Fluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of the seismogenic crust. However, the stress conditions and geological environment of fluid-pressure cycling are unclear. We address this problem by detailed structural investigation of a vein-bearing shear zone at Sagelvvatn, northern Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic quartz veins locally crosscut mylonitic fabric at a high angle and are rotated and folded with the same sense of shear as the mylonite. Chlorite thermometry indicates that both veining and mylonitization occurred at ∼315–400 °C. The vein-filled fractures are interpreted as episodically triggered by viscous creep in the mylonite, where quartz piezometry and brittle failure modes are consistent with low (18–44 MPa) differential stress. The Sagelvvatn shear zone is a stretching shear zone, where elevated pressure drives a hydraulic gradient that expels fluids from the shear zone to the host rocks. In low-permeability shear zones, this hydraulic gradient facilitates build-up of pore-fluid pressure until the hydrofracture criterion is reached and tensile fractures open. We propose that hydraulic gradients established by local and cyclic pressure variations during viscous creep can drive episodic fluid escape and result in brittle-viscous fault slip at the base of the seismogenic crust.A long-established and fundamental aspect of the deformation of geological materials is that it generates spatial and temporal variations in tectonic pressure (Casey, 1980; Mancktelow, 2002, 2008; Schmalholz and Podladchikov, 2013). Brittle fracturing and viscous flow result in pressure gradients across rheological boundaries that are crucial for driving fluid flow during rock deformation (Mancktelow, 2008). Localized viscous shear zones, which must be weaker than the surrounding material, develop a higher pressure than the host rock when they are stretched parallel to the slip direction (i.e., “positive stretching faults”; Escher and Watterson, 1974; Means, 1989). Conversely, in brittle faults, the pressure is lower than in the adjacent rocks (Mancktelow, 2006). Thus, stretching shear zones and brittle faults are “overpressured” and “underpressured” with respect to the host rock, respectively (Mancktelow, 2008). This general principle of pressure-gradient generation during deformation has been invoked to explain fluid flow at the plate scale during slab unbending (Faccenda et al., 2009, 2012; Faccenda and Mancktelow, 2010), differences in plate interface shear stress between bending and unbending slabs (Beall et al., 2021), and hydrolytic weakening in the wall rock during shear-zone widening (Oliot et al., 2014; Finch et al., 2016). We build on the concept of tectonic pressure gradients to interpret field observations of a brittle-viscous shear zone. We show that pressure gradients resulting from viscous creep in the shear zone can result in cyclic and transient hydrofracturing and fluid expulsion during ongoing creep. This creep-driven hydrofracturing