Terry L. Pavlis

Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: evidence from quartz microfabrics

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.

Fold‐fault relationships in low‐angle detachment systems

A close kinematic and timing relationship between low-angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well-exposed Neogene detachment systems, in the Simplon region of the Alps and in the Death Valley region of California. Fold amplitude is largest in the oldest mylonitic foliation of the footwall. These folds are transected by younger mylonitic foliation(s) and cataclastic detachment faults which are themselves folded about similar axes. In general, the amplitude decreases and the wavelength increases for progressively younger fold structures, consistent with a history of progressive folding during faulting. The interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes. This strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning.

Deformation during terrane accretion in the Saint Elias orogen, Alaska

The Saint Elias orogen of southern Alaska and adjacent Canada is a complex belt of mountains formed by collision and accretion of the Yakutat terrane into the transition zone from transform faulting to subduction in the northeast Pacific. The orogen is an active analog for tectonic processes that formed much of the North American Cordillera, and is also an important site to study (1) the relationships between climate and tectonics, and (2) structures that generate large- to great-magnitude earthquakes. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin of southern Alaska. Interaction between the Yakutat terrane and the North American and Pacific plates causes significant differences in the style of deformation within the terrane. Deformation in the eastern part of the terrane is caused by strike-slip faulting along the Fairweather transform fault and by reverse faulting beneath the coastal mountains, but there is little deformation immediately offshore. The central part of the orogen is marked by thrusting of the Yakutat terrane beneath the North American plate along the Chugach–Saint Elias fault and development of a wide, thin-skinned fold-and-thrust belt. Strike-slip faulting in this segment may be localized in the hanging wall of the Chugach–Saint Elias fault, or dissipated by thrust faulting beneath a north-northeast–trending belt of active deformation that cuts obliquely across the eastern end of the fold-and-thrust belt. Superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen, where the sedimentary cover of the Yakutat terrane is accreted into the upper plate of the Aleutian subduction zone. These three structural segments are separated by transverse tectonic boundaries that cut across the Yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the Saint Elias Mountains onto the coastal plain. The Malaspina fault–Pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding. Onshore, most of this boundary is buried beneath the western or “Agassiz” lobe of the Malaspina piedmont glacier. The boundary between the central fold-and-thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the Bering piedmont glacier.

The Yakutat terrane: Dramatic change in crustal thickness across the Transition fault, Alaska

We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24–27 km thick overlain by sedimentary units that are 4.5–7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ∼1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0–5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.

Structural and seismic evidence for intracontinental subduction in the Peter the First Range, central Asia

This paper examines the stratigraphy, structure, and seismicity of the Peter the First Range, an actively deforming foreland basin in Soviet Tadjikistan. The range represents a highly deformed segment of a large intracontinental basin, the Tadjik Depression, which is being shortened in response to the Cenozoic convergence between two orogenic massifs, the Pamir and the Tien Shan Ranges. The Late Jurassic to Paleogene stratigraphy of the Peter the First Range includes more than 2,000 m of shallow marine to nonmarine sediments deposited under relatively quiescent conditions, close to sea level. In contrast, the correlative units of the northernmost Pamir, directly south of the Peter the First Range, show evidence of proximity to a major, uplifted source area to the south. These Mesozoic-early Cenozoic strata are overlain by thick sequences of alluvial sandstone and conglomeratic units that coarsen upward through the Neogene and record the progressive uplift of the Pamir. Shortening within the Peter the First Range is recorded by kilometer-scale isoclinal, upright-to-over-turned folds, and imbricate, north-verging thrusts that emerge near the northern edge of the range and mark the active tectonic boundary with the Tien Shan. Most of the deformation appears to postdate the accumulation of the Mio-Pliocene molasse. The stratigraphy and structure of the region are best explained by a model of the Mesozoic Tadjik Depression as an intracontinental, foreland basin, developed in a back-arc position with respect to the active Andean or collisional orogens located in northern Afghanistan and the Pamir. Seismicity in the region is dominated by earthquakes extending from the surface to ∼12-km depth, apparently occurring within the deformed sediments of the Peter the First Range. A subhorizontal zone of relatively low activity at this depth may mark the location of a basal detachment that underlies a structurally thickened sedimentary section. This aseismic zone is in turn underlain by a south-dipping belt of seismicity that extends from about 17- to 35-km depth, and which can be traced updip to the seismogenic crystalline basement of the Tien Shan range to the north. We interpret this south-dipping structure as a zone of intracontinental subduction that may be an updip continuation of the enigmatic, intermediate-depth Wadati-Benioff Zone beneath the Pamir Range to the south.

