Vicki L. Hansen

Neoproterozoic-Cambrian basement-involved orogenesis within the Antarctic margin of Gondwana

High-grade metamorphic tectonites of the Nimrod Group in the central Transantarctic Mountains compose a major ductile shear zone that formed within the paleo-Pacific margin of Gondwana. Despite demonstrated Precambrian protoliths, the timing of metamorphism and tectonite development has been poorly constrained. Igneous rocks of diverse compositions intrude the Nimrod tectonites. Four intrusive units with incipient to well-developed ductile fabrics yield U-Pb zircon ages of 541-521 Ma, and a nondeformed pegmatite has a U-Pb zircon age of ∼515 Ma. These data show that early Paleozoic Ross magmatism was compositionally, texturally, and temporally more heterogeneous than previously recognized. Fabrics in the igneous rocks are concordant with those in their host tectonites, indicating that Nimrod tectonism was in part synchronous with plutonism. U-Pb ages of 525-522 Ma for metamorphic monazite from two pelitic tectonites support this interpretation. Thus, ductile deformation was in its peak to waning stages between about 540 and 520 Ma. This timing provides compelling evidence for transcurrent basement involvement in oblique plate convergence along the Neoproterozoic to Early Cambrian Antarctic margin of Gondwana.

Geologic mapping of tectonic planets

Geological analysis of planets typically begins with the construction of a geologic map of the planets’ surfaces using remote data sets. Geologic maps provide the basis for interpretations of geologic histories, which in turn provide critical relations for understanding the range of processes that contributed to the evolution. Because geologic mapping should ultimately lead to the discovery of the types of operative processes that have shaped a planet surface, geologic mapping must be undertaken in such a way as to allow such discovery. I argue for modifications in current planetary geologic mapping methodology that admit that tectonic processes may have contributed to the formation of a planet surface, and I emphasize a goal of constraining geologic history rather than determining global stratigraphy. To this end, it is imperative that secondary (tectonic) structures be clearly delineated from material units; each record different aspects of planet surface evolution. Neglecting such delineation can result in geologic maps and interpreted geohistories in which both the spatial limits and the relative ages of material units and suites of secondary structures are incorrect. Determination of absolute time is fundamentally difficult to constrain in planetary studies. In planetary geology the only means to estimate absolute age is the density of impact craters on a surface. Crater density surface ages are more akin to terrestrial ANd average mantle model ages, which reflect the average time at which all of a rock’s components were extracted from the Earth’s mantle. Rocks, or planetary surfaces, with very different geohistories could yield the same average model age. Dating tectonic events or determining rates of tectonic events is even more difficult. fl 2000 Elsevier Science B.V. All rights reserved.

Kinematic evolution of the Miller Range Shear Zone, Central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to Early Paleozoic tectonics of the East Antarctic Margin of Gondwana

High-grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12–15 km. Shear-sense indicators record consistent top-to-the-SE, or left-lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal-plane slip, affirming both the consistency of shear sense and high-grade syn-kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal-plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape-preferred orientation, and (3) the presence of oriented muscovite “fish” included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left-lateral (top-to-the-SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen-parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen-normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left-oblique convergence in which strain within the orogen was partitioned into deep-level strike slip and shallow-level contraction.

