Brendan McNulty

Variations in deformation fields during development of a large-volume magmatic arc, central Sierra Nevada, California

Mid- to Late Cretaceous plutons in the central Sierra Nevada magmatic arc show widely preserved magmatic foliation, whereas regionally developed solid-state foliation is absent. Relatively slow cooling of these plutons and expected strain rates (10^(−14)) suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime. Domains of solid-state ductile shear of only slightly younger age than the plutons, on the other hand, indicate a contractional regime. Timing of pluton emplacement and movement on the shear zones have been constrained using Pb-U (zircon) and ^(40)Ar/^(39)Ar (hornblende and biotite) geochronology. Both plutons and ductile shear zones become younger toward the east. The four more westerly shear zones, which were active between ca. 100 and 90 Ma, show steeply plunging stretching lineations, whereas the most easterly and/or youngest zones, active between ca. 88 and 78 Ma, show mostly oblique and/or subhorizontal stretching lineations, indicating a change in kinematics at ca. 90 Ma. The above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional. We propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid- and upper levels, probably as dikes via fractures. Slab flattening caused the neutral or weak extensional regime to move eastward away from the trench. Increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons, and domains of ductile shear formed in progressively younger plutons to the east. The above events were accompanied by an oblique convergence vector between North America and Farallon plates (Engebretson et al., 1985), which imposed a relatively small component of right-lateral shear onto the arc that increased with time. We estimate that at ca. 100 Ma the convergence vector made an angle (Φ_(obl)) ≈ 20° to the arc normal, and we suggest that around ca. 90 Ma Φ_(obl) passed through a critical value, conceivably (20° < Φ_(oblcrit) < 30°). At this juncture, the component of right-lateral shear became sufficiently large to induce significant arc-parallel strike-slip movement on the most easterly shear zones; these kinematics continued as the dominant scheme, possibly as late as ca. 78 Ma.

Multistage emplacement of the Mount Givens pluton, central Sierra Nevada batholith, California

The ca. 90 Ma Mount Givens pluton is one of the largest granodioritic to granitic intrusions in the Sierra Nevada batholith of California. Emplacement of the pluton occurred during a critical time in the tectonic evolution of the central Sierra Nevada magmatic arc, marked by a transition from regional contraction to dextral transcurrent shear. A model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility (AMS) at 351 stations. One of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton, and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain. The ∼80-km-long, 15–30-km-wide pluton crystallized from a multiphase, three-segment magma chamber marked by a bulbous northern lobe and linear central and southern segments. The pluton is interpreted to be tabular in shape with a thickness of ∼5 km. Most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets, the most important of which were steeply dipping, northwest-trending fractures formed parallel to the structural grain of the arc, and vertical, north-trending extension fractures formed in response to a component of arc-parallel dextral shear. Some of these fractures acted as magma conduits, episodically filling the pluton as source rocks became depleted in melt. An initial, voluminous intrusive event (stage 1) quickly filled the southern chamber with granodiorite magma, but only partially filled the northern and central chambers. Stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber, the latter delineated by a 20-km-long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments. Floor downdrop eventually ceased to be an effective space-making process in the northern lobe, and renewed magmatism (stage 3) led to expansion and doming of the chamber. As the northern lobe cooled, a ring fault ruptured within the viscoelastic stage 1–2 carapace, allowing ring dike intrusion (stage 4) and sinking of a central flap of consolidated material. The temporal and spatial variations in emplacement mechanisms demonstrated for the Mount Givens pluton (i.e., fracture generation, floor downdrop, underplating, inflation, ring diking) suggest that end-member models (e.g., fracture vs. diapir) are oversimplifications of the pluton assembly process.

Assembly of a dike-fed magma chamber: The Jackass Lakes pluton, central Sierra Nevada, California

The mechanisms of ascent, assembly, and emplacement of granitic magma in the crust are critical to understanding the dynamics of continental margin growth, yet these mechanisms remain controversial and poorly understood. Detailed study of structural and petrologic fabrics in the middle Cretaceous Jackass Lakes pluton-wall-rock system, central Sierra Nevada, California, coupled with U-Pb geochronology, indicates that the pluton formed via sheet-like assembly of a dike-fed magma chamber. Final emplacement of the pluton was facilitated by multiple brittle and ductile mechanisms that were active at different times and places within the system; this supports hybrid viscoelastic emplacement models as realistic alternatives to end-member models (i.e., dike versus diapir). Fracture propagation, which initiated ≈40% of the space required for emplacement, may have been facilitated by a small component of arc-parallel dextral shear that produced north-northwest-striking tension gashes. A combination of ductile wall-rock shortening during lateral expansion of sheets, and return flow of elongate, strongly deformed wall-rock septa, produced an additional ≈25% of the space required. Other mechanisms, including coeval formation of the overlying Minarets caldera and stoping in the subvolcanic part of the magma chamber, must account for the remaining ≈35% space, implying that vertical transfer of material is an important emplacement mechanism at shallow crustal levels.