Geologic consequences of plate reorganization: An example from the Eocene southern Alaska fore arc

Observation of relative timing of deformation, metamorphism, and plutonism in a high-temperature-low-pressure metamorphic belt in the eastern Chugach Mountains of Alaska leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the fore arc. Between 56 and 53 Ma, a change in the direction of the Kula-Farallon spreading halted the previous southward migration of the Kula-Farallon-North American triple junction. This forced the triple junction to migrate back north along the plate margin, enlarging the slab window beneath the accretionary margin. The expanded slab window produced a large-scale thermal manifestation now recognized as the Chugach metamorphic complex. The accretionary complex responded to plate reorganization by orogen-parallel extension associated with oblique subduction of the Kula plate. Contraction began again following passage of the triple junction and subduction of the Farallon plate.

Introduction: An overview of ridge-trench interactions in modern and ancient settings

Virtually all subduction zones eventually interact with a spreading ridge, and this interaction leads to a great diversity of tectonic processes in the vicinity of the triple junction. In the present-day Pacifi c basin, there are seven examples of active or recently extinct spreading ridges and transforms interacting with trenches. In contrast, there are only a few well-documented cases of spreading ridge interactions in the ancient geologic record, which indicates this process is grossly underrepresented in tectonic syntheses of plate margins. Analogies with modern systems can identify some distinctive processes associated with triple junction interactions, yet an incomplete understanding of those processes, and their effects, remains. Additional insights can be gained from well-documented examples of ancient ridge subduction because exhumation has revealed deeper levels of the tectonic system and such systems provide a temporal record of complex structural, metamorphic, igneous, and sedimentary events. This volume focuses on ridge-trench interactions in the Paleogene forearc record of the northern Cordillera (north of the 49th parallel). Insights from this system and modern analogs suggest that there is no single unique signature of ridge subduction events, but a combination of processes (e.g., igneous associations, changes in kinematics, motion of forearc slivers, thermal events, etc.) can be diagnostic, especially when they are time-transgressive along a plate margin. Understanding these processes in both modern and ancient systems is critical to our understanding of the creation and evolution of continental crust and provides a new framework for evaluating the evolution of the onshore and offshore tectonic history of the northern North American Cordillera.

The thermochronological record of tectonic and surface process interaction at the Yakutat–North American collision zone in southeast Alaska

We investigate the material fluxes in space and time as a result of exhumation and erosion processes at the ongoing Yakutat–North American collision in southeast Alaska. Many thermochronologic studies using a variety of sampling strategies are challenged by the widespread ice cover that limit field observations and accessibility. This paper reviews new and published low-temperature thermochronological data from southeast Alaska to give a comprehensive interpretation of the exhumation patterns through time and how they are influenced by surface processes and climate change. We find that the southeastern margin of Alaska was exhumed and eroded long before the late Miocene–Pliocene Yakutat collision, but since the beginning of the subduction of the Yakutat lithosphere in the Oligocene/early Miocene. Today there is a distinct pattern of exhumation in southeast Alaska with a localized very rapid and deep-seated exhumation at the Yakutat plate corner (St. Elias syntaxis), where strike slip motion changes to convergence. Exhumation is also rapid, but less deep along the dextral Fairweather fault, and in the evolving fold and thrust belt. We present a re-interpretation of the exhumation pattern in the fold and thrust belt and suggest that mass transport by exhumation is parallel to the observed active thrust faults and oblique to the suture zone and orogenic strike. The locus of most rapid exhumation migrated from northwest to southeast with Recent exhumation occurring near the St. Elias syntaxis. Exhumation of the Chugach terrane rocks is still active, however to a lesser degree than on the south side of the orogen where precipitation rates are much higher. The Wrangellia terrane to the north has experienced little exhumation and has essentially formed the backstop for terrane accretion in southeast Alaska since the Early Cretaceous. Apatite U-Th/He ages give the first evidence that rocks of the Wrangell Range have only been recently uplifted and eroded as a consequence of the continuing Yakutat collision. In general the thermochronology in southeast Alaska reveals that climate variations across the region as well as changes through time have a limited influence on the pattern of erosion and that the location of deep exhumation is primarily influenced by tectonic processes.

