John M. Bartley

Are plutons assembled over millions of years by amalgamation from small magma chambers

APRIL/MAY 2004, GSA TODAY ABSTRACT Field and geochronologic evidence indicate that large and broadly homogeneous plutons can accumulate incrementally over millions of years. This contradicts the common assumption that plutons form from large, mobile bodies of magma. Incremental assembly is consistent with seismic results from active volcanic areas which rarely locate masses that contain more than 10% melt. At such a low melt fraction, a material is incapable of bulk flow as a liquid and perhaps should not even be termed magma. Volumes with higher melt fractions may be present in these areas if they are small, and this is consistent with geologic evidence for plutons growing in small increments. The large melt volumes required for eruption of large ignimbrites are rare and ephemeral, and links between these and emplacement of most plutons are open to doubt. We suggest that plutons may commonly form incrementally without ever existing as a large magma body. If so, then many widely accepted magma ascent and emplacement processes (e.g., diapirism and stoping) may be uncommon in nature, and many aspects of the petrochemical evolution of magmatic systems (e.g., in situ crystal fractionation and magma mixing) need to be reconsidered.

Space-time patterns and tectonic controls of Tertiary extension and magmatism in the Great Basin of the western United States

Structural and stratigraphic relations in the Great Basin indicate widespread pre-middle Miocene crustal extension that appears to define two north-trending belts. Most extension in these belts was Oligocene age, but locally it began earlier or lasted into early Miocene time. The eastern belt straddles the Nevada-Utah border and includes the Snake Range, Nevada, area, with its southern end near 37.5°N and its western edge at the Seaman-Butte Mountains breakaway. The southern boundary of the eastern belt is occupied by the 26-15 Ma Caliente caldera complex and approximately coincides with the present east-west-trending margin of the Great Basin north of Saint George, Utah. Crust north of this boundary extended approximately east-west before volcanism began at 30-32 Ma, but to the south, extension began after about 15 Ma. This boundary may have been a rooted zone of left-slip faults that allowed the footwall of the Stampede detachment to move west relative to unextended terrain to the south. The eastern margin of the eastern belt is probably located near the present eastern edge of the Great Basin, but its northern end is poorly defined. The western belt runs from the Funeral and Grapevine Mountains, California, to the Ruby Mountains, Nevada, and north-northeast to the Albion Range, Idaho. Tens of kilometers of crustal extension occurred at least locally in both belts, but magnitude of extension is poorly known for large areas of each. Tertiary volcanism in the Great Basin began in the north in Eocene time with predominantly effusive volcanism and swept southward, ending with voluminous Oligocene-Miocene ignimbrite eruptions from calderas in an irregular, discontinuous belt between Marysvale, Utah, and Reno, Nevada. A result of the south-ward migration of volcanism is that the onset of extension in both belts was syn- or post-volcanic in the north but was pre-volcanic in the south. Late Paleogene extension and crustal magmatism coincided in both time and space only locally, where south-migrating magmatism overlapped active north-south-trending extensional belts. Most calderas in the southern Great Basin formed in previously extended belts or on their margins. Southward migration of ignimbrite sources was apparently blocked by unextended crust to the south. In contrast, volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to, or during, crustal extension. This is consistent with observations in the southern Basin and Range, where volcanism and crustal extension were generally synchronous, and volcanism was dominantly effusive. Thus, caldera formation may be controlled by the distribution of upper-crustal extension, although the physical mechanism for this control remains speculative. The space-time patterns of late Paleogene extension in the Great Basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin, but being driven by gravitational collapse of thick crust. Space-time patterns of Tertiary volcanism in the Basin and Range also appear to conform to patterns of thermal weakening or destruction of the subducted slab. Both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts, because both predict close spatial and temporal association of extension and magmatism, which is not generally observed.

