Ulrich Riller

Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes

Abstract The evolution of Andean volcanism including the formation of late Miocene to Recent collapse calderas on the Puna plateau is generally interpreted in terms of the kinematic framework of the Nazca and South American Plates. We present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first-order NW–SE-striking zones of left-lateral transtension on the local and regional scales. Consequently, ages of collapse calderas indicate activity of these fault zones which initiated at about 10 Ma on the Puna plateau. The onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen-parallel stretching upon reaching maximum crustal thickness and critical surface elevation. Horizontal magma sheets that formed at mid-crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub-vertical faults. This accounts well for the observed tectono-magmatic phenomena at surface. It follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the Andean plateau rather than to changes in the geometry of the Wadati–Benioff zone or plate boundary kinematics.

Partial-melt topology in statically and dynamically recrystallized granite

Microstructures of the Murray granite pluton (central Ontario, Canada) show evidence of both static and dynamic crystallization subsequent to partial melting. Backscattered electron analyses reveal interstitial K-feldspar and plagioclase at triple junctions of strain-free, isometric quartz grains. The geometry of the quartz-feldspar boundaries mimics the original topology of the quartz-melt contacts during crystallization. This conclusion is suggested by the occurrence of both rounded and planar faces of quartz grains, and by low (27°) dihedral angles of quartz-quartz-feldspar boundaries, similar to dihedral angles in experimentally crystallized quartz-quartz-silicic melt systems. In contrast, feldspar seams in deformed granites have high axial ratios, are usually elongated perpendicular to the foliation plane, and are located preferentially along individual grain boundaries. Quartz grains are dynamically recrystallized and occasionally transected by feldspar seams, indicating that fracturing occurred in the presence of melt during crystal-plastic deformation of quartz. The subparallel orientation in quartz grains of intragranular, feldspar-bearing fractures and interstitial feldspar seams suggests that these features originated as intragranular and intergranular fractures, respectively. Partial-melt topology was therefore controlled by intergranular and, occasionally, by intragranular fracturing.

Growth of the Central Andean Plateau by Tectonic Segmentation Is Controlled by the Gradient in Crustal Shortening

Crustal deformation, orocline formation, and growth of the central Andean plateau as well as the mutual relations of these processes with climate are a matter of international debate. Earthquake focal mechanism, fault kinematic, paleomagnetic, and cross‐section balancing data are paramount for understanding the morphotectonic evolution of this noncollisional orogen but collectively await an explanation in terms of a comprehensive kinematic model. We offer such a model that approximates simple shear in plan view and that predicts the kinematics of first‐order fault zones in the southern central Andes. The model indicates that segmentation of the upper crust into rhomb‐shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the Andes. An anastomosing network of kinematically coupled, reactivated orogen‐parallel and newly formed NE‐trending deformation zones encompassing components of sinistral and dextral strike‐slip, respectively, defines the domain boundaries. Differential strike‐slip on these boundaries accounts for the presence of both NW‐SE‐ and ENE‐WSW‐directed shortening inferred from kinematic analysis of minor faults as well as Neogene paleomagnetic rotations. Specifically, paleomagnetic rotations are interpreted in terms of both rigid and shear‐induced rotations, the latter being defined as the angular departure from originally orthogonal lines. The net rotation of both rotation components predicted by the kinematic model agrees well with observed paleomagnetic rotations. Differential strike‐slip is an integral part of deformation east of the magmatic arc that led to curvature and longitudinal stretching of the orogen. Crustal segmentation commenced in Eocene time, generated closed basins, and propagated from the central bend to the southern terminus of the plateau at a rate of ca. 26 km/Ma. Along with high sediment accumulation under arid climatic conditions, crustal segmentation into well‐defined domains contributed effectively to the horizontal and vertical growth of the central Andean plateau. This holds important consequences for the structural evolution of the eastern fold‐and‐thrust belt of the central Andes and is akin to the mechanism by which large parts of the NW Tibetan plateau experienced surface uplift.

The Time-Space Distribution of Cenozoic Volcanism in the South-Central Andes: a New Data Compilation and Some Tectonic Implications

The coincidence of late Paleogene to Neogene shortening and crustal thickening with vigorous volcanic activity in the central Andes has long invited speculation about a causal relationship between magmatism and deformation. In aid of understanding this and related issues, we present here a new compilation of radiometric ages, geographic location and dominant rock type for about 1450 Cenozoic volcanic and subvolcanic centers in the southcentral Andes (14–28° S). This paper describes variations in the timespace distribution of volcanism from 65 to 0 Ma, with emphasis on the post-30 Ma period where Andean-style shortening deformation and volcanism were most intense. The central Andes are unusual for the abundance of felsic ignimbrites and their distribution is shown separately from the intermediate to mafic volcanic centers which are here termed the “arc association”. Overall, the time-space patterns of volcanic activity for the ignimbrite and the arc association are similar but ignimbrite distribution is more patchy and more closely associated spatially with the plateau region.

