Mélanges, here defined as high-strain zones with a block-in-matrix structure, are common in exhumed subduction terranes and accommodate deformation between the overriding and subducting plates (Bebout and Penniston-Dorland, 2016). The large strength and geochemical contrasts within mélange result in unique rheological behavior (Fagereng and Sibson, Cailey B. Condit https://orcid.org/0000-0001 -5024-9101 *ccondit@uw.edu 2010) that may play a role in producing slow slip and tremor (Beall et al., 2019;Behr and Bürgmann, 2021;Hoover et al., 2022) and also influence arc magmatism (e.g., Marschall and Schumacher, 2012).
Disentangling the timing and general processes responsible for mélange formation is key to understanding the rheological significance of these units and their role in pre-, syn-, and/or post-subduction slip behaviors (e.g., slow slip, creeping, co-seismic deformation) and chemical evolution (e.g., Easthouse et al., 2025).
We present field observations, structural analysis, and microstructural data that reveal that syn-subduction mélange formation occurred at near-peak lawsonite-blueschist facies conditions in the Catalina Schist paleo-subduction terrane on Santa Catalina Island (Pimu'nga), California, USA (Fig. 1A). Folding and shearing of metasedimentary rocks coupled with feedbacks between deformation and fluid infiltration produced both a weak mélange matrix and strong mélange blocks. Rheological analysis demonstrates that fluid-assisted viscous deformation of this metasedimentary mélange did not accommodate slow slip at the typical low stresses of the plate interface but could host tectonic creeping.
The Catalina Schist comprises slices of Cretaceous subduction-related rocks (Fig. 1A; Platt, 1975;Grove and Bebout, 1995;Platt and Schmidt, 2024). These schists represent underplating of progressively cooler and younger subduction terranes between 115 Ma and 95 Ma, culminating with emplacement of the structurally lowest and youngest lawsonite blueschist unit (LBU) at peak conditions of ∼1.0 GPa and 250-350 °C at ca. 95 Ma (Grove and Bebout, 1995;Platt and Schmidt, 2024).
The LBU is exposed at Catalina (Cat) Harbor (Figs. 1B and 2A) and contains quartz vein-rich deformed metagraywackes, metasandstones, metapelites, metacherts, and (minor) metaultramafics (Fig. 2; Fig. S1 in the Supplemental Material 1 ) with a peak subduction temperature of 331 ± 21 °C (Fig. 1B; Platt and Schmidt, 2024). We focused on an ∼200-m-long exposure of block-in-matrix structures with dismembered to intact folds, veins, and high-strain foliation fabrics with field evidence consistent with simple 1 Supplemental Material. Materials and methods, including EBSD methodology and sample preparation, grain-size determination, and rheological plotting. Additional figures showing field, microstructures, and EBSD data are also included. Please visit https:// doi.org/10.1130/GEOL.S.29661134 to access the supplemental material; contact editing@geosociety .org with any questions.
shear deformation (Figs. 1B and 2). The initial foliation is approximately parallel to the original compositional layering (S1 ∼ S0 foliation) striking northwest-southeast and dipping variably to the northeast and southwest (Figs. 2B-2E). These occur as albitic quartz-mica schists that formed as a Cretaceous marine turbiditic sedimentary sequence (Grove and Bebout, 1995). S1 layering is folded by open to tight, recumbent asymmetrical southwest-verging F2 folds (one meter to tens of meters in scale) trending northwest-southeast (Figs. 2B-2D). In the metapelitic layers, these folds develop axial planar (S2) foliations (Fig. 2E) parallel to transposed earlier fabric (now S1-2) and sheared in the limbs of F2 folds (Figs. 2C-2F). Within F2 noses, millimeter-to centimeter-wide extensional quartz veins occur at a high angle to the S1 fabrics at variable orientations (Fig. 2D; Fig. S1B), which formed by outer arc extension around fold hinge areas. Progressive shearing during folding resulted in rootless quartz vein-rich asymmetrical topto-the-southwest fold noses that can occur as isolated blocks (Fig. 2F) and have consistent kinematics with other shear sense indicators that define S2 fabrics (Fig. 3C).