Structures in tessera terrain, Venus: Issues and answers

Many workers assume that tessera terrain, marked by multiple tectonic lineaments and exposed in crustal plateaus, comprises a global onionskin on Venus. Because tesserae are exposed mostly within crustal plateaus, which exhibit thickened crust, issues of tessera distribution and the mechanism of crustal plateau formation (e.g., mantle downwelling or upwelling) are intimately related. A review of Magellan data indicates that tessera terrain does not form a global onionskin on Venus, although ribbon-bearing tesserae reflect an ancient time of a globally thin lithosphere. Individual tracts of ribbon-bearing tessera terrain formed diachronously, punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600-2500 km diameter regions above hot mantle plumes. Plume-related magmatic accretion led to crustal thickening at these locations, resulting in crustal plateaus. Crustal plateau surfaces record widespread early extension (ribbon structures) and local, minor perpendicular contraction of a thin, competent layer above a ductile substrate. Within individual evolving crustal plateaus the thickness of the competent layer increased with time, and broad, gentle folds formed along plateau margins and short, variably oriented folds formed in the interior; late complex graben cut folds. Local lava flows accompanied all stages of surface deformation. In contrast to these conclusions, Gilmore et al. ( 1998) summarized post-Magellan arguments in favor of downwelling models for crustal plateau formation. In light of this discrepancy, we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models. We show that Gilmore et al. ( 1998) did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid; they disregarded shear fracture ribbons, thus invalidating their criticism of ribbon models; they misunderstood previous radargrammetric work that constrains ribbon geometry; and they relied solely on geometrical relations to constrain timing, violating kinematic analysis methodology. Their stratigraphic constraints on ribbon-fold temporal relations are invalid because they (1) misinterpreted implications of map relations; (2) did not isolate radar artifacts due to local radar slope effects from proposed material units; (3) chose a region for analysis that clearly shows the effects of younger tectonism and volcanism; and (4) presented map relations that cannot be reproduced. Their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre-Magellan and post-Magellan upwelling models. These misconceptions about the upwelling model and processes responsible for global warming (Phillips and Hansen, 1998), lead to serious errors in Gilmore et al.s (1998) criticism. Furthermore, we show that the data of Gilmore et al. ( 1998) are actually more consistent with upwelling than downwelling models, consistent with arguments that tessera terrain is not global in spatial distribution.

Tessera terrain and crustal plateaus, Venus

Many workers assume that tessera terrain—marked by multiple tectonic lineaments and exposed in crustal plateaus—comprises a global “onion skin” on Venus. A growing body of structural, mechanical, magmatic, gravitational-topographic, and geologic evidence indicates that tesserae record the local interaction of individual deep-mantle plumes with an ancient, globally thin Venusian lithosphere, resulting in local regions of thickened crust.

Tectonics and volcanism of eastern aphrodite terra, venus: no subduction, no spreading.

Eastern Aphrodite Terra, a deformed region with high topographic relief on Venus, has been interpreted as analogous to a terrestrial extensional or convergent plate boundary. However, analysis of geological and structural relations indicates that the tectonics of eastern Aphrodite Terra is dominated by blistering of the crust by magma diapirs. The findings imply that, within this region, vertical tectonism dominates over horizontal tectonism and, consequently, that this region is neither a divergent nor a convergent plate boundary.

Styles of deformation in Ishtar Terra and their implications

Ishtar Terra, the highest region on Venus, appears to have characteristics of both plume uplifts and convergent belts. Magellan imagery over longitudes 330°–30°E indicates a great variety of tectonic and volcanic activity, with large variations within distances of only a few 100 km. The most prominent terrain types are the volcanic plains of Lakshmi and the mountain belts of Maxwell, Freyja, and Danu. The belts appear to have marked variations in age. There are also extensive regions of tessera in both the upland and outboard plateaus, some rather featureless smooth scarps, flanking basins of complex extensional tectonics, and regions of gravitational or impact modification. Parts of Ishtar are the locations of contemporary vigorous tectonics and past extensive volcanism. Ishtar appears to be the consequence of a history of several 100 m.y., in which there have been marked changes in kinematic patterns and in which activity at any stage has been strongly influenced by the past. Ishtar demonstrates three general properties of Venus: (1) erosional degradation is absent, leading to preservation of patterns resulting from past activity; (2) many surface features are the responses of a competent layer less than 10 km thick to flows of 100 km or broader scale; and (3) these broader scale flows are controlled mainly by heterogeneities in the mantle. Ishtar Terra does not appear to be the result of a compression conveyed by an Earthlike lithosphere. But there is still doubt as to whether Ishtar is predominantly the consequence of a mantle upflow or downflow. Upflow is favored by the extensive volcanic plain of Lakshmi and the high geoid: topography ratio; downflow is favored by the intense deformation of the mountain belts and the absence of major rifts. Both could be occurring, or have recently occurred, with Lakshmi the most likely locus of upflow and Maxwell the main locus of downflow. But doubts about the causes of Ishtar will probably never be resolved without circularization of the Magellan orbit to obtain a more detailed gravity field.

