Alpine Fault Research Articles

Rupture direction of paleoearthquakes on the Alpine Fault, New Zealand, as recorded by curved slickenlines

We observed and further exhumed curved slickenlines on fault planes associated with paleo-surface rupture of the Alpine Fault, New Zealand’s ∼30 mm/yr continental transform plate boundary. Dynamic rupture modeling indicates that the geometry of such curvature provides a record of past earthquake rupture directions. We focused our efforts on three sites that span a region known to variably halt or allow passage of past earthquakes (an “earthquake gate”) to contribute rupture direction constraints to the fault’s spatiotemporally rich paleoseismic record. At Hokuri Creek and Martyr River, we observed both convex-up and convex-down curved slickenlines on and adjacent to principal slip surfaces, indicating past ruptures from both the northeast and southwest of these locations. At Martyr River, relationships suggest that the most recent event (inferred to correlate to 1717 CE) ruptured from the southwest. Our results demonstrate the utility of curved slickenlines as a valuable new paleoseismological tool for determining past rupture directions, applicable to surface-rupturing faults globally.

Unloading Uplift Caused by Surface Processes in New Zealand's Southern Alps

AbstractThe Southern Alps experiences rapid bedrock uplift and intense surface processes like erosion and deglaciation. We quantify how the erosion and deglaciation contribute to the ongoing vertical motions using geophysical models. The erosional unloading uplift is found to be 0.5–1.5 mm/yr throughout the central Southern Alps, whereas the recent deglaciation may locally produce uplift up to 1–3 mm/yr. The estimated unloading uplift accounts for 10%–40% of the GNSS‐observed uplift. After correcting the unloading uplift, the GNSS‐observed uplift can be explained by about 4–6 mm/yr dip‐slip motion on the Alpine fault, which is 10%–50% below previous geodetic estimates. Hence, unloading uplift must be evaluated when interpreting geodetic observations in tectonically active mountain ranges subjected to intense surface processes.

Sedimentary Characteristics and Evolution of the Late Miocene to Quaternary Tributary Channels in the Head of Bounty Channel, New Zealand

The Bounty Channel is a large-scale submarine channel system located in the eastern continental margin of New Zealand. Extending along the axis of the Bounty Trough, the channel system comprises three main tributaries (C1–C3) at its head, which merge downstream into a trunk channel leading to a terminal submarine fan. In this study, we use high-quality two-dimensional multichannel seismic data to investigate the formation and evolution of tributary channels C1 and C2. Four types of seismic facies are identified in the tributary channels: fill-type, mounded divergent, wavy, and subparallel facies. These seismic facies are correspondingly interpreted as topographic depression or channel fills, levees, sediment waves, and hemipelagic deposits. The Late Miocene tributary channels were developed above a pre-existing NE–SW-oriented depression. The Pliocene to Quaternary tributary channels are characterized by preferential development of higher levees on their left hand, and the presence of sediment waves on the lower levees of their right-hand, signaling an effect of the Coriolis force. The formation and evolution of the tributaries are primarily linked to regional tectonics, including increased convergence rate between the Pacific and Australian plates along the Alpine Fault in the Late Miocene and enhanced uplift and erosion at the Southern Alps during the Pliocene.

Velocity and Temperature Dependence of Steady‐State Friction of Natural Gouge Controlled by Competing Healing Mechanisms

AbstractThe empirical rate‐ and state‐dependent friction law is widely used to explain the frictional resistance of rocks. However, the constitutive parameters vary with temperature and sliding velocity, preventing extrapolation of laboratory results to natural conditions. Here, we explain the frictional properties of natural gouge from the San Andreas Fault, Alpine Fault, and the Nankai Trough from room temperature to ∼300°C for a wide range of slip‐rates with constant constitutive parameters by invoking the competition between two healing mechanisms with different thermodynamic properties. A transition from velocity‐strengthening to velocity‐weakening at steady‐state can be attained either by decreasing the slip‐rate or by increasing temperature. Our study provides a framework to understand the physics underlying the slip‐rate and state dependence of friction and the dependence of frictional properties on ambient physical conditions.

