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Low‐angle normal faults (LANF), typically regarded as accommodating crustal or lithospheric extension, may also form during lithospheric shortening. The best‐studied system of syn‐contractional LANFs is the South Tibetan detachment system, a network of low‐angle normal sense faults and shear zones that formed coevally with and parallel to south‐vergent thrusts during lithospheric shortening accompanying development of the Himalayan orogen. In the eastern Himalaya, there are several across‐strike exposures of the South Tibetan detachment system. We present new structural and thermometry data from the eastern Himalaya that demonstrate that the South Tibetan detachment system cannot have formed as a single progressive structure. We characterize and distinguish two distinct structural and tectonic components within the currently recognized system: (1) an extensive diffuse, sheared layer that formed the boundary between strong upper crust and weak, southward‐flowing middle crust, and (2) a network of brittle‐ductile LANFs that locally exhume, partly excise and overprint the earlier mylonite zone at the topographic break between the Himalayan orogen and the Tibetan plateau. The sheared layer, not a LANF, formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the Himalaya at the base of klippen of upper crustal rocks preserved in Bhutan, along the crest of the Himalaya where it has been excised and exhumed by the brittle‐ductile extrusion LANFs, and bounding the cores of the North Himalayan gneiss domes. Key Points The South Tibetan detachment system is not a single fault system The lower structure is a sheared layer bounding mid and upper crust The extensive sheared layer is locally cut by low‐angle normal faults

Despite decades‐long debate over the mechanics of low‐angle normal faults (LANFs) dipping less than 30°, many questions about their strength, stress, and slip remain unresolved. Recent geologic and geophysical observations have confirmed that gently dipping detachment faults can slip at such shallow dips and host moderate‐to‐large earthquakes. Here, we analyze the first 3D dynamic rupture models to assess how different stress and strength conditions affect rupture characteristics of LANF earthquakes. We model observationally constrained spontaneous rupture under different loading conditions on the active Mai'iu fault in Papua New Guinea, which dips 16°–24° at the surface and accommodates ∼8 mm/yr of horizontal extension. We analyze four distinct fault‐local stress scenarios: (1) Andersonian extension, as inferred in the hanging wall; (2) back‐rotated principal stresses inferred paleopiezometrically from the exhumed footwall; (3) favorably rotated principal stresses well‐aligned for low‐angle normal‐sense slip; and (4) Andersonian extension derived from depth‐variable static fault friction decreasing toward the surface. Our modeling suggests that subcritically stressed detachment faults can host moderate earthquakes within purely Andersonian stress fields. Near‐surface rupture is impeded by free‐surface stress interactions and dynamic effects of the gently dipping geometry and frictionally stable gouges of the shallowest portion of the fault. Although favorably inclined principal stresses have been proposed for some detachments, these conditions are not necessary for seismic slip on these faults. Our results demonstrate how integrated geophysical and geologic observations can constrain dynamic rupture model parameters to develop realistic rupture scenarios of active faults that may pose significant seismic and tsunami hazards to nearby communities. Plain Language Summary Movement across faults allow parts of the Earth's crust to move past each other in response to forces driven by tectonic plate motions and can occur during large, devastating earthquakes. The orientation of a fault relative to the direction of the forces and stresses loading determines how easily it can “slip” in any given direction and whether it will continue to slip or if new fractures and faults will form instead. Some faults appear to be geometrically misoriented and thus “locked” relative to their local forces, but nonetheless continue to move on the scale of mm per year and accommodate crustal motions. Here, we develop data‐constrained computer models to test how different forces at depth affect the movement and associated potential earthquake magnitude of one of these misoriented faults: the Mai'iu normal fault in Papua New Guinea. Our results suggest these faults can indeed slip in large earthquakes under tectonic crustal stress conditions, and that locally favorably rotated stresses would generate even larger earthquakes. We find that seismic slip does not always reach the Earth's surface and explore various physical mechanisms limiting near‐surface slip on these faults. Key Points We perform the first 3D dynamic rupture simulations of low‐angle normal fault (LANF) earthquake scenarios constrained by laboratory and field evidence Large LANF earthquakes are dynamically viable under various stress conditions including perfectly Andersonian extension Shallow slip is limited by the stabilizing effects of shallow fault geometry, velocity‐strengthening gouges, and free‐surface interactions

