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The Late Cretaceous paleogeography of Southern California potentially plays a central role in resolving conflicting models for postulated large-magnitude dextral translations along the western margin of North America (the Baja-BC hypothesis) and the beginning of the Laramide orogeny. The Mt. Pinos sector of the Southern California Batholith provides a unique window into this time because it preserves evidence for a kinematically and temporally partitioned fault system that includes a ductile shear zone (the Tumamait shear zone) and a ductile-to-brittle thrust fault (the Sawmill thrust). These two structures accommodated intra-arc strain during the Late Cretaceous to Paleocene during three phases of deformation (D3-D5) that are superimposed on older (D1 and D2) structures. D1 structures only occur in Pre-Mesozoic rocks and provide a reference frame for understanding subsequent deformation phases. D2 structures form part of a previously unmapped dextral-normal shear zone that predates the Tumamait shear zone. The initiation of displacements within the Tumamait shear zone is recorded by the formation of D3 mylonites which everywhere record reverse-sinistral movement. Petrochronology of syn- D3 titanites give lower-intercept 206Pb/238U dates ranging from 77.0 to 74.0 Ma and upper amphibolite-facies temperatures ranging from 699 to 718°C. Subsequent folding of the D3 mylonites during D4 was synchronous with late-stage, peraluminous magmatism at ca. 70 Ma. Near the Sawmill thrust, the D4 event resulted in a S4 crenulation cleavage and asymmetric, overturned folds that record top-to-the-NE tectonic displacements. NE-directed thrusting along the Sawmill thrust occurred at 67-66 Ma is interpreted to have been kinematically linked to D4 deformation. This thrust placed upper plate rocks of the Southern California Batholith above the Late Cretaceous Pelona schist. We interpret deformational fabrics in the Mt. Pinos area to record a kinematically partitioned, transpressional system that involved sinistral-reverse shearing (D3) closely followed by folding and arc-directed thrusting (D4-D5). We speculate that D3 structures developed in response to opening of the Kula-Farallon plate boundary and we hypothesize that the Kula-Farallon-North American plate triple junction was located at the present-day location of the Garlock Fault at ca. 85 Ma thereby segmenting the arc at this location. This geometry resulted in in dextral shearing in the Sierra Nevada Batholith (and northward) and sinistral shearing in the Southern California Batholith and Baja California. Continued subduction of the Farallon plate beneath the Southern California Batholith led to a major arc flare-up event from 90-70 Ma which was associated with D3 sinistral transpression. We interpret D3-D5 structures to record oblique convergence and the underthrusting of the Hess oceanic plateau beneath the Southern California Batholith at ca. 70-66 Ma. Our model for the segmentation of the California arc is compatible with a moderate (1000-1600 km), ‘Sierra-BC’ translation model in which the Insular superterrane was located north of the Southern California Batholith in the Late Cretaceous. 

Structural analyses combined with U‐Pb zircon petrochronology show the influence of arc magmatism on the evolution of two transpressional shear zones in the deep root of the Late Cretaceous Southern California batholith. The mid-crustal Black Belt and lower-crustal Cucamonga shear zones (eastern San Gabriel Mountains) formed at ~84 Ma shortly after a large mass of tonalite and granodiorite intruded the lower crust. Both shear zones were active until at least ~74 Ma and probably until 72-70 Ma. In the mid-crustal shear zone, rheological contrasts between mingling magmas localized deformation at dike margins. The deformation began as hypersolidus flow in partially crystallized dikes and then transitioned to deformation below the solidus when alternations between viscous creep and brittle faulting produced interlayered pseudotachylyte, cataclasite, and mylonite. As the dikes solidified, strain hardening drove shear zone growth and created thin (10-30 m) high-strain zones and faults that are widely spaced across ~1 km. In contrast, the lower-crustal Cucamonga shear zone was magma-starved, lacks the variety of shear zone fabrics exhibited by its mid-crustal counterpart, and formed by the reactivation of a pre-existing fabric that records pure reverse displacements at 124-93 Ma. The two shear zones created a partitioned style of intra-arc transpression where sinistral-reverse (mostly arc-parallel with some arc-oblique) displacements were accommodated on moderately dipping faults and shear zones and arc-normal shortening was accommodated by coeval folds. This study shows how a magmatic surge influenced the architecture and style of Late Cretaceous transpression in the Southern California batholith, including the evolution of high-strain zones that record alternating episodes of brittle, ductile, and hypersolidus deformation. The results illustrate how magmatism localizes strain on deep-crustal faults during orogenesis and oblique convergence. 