Dynamical Modeling of Fault Slip Rates at the New Zealand Plate Boundary Indicates Fault Weakness

AbstractWe construct a thin‐sheet dynamical model of the New Zealand plate boundary that includes faults. Our model fits fault slip rates, style of distributed deformation, and is constrained by relative plate boundary motion. We assume a pseudo‐plastic rheology and achieve a best fit to slip rate observations with a deviatoric stress magnitude of 20 MPa. Modeled local forces are significant at Puysegur and Hikurangi subduction zones, and smaller forces are related to mantle downwelling beneath South Island and Havre Trough mantle upwelling. Modeled tractions on faults are mostly 5–20 MPa, similar to or slightly smaller than stress magnitudes adjacent to faults. Modeled shear tractions are generally 2–10 MPa, comparable to stress drops during earthquakes. Modeled stress orientations and fault dips suggest that many faults are not optimally oriented for their style of faulting. Notably small traction magnitudes of <5 MPa and shear tractions of <0.5 MPa are modeled for faults in the central North Island Dextral Fault Belt (NIDFB), which we infer to be very poorly oriented. Friction coefficients on faults (ratio of shear stress to effective normal stress) are in the range 0.1–0.3 for major crustal faults such as the Alpine Fault and Marlborough faults, but subduction zones and the NIDFB have values <0.1. We propose that low values of long‐term fault strength, shear stress resolved onto the fault, and overall magnitudes of deviatoric stress in the crust are a consequence of dynamic weakening of faults during fault slip.

Mega‐Folding of a Basement During Incipient Intra‐Plate Continental Subduction (Alpine Central Iberia)

AbstractMetamorphic basements are usually considered rigid and isotropic at a large scale. However, basements contain inherited weaknesses that may potentially accommodate superimposed contraction (e.g., fault reactivation), and that favor fold nucleation (e.g., penetrative foliations). If these conditions are met, what could be the factors that impede the development of basement folds or their recognition? Actual basement folding is rarely documented, especially for large dimensions. Here we provide a case example, discussed from the perspective of structural analysis of surface data and sustained by geophysical data. The basement of the Spanish‐Portuguese Central System is defined by an Alpine mega‐fold (Hiendelaencina Antiform) that trends parallel to this mountain range and affects the basement and its sedimentary cover, collectively. The wavelength of this fold matches or even surpasses the thickness of the crust that hosts it (36–41 km). The Moho under this mega‐fold is displaced by an Alpine fault that accounts for incipient intraplate continental subduction. The topography of the mantle may reflect an up‐warping compatible with the mega‐fold observed on the surface. Mega‐folding is observed in the hanging wall of the Berzosa Fault, which emerges as a SE‐dipping, Variscan (Paleozoic), extensional fault reactivated as a basal decollement upon Alpine (Cenozoic) contraction. The mega‐fold was formed after well‐oriented planar anisotropies in the basement (foliation and bedding). The development of this fold was assisted by heterogeneous shearing (coeval thrusting) plus the buttressing effect of pre‐existing, near‐vertical, crustal‐scale faults (Somolinos and Somosierra), which inhibited slip‐upsection through the basal decollement (Berzosa Fault).

Inverting Geodetic Strain Rates for Slip Deficit Rate in Complex Deforming Zones: An Application to the New Zealand Plate Boundary

AbstractThe potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike‐slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike‐slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting.

Geodetic Strain Rates for the 2022 Update of the New Zealand National Seismic Hazard Model

ABSTRACT Geodetic data in plate boundary zones reflect the accrual of tectonic strain and stress, which will ultimately be released in earthquakes, and so they can provide valuable insights into future seismic hazards. To incorporate geodetic measurements of contemporary deformation into the 2022 revision of the New Zealand National Seismic Hazard Model 2022 (NZ NSHM 2022), we derive a range of strain-rate models from published interseismic Global Navigation Satellite Systems velocities for New Zealand. We calculate the uncertainty in strain rate excluding strain from the Taupō rift–Havre trough and Hikurangi subduction zone, which are handled separately, and the corresponding moment rates. A high shear strain rate occurs along the Alpine fault and the North Island dextral fault belt, as well as the eastern coast of the North Island. Dilatation rates are primarily contractional in the South Island and less well constrained in the North Island. Total moment accumulation derived using Kostrov-type summation varies from 0.64 to 2.93×1019 N·m/yr depending on method and parameter choices. To account for both aleatory and epistemic uncertainty in the strain-rate results, we use four different methods for estimating strain rate and calculate various average models and uncertainty metrics. The maximum shear strain rate is similar across all methods, whereas the dilatation rate and overall strain rate style differ more significantly. Each method provides an estimate of its own uncertainty propagated from the data uncertainties, and variability between methods provides an additional estimate of epistemic uncertainty. Epistemic uncertainty in New Zealand tends to be higher than the aleatory uncertainty estimates provided by any single method, and epistemic uncertainty on dilatation rate exceeds the aleatory uncertainty nearly everywhere. These strain-rate models were provided to the NZ NSHM 2022 team and used to develop fault-slip deficit rate models and scaled seismicity rate models.