The brittle‐plastic transition (BPT), the strongest part of the crust, is critical to continental geodynamics but is poorly understood relative to simpler crust above and below. It is typically represented as a depth transition from brittle/frictional to plastic/viscous deformation controlled by temperature and pressure. Footwalls of low‐angle normal faults (LANFs) exhumed through the BPT provide rock records that challenge this view. Three well‐studied LANF footwalls are reviewed. All record geochemical, mineralogical and fluid‐related controls on embrittlement, not just monotonic P‐T decrease. Two quartz‐rich examples record embrittlement at unexpectedly high T (≥450–500°C) that was modulated by wetting characteristics of fluids. One had an inverted BPT: brittle fracture beneath contemporaneous mylonites. In another study, a brittle LANF grew from plastic mylonites due to mineralogic changes that strengthened parts, causing initial frictional slip and cataclasis on weak planes that ultimately linked. In all, geologically abrupt small‐scale processes controlled behavior at kilometer scales. Similar processes likely affect other tectonic settings and seismic cycles. Such processes offer fertile research opportunities in continental geodynamics; they will be increasingly tractable as computational abilities improve. Adaptive, multi‐scale approaches including the effects of fluid‐rock geochemistry and mineralogical changes on rock strength and deformation are needed. Thoughtful modeling approaches may yield key insights into the positive and negative feedbacks that are likely. Discontinuous deformation is probably needed explicitly along with exploration of initial and boundary conditions. Plain Language Summary The strongest part of continental crust is the brittle‐plastic transition (BPT) in the midcrust at ∼10–20 km depth. There, brittle behavior (fault slip, fracturing, fragmentation), which increases in strength downward, gives way to “ductile” behavior dominated by crystal plasticity of quartz, which decreases in strength downward. The BPT is less well understood than either brittle crust above or plastic crust below. The transition is generally modeled and thought of as being dominated by a downward increase in temperature and pressure, but studies of faults that evolved from being plastically deforming shear zones to brittle slip surfaces suggest that abrupt and localized geochemical, mineralogical and fluid processes are also very important. Such processes are currently little represented in numerical geodynamic models of the crust. Their inclusion in future models will probably lead to important insights into crustal geodynamics. Key Points A detailed study of the evolution of three large‐slip low‐angle normal faults in the continental brittle‐plastic transition shows that, in addition to monotonic decreases in pressure and temperature, diverse, localized, and/or geologically abrupt geochemical, fluid‐related, and mineralogical processes were critical to fault evolution These processes commonly occurred over small spatial (sub‐millimeter to several meter) and temporal (seconds to years?) scales but affected geodynamic factors such as crustal strength and deformation style on larger and longer (kilometer and Myr) scales Such processes are presently only rarely incorporated into geodynamic models. Anticipated improvements in modeling methods and computational ability should allow their future incorporation, which will likely provide fresh insights into crustal geodynamics

Using a case study from the island of Elba, Italy, we seek to test the hypothesis that the presence of minerals with low frictional strengths can explain prolonged slip on low‐angle normal faults. The central core of the Zuccale low‐angle normal fault contains a distinctive fault rock zonation that developed during progressive exhumation. Most fault rock components preserve microstructural evidence for having accommodated deformation entirely, or partly, by frictional mechanisms. One millimeter thick sample powders of all the major fault rock components were deformed in a triaxial deformation apparatus under water‐saturated conditions, at room temperature, and at constant effective normal stresses of 25, 50, and 75 MPa. Pore fluid pressure was maintained at 50 MPa throughout. Overall, the coefficient of friction (μ) of the fault rocks varies between 0.25 and 0.8, emphasizing the marked strength heterogeneity that may exist within natural fault zones. Also, μ is strongly dependent on fault rock mineralogy and is <0.45 for fault rocks containing talc, chlorite, and kaolinite and >0.6 for fault rocks dominated by quartz, dolomite, calcite, and amphibole. Localization of frictional slip within talc‐rich portions of the fault core can potentially explain movements along the Zuccale fault over a wide range of depths within the upper crust, although the mechanical importance of the talc‐bearing fault rocks likely decreased following their dismemberment into a series of poorly connected fault rock lenses. Additionally, slip within clay‐bearing fault gouges with μ between 0.4 and 0.5 may have facilitated movements in the uppermost (<2 km) crust. For several other fault rock components, μ varies between 0.5 and 0.8, and mineralogical weakening alone is insufficient to account for low‐angle slip. In the latter fault rock components, other weakening mechanisms such as the development of high fluid pressures, or dissolution‐precipitation creep, may have been particularly important in reducing fault strength.