The tenuous connection between high-silica rhyolites and granodiorite plutons

The trace element compositions of aplite dikes in the Sierra Nevada batholith of California differ profoundly from high-silica rhyolites (HSRs) and contradict a genetic connection to them. The aplites are strongly depleted in all middle rare earth elements (REEs), whereas HSRs are strongly depleted only in Eu and enriched in other REEs; the aplites are strongly depleted in Y and variably enriched in Sr, whereas HSRs are enriched in Y and strongly depleted in Sr. Volcanic rocks with the trace element characteristics of these aplites are rare to absent in the geologic record. Aplite REE patterns are likely controlled by titanite, which has large distribution coefficients for REEs, whereas HSRs cannot have equilibrated with titanite. Titanite may crystallize late in dacitic magma and thus HSR may be extracted before titanite saturation is reached; aplites would form after titanite appears, but when the melt percentage is too low and the water content of the melt too high (at fluid saturation) for the magma to ascend without rapid crystallization, thus preventing eruption. HSRs could also form by low-degree partial melting of granodiorite plutons in which titanite melts out early. Alternatively, HSRs may be extracted from silica-rich plutons that lack titanite; leucogranite plutons with REE contents that could be complementary to HSRs are present but uncommon in the Sierra Nevada.

Sheeted intrusion of the synkinematic McDoogle pluton, Sierra Nevada, California

Field, microstructural, and geochronologic evidence indicates that the Late Cretaceous McDoogle pluton, located near the eastern margin of the Sierra Nevada batholith, was emplaced as a subvertically sheeted complex into a steep reverse-sense shear zone. Evidence of internal subvertical sheeting includes abundant concordant wall-rock inclusions and screens that separate the pluton from adjacent Jurassic plutons, preservation of a ghost tectonostratigraphy in the distribution of the inclusions, and rare interphase contacts. The solid-state tectonic fabric in the wall rocks and the magmatic, submagmatic, and weak solid-state fabrics in the McDoogle pluton all are concordant and record northeast-southwest horizontal shortening, subvertical extension, and a significant component of northeast-side-up simple shear. Intrusive contacts generally are concordant with the fabrics but, where discordances occur, the wall-rock fabric invariably is truncated by the contact. However, late synkinematic emplacement of the McDoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation. Zircon U-Pb isotope dates were obtained from the quartz monzodiorite central phase of the McDoogle pluton (94 ± 4 Ma), the mafic granodiorite border phase (94.8 ± 0.6 Ma), and an older hornblende granodiorite included in the central phase (97.6 ± 0.4 Ma). Granodiorite orthogneiss in the wall rock yielded a U-Pb zircon date of 164 +2.1/–6.8 Ma, which we interpret to be the crystallization age and which is indistinguishable from a new age of 164.5 ± 2.3 Ma for the Twin Lakes pluton. A U-Pb titanite date of 94.4 ± 0.9 Ma from the 164 Ma orthogneiss sample may reflect thermal effects of the intruding McDoogle pluton, synkinematic growth of titanite in the shear zone, or perhaps both. The age of the Sawmill Lake shear zone that hosts the McDoogle pluton is bracketed between 148 Ma, the age of the Independence dike swarm, and 92 Ma, the age of the Lamarck pluton that crosscuts both the McDoogle pluton and the shear-zone fabrics. Regional evidence suggests a shear-zone age younger than 110 Ma, but older deformation is possible. The predominance of horizontal shortening over transcurrent motion and an age older than 90 Ma exclude a direct relationship to the dextral Sierra Crest shear system, which has been proposed to pass near the study area. The location, age constraints, and kinematics of the shear zone are consistent with both the later stages of movement in the East Sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate.

Is stoping a volumetrically significant pluton emplacement process

Magmatic stoping is widely called upon to explain discordant pluton contacts, but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons. First, xenoliths rarely make up more than a few percent of most plutons; this sparseness, even on well-exposed pluton floors, argues that few xenoliths make it into plutons. Second, piles of rock fragments have sufficiently high porosities (∼50%) that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much, if not all, of the available magma, producing pluton-scale magmatic breccias that are not observed. Third, although settling calculations show that large (meter-scale or larger) xenoliths could sink rapidly in magma, natural fragmentation processes are likely to produce a fractal distribution of particle sizes, with small particles far more abundant than large ones. The absence of small xenoliths at pluton contacts, where magma cools most rapidly, thus argues against stoping. Fourth, although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common, geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks. Finally, there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally. Stoping cannot be volumetrically important in incrementally emplaced plutons, because blocks can only fall through the volume that is molten at any given time. Where xenoliths are abundant in plutons, they may commonly represent isolation of wall-rock bodies between successive intrusive increments rather than wall-rock fragments that were engulfed by magma. However, even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber. Although stoping undoubtedly occurs during the emplacement of some plutons, it is unlikely to be a volumetrically significant process.