Comparison of Ground-Shaking Hazard for Segmented versus Multifault Earthquake-Rupture Models in Aotearoa New Zealand

Abstract Multifault ruptures are common for historical earthquakes, and here we consider their impact on seismic hazard. We compare ground-shaking hazard forecasts from the 2022 Aotearoa New Zealand National Seismic Hazard Model (NZ NSHM 2022), which incorporates many multifault ruptures (referred to as the multifault model) with modeled hazard from a simpler model of characteristic earthquakes on individual faults or fault segments (referred to as the segmented model). The multifault model includes very-low-probability rupture lengths of up to ∼1100 km and a mean of 221–234 km, whereas the segmented model primarily comprises rupture lengths of <200 km (mean, 43–51 km) and the maximum of 414 km. The annual rates of Mw 6.9–7.5 earthquakes are more than an order of magnitude higher for the segmented model (0.132–0.24/yr; recurrence times ∼4–7 yr) than the multifault model (0.027/yr; recurrence times 37 yr). Conversely, the rates of earthquakes are similar for segmented and multifault models at Mw>7.5 (0.018–0.031/yr; recurrence times 32–56 yr). Despite differences in rupture lengths and annual rates of earthquakes, the calculated ground-shaking hazard at 10% probability of exceedance (PoE) in 50 yr for the segmented model differs by <55% compared with the multifault model for 95% of sites across Aotearoa New Zealand. For 50% of sites, the modeled hazard differs by <20% between the two models. If a distributed seismicity model (DSM) is included in the hazard calculations, 95% of sites differ in modeled hazard by <18%, and 50% of sites differ by <2.2%. In most areas, seismic hazard at 10% PoE in 50 yr is greater for the segmented model than the multifault model, with notable exceptions along the central Alpine fault in the western South Island and the Taupō volcanic zone in the central North Island.

A window into thousands of earthquakes: Results from the Deep Fault Drilling Project (DFDP)

In 1941, two geologists, Harold Wellman and Richard Willett, traversed the length of Westland, mapping what would become one of the most influential continental faults in the world, New Zealand’s Alpine Fault (Wellman & Willet 1942). The Alpine Fault strikes down the western edge of the Southern Alps, a youthful mountain range on New Zealand’s South Island (Fig 1). Here, collision between the Australian and Pacific Plates forms peaks over 3000 m in elevation which trap rain-laden clouds, resulting in 5–15 m of precipitation a year in central and south Westland. Driven by gravity, the rain and snow migrate into fractures and voids along the Alpine Fault, becoming heated and saturated with reactive ions along the way. Within the fault zone, these fluids play a fundamental role in the processes that drive earthquake nucleation and rupture propagation. Measurements made, and rocks recovered, from boreholes drilled during phases one and two of the Deep Fault Drilling Project (DFDP) have enabled scientists to document and quantify these processes for the first time.

The Development of Fault Networks at the Termination of Continental Transform Faults When Their Connecting Plate Boundary Is “Misaligned”

AbstractWe report the results of a series of scaled laboratory experiments that investigate the development of fault systems in plate boundary transition zones, where deformation is distributed between a continental transform fault (e.g., Alpine Fault, New Zealand; North Anatolian Fault (NAF), Turkey; San Andreas Fault (SAF), USA) and its connecting plate boundary. In these transition zones, continental transform faults are observed to branch into multiple subsidiary faults. Here we show that large‐scale transition zone fault networks comprise crustal‐scale Riedel shears that develop sequentially outwards and away from the parent transform fault. Such fault networks form within brittle upper crust that overlies a ductile lower crust that deforms by large‐scale distributed simple shear. We argue that large‐scale distributed deformation of the lower crust occurs when continental transform faults are “misaligned” relative to their connecting plate boundaries. This misalignment may occur: (a) where a transform fault does not directly connect with a convergent or divergent plate boundary, as in the northern termination of the Alpine Fault and the western termination of the NAF; and (b) where a significant bend in the transform fault occurs in the plate boundary transition zone, as in the southern termination of the SAF. The development of such plate boundary misalignments appears to occur when a plate boundary transition zone develops as a continental transform fault propagates toward a different type of “connecting” plate boundary.