The mechanics of low‐angle normal faulting and metamorphic core complexes continue to be a subject of debate. We investigate the conditions, timing, and kinematics of slip in the late, upper‐crustal stages of core complex evolution of the Ruby Mountains detachment fault at the well‐exposed Secret Pass locality with an X‐ray diffraction (XRD) and Ar‐Ar study of clay gouge samples from three separate faults, two from the low‐angle detachment system and one from a high‐angle normal fault that soles into the main detachment fault system. XRD analysis and modeling of XRD analysis show that authigenic illite‐rich illite/smectite (I/S) in gouge at Secret Pass is distinguishable from clay phases in hanging wall rocks because the I/S in the gouges contains only one‐water layer as opposed to the more common two‐water I/S phases found in both the hanging wall and footwall. Ar‐Ar ages for the monomineralic one‐water I/S found in the hanging wall high‐angle fault, the main detachment, and a low‐angle normal fault structurally above the main detachment are 11.6 ± 0.1 Ma, 12.3 ± 0.1 Ma, and <13.8 ± 0.2 Ma, respectively. The not‐quite‐flat Ar‐Ar spectra indicate the gouge illites grew over some interval of time and not in discrete events. The nearly overlapping ages indicate that gouge formation and thus the last major period of activity on the detachment were at 11–13 Ma and were active coevally as part of a kinematically linked fault system with the main detachment active at dips <45° and possibly as low as 22°.

The existence of active low‐angle normal faults is much debated because (1) the classical theory of fault mechanics implies that normal faults are locked when the dip is less than 30° and (2) shallow‐dipping extensional fault planes do not produce large earthquakes (M > 5.5). However, a number of field observations suggest that brittle deformation occurs on low‐angle normal faults at very shallow dip. To reconcile observations and theory, we use an alternative model of fault reactivation including a thick elasto‐plastic frictional fault gouge, and test it at large strain by the mean of 2D mechanical modeling. We show that plastic compaction allows reducing the effective friction of faults sufficiently for low‐angle normal faults to be active at dip of 20°. As the model predicts that these faults must be active in a slip‐hardening regime, it prevents the occurrence of large earthquakes. However, we also evidence the neoformation of Riedel‐type shear bands within thick fault zone, which, we believe, may be responsible for repeated small earthquakes and we apply the model to the Gulf of Corinth (Greece). Key Points Slip on low‐angle normal faults is possible for peak friction as high as 0.4 Low‐angle normal faults are not themselves generating earthquakes when they slip Slip on Riedel shears within LANFs can generate the micro‐seismicity observed

Minidetachments (MDs) found in the uppermost footwall of the Whipple low‐angle normal fault record physical and chemical conditions of LANF formation and early history. MDs are subparallel to the Whipple LANF and mimic features of that fault on a small scale. Principal slip surfaces and R1 Riedel shear fractures parallel C and C′ planes, respectively, in adjacent mylonites. Thus, MDs likely formed subparallel to planes of maximum shear stress and were not severely misoriented during initial rupture of intact rock. Damage zones contain secondary epidote, titanite, chlorite, calcite, and felspars. Breccias record volume gains via enrichment in all elements relative to immobile Fe‐Ti‐Zr‐P, and ultracataclasites record volume losses. Epidote and titanite are locally porphyroclastic in mylonites; cataclasites contain both old shattered fragments and new euhedral grains of these minerals. Pseudosections constrain alteration, the end of mylonitization, and cataclasis to T = 380–420°C. Fluid inclusions with 17–20 wt% CaCl2 were entrapped at 270–290, 170–200, and 80–130 MPa, consistent with a drop from lithostatic toward hydrostatic Pfluid at ∼9.5 km depth. MDs thus record (1) infiltration of reactive fluids into a mid‐crustal shear zone; (2) reaction strengthening at the locus of maximum infiltration and sealing; (3) brittle fault slip triggered by fluid overpressure; and (4) permanent embrittlement following reduction of Pfluid. The brittle‐plastic transition and crustal strength maximum were strongly modified by fluid‐ and reaction‐driven mineralogical changes. At any given point in space or time, this “transition” may thus be very thin, corresponding to the thickness of the altered zones surrounding nascent LANFs. Key Points Minidetachments constrain conditions low‐angle normal fault initiation and slip Fluid infiltration into mylonites controlled locus of brittle‐plastic transition LANFs are neither misoriented nor weak at time of formation

