the same fluid source may also play a significant role in extension in the Apennines. The east dipping active ATF is situated within the transition zone between the accretionary foreland and the extensional hinterland, and can be resolved in seismic reflection data to the brittle ductile transition at approximately 13 km, penetrating the thickened crust of the Apennines accretionary prism (Pauselli et al., 2006). The geometry of the ATF thus provides a spatial link between the GuF and sediments undergoing burial diagenesis and metamorphism in the accretionary prism. Closer to the surface, two boreholes drilled through the ATF encountered elevated CO 2 pressure at the ATF footwall; in contrast, the Civitello borehole ( Figure 5) sampled the deep ATF hangingwall did not yield elevated fluid pressures. These contrasting borehole-determined fluid pressures from above and below the detachment suggest that the rocks bounding the ATF are impermeable, forming a fluid migration pathway along the damage zone. Our observations of elevated fluid temperatures, and previous observations of closed source fluids (e.g., Bussolotto et al., 2007) within the GuF suggest that this fault and possibly other epidetachment faults act as conduits for fluid migrating along the ATF from the subduction and related dehydration or metamorphic reactions. Our conceptual model of the migration path of fluids derived in the accretionary prism emerging to the near surface in the extensional hinterland ( Figure 5), is a mirror image of the interpretation of Meneghini et al. (2012) but applied to the proximal Apennines extensional hinterland. The tectonic implications of this interpretation are that subduction, and concomitant metamorphism and diagenesis, exert a spatial control on near surface seismicity and the style of extension in the hinterland. Seismicity within the ATF hanging-wall is largely concentrated along faults located closest to the foreland, such as the GuF (Chiaraluce et al., 2017); in contrast, older epidetachment faults (e.g., Corciano fault; Figure 1B) are situated far from the source of subduction-related metamorphic fluids and are less active. On a larger scale, our interpretation implies that proximity to the subduction zone moderates the distribution of low-angle normal faulting, which Sibson (1985) suggests is viable provided sufficient fluid pressures. NE migration of large-scale low angle extensional structures such as the ATF has persisted since the Miocene, migrating in tandem with the retreating subduction front, leaving exhumed inactive structures which become progressively older towards the hinterland (Collettini and Holdsworth, 2004). The ATF is the youngest and only active LANF in the Northern Apennines and is the closest to the Apennine foreland; we suggest that its current microseismic activity (Chiaraluce et al., 2007;Chiaraluce et al., 2014) is therefore owed to its proximity to the source of subduction-related fluids.

CONCLUSIONS
Field and micro-scale calcite vein fabrics within the Gubbio Fault damage zone record the pervasive role of fluids during faulting. Our clumped/stable isotopic measurements and microstructural observations are inconsistent with a near-surface source of these fluids. We calculate that fluid temperatures were in some places in excess of 150°C and show that fluid compositions are inconsistent with fluid-rock interaction for any of the lithological units in the ATF hanging-wall, but are instead compatible with a deep external source. We therefore interpret these fluids as deeply sourced fluids and suggest that the structurally-connected ATF is a likely conduit, providing a pathway for fluids derived from processes associated with the nearby Apennine subduction zone. Previous radiometric crystallization ages and borehole fluid pressure measurements suggest that these fluids have persisted within this system throughout at least Quaternary time. This interpretation implies that the proximity of the subduction zone is a key factor in driving low-angle normal faulting and associated epidetachment faulting in the northern Apennines, a conclusion supported by the geologic record of NE migrating low angle normal faulting in the wake of the retreating Apennines subduction zone from the early Miocene to the Present.

DATA AVAILABILITY STATEMENT
The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: EarthChem database doi:10.26022/IEDA/111720.

AUTHOR CONTRIBUTIONS
All authors participated in the field study and collected samples, and all contributed to manuscript discussions. HR carried out calcite texture analysis and contributed to writing. GC gave guidance for calcite textures and contributed to writing. FM provided regional expertise and contributed to writing. EH carried out clumped/stable isotope analyses and contributed to writing. LT carried out initial field and microstructural studies.

ACKNOWLEDGMENTS
Clumped isotope data is available under the EarthChem database 10.26022/IEDA/111720. Graduate support provided by the University of Perugia. We thank S. Cirilli (Perugia University) for helpful suggestions. We thank two reviewers for their thoughtful comments which helped us improve the article, and Associate Editor Andrea Billi for editorial support.