Constitutive Behavior of Rocks During the Seismic Cycle

AbstractEstablishing a constitutive law for fault friction is a crucial objective of earthquake science. However, the complex frictional behavior of natural and synthetic gouges in laboratory experiments eludes explanations. Here, we present a constitutive framework that elucidates the rate, state, and temperature dependence of fault friction under the relevant sliding velocities and temperatures of the brittle lithosphere during seismic cycles. The competition between healing mechanisms, such as viscoelastic collapse, pressure‐solution creep, and crack sealing, explains the low‐temperature stability transition from steady‐state velocity‐strengthening to velocity‐weakening as a function of slip‐rate and temperature. In addition, capturing the transition from cataclastic flow to semi‐brittle creep accounts for the stabilization of fault slip at elevated temperatures. We calibrate the model using extensive laboratory data on synthetic albite and granite gouge, and on natural samples from the Alpine Fault and the Mugi Mélange in the Shimanto accretionary complex in Japan. The constitutive model consistently explains the evolving frictional response of fault gouge from room temperature to 600°C for sliding velocities ranging from nanometers to millimeters per second. The frictional response of faults can be uniquely determined by the in situ lithology and the prevailing hydrothermal conditions.

A tectonic reconstruction model for Aotearoa-New Zealand from the mid-Late Cretaceous to the present day

ABSTRACT We present a mid-Late Cretaceous to present day tectonic reconstruction model for Aotearoa-New Zealand. Our GPlates model comprises 50 rigid crustal blocks grouped into regions with common deformation histories set within a well-defined Australia-Pacific-Antarctica plate circuit tied to a published global paleomagnetic absolute reference frame. Within the model, four distinct periods of deformation are recognised from both near- and far-field observations. A key model assumption is the continuity of basement terranes between North and South Zealandia prior to Middle Eocene rifting and Late Oligocene initiation of transform motion. To complement the rigid crustal block model, continuously closing polygons show a ∼25% decrease in plate boundary area since the Middle Eocene that has been compensated for by corresponding increases in crustal thickness. A kinematic fault-propagation fold model demonstrates the plausibility of post-Late Oligocene asymmetric oroclinal folding on both sides of the evolving transform boundary. ‘Missing’ areas of map section common to previous tectonic reconstructions can be reconciled through contraction, elongation and vertical-axis rotation of continental crust within the deformation zone ahead of northward propagation of the Alpine Fault. This tectonic reconstruction model provides an open, accessible, and testable foundation for current and future paleogeographic and tectonic studies across Zealandia.

Islands of chaos in a sea of periodic earthquakes

Long paleoseismic records on mature faults suggest potentially chaotic recurrence patterns with cycles of strain accumulation and release that challenge simple slip- or time-predictable recurrence models. In apparent contradiction, the relatively small variability of earthquake recurrence times on these faults is often characterized as quasi-periodic, implying much regularity in the underlying mechanics. To reconcile these observations, we simulate one of the longest paleoearthquake records – the 24-event record from the Hokuri Creek site on the Alpine fault in New Zealand – using a physical model of rate- and state-dependent friction. In a parameter space formed by three non-dimensional parameters, a sea of parameters produces periodic earthquake recurrence behavior. Only a few models are characterized by fundamentally aperiodic recurrence patterns, in parametric islands of chaos. Complex models that produce partial and full ruptures of the Alpine fault can explain the earthquake recurrence behavior of the Alpine fault, reproducing up to 11 consecutive events of the Hokuri Creek paleoseismic record within uncertainties. The breakdown of the slip- and time-predictable recurrence patterns occurs for faults that are much longer than the characteristic nucleation size. The quasi-periodicity of seismic cycles is compatible with the nonlinear and potentially chaotic underlying mechanical system, posing an inherent challenge to long-term earthquake prediction.

Superimposed brittle structures in polyorogenic contexts: Variscan and Alpine faults in the Duje Valley (Picos de Europa, Cantabrian Mountains, NW Spain)

Abstract Detailed geological mapping of a highly fractured Paleozoic (pre-Permian) basement and the scarce outcrops of overlying Permian–Mesozoic cover in the surroundings of the Duje Valley (Picos de Europa Unit, Cantabrian Mountains, NW Spain), together with new field data, have allowed the separation of four genetic fault sets in a polyorogenic area, affected by the Variscan and Alpine cycles. These fault sets are, from oldest to youngest: Variscan thrusts (late Carboniferous), late Variscan strike-slip faults (late Carboniferous–earliest Permian), Alpine normal faults (Permian–Mesozoic) and Alpine reverse faults (Cenozoic). A structural analysis is reported here, based on the joint use of geometric, kinematic and deformational features, cross-cutting and tectono – sedimentary relationships between the structures. This analysis has allowed the recognition and full characterization of the four fault sets. These types of structural analyses are useful for unravelling complex tectonic histories in regions where massive limestone lithologies make reconstructing the timing of fault activity difficult, especially if the basement is affected by late deformation events that are not recorded by cover outcrops.