may contribute to explaining the cyclic frictional-viscous deformation along single structures commonly inferred from the geological and seismological record of the fluid-rich, seismogenic subduction environment (Fagereng and Sibson, 2010; Audet and Schaeffer, 2018).The studied shear zone is situated in the Ordovician–Silurian Balsfjord Group of the Lyngen nappe, in the Uppermost Allochthon thrust sheets of the Norwegian Caledonides (Fig. 1A). The Lyngen nappe consists of ophiolites including metavolcanic and metasedimentary rocks of the Iapetus Ocean (Bergh and Andresen, 1985). In the study area, the Hølen Conglomerate of the Balsfjord Group developed east-southeast–vergent, open to tight folds during the collisional stage of the Caledonian orogeny at ∼400 °C and 300–400 MPa (Bergh and Andresen, 1985) (Fig. 1B; Fig. S1 in the Supplemental Material1). The kinematics and conditions place the Balsfjord Group in the orogenic wedge at an approximate depth of 10–15 km during the Caledonian continental collision. We analyzed a 3-m-thick shear zone in the Hølen Conglomerate exposed west of Sagelvvatn. In this exposure, long, gently dipping limbs of the asymmetric folds are sheared in discrete shear zones with a top-to-the-east-southeast sense of shear (Fig. 1B). Limbs of tight folds are common locations for the development of stretching shear zones (Means, 1989).The shear zone developed predominantly in the folded metaconglomerate but also contains bands of chlorite-muscovite schists as much as 0.5 m thick both at its base and in the shear-zone interior (Fig. 1B). The shear zone developed an internal strain partitioning with alternating low-strain protomylonitic and high-strain mylonitic domains (Figs. 1B and 1C). The mylonitic metaconglomerate contains highly elongate quartzitic pebbles in a quartz- and carbonate-rich cement. Quartzitic pebbles lack macroscopic fractures and boudinage. Although the shear zone is dominantly ductile (defined as spatially continuous deformation at the scale of observation), 1–5-cm-thick sigmoidal quartz veins, arranged en echelon, locally crosscut the mylonitic fabric, predominantly at a high angle (Figs. 1B and 1C). The host rock does not contain veins (Fig. 1B), although the tips of some shear zone-hosted veins extend into the host rock (Fig. 1C).The mylonitic foliation dips gently southwest and contains a gently west-northwest–plunging stretching lineation (Fig. 1D). Lower-strain domains within the shear zone preserve decimeter-scale asymmetric, east- to east-southeast–vergent, subhorizontal parasitic folds (Figs. 1C and 1E). Higher-strain domains include SC′ foliations and interconnected chlorite folia (Fig. S3C). Veins tips are oriented 50°–80° to the shear zone margins and are rotated, typically by 30°–55° relative to the vein center, with the same sense of shear as the mylonite (Fig. 1C; Fig. S2). Vein rotation occurred only inside the high-strain zone, whereas vein tips extending into the host rocks are straight (Fig. 1C). The