The eastern part of Mykonos island (Cyclades, Greece) shows the detailed internal structure of a two‐branch shallow‐dipping extensional detachment system of Miocene age. This paper shows that the last stage of slip along the detachment, during the deposition of syn‐rift sediments, occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension. Extensional deformation during and after a 13 Ma old granite intrusion is taken up by two main shallow‐dipping shear zones: (1) The lower Livada detachment, which extends laterally across Tinos island, is a ductile structure located at the interface between the granite and the Upper Cycladic Nappe metabasites. A pervasive top‐to‐the‐NE shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact. Later brittle faults, shallowly and steeply dipping, rework the ductile deformation with the same overall shear sense. (2) The upper Mykonos detachment is brittle and separates the metabasites from late Miocene sandstones and conglomerates. The detachment fault dips 12–15° toward the NNE, and its sense of shear is consistent with that of the Livada detachment. Soft‐sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation. Rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone, indicating that the overall direction of extension is NNE/SSW with a sense of shear toward the NNE and the dip of the fault is throughout very low. The two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the Aegean Sea, which emphasizes the importance of low‐angle extensional faults and shear zones in extensional tectonics.

Several active graben systems in Tibet and the Himalaya are the expression of ongoing east-west extension, but the significance and history of normal faulting in this large region are still debated. Here, we present geo- and thermochronological data for a granite intrusion in the footwall of an active high-angle normal fault at the Tangra Yumco graben to constrain the onset and history of normal faulting. Crystallization of the granitic rocks occurred at 87±1 Ma, as revealed by U/Pb zircon dating. After an initial phase of rapid cooling from magmatic temperatures, a later phase of slow cooling is recorded by Rb/Sr biotite ages between ∼72 and ∼60 Ma. The elevation dependence of the Rb/Sr ages suggests that cooling was controlled by erosion, which proceeded at a rate of ?0.05 km/My during the latest Cretaceous and early Paleocene. The subsequent history of normal faulting is recorded by zircon (U-Th)/He ages of 12.5±1.1 and 9.7±0.7 Ma, apatite fission-track ages between 10.8±1.7 and 7.8±1.2 Ma, and apatite (U-Th)/He ages from 4.9±0.4 to 3.0±0.2 Ma. Thermokinematic modeling of these age data indicates that normal faulting started at 14.5±1.8 Ma at a rate of ∼0.3 km/My and accelerated to ∼0.7 km/My in the Pliocene. Our age constraint for the initiation of faulting supports a widespread onset of rifting in Tibet at ∼15-10 Ma, as reported for other graben systems. Finally, we suggest that the distribution of high-angle and low-angle normal faults is controlled by their position relative to the India-Asia convergence vector and by lateral variations in the thermal state of the lithosphere.

The nucleation, propagation, and associated folding of ramp‐flat‐ramp normal faults were analyzed from field examples developed in a brittle/ductile multilayer sequence of the Huércal‐Overa Basin (SE Spain). Gently dipping sandy silt layers, which display a low cohesive strength (C0 = 7 kPa, μ= 34°), favor the development of extensional detachments. A tectonic origin instead of a possible gravitational origin is supported by the perpendicularity between the paleoslope direction of the fluvial‐deltaic environment inferred from imbricated pebbles, and the senses of movement deduced from fault slicken‐lines. The link between high‐angle normal faults (HANFs) —formed at different levels in the layered sequence— with horizontal fault segments comes to develop ramp‐flat‐ramp normal faults with associated roll‐over in the hanging wall. Observed extensional duplexes are formed by parallel detachments connected through synthetic Riedel faults. These Riedel faults would produce the back‐rotation of the individual blocks (horses), i.e., extensional folding of the originally subhorizontal layers. There is no correlation between the analyzed ramp‐flat‐ramp normal faults, accommodating south‐southeastward extension during Serravallian‐lower Tortonian, and either the regional Alpujarride/Nevado‐Filabride west‐directed extensional shear zone or the top‐to‐the‐north detachments within Alpujarride units, which are clearly sealed by Serravallian‐lower Tortonian sediments. Therefore, the studied normal faults are restricted to the brittle/ductile multilayer fluvio/deltaic sequence and accommodate moderate late extension instead of belonging to a large crustal extensional system connected with a regional detachment at depth. Therefore, the basin formed in a moderate crustal thickness context where small and medium‐scale extensional systems were subordinate structures. These natural examples support the development of low‐angle normal faults at very shallow crustal levels in multilayer sequences with suitable rheological conditions. Key Points We analyze near‐surface LANFs in sedimentary rocks They show parallel detachments linked through Riedel faults They are unconnected to basement detachments