The effect of temperature on the nonlinear elasticity of a fault rock in dynamic acoustoelastic testing (DAET) experiments

Summary Dynamic nonlinear elasticity in rocks may play an important role in earth processes, such as earthquake nucleation. In order to understand how nonlinear elasticity occurs within the shallow crust, experiments are required that simulate the in situ conditions of intact crustal rocks. Additionally, exploring the behaviour of nonlinear elasticity in response to changes in external parameters (e.g., temperature and wave frequency) acts as a means to further illuminate the complex mechanisms which give rise to nonlinear elasticity in rocks. In this study, we perform dynamic acoustoelastic testing (DAET) experiments on an intact cataclasite from the damage zone of the Alpine Fault, New Zealand. By performing pump-probe DAET experiments inside a temperature-controlled chamber, we are able to investigate a rich variety of nonlinear behaviour as a function of temperature. We find that the magnitude of averaging softening, cubic nonlinearity, and hysteresis tend to increase as temperature increases from 20 to 110○C. In contrast, quadratic nonlinearity decreases with increasing temperature. These observations support the hypothesis that at least two distinct mechanisms control nonlinear phenomena in rocks. Nonlinear parameters show little to no dependence on frequency over the 200-600 Hz pump range, although values of the nonlinear parameter α are found to be nearly two orders of magnitude smaller than those determined using ultrasonic perturbations. Additionally, an analysis using different time windows shows that the surface waves of the ultrasonic probe sense greater nonlinearity compared to the direct P-wave due to differences in the polarisation and propagation paths. As well as providing further insight into the mechanisms responsible for nonlinear elasticity in rocks, our experiments show that nonlinear softening will increase as temperature increases in the damage zones of faults. This has potential implications for understanding earthquake nucleation processes.

Structure and topology of a brittle-ductile fault swarm at Crawford Knob, Franz Josef, New Zealand

ABSTRACT We present surface and structural models of a swarm of dm-scale subparallel faults exposed in a ∼2000 m2 glaciated outcrop near Franz Josef Glacier, in the Southern Alps of New Zealand. These structures are inferred to have slipped at ∼20 km depth in the hanging-wall Alpine Schist of the Alpine Fault under conditions that were variably brittle to ductile as the Pacific Plate was tilted and uplifted. Using field mapping, real-time kinematic GPS and digital images from a remotely piloted aircraft system, we have created a digital surface model of the outcrop and orthophotographs at a ground resolution of ∼1 cm to map compositional layering in the metasediments and the array of brittle-ductile faults displacing them. In order of decreasing relative age (and average thickness), displaced markers in the schist include primary psammite and pelite beds, quartz veins, and a deformational foliation. The surface models have been used to create 2-D transects and a 3-D model where faults are projected down-dip, to determine the connectivity, topology and intersection types of the fault swarm. Lithological variations, particularly the interface between pelitic and psammitic schist, were a primary control on the topology of the fault network and the spacing between faults.

Alluvial fan response to Alpine Fault earthquakes on the Westland piedmont, Whataroa, Aotearoa‐New Zealand

AbstractWe examined the stratigraphy of alluvial fans formed at the steep range front of the Southern Alps at Te Taho, on the north bank of the Whataroa River in central West Coast, South Island, New Zealand. The range front coincides with the Alpine Fault, an Australian‐Pacific plate boundary fault, which produces regular earthquakes. Our study of range front fans revealed aggradation at 100‐ to 300‐year intervals. Radiocarbon ages and soil residence times (SRTs) estimated by a quantitative profile development index allowed us to elucidate the characteristics of four episodes of aggradation since 1000 CE. We postulate a repeating mode of fan behaviour (fan response cycle [FRC]) linked to earthquake cycles via earthquake‐triggered landslides. FRCs are characterised by short response time (aggradation followed by incision) and a long phase when channels are entrenched and fan surfaces are stable (persistence time). Currently, the Te Taho and Whataroa River fans are in the latter phase. The four episodes of fan building we determined from an OxCal sequence model correlate to Alpine Fault earthquakes (or other subsidiary events) and support prior landscape evolution studies indicating ≥M7.5 earthquakes as the main driver of episodic sedimentation. Our findings are consistent with other historic non‐earthquake events on the West Coast but indicate faster responses than other earthquake sites in New Zealand and elsewhere where rainfall and stream gradients (the basis for stream power) are lower. Judging from the thickness of fan deposits and the short response times, we conclude that pastoral farming (current land‐use) on the fans and probably across much of the Whataroa River fan would be impossible for several decades after a major earthquake. The sustainability of regional tourism and agriculture is at risk, more so because of the vulnerability of the single through road in the region (State Highway 6).