The beginning of the Laramide orogeny is a pivotal time in the geological development of the western United States, but the driving mechanism responsible for mountain building, basin formation and ore mineralization is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith sector of western North America at ca. 88 Ma which caused the angle of subduction beneath the continent to shallow. This subhorizontal (flat) subduction is thought to have led to shut-down of the arc, crustal cooling, and the formation of deep, basement-involved thrust faults that penetrated far into the continental interior. In contrast to these predictions, we show that the Southern California Batholith experienced a magmatic surge from 90 to 70 Ma, the lower crust was hot (835-750°C) and partially molten, and cooling occurred after 75 Ma. These data contradict plateau underthrusting as the driving mechanism for early Laramide deformation at 90-80 Ma; therefore, the Laramide orogeny cannot have been initiated by flat-slab subduction. We propose that the Laramide orogeny is best explained as a two-stage orogeny consisting of: 1) an arc magmatic ‘flare-up’ phase associated with sinistral-reverse ductile shearing in the Southern California Batholith from at 90-75 Ma and coeval dextral-transpression north of the Garlock fault, and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma. Only that latter phase is linked to flat-slab subduction beneath the Southern California Batholith. 

Ancient basement rocks in Southern California contain mechanical anisotropies that may influence the architecture of Quaternary faulting. We study exposed basement rocks found within the southeastern San Gabriel lithotectonic block with the intention of reconciling the relationship between inherited ductile fabrics and the geometry of Quaternary faults that are part of the San Andreas Fault system. By focusing our study on the southeastern corner of the San Gabriel block we can study the exposed lower- to middle crustal shear zone fabrics near where the Cucamonga Fault and the San Jacinto Fault intersect. The brittle Quaternary Cucamonga Thrust Fault strikes E-W and dips to the north-northeast (35-25°) and is localized at the range front and cuts these older fabrics, however there is also brittle deformation distal from the fault that also affects the sequence of lower- to middle crustal (6-8 kbar) granulite- to upper amphibolite facies mylonite and granulite-facies metasedimentary rocks. Near the Cucamonga Fault, mylonitic fabrics strike E-W and dip northeast (40-50°). Quaternary brittle faults that strike E-W and dip northeast (30-40°) reactivate the mvlonites and slickenlines and record a sinistral, top-to-the-west sense of shear. Investigation of host rocks indicates that they formed in the roots of a continental arc which was active from the Middle Jurassic to Late Cretaceous (172-86 Ma) at 740-800°C. Ductile deformation was associated with granulite-facies metamorphism at approximately 30 km depth during the Late Cretaceous (88-74 Ma) at 730-800 °C. Our work shows that the exhumed Late Cretaceous mylonitic fabrics may have operated as stress guides during Quaternary faulting in the Cucamonga Fault zone. We conclude that these lower crustal fabrics influence the geometry and kinematics of late Cenozoic faulting of the Cucamonga and San Jacinto fault zones. 

We present >90 new igneous and metamorphic zircon and titanite petrochronology ages from the eastern Transverse Ranges of the Southern California Batholith (SCB) to investigate magmatic and tectonic processes in the frontal arc during postulated initiation of Late Cretaceous shallow-slab subduction. Our data cover >4000 km2 in the eastern Transverse Ranges and include data from Mesozoic plutons in the Mt. Pinos, Alamo Mountain, San Gabriel Mountain blocks, and the Eastern Peninsular mylonite zone. Igneous zircon data reveal 4 discrete pulses of magmatism at 258-220 Ma, 160-142 Ma, 120-118 Ma, and 90-66 Ma. The latter pulse involved a widespread magmatic surge in the SCB and coincided with garnet-granulite to upper amphibolite-facies metamorphism and partial melting in the lower crust (Cucamonga terrane, eastern San Gabriel Mountains). In this region, metamorphic zircons in gneisses, migmatites and calc-silicates record high-temperature metamorphism from 91 to 74 Ma at 9–7 kbars and 800–730°C. The Late Cretaceous arc flare-up was temporally and spatially associated with the development of a regionally extensive oblique sinistral-reverse shear system that includes from north to south (present-day) the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. Syn-kinematic, metamorphic titanite ages in the Tumamait shear zone range from 77–74 Ma at 720–700°C, titanites in the Black Belt mylonite zone give an age of 83 Ma, and those in the eastern Peninsular Ranges mylonite zone give ages of 89–86 Ma at 680–670°C. These data suggest a progressive northward younging of ductile shearing at amphibolite- to upper-amphibolite-facies conditions from 88 to 74 Ma, which overlaps with the timing of the Late Cretaceous arc flare-up event. Collectively, these data indicate that arc magmatism, high-temperature metamorphism, and intra-arc contraction were active in the SCB throughout the Late Cretaceous. These observations appear to contradict existing models for the termination of magmatism and refrigeration of the arc due to underthrusting of the conjugate Shatsky rise starting at ca. 88 Ma. We suggest that shallow-slab subduction likely postdates ca. 74 Ma when high-temperature metamorphism ceased in the SCB. 