Role of plate kinematics and plate-slip-vector partitioning in continental magmatic arcs: Evidence from the Cordillera Blanca, Peru

New structural and geochronological data from the Cordillera Blanca batholith in the Peruvian Andes, coupled with Nazca–South American plate-slip-vector data, indicate that oblique convergence and associated strike-slip partitioning strongly influenced continental magmatic arc evolution. Both the strain field and mode of magmatism (plutonism vs. volcanism) in the late Miocene Peruvian Andes were controlled by the degree to which the arc-parallel component of the plate slip vector was partitioned into the arc. Strong strike-slip partitioning at ca. 8 Ma produced arc-parallel sinistral shear, strike-slip intercordilleran basins and east-west–oriented tension fractures that facilitated emplacement of the Cordillera Blanca batholith (ca. 8.2 ± 0.2 Ma). Periods during which the strike-slip component was not partitioned into the arc (ca. 10 and ca. 7 Ma) were associated with roughly arc-normal contraction and ignimbrite volcanism. The data thus support the contention that contraction within continental magmatic arcs favors volcanism, whereas transcurrent shear favors plutonism. The tie between oblique convergence and batholith emplacement in late Miocene Peruvian Andes provides a modern analogue for batholiths emplaced as the result of transcurrent shear in ancient arcs.

Microgranitoid enclave swarms in granitic plutons, central Sierra Nevada, California

Abstract Microgranitoid enclaves are common in granitic plutons worldwide, occurring individually and in homogeneous or heterogeneous swarms. Three plutons in the central Sierra Nevada batholith contain swarms with mostly heterogeneous suites of enclaves in the intermediate composition range, and occur in a number of two-dimensional shapes, specifically as dikes, small rafts, lenses, pipe/vortices and large massive shapes. Swarms are characterized by various features, including the nature of their boundary with the host, their planar or non-planar character, internal geometry, density of enclave packing, presence or absence of schlieren and crystal aggregates, and axial ratios and degree of preferred alignment of enclaves. We propose that heterogeneous enclave swarms form by one, or some combination of, the following mechanisms: (1) velocity-gradient sorting parallel or normal to the flow, (2) gravitational sorting or (3) break-up of heterogeneous dikes. Common sites where enclave swarms form include pluton margins or internal viscosity walls, within fractures, and near the pluton roof.

Active detachment faulting above the Peruvian flat slab

The Cordillera Blanca detachment fault in the Peruvian Andes is, to our knowledge, the first active detachment to be documented above a modern flat slab. Crustal detachment has unroofed the ca. 8 Ma Cordillera Blanca batholith, now the backbone of the highest mountain range in Peru. Large-magnitude slip along the fault was thermally enhanced by emplacement of the batholith, the penultimate magmatic event prior to flattening of the Nazca slab. However, extensional models based on arc magmatism and crustal thickening alone do not adequately explain the scale or structural asymmetry of a series of young, deep-seated, west-dipping normal faults across Peru. Here we show that the onset of detachment faulting coincided with subduction of the aseismic Nazca Ridge and consequent flattening of the Nazca slab. We propose that slab buoyancy from ridge subduction triggered extensional collapse of the prethickened continental crust, and that this buoyancy drove footwall uplift that exceeds basin subsidence. The west-dipping asymmetry of late Cenozoic extensional faults in Peru may be controlled by a preexisting crustal anisotropy (older thrusts), and/or formation of Riedel-like shears kinematically linked to the flat Nazca slab.

Pseudotachylyte generated in the semi-brittle and brittle regimes, Bench Canyon shear zone, central Sierra Nevada

Pseudotachylyte cuts Cretaceous plutonic and volcanic rocks in the Bench Canyon shear zone, central Sierra Nevada, California. As documented by optical and scanning electron microscopy, the pseudotachylyte displays evidence for a friction melt origin, including vesicles, amygdules, crystallites, flow fabrics and embayed crystal fragments. Based on fault-related rock association, vein morphology, and inferred crustal level of generation, two distinct types of pseudotachylyte are documented in the shear zone. Pseudotachylyte associated with cataclasite was generated at shallow crustal levels (~1–10 km) via unstable, velocity-weakening, abrasive wear friction. Pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels (~10–15 km) most likely via stable, velocity-strengthening, adhesive wear friction; this pseudotachylyte underwent ductile deformation in the solid state. Interlayered pseudotachylyte and mylonite records cyclic aseismic/seismic slip at a transitional crustal level between the semi-brittle field of quartzofeldspathic rocks and the base of the seismogenic zone. The cyclicity is attributed to strain rate fluctuation effected by strain-resistant phacoids, pore-fluid pressure variation, ductile instabilities, or downward fault propagation. The mode of deformation within the shear zone progressed from (i) production of mylonite during amphibolite to greenschist facies conditions, to (ii) generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi-brittle field (~300–400 °C), to (iii) generation of pseudotachylyte through seismic events in the brittle (< ~300 °C) during uplift/exhumation and cooling; some of the latter may have been generated at shallow enough crustal levels (< ~4 km) to allow vesicle formation. The exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi-brittle flow to seismogenic rupturing.

