Post-Caledonian Brittle Fault Zones on the Hyperextended SW Barents Sea Margin: New Insights into Onshore and Offshore Margin Architecture

Onshore-offshore correlation of brittle faults and tectonic lineaments has been undertaken along the SW Barents Sea margin off northern Norway. The study has focused on onshore mapping of fault zones, the mapping of offshore fault complexes and associated basins from seismic interpretation, and the linkage of fault complexes onshore and offshore by integrating a high-resolution DEM, covering both onshore and offshore portions of the study area, and processed magnetic anomaly data. This study shows that both onshore and offshore brittle faults manifest themselves mainly as alternating NNE–SSW- and ENE–WSW-trending, steeply to moderately dipping, normal fault zones constituting at least two major NE-SW-trending fault complexes, the Troms-Finnmark and Vestfjorden-Vanna fault complexes. These fault complexes in western Troms bound a major basement horst (the West Troms Basement Complex), run partly onshore and offshore and link up with the offshore Nysleppen and Måsøy fault complexes. Pre-existing structures in the basement, such as foliation, lithological boundaries and ductile shear zones are shown, at least on a local scale, to have exerted a controlling effect on faulting. On a larger scale, at least two major transfer fault zone systems, one along the reactivated Precambrian Senja Shear Belt and the other, the Fugløya transfer zone, accommodate changes in brittle fault polarity along the margin. Our results suggest that distributed rifting during Carboniferous and Late Permian/Early Triassic time was followed by a northwestward localisation of displacement to the Troms–Finnmark and Ringvassøy–Loppa fault complexes during the Late Jurassic/Early Cretaceous, resulting in the formation of a short-tapered, hyperextended margin with final break-up at ~55 Ma. An uplift of the margin and preservation of the West Troms Basement Complex as a basement outlier is suggested to be due to unloading and crustal flexure of the short-tapered margin in the region

Post-Caledonian brittle faults along the SW Barents Sea Margin: onshore-offshore margin architecture and fault rock-forming conditions

The thesis aims to unravel the evolution and finite stage architecture of the SW Barents Sea Margin, which formed as a part of the rifting of present day Greenland and Scandinavia and the opening of the North Atlantic Ocean (cf. Faleide et al., 2008). The SW Barents Sea Margin studied in this thesis starts just north of the Lofoten-Vesterålen archipelago, continues northward outboard northern Troms and into western Finnmark. This thesis focus on (i) if and how onshore Post-Caledonian brittle faults correlate with major offshore, basin-bounding fault complexes and (ii) under which conditions onshore faulting occurred

The Tromsø Nappe Contact with the Nakkedal Nappe Complex NE of Tromsdalstind : Shear kinematics and relationship with metamorphism.

The tectonic contact between the Nakkedal Nappe Complex and the Tromsø Nappe crops out NE of Tromsdalstind, 5 km E of Tromsø, Norway. Petrological- and structural analysis of the contact have been undertaken in order to contribute to a better understanding of the tectonometamorphic history of the Uppermost Allochton in the Scandianavian Caledonides. Methods such as P-T pseudosection analysis, Computer-Integrated Polarization microscopy (CIP) and quartz grain size and -shape analysis have been combined with classic polarizing microscopy to obtain shear sense, P-T-conditions, flow stress and strain rate of deformation. A minimum P-T condition of a Scandian prograde metamorphism is obtained from a metapelite in the Tromsø Nappe at 0.9 GPa and 680°C. A widespread distribution of quartz deformation microstructures indicate that deformation was continuous and took place over a large span of temperatures during final retrograde conditions from more than 700°C to less than 400°C. Flow stress and strain rate during deformation are estimated to ~30 MPa and 10-11 s-1. Chessboard patterns in quartz are observed locally and c-axis orientation images and pole figures of a quartz vein within the Tromsø Nappe show prism [c]- and basal -slip indicative c-axis preferred orientions (CPOs) in completely recrystallized grains. Prism [c]- and basal -slip pole figures from the same microstructure have previously only been reported in large submagmatic quartz grains. The implications of the new findings for the current theories on the development of prism [c]-slip are discussed. A new unit within the Skattøra Migmatite Complex (SMC) is identified and evidence of retro-eclogites and partial melting in the Tromsø Nappe is presented. A switch in the direction of nappe translation during a late- to post-Scandian retrograde metamorphism from either SW or NE to SSE is identified in the Tromsø Nappe. The results are set into context with previous work in the area and it is concluded that the Tromsø Nappe was subjected to at least two major prograde metamorphic events followed by uplift, partial melting and retrogradation interpreted to reflect the Taconian- and the Scandian phase, respectively. The SMC suffered extensive partial melting in connection with the Taconian phase, was relatively unaffected by Scandian deformation and did not come into contact with the Tromsø Nappe until late in the Scandian phase

Early Cretaceous synrift uplift and tectonic inversion in the Loppa High area, southwestern Barents Sea, Norwegian shelf