Over 500 km2 of rock exposure in Fiordland, New Zealand records strain localization processes accompanying the formation of a steep, transpressional shear zone within the root of a Cretaceous continental magmatic arc. Here, we pair field observations with microstructural and petrographic analyses of the George Sound shear zone (GSSZ) to investigate how metamorphism and compositional variability influenced shear zone evolution in the lower continental crust. The northern portion of the 50 km-long GSSZ deforms a monzodioritic pluton where superposed mineral fabrics record a narrowing of the shear zone width over time. Early stage deformation was accommodated mostly by dynamic recrystallization of pyroxene and plagioclase, forming a steep zone of coarse, gneissic foliations over 10 km wide. Subsequent deformation created a 2 km-thick zone of mylonite containing fine-grained plagioclase, hornblende, biotite, and quartz. The latter three minerals formed during the hydration of older minerals, including igneous pyroxene. The change in mineralogy and grain size also produced thin (< 1 mm), weak layers that localized deformation in shear bands in the highest strain zones. The southern ~35 km of the GSSZ deforms a heterogeneous section of granite, diorite, and metasedimentary rock. In this area, the hydration of igneous assemblages also is pervasive but is not restricted to high-strain zones. Instead, the shear zone branches into four ≤1 km-wide strands that closely follow lithologic contacts. The thinnest branch occurs at the contact of a coarse-grained, dioritic pluton and a fine-grained granitic pluton. These patterns suggest that the factors that controlled strain localization in the GSSZ vary along its length. In the north, where its host rock is homogeneous, retrograde metamorphism helped localized strain into shear bands at the micro scale, mirroring a narrowing at the km scale. In the south, lithologic contacts created weak zones that appear to have superseded the effects of metamorphism, creating a series of thin, branching high-strain zones. These results suggest that the rheology of lower-crustal shear zones also varies significantly along their length and over time. Both of these factors can be used to generate improved models of continental deformation. 

The quantification of strain in three-dimensions is a powerful tool for structural investigations, allowing for the direct consideration of the localization and delocalization of deformation in space, and potentially, in time. Furthermore, characterization of the distribution of strain in three-dimensions may yield information concerning large-scale kinematics that may not be obtained through the traditional use of asymmetric fabrics. In this contribution, we present a streamlined methodology for the calculation of three-dimensional strain using objective approaches that allow for consideration of error assessment. This approach begins with the collection of suitable samples for strain analysis following either the Rf/ϕ or normalized Fry techniques. Samples are cut along three mutually perpendicular orientations using a set of jigs designed for use in a large oil saw. Cut faces are polished and scanned in high resolution. Scanned images are processed following a standard convention. The boundaries of objects are outlined as “Regions Of Interest” in the open-source program ImageJ and saved. A script reads the saved files of object outlines and statistically fits an ellipse to each digitized object. The parameters of fitted objects are then extracted and saved. Two-dimensional strain analyses are completed following the normalized Fry method or the Rf/ϕ technique following a bootstrap statistical approach. For the normalized Fry method, an objective fitting routine modified from Mulchrone (2013) is used to determine the parameters of the central void. For the Rf/ϕ method, an inverse straining routine is applied and tests the resulting object orientations to a random uniform distribution following a Kolmogorov–Smirnov test in order to obtain the sectional strain ratio and orientation. Bootstrap sampling of Fry coordinates or objects results in a distribution of possible sectional strains that can be sampled for fitting of strain ellipsoids following the technique of Robin (2002). As such, the parameters of three-dimensional strain including Lode parameter and octahedral shear strain can be contoured based on confidence intervals for each sample processed. The application of the objective approach is presented in a corresponding poster.