Alexander R. Cruden

Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden

Abstract The N-S-trending so-called Protogine Zone in the Baltic Shield of south-central Sweden is usually considered to mark a tectonic boundary between the rocks of the Transscandinavian Igneous Belt (TIB) in the east and the Sveconorwegian orogen in the west. Detailed structural mapping in the Karlskoga-Kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally N-S strike extends ca. 40 km east of the traditional “Protogine Zone”. Furthermore, the western boundary of this ductile deformation and the TIB is not constrained in the Kristinehamn area. Reconnaissance studies indicate that they both extend westwards towards the so-called Mylonite Zone. It is suggested that the eastern limit of the Sveconorwegian orogen is located some 40 km east of the present boundary and that the “Protogine Zone” concept is obsolete. The term Sveconorwegian Frontal Deformation Zone (SFDZ) is proposed as a more appropriate alternative in southern Sweden (south of lake Vattern) and to correspond to a younger set of oblique ductile deformation zones with reverse and right-lateral components of movement in the easternmost part of the orogen farther to the north. Ductile deformation zones older than the SFDZ in the Karlskoga-Kristinehamn area display a fan-like geometry in an E-W cross-section, with steep westerly dips in the eastern part of the section, vertical dips farther west and moderate easterly dips in the western part of the section. Kinematic analysis indicates that dip-slip movements predominate with a consistent top-to-the-east sense of movement across the entire fan-like structure. In their present orientation, deformation zones are characterized by reverse movements in the eastern part and normal movements in the western part of the structure. Between Kristinehamn and the Mylonite Zone, the main foliation is gently dipping to subhorizontal, indicating that the regional structure is strongly asymmetric, and that the fan-like structure occurs close to the foreland of the orogen. Deformation zones are spaced to semi-penetrative in the eastern part of the fan-like structure, whereas the deformation is more or less penetrative and the TIB rocks are transformed to orthogneisses west of Kristinehamn. This east to west increase in bulk strain is in accordance with an increase in syn-deformational metamorphic grade across the structure. Younger ductile deformation zones belonging to the SFDZ are responsible for a major change in orientation of the older deformation zones in the easternmost part of the structure. The fan-like structure is best explained by models involving the interference of two separate tectonic events. Deformation occurred after ∼ 1.57 Ga and prior to deposition of Neoproterozoic and younger cover sedimentary rocks. It is not yet clear whether the initial phase of deformation (early Sveconorwegian or older) was related to the build-up of an imbricate thrust stack in a compressional regime, as favoured here, or to regional E-W extension. The younger deformation phase was related to rotation of these older structures into the compressional, late Sveconorwegian SFDZ.

Analogue benchmarks of shortening and extension experiments

Abstract We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittleviscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscious extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in close vicinity to the basal velocity discontinuity. Measurements of fault dip angles and fault spacing vary among laboratories. Comparison of experimental results indicates an encouraging overall agreement in model evolution, but also highlights important variations in the geometry and evolution of the resulting structures that may be induced by differences in modelling materials, model dimensions, experimental set-ups and observation location.

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.

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian orogen, Baltic Shield, southwestern Sweden

Abstract The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vanern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north. Structural studies in three areas north of lake Vanern (Varmlandsnas, Kil and Torsby-Charlottenberg) suggest that the MZ displays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist-facies conditions. In the northernmost Torsby-Charlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defines the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within this thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorwegian Frontal Deformation Zone (SFDZ) in the frontal part of the orogen. The MZ is inferred to have formed in an extrusive (dispersive) tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and shortening was absorbed, at this stage, by crustal thickening. Late, possibly out-of-sequence thrusting with a right-lateral, strike-slip sense of shear along the SFDZ marked the waning stages of crustal shortening in the Sveconorwegian orogen.

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.

Surface topography and internal strain variation in wide hot orogens from three-dimensional analogue and two-dimensional numerical vice models

Abstract The post-accretionary deformation of wide, hot orogens is characterized by pure-shear or transpressional shortening of relatively weak lithosphere (the orogen) between converging stronger blocks (the vice). We report on a series of analogue vice models and compare the resulting three-dimensional strain fields and surface topographies to equivalent two-dimensional numerical experiments. In the analogue models a rheologically stratified (frictional/viscous) weak orogenic lithosphere overlying a viscous asthenosphere is squeezed between converging strong lithospheric blocks. Ductile lower crust and mantle in the weak lithosphere is free to flow laterally, parallel to the orogen. The Argand number describes the model dynamics and strongly controls both the orogenic relief and the degree of lower crustal orogen parallel stretching in the analogue models. Cross sections of numerical and analogue experiments display consistent geometries in which upper crustal deformation is characterized by upright folding compared to apparently decoupled horizontal strains in the lower crust. The relative buoyancy and degree of orogen parallel flow in the lower crust of the analogue models has a dramatic influence on three-dimensional strain fields and the kinematics of upper crustal curvilinear shear zones. The analogue and numerical results demonstrate the importance of three-dimensional effects in determining the structure of natural orogens and compare favourably to field and geophysical observations of large hot orogens in the geological record.