Tectono‐Thermal Evolution of the Hope‐Kelly Fault System, Southern Alps, New Zealand: Insights From Topographic Analysis and (U‐Th)/He Thermochronology

AbstractThe fast‐slipping Alpine (∼30 mm/yr), Hope (∼10–20 mm/yr) and Kelly (∼6 mm/yr) faults in the South Island of New Zealand form a complex intersection zone that accommodates tectonic strain along the Australian‐Pacific plate boundary. Analysis of digital topography reveals evidence for stream capture, drainage divide migration, landscape responses to incipient fault development, and preserved enclaves of relic topography that collectively reflect complex interplays between active faulting and landscape evolution. (U‐Th)/He thermochronology of zircon (ZHe) and apatite (AHe) is used to investigate the low‐temperature thermal evolution of rocks in the intersection zone. Weighted mean sample ages for ZHe single grain ages (n = 13 samples) range from ∼9 to 2 Ma, and AHe multi‐grain and single grain aliquot ages (n = 9 samples) range from ∼1.5 to 0.5 Ma. Inverse and forward thermal history modeling reveals distinct spatiotemporal variations in thermal histories. Late Miocene exhumation rates (∼0.6–3.5 km/Myr, assuming geothermal gradients of 33–40 °C/km) through crustal depths of approximately 5–6 km, are interpreted to be controlled by proximity to the Alpine fault, with rocks proximal to the fault recording faster exhumation rates relative to distal samples. Establishment of the Hope‐Kelly fault system in the Quaternary structurally juxtaposed rocks with discordant cooling histories. Rocks throughout the study region record increased cooling rates from ∼2 Ma. Possible causal mechanisms include, spatial changes in rock uplift associated with transport toward the Alpine Fault, increased erosion rates associated with Quaternary climate change, or increased rock mass erodibility associated with development of the Hope‐Kelly fault system.

Interdisciplinary Approach and Geodynamic Implications of the Goutitir Geothermal System (Eastern Meseta, Morocco)

Morocco has an important geothermal potential materialized by its several thermal springs which constitute an essential surface geothermal indicator. These springs are dispersed throughout the country and present in every major structural domain. However, a significant amount is concentrated in the northern and northeastern areas. Associated with the great hydrothermal system of eastern Morocco, the thermal spring of Goutitir emerges in the Meso-Cenozoic sedimentary formations located east of the Guercif Basin, composed of a mixture of clays, carbonates, and marls, covered in unconformity by Quaternary tabular molasses. The upflow of the thermal water is dependent of Alpine faults systems with N30 and N100 directions, which are probable reactivated Hercynian structures that facilitate its circulation to the surface. The Goutitir spring has been studied by an interdisciplinary approach to identify the origin of the thermal water, the rock–water interactions, and the reservoir temperatures, contributing to the establishment of the conceptual model of the associated hydrothermal system. This thermal water is of chloride-sodium type with a hyperthermal character (43–47 °C). The isotopic composition (δ18O = −8.7 to −8.35‰; δ2H = −58.6 to −54.3‰) indicates a meteoric origin and a recharging zone located at around 2000 m of altitude. The chemical composition allows to classify the water as chloride-sodium hydrochemical facies, stabilized at ~100 °C in crystalline basement rocks, which, according to seismic data, are located at ~3 km depth. The concentrations, patterns, and correlations of trace elements point out water–rock interaction processes between the deep water and basic magmatic rocks. The integration of the chemical and isotopic data and the surface geological context shows that the Goutitir water flows within a hydrothermal zone were basic to ultrabasic lamprophyres rich in gabbroic xenoliths outcrop, witnessing the existence, at depth, of basic plutons. Moreover, near the source, these veins are strongly altered and hydrothermalized, showing late recrystallization of centimetric-sized biotites. The chloride-sodium composition of this water may also be a testimony to the presence and reaction with the overlying Triassic saline and gypsiferous and Meso-Cenozoic mainly carbonated formations.