gently south-southwest–plunging fold hinges of the veins and the stretching lineation of the mylonite are approximately perpendicular (Figs. 1E and 1F). This indicates that the shear strain imposed on the veins was not sufficient to significantly rotate the hinge lines toward the stretching direction (Fig. 1E). Locally, however, ≤75° rotation results in a few veins being considerably thinned and lengthened in the finite stretching field (Fig. 1B; Fig. S2). Vein quartz crystals are coarse and elongate-blocky with long axes oriented 70°–80° to the vein margins and subparallel to the mylonite stretching lineation (Fig. 1G; Fig. S3).In summary, the veins reflect the same kinematics as the mylonitic shear zone and formed and deformed during bulk viscous flow. Thus, the Sagelvvatn shear zone is an example of a relatively weak zone that is stretched parallel to its length through dominantly viscous but locally brittle mechanisms, illustrated by subparallel mylonitic stretching lineations and vein opening vectors.The matrix of the mylonitic metaconglomerate consists of large (50–200 μm) polygonal grains of quartz mixed with calcite, dolomite, biotite, and epidote. The mylonitic pebbles are predominantly quartzitic. Quartz grain boundaries are generally straight, and the grains are equant; however, irregular and lobate grain boundaries locally occur, indicating a component of grain-boundary migration recrystallization (Fig. 2A). Larger grains contain optically visible subgrains of 50–100 μm (Fig. 2B). Vein quartz grains are ∼200 μm to ∼1.5 cm long and show evidence of intracrystalline deformation in the form of arrays of blocky and elongated subgrains (Figs. S3A and S3B).The temperatures of veining and mylonitic deformation were estimated with chlorite thermometry (Lanari et al., 2014) assuming a pressure of 350 MPa (Bergh and Andresen, 1985). Chlorite composition was measured from SC′ fabrics indicating top-to-the-east-southeast shear in the schists and chlorite grains within the quartz veins (Fig. S3). Chlorite thermometry yields an average temperature of mylonitization of 360 ± 26 °C and a temperature range of 313–400 °C for the quartz veins (Table S1).The quartz c-axis crystallographic preferred orientation in the mylonitic pebbles is an incomplete asymmetric crossed girdle consistent with top-to-the-east-southeast sense of shear (Fig. 2C). Low-angle boundaries are common in the larger grains and define subgrains of 50–100 μm in size (Fig. 2D; Fig. S4). The microstructure and electron backscatter diffraction (EBSD) analyses indicate that pebble elongation was accommodated by dislocation creep in quartz, which recrystallized by subgrain rotation with a contribution of grain-boundary migration.Following the approach of Cross et al. (2017), we used the grain orientation spread to separate recrystallized from relict grains in our samples, with a threshold of 0.98°. The resultant average recrystallized grain size is 97 ± 59 μm (root mean square ± one standard deviation; Fig. 