Structural expression of a rolling hinge in the footwall of the Brenner Line normal fault, eastern Alps

The kinematic and temporal sequence of structures observed to overprint mylonites along the Brenner Line low-angle normal fault may record passage of the footwall through two rolling hinges, at the top and bottom of a ramp in the shear zone. The structures comprise west down brittle and brittle-ductile structures and east down brittle structures. PT conditions of formation (250° to >400°C and 2–23 km depth), obtained from analysis of oriented fluid inclusion planes, indicate that west down structures were formed at greater depths and temperatures, and therefore earlier, than the east down structures. These data suggest that the brittle structures formed under conditions that permit crystal-plastic deformation at long-term geologic strain rates and therefore probably reflect transient rapid strain rates and/or high fluid pressure. Structures inferred to have formed at a lower hinge are consistent with viscous flow models of rolling-hinge deformation and support the concept of a crustal asthenosphere. Such high temperatures at shallow crustal depth also suggest significant upward advection of heat by extensional unroofing of warm rocks, which may have reduced the flexural rigidity of the footwall and thus affected mechanical behavior at the upper rolling hinge. Exposed mylonitic foliation within a few hundred meters of the Brenner line and on top of the east–west trending anticlines in the footwall dips ∼15° west. Our data favor a ramp dip of ∼25° but permit a dip as great as 45°. Fluid inclusion data suggest that structures related to the hinge at the base of the ramp formed at depths of 12–25 km. If the average dip of the Brenner shear zone to those depths was 20°, intermediate between the favored ramp dip and the dip of exposed foliation, then the horizontal component of slip could be as high as 33–63 km. The two discrete sets of structures with opposite shear senses, formed in the temporal sequence indicated by PT data, are consistent with subvertical simple shear models of rolling-hinge strain. This kinematic pattern is not predicted by the flexural-failure model for rolling hinges. However, the predominance of normal slip at the upper hinge, which extends rather than shortens the mylonitic foliation, fails to match the subvertical simple shear model, which predicts shortening of the foliation there. One possible solution is that superposition of regional extension upon hinge-related stresses modified the rolling-hinge kinematics. Such a modified subvertical shear model can account for the observed small foliation-parallel extensional strains if the foliation was bent <5°–10° passing through the upper hinge. If more bending than that occurred, the data suggest rolling-hinge kinematics in which deformation is achieved by uniform-sense simple shear across the shear zone as in the subvertical simple shear model but in which material lines parallel to the shear-zone foliation and the detachment fault undergo very small length changes, presumably indicating that footwall rocks retained significant resistance to shear and underwent minimal permanent strain. The mechanics that would generate such a rolling hinge are uncertain but may incorporate aspects of both subvertical simple shear and flexural failure. An important kinematic consequence of such a rolling hinge is that all of the net slip across a normal fault, not only its horizontal component, is converted into horizontal extension. This implies a significantly larger magnitude of crustal extension across dipping normal faults whose footwalls passed through a rolling hinge than for those that did not develop along with a hinge.

Constrictional strain in a non-coaxial shear zone: implications for fold and rock fabric development, central mojave metamorphic core complex, california

Finite strain and fold analyses of footwall mylonites in the central Mojave metamorphic core complex (CMMCC) reveal two distinct deformation paths that generated non-coaxial constrictional strain. The first path, recorded in the Waterman Hills, accomplishes constrictional strain through the formation of L-tectonites at the grain scale. The second path, recorded in the Mitchel Range, involves a combination of plane strain at the grain scale and Y-axis shortening through syn-mylonitic folding. The ductile shear zone in the Waterman Hills is approximately 500 m thick and is developed entirely within a Miocene syn-kinematic granodiorite. Strain magnitude decreases structurally downward. The mylonites contain abundant NE-directed non-coaxial microstructures yet are L-tectonites. Rf/o analysis of quartz-ribbon grain shapes indicate prolate strains (K=6) and bulk chemical data suggest that mylonitization was isochemical or involved volume loss. Therefore, ductile shearing in the Waterman Hills involved true constrictional strain at the grain scale. Immediately to the southeast in the Mitchel Range, the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre-Tertiary basement. The mylonitic fabric comprises a well-developed foliation and NE-trending stretching lineation. Rf/o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite. In contrast to the Waterman Hills, finite strains in these L-S-tectonites approximate plane strain (K = 1.1). However, the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation. The coaxial folds range from open to isoclinal. We interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished Y-axis shortening in the shear zone. Macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone, suggesting that Y-axis shortening continued after the fault zone had reached the brittle regime. Corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in Cordilleran metamorphic core complexes. Therefore, constrictional strain may be common in core complexes. Strain compatibility with upper plate rocks in core complexes may be maintained by Y-axis shortening along conjugate strike slip faults. A nearly uniaxial stress field (σ3 < σ2 ≈ σ1) is consistent with most of the structures formed in Basin and Range extension and could generate the observed constrictional strain.