Structure, magnetic fabric and emplacement of the Archean Lebel Stock, SW Abitibi Greenstone Belt

Abstract The anisotropy of magnetic susceptibility (AMS) has been determined in a suite of samples from the ca 2680 Ma old, sub-circular, syenitic Lebel Stock which is bound to the north by a major late-Archean shear zone (Larder Lake-Cadillac deformation zone; LCDZ). The AMS defines a magnetic foliation which is parallel to a strong, planar preferred mineral orientation fabric considered to have been acquired during flow of a crystalladen magma during emplacement. The geometry of the foliation indicates that the stock has an horizontal dish shape at depth. The syenites also contain a well defined magnetic lineation which consistently points toward the LCDZ apart from localities in the south where it trends E-W, parallel to the stocks margin. AMS ellipsoids are dominantly of oblate type except in samples showing solid-state strain and alteration overprints, which often have plane to prolate ellipsoids. The intensity of the magnetic fabric shows a weak spatial variation, being strongest at the southern margin. The magnetic and magmatic fabric pattern, AMS parameter variation and the sense of displacement of the western wall-rocks are used to propose an emplacement mechanism involving southward migration of a relatively thin sheet of syenitic magma from a linear source corresponding to the present location of the LCDZ.

Magnetic fabric evidence for conduit‐fed emplacement of a tabular intrusion: Dinkey Creek Pluton, central Sierra Nevada batholith, California

Anisotropy of magnetic susceptibility measurements in the Cretaceous Dinkey Creek Pluton (DCP), central Sierra Nevada batholith, reveal a well-defined structural pattern and zonation in bulk susceptibility (K). Outward decrease in K within the pluton is due to a decrease in ferrimagnetic contribution, corresponding to a compositional zoning from tonalite to granite. Magnetic fabrics in the pluton formed during supersolidus flow of a crystal-rich magma, except in the eastern domain where subvertical magnetic lineations and NW striking magnetic foliations are attributed to deformation associated with a regional shear zone. Magnetic foliations in the central domain of the DCP define a NNW trending synformal structure, passing structurally upward into a dome beneath a small roof pendant, interpreted to represent different structural levels in a zone of magma upwelling. Arcuate foliation trajectories in the plutons SW lobe resemble deformed passive markers in two-dimensional, NW to SW, horizontal, channel flow of non-Newtonian magma. Analysis of the fabric pattern in cross-section estimates the preerosion thickness of the lobe to between 915 and 3660 m. The DCP was emplaced as a tabular body with vertical sides and a gently inclined roof. Magma with increasingly more mafic compositions entered the chamber continuously or episodically via a NNW trending conduit. Space for the pluton was created by vertical inflation, probably accommodated by depression of its floor. These results suggest that the mid crust to upper crust of magmatic arcs may be constructed by vertical stacking of tabular granitic plutons with high width/thickness ratios like the DCP.

Diapiric basal entrainment of mafic into felsic magma

Abstract One consequence of partial melting of the lower crust by heat transfer from a mantle-derived underplate is that the resultant buoyant, felsic magma layer (density ϱ2, viscosity μ2, thickness h2) will overllie a denser mafic layer (density ϱ3, viscosity μ3, thickness h3, which can be fully liquid to completely solid, depending on its thermal history. Laboratory experiments and finite-difference numerical models have been used to determine the conditions that favour the entrainment of the mafic layer into the overlying felsic magma as it ascends diapirically. Large amounts of entrainment occur when R = (ϱ 3 − ϱ 1 ) (ϱ 1 − ϱ 2 ) ≈ 0 (where ϱ1 is the density of the crust), m = μ 3 μ 2 = 1−0.001 , and t r = h 3 h 2 > 1 . When these conditions occur, the buoyancy and viscous effects acting to maintain the stability of the felsic-mafic layer interface are minimized. The role of m is much more important in the diapiric entrainment phenomenon than in the comparable problem of axial withdrawal from a density- and viscosity-stratified magma chamber with rigid walls. Favourable conditions for entrainment are likely to occur during the evolution of many lower crustal felsic magma source regions with a mafic underplate. Low amounts of entrainment result in minimal interaction (i.e., mixing) between the felsic host and entrained mafic material. If a large amount of entrainment occurs, our models combined with other studies show that mafic magma can remain in the centre of the conduit (low to high Reynolds number (Re), m ≈ 1), become fully mixed with the felsic host (high Re, low m), or become encapsulated by the felsic magma (low Re, m

Emplacement of rapakivi granite and syenite by floor depression and roof uplift in the Palaeoproterozoic Ketilidian orogen, South Greenland

In the Palaeoproterozoic Ketilidian orogen of South Greenland, tabular intrusions of the rapakivi granite suite exposed at Graah Fjelde, Lindenow Fjord and Qernertoq, and a Mesoproterozoic syenite of the Gardar province exposed at Paatusoq, were emplaced by a combination of roof uplift and floor depression. The strain associated with the emplacement of the intrusions mainly involved redistribution of mass vertically within the lithosphere. Space for the emplacement of the rapakivi plutons was not created during regional extension on low-angle, ductile shear zones as claimed by some previous workers and there is no evidence that emplacement of the intrusions coincided with extensional collapse of the orogen following crustal thickening. The rapakivi granites post-date peak metamorphism in the orogen by 35–46 Ma and their contacts cross-cut and clearly post-date intense, flat-lying D1/D2 fabrics that formed by partitioning of deformation into arc-normal and arc-parallel components during oblique convergence.