2E), which yields a differential stress during mylonitic flow of 18–44 MPa using the Cross et al. (2017) piezometer. Although caution is needed in applying grain-size piezometry to grains that show a contribution of grain-boundary migration (Cross et al., 2017), the presence within the relict grains of subgrains with a similar size to the recrystallized grains (Figs. 2B and 2D) suggests that the grains did not grow significantly after recrystallization.The en echelon quartz veins are interpreted to have been triggered and controlled by viscous creep in a stretching shear zone (Fig. 1). Stretching shear zones develop a higher pressure than in the host rock, and the resulting hydraulic gradient drives expulsion of fluids from within the shear zone (Mancktelow, 2006, 2008; Finch et al., 2016). If fluids are trapped in the shear zone because of low-permeability horizons at its margins, dynamic fluid pressure may locally approach, or in thrust faulting regimes even exceed, the lithostatic pressure (Fig. 3), triggering dilatant fracturing if differential stress is low (Sibson, 1998; Cox, 2010). In the Sagelvvatn shear zone, quartz microstructure and piezometry confirm that differential stress was sufficiently low (<50 MPa) for tensile fracture in frictionally strong and cohesive quartz-rich metaconglomerate (Figs. 2 and 3). Low-permeability horizons are represented by chlorite-rich schists (Fig. 1B), which typically form low-porosity seals in metamorphic environments (Ganzhorn et al., 2019). Under the same differential stress conditions as dislocation creep and tensile failure in the metaconglomerates, these frictionally weak chlorite-rich horizons likely experienced frictional sliding in interconnected chlorites (Fig. 3; Okamoto et al., 2019), although direct observational evidence of this is missing in our example.We propose a model in which tensile brittle failure occurs spontaneously within a creeping shear zone as a consequence of hydraulic gradients established by local pressure variations. In this model, the timing of fracturing is governed primarily by the rate of dynamic fluid-pressure increase during shear (Fig. 4, stage 1) until a maximum sustainable fluid overpressure is achieved and hydrofracturing occurs in frictionally strong materials (Fig. 4, stage 2). The transient fractures would be underpressured relative to the surrounding creeping matrix (Mancktelow, 2006) and represent sinks for the intergranular fluid. This decreases the pore-fluid pressure and allows creep to continue without further tensile failure (Fig. 4, stage 3) until fluid pressure builds up to trigger a new hydrofracturing episode (Fig. 4, stage 4). This process is expected to be cyclical as long as fluid pressure rises fast enough for the hydrofracture criterion to be reached before shear failure is triggered by increasing tectonic stresses. These fluid-pressure cycles originate within the creeping shear zone itself and are driven by local pressure gradients rather than external fault valving (Sibson, 1998) or local viscosity contrasts between weak matrix and strong inclusions (e.g., Hayman and Lavier, 2014; Behr et al., 2018; Beall et al., 2019).The creep-driven hydrofracturing model might be particularly relevant to the frictional-viscous transition at the base of the seismogenic zone, where the progressive increase in the effective area of contact along a fault surface facilitates the build-up of pore-fluid pressure (Hirth and Beeler, 2015). Pressure gradients resulting from viscous creep would contribute to the build-up of fluid pressure until a brittle failure criterion is reached and the overpressured fluid is driven into vein and/or fault systems.Fluid-pressure cycles are commonly invoked to explain mixed aseismic-seismic fault slip behavior at the base of the seismogenic zone, particularly in subduction zones and accretionary wedges, suggesting a link between fault-zone hydrology and fault-slip behavior (e.g., Audet and Bürgmann, 2014; Fagereng et al., 2018; Kotowski and Behr, 2019; Gosselin et al., 2020). Our observations (Figs. 1 and 2) and considerations of brittle failure modes as a function of fluid pressure and differential stress (Fig. 3) suggest that in stretching shear zones where differential stress is low and fluids are confined, fluid overpressure increasing rapidly relative to tectonic stresses leads to episodic fracture events as a direct consequence of creep (Fig. 4). Vein-filled tensile fractures can be directly observed in the Sagelvvatn shear zone. However, frictional sliding would also be triggered on frictionally weak, lower-cohesion, phyllosilicate-rich horizons at the same conditions we estimate for mylonitic shear and tensile vein opening in the metaconglomerate (Fig. 3). We cannot observationally constrain whether this frictional sliding was episodic or continuous, but it would have occurred only when a certain minimum fluid pressure was reached that allowed frictional resistance to be overcome (Fig. 3). It is therefore conceivable that hydrofracturing in the Sagelvvatn shear zone also involved transient frictional shear deformation, analogous to experiments in serpentine where reaction-induced extension and shear fractures developed simultaneously (Zheng et al., 2019).We propose that tectonically induced pressure gradients can control fluid-pressure cycling characteristic of the brittle-viscous fault-slip behavior commonly observed at the base of the seismogenic zone. This effect is likely most pronounced along the subduction thrust interface, where low-permeability phyllosilicates are common, fluid release occurs from prograde dehydration, and low differential stresses prevail. These conditions fulfill all the criteria for our proposed model of creep-driven hydrofracturing, which may involve associated frictional sliding on weak, phyllosilicate-rich foliations (Figs. 3 and 4; e.g., Fagereng et al., 2018). These effects are a direct consequence of the tectonic pressure gradients that occur from outcrop to plate-boundary