Structural and kinematic evolution of the teslin suture zone, Yukon: record of an ancient transpressional margin

Abstract The Teslin suture zone (TSZ), Yukon, which forms the fundamental boundary between rocks deposited along the ancient margin of North America, and allochthonous terranes to the west, preserves a complex history of pre-mid-Jurassic convergence and dextral strike-slip translation which is overprinted locally by late Cretaceous dextral strike-slip shear. The TSZ, comprised of a 15–20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata, basalt, peridotite and granitoids, is divisible into distinct elongate structural domains based on the distribution of two well-defined but differently oriented stretching lineations, Lds and Lss, Lds and Lss both formed during non-coaxial ductile deformation; Lds trends E-W and plunges down-dip, whereas Lss trends NNW-SSE and plunges shallowly. Two 1–2 km wide, NNW-trending anastomosing shear zones of Lss cross-cut Lds structures, indicating Lss is younger. Other field relations indicate further that Lds began forming earlier than Lss, followed by a period of coeval Lds and Lss formation; latest motion was dominantly parallel to Lss. Synkinematic mineral assemblages associated with both Lds- and Lss-related structures record greenschist to albite-epidote amphibolite facies conditions. Lds tectonites record a complex movement history: to the west textural asymmetries indicate west-side-down, or normal shear, whereas in eastern domains top-to-the-east thrust-style asymmetries dominate. Lss tectonites consistently record dextral, or top-to-the-north, shear parallel to Lss. Tectonic motion began as chiefly penetrative dip-slip shear (Lds) at a high angle to the present trend of the TSZ, and evolved to dominantly dextral strike-slip shear (Lss) parallel to the trend of the zone. The geometry, kinematics and sequence of deformation in TSZ tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin-parallel translation.

Geologic evolution of southern Rusalka Planitia, Venus

Geologic mapping of southern Rusalka Planitia, Venus, reveals interactions of volcanism, tectonism, and topography. We recognize three regional plains units (prR1, prR2, and prR3) based on crosscutting structural relations, embayment patterns, radar brightness, and surface roughness data. Delineation of secondary (tectonic) structures allows us to constrain the relative temporal relations between the three material units. Unit prRl, a radar dark smooth unit exposed in local topographic highs, hosts NE trending extension fractures. Low-viscosity lava flows of prR2, the most areally extensive unit, fill local topographic lows and the NE trending fractures. A shield-sourced lava unit, prR3, overlies prR2 on the basis of embayment relations and radar brightness. NW trending wrinkle ridges deform all three plains units and record regional contraction. Locally, flood lava flows that fill NE trending fractures are structurally inverted to form short, stepped NE trending wrinkle ridges. Map patterns indicate that prR2 comprises a thin layer (<50 m thick), much thinner than previous estimates of 1–3 km. Therefore previously proposed estimates of plains flood lava flow volumes and effusion rates are much too high. The local geologic history of southern Rusalka Planitia is inconsistent with global stratigraphy models. Our study supports the view of plains evolution occurring through discrete volcanic processes working at local and regional (but not global) scales.

Subduction origin on early Earth: A hypothesis

I propose a hypothesis for the origin of subduction on early Earth that is directly coupled with mantle upwelling and the formation of mafi c crust. The hypothesis invokes spatial and temporal overlap of both endogenic and exogenic processes, broad quasi-cylindrical mantle upwelling and large bolide impact, respectively, leading to subduction and spreading, two signature processes of modern plate tectonics. The spatial and temporal intersection of these processes could have occurred variably across early Earth, and thus subduction could have begun at different locations and times globally. The hypothesis postulates that the ability of a terrestrial planet to evolve plate tectonics results from a balance between the strength of its lithosphere and the size bolide it can attract and survive; both factors, to a fi rst order, are a function of planet size.