To constrain the tectonic processes that lead to mélange formation and its unique rheology, we combined structural observations with electron backscatter diffraction (EBSD) data. We focused on a rootless fold nose (block) and the high-strain matrix, which contain albite-quartzwhite mica-stilpnomelane-apatite ± chlorite.
Within fold noses, the foliation (S0 ∼ S1; Figs. 3B and 3C) is crosscut at high angles by coarse-grained quartz veins (Figs. 2C and 3A; Figs. S1C and S2A). Quartz in the veins show limited crystal-plastic deformation with minor bulging to incipient subgrain microstructures (Fig. 3A), consistent with deformation at 300-350 °C (Stipp et al., 2002) at peak subduction temperatures (Platt and Schmidt, 2024). The foliation is defined by layers of variably kinked phyllosilicates and quartzofeldpathic microlithons (Figs. 3B and 3C). In the foliation, quartz and albite grains are ∼50 μm and 60 μm in diameter, respectively (see the Supplemental Material), and show little to no evidence of crystallographic preferred orientation [CPO] or intragranular misorientations [IM] (Fig. 3D).
Within the matrix fabric (S2), to first order, we observe similar microstructures as the fold noses, including alternating quartzofeldspathic and phyllosilicate layers, lack of quartz ± albite CPO, and little evidence of IM (Fig. 3E; Fig. S2B). However, in contrast to the fold noses, the grain sizes in the matrix fabrics are significantly smaller and pinned by phyllosilicates, with ∼20 μm for quartz and albite (Fig. 3F; see the Supplemental Material). Additionally, asymmetrical mica fish indicate a top-to-the-southwest sense of shear (Fig. 3F; Fig. S2B), consistent with macrostructural observations (Fig. 2F).
Based on our microstructural observations of little to no CPOs, small grain sizes, and limited IMs, S2-dominated high-strain fabrics (F2 fold limbs S1-2 and axial planar S2 fabrics) deformed by pressure solution creep of fine-grained quartz and albite. In contrast, the fold noses experienced minor deformation by dislocation creep in coarse-grained quartz veins and some pressure solution of coarse-grained albite ± quartz. Phyllosilicates acted as pinning phases and provided faster pathways for fluid transport (Hickman and Evans, 1995). Kinking of these phases in the fold noses (Fig. 3C) and a lack of large, interconnected mica zones in the matrix indicate that basal sliding did not accommodate high strains in either setting. Deformation of the fold veins occurred at lawsonite-blueschist facies conditions of ∼330 °C and ∼1.0 GPa, based on the peak mineral assemblage, thermometry (Grove and Bebout, 1995;Platt and Schmidt, 2024), and the undulose extinction and IM (e.g., Figs. 3A and 3E), which are clear quartz dislocation creep microstructures. Guided by these deformation mechanisms, we modeled the strength of these mélange components (Fig. 4A). We used the experimentally constrained flow laws for quartz and albite dislocation creep (Hirth et al., 2001;Offerhaus et al., 2001;Tokle et al., 2019) and pressure solution and/or diffusion creep (Den Brok, 1998;Offerhaus et al., 2001).
Collectively our micro-and meso-structural and petrographic observations show that quartz ± albite controls the contrasting strength in these rocks, and this contrast manifests in the formation of the block-in-matrix mélange structure. Quartz veins in fold noses formed the observed blocks (Figs. 2A and 2D-2F). These veins have coarse grain sizes (>100 μm) and were minorly deformed by the grain-size-insensitive mechanism of dislocation creep (Fig. 4A). In contrast, the fine grain size (∼20 μm) of the highly strained fold limbs turned-matrix facilitated deformation via grain-size-sensitive pressure solution creep. The larger grain sizes in the fold noses result in the pressure solution requiring higher stresses than the matrix (Fig. 4A). Thus, at these temperatures, the fold noses deforming by dislocation creep of quartz are stronger than the fold limbs, which deformed by grainsize-sensitive pressure solution creep at the rela-tively low stresses (less than tens of MPa) of the subduction interface (Fig. 4A; Stöckhert, 2002;Behr and Platt, 2013;Platt et al., 2018;Condit et al., 2022). Together, the fracturing and mineral precipitation kept the coarse-grained fold noses strong, while continued shearing, fluid flow, and pressure solution creep in the fine grains of the fold limbs led to a weak matrix (Fig. 4A). This is consistent with our macrostructural observations (e.g., Figs. 3D-3F).