Second look at suspect terranes in southern Mexico

The boundary between the Xolapa and the Guerrero, Mixteca, and Juarez (or Oaxaca) terranes is a zone of normal faulting indicating north-south subhorizontal extension. Stratigraphic and geochronometric evidence dates tectonic uplift of the Xolapa terrane as Late Cretaceous and Tertiary. We propose that the Xolapa terrane represents a late Mesozoic-early Tertiary magmatic arc built near or on North American continental crust, and we discuss, as possible tectonic uplift mechanisms, (1) extension associated with back-arc rifting, (2) extension during gravitational spreading of the upper and middle crust, and (3) transtension within a strike-slip regime established during formation of the Caribbean. Both far- and near-field deformations indicate distributed transtension. Therefore, a single regional tectonic framework can account for the Mesozoic and Cenozoic geologic history of these terranes.

Mid-crustal deformation at the southern flank of the Sudbury Basin, central Ontario, Canada

A field-based structural analysis was made in the southern footwall of the Sudbury Basin to facilitate a reassessment of the igneous emplacement mechanism of the 1.85 Ga Sudbury norite (i.e., synfold intrusion, impact-melt ponding). More specifically, we wanted to discern whether attitude and curvature of the norite shell were primary or produced mainly by noncylindrical folding. The southern footwall of the Sudbury Basin is composed of Paleoproterozoic metavolcanic rocks of the lower Huronian Supergroup that host the 2.3 Ga Creighton and Murray plutons. The geometry of foliation trajectories and other structural evidence reveal that the plutons were emplaced into ductilely deforming host rocks during the Blezardian tectonic pulse, 2.4–2.2 Ga. This deformation led to a southward overturn of Huronian metavolcanic rocks, and was accompanied by amphibolite facies metamorphism. Static greenschist facies metamorphism in metavolcanic and granitoid rocks was prompted by Penokean northward thrusting, after subconcordant emplacement of the norite. The lack of evidence for Penokean folding of Huronian footwall rocks suggests that the curvilinear shape of the norite shell is primary. This in turn constitutes evidence against impact-melt ponding and lends indirect support to an intrusive origin.

Left-lateral transtension along the Tierra Colorada deformation zone, northern margin of the Xolapa magmatic arc of southern Mexico

Resume Structural analysis of steeply NNW-dipping tectonites along the northern margin of the Xolapa magmatic arc, southern Mexico, reveals progressive deformation involving ductile and brittle deformation mechanisms. Ductile deformation detached Cretaceous cover rocks from the Xolapa basement along a crustal-scale mylonite zone with normal fault geometry. Normal faults dissected the mylonite zone into blocks which rotated a minimum of 35° to the north. Stress tensors calculated from fault-striae data show subhorizontal, roughly N/S-trending principal extension. Deformation resulted from differential uplift of the Xolapa magmatic arc with respect to its northern hinterland (Mixteca terrane). The oblique normal fault geometry of the mylonites conforms with strike-slip and dip-slip movements along the faults. Left-lateral transtension commenced ductilely between 90 Ma (age of deformed cover rocks) and 34 Ma (U/Pb zircon age of an undeformed pluton cutting the mylonite zone) and continued brittlely into the late Tertiary (tilted Miocene volcanic rocks). We argue that deformation resulted from the interaction of a left-lateral strike-slip regime established during formation of the Caribbean, and an extensional collapse of the Xolapa magmatic arc resulting from a change in steady-state plate-boundary conditions in the early Tertiary.

Origin of large-volume pseudotachylite in terrestrial impact structures

Large-volume pseudotachylite bodies in impact structures are dike like and consist of angular and rounded wall-rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt. Knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation. Most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process. Based on the structure of pseudotachylite bodies at Sudbury (Canada) and Vredefort (South Africa), we show that these processes differ in time and space. We demonstrate that the centimeter- to kilometer-scale bodies are effectively fragment- and melt-fi lled tension fractures that formed by differential rotation of target rock during cratering. Highly variable pseudotachylite characteristics can be accounted for by a single process, i.e., drainage of initially superheated impact melt into tension fractures of the crater fl oor.

The formation of peak rings in large impact craters

Drilling into Chicxulubs formation The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces. Science, this issue p. 878; see also p. 836 Rock samples from IODP/ICDP Expedition 364 support the dynamic collapse model for the formation of the Chicxulub crater. Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Magnetic fabric and microstructural evidence for a tectono-thermal overprint of the early Proterozoic Murray pluton, central Ontario, Canada

Abstract The 2.38 Ga old Murray granite pluton intruded the southern footwall of the Sudbury Igneous Complex and was subjected to repeated ductile deformation. Its NW and SE domains display differences in strain state, AMS signature and the nature and distribution of Fe-oxide minerals. The magnetic foliation is parallel to the visible foliation in the NW domain, but discordant to it in the SE domain. Microstructural data show that the magnetic fabric in the NW domain coincides with a foliation formed at high temperature, probably in response to thermal softening during the emplacement of the adjacent Sudbury Igneous Complex. This tectono-thermal overprint affected pre-existing shape fabrics and was facilitated by incipient melting of the granite near the intrusive contact with the Sudbury Igneous Complex. Rocks of the SE domain are only partially overprinted and two mineral subfabrics are preserved. Prolate susceptibility ellipsoids and clusters of maximum susceptibility directions at the intersections of the two planar fabrics suggest oblique superposition of strain increments. Increases in bulk magnetic susceptibility and magnetic anisotropy degree towards the intrusive contact with the Sudbury Igneous Complex can be explained by respective increases in the proportion of magnetite over ilmenite and strain intensity in the Murray pluton.