scale in stretching shear zones.This work was supported by the EU's Seventh Framework Programme (FP7) Marie Curie Career Integration Grant 618289 “EVOCOS” to L. Menegon and by the European Research Council Horizon 2020 Starting Grant 715836 “MICA” to Å Fagereng. We thank Francesca Prando for acquiring the electron microprobe analyzer data, Glenn Harper for support during scanning electron microscope work, and François Renard for discussions. Neil Mancktelow, Nick Hayman, Melanie Finch, and an anonymous reviewer provided constructive reviews that greatly improved the manuscript. Steffen Bergh and Holger Stünitz are thanked for having introduced Menegon to the study area.

中文翻译：

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粘性蠕变过程中的构造压力梯度驱动发震带底部的流体流动和脆性破坏

流体压力循环通常被用来解释地震地壳底部的交替摩擦和粘性变形。然而，流体压力循环的应力条件和地质环境尚不清楚。我们通过对挪威北部 Caledonides 的 Sagelvvatn 含脉剪切带进行详细的结构研究来解决这个问题。在这个主要的粘性剪切带中，同运动石英脉以大角度局部横切糜棱岩织物，并以与糜棱岩相同的剪切感旋转和折叠。绿泥石温度测定表明脉纹和糜棱化都发生在~315-400°C。充满静脉的裂缝被解释为由糜棱岩中的粘性蠕变偶然触发，其中石英测压法和脆性破坏模式与低 (18-44 MPa) 差分应力一致。Sagelvvatn 剪切带是一个拉伸剪切带，其中升高的压力驱动水力梯度，将流体从剪切带排出到主岩。在低渗透剪切带中，这种水力梯度有利于孔隙流体压力的积累，直到达到水力压裂标准并张开裂缝。我们提出，在粘性蠕变期间由局部和循环压力变化建立的水力梯度可以驱动间歇性流体逃逸并导致发震地壳底部的脆性粘性断层滑动。地质材料变形的一个长期确立的基本方面是它会产生构造压力的空间和时间变化（Casey，1980；Mancktelow，2002，2008年；Schmalholz 和 Podladchikov，2013 年）。脆性压裂和粘性流导致跨流变边界的压力梯度，这对于在岩石变形过程中驱动流体流动至关重要（Mancktelow，2008 年）。局部粘性剪切带必须比周围材料弱，当它们平行于滑动方向拉伸时，会产生比主岩更高的压力（即“正拉伸断层”；Escher 和 Watterson，1974 年；Means，1989 年） . 相反，在脆性断层中，压力低于相邻岩石中的压力（Mancktelow，2006 年）。因此，相对于主岩，拉伸剪切带和脆性断层分别是“超压”和“负压”（Mancktelow，2008）。变形过程中压力梯度产生的一般原理已被用来解释板坯展开过程中板尺度的流体流动（Faccenda 等人，2009 年，2012 年；Faccenda 和 Mancktelow，2010 年）、弯曲和弯曲之间的板界面剪应力差异。不弯曲板（Beall 等人，2021 年），以及剪切带加宽期间围岩的水解弱化（Oliot 等人，2014 年；Finch 等人，2016 年）。我们基于构造压力梯度的概念来解释脆性粘性剪切带的现场观察结果。我们表明，由剪切带中的粘性蠕变引起的压力梯度会在持续蠕变期间导致循环和瞬态水力压裂和流体排出。这种蠕变驱动的水力压裂可能有助于解释沿单个结构的循环摩擦粘滞变形，这通常是从富含流体的地震俯冲环境的地质和地震记录推断出来的（Fagereng 和 Sibson，2010 年；Audet 和 Schaeffer，2018 年）。研究的剪切带位于 Lyngen 推覆的奥陶纪-志留纪 Balsfjord 群，在挪威 Caledonides 的最上层 Allochthon 冲断层中（图 1A）。Lyngen 推覆岩由蛇绿岩组成，包括 Iapetus 洋的变火山和变沉积岩（Bergh 和 Andresen，1985）。在研究区，Balsfjord Group 的 Hølen 砾岩在加里东造山运动的碰撞阶段在~400 °C 和 300-400 MPa 期间发育东-东南-辐合，向紧密褶皱开放（Bergh 和 Andresen，1985）（图. 1B; 补充材料中的图 S11)。在加里多尼亚大陆碰撞期间，运动学和条件将 Balsfjord 群置于大约 10-15 公里深度的造山楔中。我们分析了 Sagelvvatn 以西出露的 Hølen 砾岩中 3 米厚的剪切带。在这次暴露中，不对称褶皱的长而轻微倾斜的分支在离散的剪切带中被剪切，具有从顶部到东南向的剪切感（图 1B）。紧密褶皱的边缘是伸展剪切带发育的常见位置（Means，1989）。剪切带主要在褶皱变砾岩中发育，但在其底部和底部也包含厚度达 0.5 m 的绿泥石-白云母片岩带。剪切带内部（图 1B）。剪切带形成了内部应变分区，具有交替的低应变原糜棱和高应变糜棱域（图 1B 和 1C）。糜棱质变砾岩在富含石英和碳酸盐的胶结物中含有高度细长的石英质卵石。石英卵石没有宏观的裂缝和布丁。虽然剪切带主要是延展性的（定义为观察尺度上的空间连续变形），但 1-5 厘米厚的 S 形石英脉，排列成梯形，局部横切糜棱岩结构，主要以高角度（图 1B）和 1C)。主岩不包含脉（图 1B），尽管一些剪切带主脉的尖端延伸到主岩中（图 1C）。糜棱片理向西南方向倾斜，并包含一个温和的西-西北-俯冲伸展划线（图1D）。剪切带内的低应变域保留了分米级的不对称、东-东南-辐合、亚水平寄生褶皱（图 1C 和 1E）。更高应变域包括 SC' 叶理和相互连接的绿泥石叶 (图 S3C)。静脉尖端与剪切带边缘成 50°–80° 方向并旋转，通常相对于静脉中心旋转 30°–55°，具有与糜棱岩相同的剪切感（图 1C；图 S2）。脉旋转仅发生在高应变带内，而延伸到主岩中的脉尖是直的（图 1C）。脉的轻微的南-西南-俯冲褶皱铰链和糜棱岩的拉伸线理大致垂直（图1E和1F）。这表明施加在静脉上的剪切应变不足以使铰链线朝着拉伸方向显着旋转（图 1E）。然而，在局部，≤75° 旋转会导致一些静脉在有限拉伸场中显着变细和变长（图 1B；图 S2）。脉石英晶体粗糙且细长块状，长轴与脉边缘成 70°–80°，与糜棱岩伸展线状平行（图 1G；图 S3）。