Folding, fracturing, precipitation, veining, and progressive shearing lead to block-in-matrix development within the Cat Harbor mélange (Fig. 4B i-iv). Deformation fabrics are defined by the peak mineral assemblage, indicating mélange formation occurred during near-peak prograde or earliest retrograde subduction processes (Fig. 2B). Fluid-mediated deformation created a blockin-matrix structure with block strengthening and matrix weakening governed by the grain size of quartz and its resulting deformational response (Fig. 4). The extensional vein mesh networks that strengthened blocks form due to orthogonal flexure and outer arc fold extension. Both vein formation and pressure solution creep require ample free fluids, yet depending on the structural context (i.e., fold nose versus limb), these processes will lead to (1) strong blocks through coarse-quartz-grain growth in veins, or (2) weak matrix through pressure solution creep in fine grains.
Fluid flow within the matrix of mélange zones facilitates deformation by pressure solution creep due to the fine grain sizes of the matrix quartz ± albite, which are maintained and augmented by the pinning of phyllosilicates (e.g., Behr and Platt, 2013;Condit et al., 2022). This is in stark contrast to the way fluids lead to strengthening via coarse-grained vein precipitation in fold noses; because of the coarse grain sizes in the fold foliation, pressure solution is also stronger in the blocks (Fig. 4A). Thus, fluid-mediated deformation leads to both a net strengthening of blocks and a weakening in the matrix (Fig. 4B). These processes lead to block-in-matrix mélange formation without large initial rheological contrasts imparted by variable lithologies, as observed in some olistostromal mélanges (e.g., Festa et al., 2022). Our model does not explain all mélange occurrences. However, we show that these block-inmatrix structures can develop due to fluid-mediated syn-subduction deformation of a single metasedimentary package (Fig. 4B). Folding, simple shear, and fluid infiltration are common processes along the subduction plate interface, as is the development of quartz vein networks (Fisher et al., 1995;Fagereng, 2011;Ujiie et al., 2018).
Our rheological analysis indicates that shear stresses were, at maximum, ∼30 MPa in order to satisfy the observed deformation mechanisms (Fig. 4A). These stresses readily accommodate strain rates of ∼10 -13 s -1 to 10 -14 s -1 and slower. For a reasonable thickness of a mélange unit that ranges from tens to hundreds of meters, these stresses correspond to slip rates of ∼10 -13 m/s to 10 -12 m/s, consistent with tectonic creep rates (French and Condit, 2019). This clearly indicates that the deformation mechanisms responsible for the microstructures in these mélanges cannot host slow earthquakes; given similar thicknesses as above, slow slip rates of ∼10 -8 m/s to 10 -7 m/s (e.g., Hirose and Obara, 2005) correspond to strain rates of 10 -10 s -1 to 10 -8 s -1 . These slow-slip strain rates for quartz require minimum stresses of >200 MPa to be accommodated by dislocation creep and >>10 4 MPa to be accommodated by pressure solution creep (Fig. 4A). Given the low stresses inferred at slow slip conditions, these rocks are unable to host slow slip deformation. Consistent with this analysis, we do not see evidence for stress amplifications in these blocks, as expected dur-ing deformation at slow-slip, high-strain rates (e.g., Beall et al., 2019;Hoover et al., 2022). We suggest that only tectonic creeping was hosted in these rocks by pressure solution creep. Slow slip may not have occurred within the relatively cold LBU (cf., Condit et al., 2020), but if it did, it was likely in stress-sensitive lithologies like talc-or chlorite-schists (French and Condit, 2019).
Syn-subduction fluid-mediated deformation led to simultaneous strengthening and weakening of metasedimentary rocks during the development of block-in-matrix mélange structures in the lawsonite blueschist unit of the Catalina Schist. Quartz veins formed during folding led to block formation in rootless fold noses, while fine grain sizes and pressure solution creep led to weakening and matrix formation from the fold limbs at near to peak subduction conditions. This did not require initial large lithologic or strength heterogeneity because quartz acts as both a strain-hardening and strain-softening phase dependent on grain size and the effective deformation mechanism. Our rheological analysis indicates that this quartz-and albitedominated metasedimentary mélange matrix cannot host slow slip and likely accommodated tectonic creeping.