Tectonic inversion of rift basins is most commonly reported in the literature to occur after rifting has ceased. In contrast, we present evidence for synrift, localized tectonic inversion from the Loppa High area, southwestern Barents Sea and present a model for the formation of inversion structures as a result of differential uplift. The structures are of early Barremian to mid-Albian age (c. 131 – 105 Ma) and are focused in or near pre-existing extensional boundary faults along the margins of the Loppa High. Inversion is interpreted to be the result of uplift of the high along its inclined boundary faults, leading to space accommodation problems as uplift was not properly compensated by extension in the region. The model constrains the initiation of uplift of the Loppa High to the early Barremian and shows that the asymmetric margin configuration of the high may have led to a bulk clockwise rotation of the high around a vertical axis during uplift. The cause of uplift is not fully understood, but is suggested to be linked to contemporaneous extreme lithospheric thinning in neighbouring basins to the west. Processes involved may include isostatic flexure, thermal heating, lithological phase changes and/or far-field stresses, although these aspects need to be further tested. The final version of this reseach has been published in the Journal of the Geological Society. © 2016 Geological Societ

Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas fault, southern California

Miocene–Pliocene sedimentary rocks in the Mecca Hills, southern California, were uplifted and deformed by transpression along a restraining bend in the San Andreas fault trace between the Orocopia and San Bernardino Mountains in Pleistocene time. This paper presents field evidence for three stages of structural evolution of a complex, asymmetric wedge-like flower structure, expressed as: (1) subhorizontal en échelon folds and faults oblique to the San Andreas fault; (2) steeply plunging folds subparallel to the San Andreas fault; and (3) folds and thrust faults fully parallel to the San Andreas fault. We argue that the resulting flower-structure deformation formed successively from early distributed transpression through full (?) strain partitioning, rather than from active, synchronous, strike-slip–forming movements, as expected. The model is supported by crosscut relations of major folds and faults and strain estimates from minor conjugate shear fracture sets. The polyphase evolution initiated on a steep right-lateral strand of the San Andreas fault, producing thick fault gouge. Then, the adjacent Neogene strata were folded en échelon outward in a uniformly distributed simple shear strain field. The subsidiary Skeleton Canyon fault formed along a restraining bend that localized right-lateral shearing along this fault, and reshaped the en échelon folds into steeply plunging folds almost parallel to the San Andreas fault in a nascent partly partitioned strain field. The final kinematic stage generated SW-verging folds and thrust faults trending parallel to the San Andreas fault and decapitated the en échelon folds and faults. The switch from early, distributed strike-slip to late-stage regional slip-partitioned shortening (fold-thrust) deformation may have been locally induced by the bending geometry of the fault. The polyphase structures were active in successive order to balance the driving forces in one or more critical-angled transpressional and fold-and-thrust uplift wedges. Fault-related shortening, uplift, and erosion are still controlled in the Mecca Hills by combining and adjusting the wedges with low convergence angle, transpression, and lateral crustal motion in a San Andreas fault plate scenario. Our model, therefore, addresses a more nuanced view of a polyphase flower-structure system and highlights the need to more carefully sort out spatially and temporally different kinematic data as a basis for analog and numerical modeling of transpressional uplift areas

Crustal scale subsidence and uplift caused by metamorphic phase changes in the lower crust: A model for the evolution of the Loppa High area, SW Barents Sea, from late Paleozoic to Present.

The Loppa High area has been subject to several events of uplift and subsidence from the Late Paleozoic to Present. The driving mechanisms behind the vertical movements, however, are not fully understood. We propose that uplift and subsidence were influenced by the combination of density changes caused by metamorphic phase changes in a 90 × 140 km wide mafic lower crustal body below the Loppa High and local (rift-related) and far-field stress. Through a numerical modelling approach we analyse the tectonically induced variations in pressure and temperature in the lower crust, their influence on phase changes in the mafic body and the affiliated vertical movements. Results show that (1) densification of the mafic body caused by far-field compression associated with the late Triassic westward translation of Novaya Zemlya could cause surface subsidence, (2) heat and fluid influx provided by early Cretaceous rifting could trigger density reduction and surface uplift and (3) the present-day geometry of the Loppa High as observed in seismic data can be reproduced by combining the modelled effect of rift flank uplift and phase changes in the mafic body. Phase change-driven vertical movements may also have affected other structural highs in the western Barents Sea, including the Stappen High. The final version of this reseach has been published in the Journal of the Geological Society. © 2017 Geological Societ

Fig. S1: Map. Geomorphology and topography of relict surfaces: the influence of inherited crustal structure in the northern Scandinavian Mountains

A map of polygons of all surface heights within the study area

Relict Surfaces Table. Geomorphology and topography of relict surfaces: the influence of inherited crustal structure in the northern Scandinavian Mountains

Data on the location of median centres of polygons, assigned attributes and selected zonal statistic