总之，脉反映了与脉缘相同的运动学糜棱剪切带，并在大量粘性流动过程中形成和变形。因此，Sagelvvatn 剪切带是一个相对薄弱的区域，它通过主要的粘性但局部脆性机制平行于其长度拉伸，由近平行的糜棱岩拉伸线和静脉开口矢量说明。糜棱岩变砾岩的基质由大 (50-200 微米) 多角石英颗粒与方解石、白云石、黑云母和绿帘石混合而成。糜棱岩卵石主要是石英岩。石英晶界一般是直的，晶粒是等长的；然而，局部出现不规则和叶状晶界，表明晶界迁移再结晶的组成部分（图 2A）。较大的晶粒包含 50-100 μm 的光学可见亚晶粒（图 2B）。脉石英晶粒长约 200 微米至约 1.5 厘米，显示出呈块状和细长亚晶粒阵列形式的晶内变形证据（图 S3A 和 S3B）。假设压力为 350 MPa（Bergh 和 Andresen，1985），脉纹和糜棱变形的温度是用绿泥石温度测定法（Lanari 等，2014）估计的。绿泥石成分是从 SC' 织物测量的，表明在石英脉内的片岩和绿泥石颗粒中从顶部到东南向剪切（图 S3）。绿泥石温度测定法产生糜棱岩化的平均温度为 360 ± 26 °C，石英脉的温度范围为 313–400 °C（表 S1）。糜棱岩鹅卵石中的石英 c 轴晶体优先取向是不完全不对称的交叉腰围与自上而东-东南的剪切感一致（图 2C）。小角度边界在较大的晶粒中很常见，并且定义了尺寸为 50-100 μm 的亚晶粒（图 2D；图 S4）。微观结构和电子背散射衍射 (EBSD) 分析表明，石英中的位错蠕变调节了卵石的伸长率，石英中的位错蠕变通过亚晶旋转再结晶，并有助于晶界迁移。遵循 Cross 等人的方法。(2017)，我们使用晶粒取向扩展将样品中的再结晶晶粒与残余晶粒分开，阈值为 0.98°。由此产生的平均再结晶晶粒尺寸为 97 ± 59 μm（均方根 ± 一个标准偏差；图 2E），使用 Cross 等人在糜棱岩流动过程中产生 18-44 MPa 的差异应力。(2017) 压力计。尽管在将晶粒尺寸测压法应用于显示出晶界迁移贡献的晶粒时需要谨慎（Cross 等，2017），与再结晶晶粒尺寸相似的亚晶粒残存晶粒内的存在（图 2B 和 2D）表明晶粒在再结晶后没有显着生长。 梯级石英脉被解释为由粘性蠕变触发和控制在拉伸剪切区（图 1）。拉伸剪切带产生比主岩中更高的压力，由此产生的水力梯度驱使剪切带内的流体排出（Mancktelow，2006 年，2008 年；Finch 等，2016 年）。如果流体由于其边缘的低渗透层而被困在剪切带中，动态流体压力可能会局部接近，或者在逆冲断层状态下甚至超过岩石静压力（图 3），如果差应力较低，则会触发剪胀压裂（西布森，1998 年；考克斯，2010 年）。在 Sagelvvatn 剪切带中，石英微观结构和测压法证实，对于摩擦力强且具有粘性的富含石英的变砾岩，差应力足够低（<50 MPa）以实现拉伸断裂（图 2 和图 3）。低渗透层以富含绿泥石的片岩为代表（图 1B），通常在变质环境中形成低孔隙度封层（Ganzhorn 等，2019）。在与变砾岩中的位错蠕变和拉伸破坏相同的不同应力条件下，这些摩擦力弱的富含绿泥石的层很可能在相互连接的绿泥石中经历了摩擦滑动（图 3；Okamoto 等人，2019），尽管对此的直接观察证据是在我们的例子中缺失。我们提出了一个模型，其中由于局部压力变化建立的水力梯度，在蠕变剪切带内自发发生拉伸脆性破坏。在该模型中，压裂时间主要由剪切期间流体压力的动态增加速率决定（图 4，阶段 1），直到达到最大可持续流体超压，并且在摩擦力强的材料中发生水力压裂（图 4，阶段 2)。相对于周围蠕动基质（Mancktelow，2006 年），瞬态裂缝将处于欠压状态，并代表粒间流体的汇。这降低了孔隙流体压力并允许蠕变继续而不会进一步拉伸破坏（图 4，第 3 阶段），直到流体压力增加以触发新的水力压裂事件（图 4，第 4 阶段）。只要流体压力上升得足够快以在构造应力增加引发剪切破坏之前达到水力压裂标准，该过程预计将是循环的。这些流体压力循环起源于蠕变剪切带本身，由局部压力梯度驱动，而不是外部断层阀（Sibson，1998 年）或弱基质和强包裹体之间的局部粘度对比（例如，Hayman 和 Lavier，2014 年；Behr 等人） al., 2018; Beall et al., 2019).蠕变驱动的水力压裂模型可能与孕震带底部的摩擦-粘性转变特别相关，其中沿断层的有效接触面积逐渐增加表面促进孔隙流体压力的建立（Hirth 和 Beeler，2015 年）。由粘性蠕变引起的压力梯度将有助于流体压力的积累，直到达到脆性破坏标准并且超压流体被驱入静脉和/或断层系统。流体压力循环通常被用来解释混合地震-地震地震带底部的断层滑动行为，特别是在俯冲带和增生楔中，表明断层带水文与断层滑动行为之间存在联系（例如，Audet 和 Bürgmann，2014 年；Fagereng 等人，2018 年；Kotowski 和Behr，2019 年；Gosselin 等人，2020 年）。我们的观察（图 1 和 2）以及脆性破坏模式作为流体压力和差应力的函数（图 3）的考虑表明，在差应力低且流体受限的拉伸剪切区中，相对于构造应力快速增加的流体超压导致作为蠕变的直接结果的偶发性破裂事件（图 4）。在 Sagelvvatn 剪切带中可以直接观察到充满静脉的拉伸断裂。然而，在我们估计变砾岩中糜棱剪切和张性脉张开的相同条件下，摩擦力较弱、凝聚力较低、富含页硅酸盐的层位也会触发摩擦滑动（图 3）。我们无法从观察上限制这种摩擦滑动是偶发的还是连续的，但只有在达到允许克服摩擦阻力的某个最小流体压力时才会发生（图 3）。因此可以想象，Sagelvvatn 剪切带的水力压裂也涉及瞬时摩擦剪切变形，类似于蛇纹石实验，其中反应诱导的伸展和剪切裂缝同时发展（Zheng et al., 2019）。我们提出构造诱导的压力梯度可以控制脆性粘性断层滑动行为的流体压力循环特征，通常在地震带的基础。这种效应可能在俯冲推力界面最明显，在那里低渗透率的页硅酸盐很常见，流体因顺行脱水而释放，并且低微分应力占主导地位。这些条件满足我们提出的蠕变驱动水力压裂模型的所有标准，这可能涉及弱的、富含页硅酸盐的叶理上的相关摩擦滑动（图 3 和 4；例如 Fagereng 等，2018）。这些影响是拉伸剪切带中从露头到板块边界尺度发生的构造压力梯度的直接后果。这项工作得到了欧盟第七框架计划 (FP7) 玛丽居里职业整合赠款 618289 “EVOCOS”的支持。 Menegon 和欧洲研究委员会地平线 2020 年启动赠款 715836 “MICA”给 Å Fagereng。我们感谢 Francesca Prando 获取电子探针分析仪数据，感谢 Glenn Harper 在扫描电子显微镜工作期间的支持，以及 François Renard 的讨论。Neil Mancktelow、Nick Hayman、Melanie Finch 和一位匿名审稿人提供了建设性的意见，极大地改进了手稿。感谢 Steffen Bergh 和 Holger Stünitz 将 Menegon 引入研究领域。