Post-Middle Oligocene origin of paleomagnetic rotations in Upper Permian to Lower Jurassic rocks from northern and southern Peru

Abstract We report paleomagnetic data from 28 sites of Upper Permian to Lower Jurassic strata from northern and southern Peru. In northern Peru (6°S), a stable magnetic component from six Permo-Triassic sites passes fold and reversal tests. The overall mean pole agrees well with Late Permian to Triassic poles from cratonal South America, suggesting this part of Peru has experienced neither significant rotation nor latitudinal transport since the Permo-Triassic. In southern Peru (13 to 16°S), thermal demagnetization isolates stable magnetic components in 16 of 22 Upper Permian to Lower Jurassic sites collected along the transition between the Altiplano and the Eastern Cordillera. These 16 sites are rotated 14 to 147° counterclockwise and pass an inclinations-only fold test. Within the same structural zone, three other Permo-Triassic sites as well as 10 Paleocene sites also show important counterclockwise rotations [Roperch and Carlier, J. Geophys. Res. 97 (1992) 17233–17249; Butler et al., Geology 23 (1995) 799–802]. The large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component. No correlation exists between rotation amount and rock age, suggesting the rotations are post-Paleocene in age. Because the rotations occur along the fringe of the Eastern Cordillera, they were likely produced during its structural formation, hence from the Late Oligocene to Present. Sinistral shear acting in the northern part of the Bolivian Orocline appears much more pronounced than that north of the Abancay Deflection, which likely arises from differences in convergence obliquity.

Paleomagnetic evidence for rapid vertical-axis rotation in the Peruvian Cordillera ca. 8 Ma

Paleomagnetic results from 31 Neogene sites in the Peruvian Andes yield primary magnetizations, as demonstrated by positive fold and reversal tests. Strata dated as 18–9 Ma record a significant counterclockwise rotation (−11° ± 5°), whereas unconformably overlying younger strata (7–6 Ma) are not rotated. The age of rotation thus is between 9 and 7 Ma, a period that coincides with the widespread Quechua 2 deformation phase. Moreover, eight independent studies on 107–9 Ma rocks from Peru between 9°S and 15°S reveal similar and significant rotations (−15° ± 6°). This suggests that the region rotated during a 2 m.y. period of deformation ca. 8 Ma, when the Andes underwent rapid uplift and important deformation commenced in the Subandean zone.

Shear zone development during magmatic arc construction: The Bench Canyon shear zone, central Sierra Nevada, California

The Bench Canyon shear zone is a 20-km-long, moderately to steeply dipping zone of ductile to brittle strain that cuts Cretaceous plutonic and volcanogenic rocks located in the central Sierra Nevada, California. This zone is one of several Cretaceous-age ductile shear zones that record fluctuating strain fields associated with the tectonic/magmatic evolution of the central Sierra Nevadan magmatic arc. As determined by field relations, fabric analysis, and U-Pb and 40 Ar/ 39 Ar geochronology, the Bench Canyon shear zone underwent an episodic deformation history involving both contractional and extensional strain over a period of ∼17 m.y. or longer. Deformation is divided into early (ca. 101?–95 Ma), main (ca. 95–90 Ma), and late (ca. 90–78 Ma) phases. Temperature during main-phase deformation followed a retrograde T - t path of ∼600–300 °C after emplacement of multiple plutons. Main-phase deformation involved ductile thrust movement in both the 95 ± 1 Ma Red Devil Lake pluton and Lower to mid-Cretaceous volcanic rocks, where thrusting was facilitated by heat and fluids associated with plutonism. Fingerlike sills associated with the Red Devil Lake pluton were emplaced syntectonically with respect to main-phase deformation, but cooled rapidly, undergoing heterogeneous strain more typical of “pre-tectonic” emplacement. The ca. 90 Ma Mount Givens pluton cuts the shear zone fabric in the wall rock, limiting the bulk of deformation to have occurred prior to that time. Domainal and diachronous late-phase reactivation(s) along the zone involved weak, fluid-enhanced ductile (∼500 > T > 300 °C) deformation, and generation of abundant cataclasite and pseudotachylyte. The long-lived, episodic and diachronous deformation history of the Bench Canyon shear zone illustrates the complexities to be expected within shear zones during continental magmatic arc evolution.