Large-magnitude continental extension: An example from the central Mojave metamorphic core complex

The central Mojave metamorphic core complex is defined by a belt of Miocene brittle-ductile extension and coeval magmatism. The brittle-ductile fault zone defines a basin-and-dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes. One fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km. The axial surfaces of these folds can be traced from the footwall mylonites, through the brittle detachment, and into hanging-wall strata. However, folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment, suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime, possibly after movement across the fault zone ceased. Mesoscopic fabrics related to the transport-parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction. We interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large-magnitude extension. The other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km. Mesoscopic fabrics related to these folds include northwest-striking joints, kink bands, and en echelon tension-gash arrays. These fabrics formed after mylonitization and record both layer-parallel extension and northeast-side-up subvertical shear. The postmylonitic fabrics are kinematically compatible with rolling-hinge-style isostatic rebound of the footwall following tectonic denudation. The relative timing of extension-related magma intrusion and ductile deformation varies through the central Mojave metamorphic core complex. On the scale of the small mountain ranges that make up the central Mojave metamorphic core complex, no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension-related igneous rocks. This suggests that models that invoke a single upper-crustal genetic relationship, such as magmatism triggering extension or vice versa, do not apply to the central Mojave metamorphic core complex. Systematic variation in the relative timing of dike emplacement and mylonitization suggests that, at the time of dike emplacement, rocks in the Mitchel Range were below the brittle-ductile transition while those in the Hinkley Hills were above it. The Hinkley Hills and Mitchel Range are separated by ∼2 km in the dip direction of the fault zone, which suggests that the vertical thickness of the brittle-ductile transition probably was between 100 and 950 m.

Volume loss, fluid flow and state of strain in extensional mylonites from the central Mojave Desert, California

Abstract Compositional changes in mylonitic rocks from the central Mojave metamorphic core complex, California, indicate that large volume loss (20–70%) attended formation of mylonite along the Waterman Hills detachment fault. Strong silica depletion and significant mobility of normally immobile elements imply large fluid/rock ratios during mylonitization, possibly as a result of multiple-pass hydrothermal convection connecting near-surface and mid-crustal alteration regimes. Dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica, and therefore was probably not magmatic or connate. Large volume loss in a shear zone should result in apparent flattening strains, yet mylonites throughout the complex, including the mylonites that record large volume loss, exhibit constrictional strains. This inconsistency raises questions about the applicability of the shear-zone model to Cordilleran metamorphic core complexes.

Magnitude and significance of Miocene crustal extension in the central Mojave Desert, California

The newly recognized Waterman Hills detachment fault (WHDF) of the central Mojave Desert, California, is significant because it provides the first unambiguous evidence for large-scale core complex-style crustal extension in the central Mojave Desert, and because it has significantly rearranged the pre-Miocene paleogeography of the Mojave Desert. The WHDF places steeply dipping to overturned Miocene volcanic and sedimentary rocks upon mylonitic pre-Tertiary basement. The mylonites, which apparently formed during extension, are predominantly L-tectonites which manifest top-to-northeast shear. The WHDF dips to the northeast beneath dominofaulted ranges of the central Mojave Desert and detachment faults of the Colorado River trough, forming an imbricated early Miocene system of detachment faults. Extension continued in the Colorado River trough after extension had ceased in the central Mojave Desert. Tentative correlations of Mesozoic intrusions suggest about 40 km of slip across the WHDF, which carries eugeoclinal Paleozoic rocks in its hanging wall and cratonal/miogeoclinal Paleozoic rocks in its footwall. Restoration of 40 km of slip (1) removes a prominent kink in the boundary between eugeoclinal and cratonal/miogeoclinal facies, (2) aligns cratonal/miogeoclinal strata near Victorville more closely with the late Paleozoic continental margin farther north, (3) places cratonal/miogeoclinal rocks structurally beneath eugeoclinal rocks, implying that the facies were stacked by thrusting, and (4) straightens the western margin of the Late Jurassic Independence dike swarm.