Othmar T. Tobisch

A review of criteria for the identification of magmatic and tectonic foliations in granitoids

Abstract Foliations in granitoids can form by magmatic flow, ‘submagmatic flow’, high-temperature solid-state deformation and moderate- to low-temperature solid-state deformation. A review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent, diapiric emplacement and expansion, emplacement during regional deformation, or regional deformation post-dating emplacement. However, a magmatic origin is favoured for foliations defined by the alignment of igneous, commonly euhedral minerals, particularly where the foliation is parallel to internal or external pluton contacts. Foliations formed during expansion or ‘ballooning’ of diapirs may be strictly magmatic in origin, although some studies suggest that solid-state deformation also may occur. If so, we would hope to find evidence of deformation of crystal-melt systems, and that the solid-state deformation occurred at high temperatures. The inference of syntectonic foliations is most convincing where magmatic and high-temperature solid-state foliations are subparallel, these foliations are continuous with regionally developed foliations in the wall rocks, synkinematic porphyroblasts are present in the wallrocks, and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage. A strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals, no alignment of igneous minerals occurs, the foliation is locally at high angles to pluton-wallrock contacts, and the foliation is continuous with a regionally developed cleavage.

Rates of processes in magmatic arcs: implications for the timing and nature of pluton emplacement and wall rock deformation

Abstract The construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions. It is critical, therefore, to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton-wall rock systems. Our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following: (1) crystal growth rates in magma of 10 −4 cm year −1 ; (2) growth rates of metamorphic porphyroblasts between 10 −5 and 10 −2 cm year −1 ; (3) long-term magma supply rates of 10 −1 km 3 year −1 and short-term rates of up to 350 km 3 year −1 ; (4) diapiric ascent rates for mafic plutons of 1–3 m year −1 ; (5) cooling of plutons to ambient wall rock temperatures in 10 5 –10 6 years; (6) final crystallization of plutons in a small fraction of the time needed for complete cooling; (7) fault displacements of 3 cm year −1 ; (8) development of cleavages in fault zones in less than 10 6 years at strain rates of 10 −13 s −1 or higher; and (9) the development of regional cleavages in 10 6 years at strain rates of 10 −14 s −1 These rates indicate that processes operating in magmatic arcs are relatively rapid: pluton emplacement, cleavage development, etc., occur over time spans of tens of thousands to no more than a few million years. However, the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed. At deeper levels, or in zones undergoing faster strain rates, the time spans of the various processes approach one another. Thus plutons, with otherwise similar characteristics, emplaced in regions undergoing fast strain rates, or at deeper crustal levels, may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates. Comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered. Thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration. Finally, we argue that the structural and other characteristics of pluton-wall rock systems will depend on rates of various processes involved and that these rates at the very least influence, and sometimes invalidate, the timing criteria previously published by us and others.

Using pluton ages to date regional deformations: Problems with commonly used criteria

Studies of pretectonic plutons in the Foothills terrane, central Sierra Nevada, California, along with a review of studies on syntectonic and post-tectonic plutons, indicate that no single criterion can establish the relative timing of pluton emplacement and regional deformation. Criteria most often used are whether a pluton is deformed, cuts regional structures, or is associated with porphyroblasts that postdate regional structures. However, all three types of plutons may (1) appear deformed or nondeformed, (2) have contacts that appear to cut structures in the wall rocks, and (3) have porphyroblasts in their aureoles that display varied timing relations. The ambiguity of these criteria emphasize the need for careful studies of the structures present throughout the pluton and surrounding wall rock, with particular attention paid to regions where the pluton contact is at high angles to regional structures. Porphyroblast-cleavage relations are an important tool in this regard, but also must be used with caution.

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.

Analysis and interpretation of composite foliations in areas of progressive deformation

Abstract In areas of progressive deformation, where successive structures develop during a relatively continuous deformation within a geologically short time period, traditional chronological notations of structural elements (e.g. S 1 , S 2 , etc.) can give erroneous impressions of how large rock masses evolve in time and space. We demonstrate from field examples that successive structures can develop which: (a) are comparable in morphology and orientation but of different ages; (b) are different in morphology but of comparable age; and (c) show rapid morphological changes over short distances. Under such conditions, correct identification of the relative age of structures is often difficult to impossible. We consider the concepts of a composite foliation , and of Transposition Cycles as vehicles to objectively evaluate the significance of different sets of structures in the evolution of larger rock masses. We suggest that: (1) structural elements be labeled using morphological notation, adding numerical subscripts only when independent evidence is available; (2) geologists more fully acknowledge and integrate the concept of deformation partitioning into their models; and (3) when analyzing areas of multiple deformation, more emphasis is placed on the relationship between domains of differing complexity. Integration of these three perspectives in the analysis will lead to a more realistic basis upon which to model the structural evolution of large rock masses.

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.

Fracture-controlled magma conduits in an obliquely convergent continental magmatic arc

Magnetic fabric patterns of two mid-Cretaceous nested plutons (102 ± 1 and 96 ± 3 Ma) in the central Sierra Nevada batholith provide evidence that felsic magma emplacement (and ascent?) occurred via north-trending, steeply dipping, planar fracture conduits oriented obliquely to the arc. U-Pb geochronology data indicate that emplacement of the plutons was separated by 2 to 10 m.y. and that they were emplaced in part via the same conduit. Magnetic fabrics in the younger pluton are related to its final emplacement, which was strongly influenced by a system of host-rock joints. Formation of the north-trending conduits at ≈100 Ma can be related to a small, arc-parallel, dextral-shear component that produced tension fractures and that was associated with high-angle oblique convergence. By ≈90 Ma, convergence had become sufficiently oblique for the development of nearly arc-parallel structures, which were then favored as magma conduits.

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.

Nature and timing of deformation in the Foothills terrane, central Sierra Nevada, California: Its bearing on orogenesis

Detailed mapping and structural analysis combined with new age dates, using U-Pb and ^(40)Ar/^(39)Ar techniques, have allowed us to constrain the timing of pre-ductile and ductile deformation in the Foothills terrane of the central Sierra Nevada. By using strain and other data, it can be shown that rigid rotation of beds (folding/faulting) predated the onset of ductile deformation and probably occurred between 160 and 151 Ma. Ductile structures, consisting of continuous and secondary cleavages and associated folds and lineations, started forming ca. 151 Ma in the Bear Mountains fault zone and then ca. 145 Ma, began moving away from the fault zone, forming diachronously over an ∼30-m.y. period. The last documented ductile structure formed ca. 123 Ma, although some secondary structures may be younger. Metamorphism of these rocks is generally upper greenschist facies, although higher-grade belts (one bearing staurolite, andalusite, and sillimanite) are present. Strain was preferentially partitioned into one of these belts of higher metamorphic grade (and sporadically elsewhere). The structural history here is much more complex, and at least one and locally two complete transpositions of the original cleavage have occurred. In these zones of complex deformation, it is in most cases possible only to identify a composite foliation consisting of new continuous cleavage and relicts of earlier phases, all lying mutually parallel. Timing constraints indicate that the pre-ductile structures may correspond to a very late stage of the Nevadan orogeny (that is, 155 ± 3 Ma), but the ductile structures postdate that orogeny (as defined) by as much as about 25-30 m.y. Models which relate the ductile structures in the central Foothills terrane to Nevadan plate-tectonic events are untenable. In addition, recent work indicates that Late Cretaceous ductile deformation in the central and southern Sierra Nevada may be relatively widespread, indicating that tectonic models for the Sierra Nevada need to be reassessed.