Structural history of the Chugach metamorphic complex in the Tana River region, eastern Alaska: A record of Eocene ridge subduction

The Chugach metamorphic complex of southern Alaska is an Eocene high-temperature ( T ), low-pressure ( P ) fore-arc metamorphic belt related to subduction of the Kula-Farallon spreading center beneath western North America. The Chugach metamorphic complex has a three-phase ductile deformational history that records major changes in kinematic axes during a short interval of geologic time (∼8 m.y.). The earliest deformation (D 1 ) is a regional event recognized throughout the flysch subterrane of the Chugach terrane. D 1 is a regional layer-parallel slaty/phyllitic cleavage developed during accretion and subsequent shortening. In the Chugach metamorphic complex, D 1 predates high-temperature metamorphism. During prograde metamorphism, there were two major structural events. D 2 records orogen-parallel extensional accompanied by vertical shortening with components of pure shear and top-to-the-east simple shear. D 2 is synchronous with melt injections (Ti 2 ) in the gneissic core of the complex and large plutons throughout the complex. D 3 records a return to subhorizontal contraction perpendicular to the margin and is interpreted as a dextral transpressional event. D 3 contraction produced a dramatic thickening of the complex in a regional-scale D 3 anticlinorium. In the gneissic core, the presence of melt (Ti 3 ) strongly influenced D 3 . Finite strain data and field observations indicate that both F 2 and F 3 have axes that are parallel to the stretching direction, yet these are not sheath folds because strains are too low. Instead the structures are examples of folds that developed with their axes parallel to the elongation axis. Together these observations provide further evidence for our previous interpretations that the Chugach metamorphic complex is a manifestation of an Eocene plate reorganization at ca. 56–52 Ma. Plate models predict that before 56 Ma the Kula–Farallon–North American triple junction migrated southward and is associated with a time-transgressive fore-arc plutonic belt. After plate reorganization, the triple junction either backtracked northward (Kula Plate model) or continued southward with intermittent northward motion (Pacific-Farallon model). We interpret the D 2 -to-D 3 progression as either a result of highly oblique subduction of the Kula plate followed by more orthogonal—but still dextral-oblique—convergence of the Farallon plate (Kula Plate model), or a special case of Pacific–Farallon–North American interaction.

Geophysical insights into the Transition fault debate: Propagating strike slip in response to stalling Yakutat block subduction in the Gulf of Alaska

On the basis of faulting mapped on seismic reflection and bathymetric data, seismicity, current plate motions, and evidence that the Yakutat block may be anomalously thick, we propose a tectonic model for Yakutat-Pacific interactions, including the often-debated Transition fault. To the east, deformation associated with the Queen Charlotte–Fairweather fault system is extending offshore, facilitating westward propagation of strike-slip motion along the eastern segment of the Transition fault. To the west, the oblique-slip Pamplona zone and Transition faults merge at an embayment in the continental margin, where a north-south dextral strike-slip fault within the Pacific plate, illuminated by the 1987–1992 earthquake swarm, intersects the Pacific-Yakutat tectonic boundary. These fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the Yakutat block, a possible moderate-sized oceanic plateau.