Anna K. Ksienzyk

Post-Caledonian extension in the West Norway–northern North Sea region: the role of structural inheritance

Abstract The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting. Several of the reactivated major Devonian extensional structures can be extrapolated offshore into the rift, where they appear as bands of dipping reflectors. They coincide with large-scale boundaries separating 50–100 km-wide rift domains of internally uniform fault patterns. Major north–south-trending rift faults, such as the Øygarden Fault System, bend or terminate against these boundaries, clearly influenced by their presence during rifting. Hence, the North Sea is one of several examples where pre-rift basement structures oblique to the rift extension direction can significantly influence rift architecture, even if most of the rift faults are newly-formed structures.

From orogen to passive margin: constraints from fission track and (U–Th)/He analyses on Mesozoic uplift and fault reactivation in SW Norway

Abstract The post-Caledonian tectonic history and landscape evolution of southwestern Norway are poorly understood, primarily owing to the lack of onshore post-Devonian sediments. To bridge this knowledge gap, low-temperature thermochronological techniques were applied to investigate vertical movements in the upper crust. New apatite fission track and apatite and zircon (U–Th)/He analyses on samples from southwestern Norway yielded Permian to Jurassic, Triassic to Cretaceous and Carboniferous to Triassic ages, respectively. Thermal history modelling indicates relatively high cooling rates (2–3 °C Ma−1) throughout Permian to early Jurassic times. Since the Jurassic, samples from coastal areas have remained close to the surface and were reheated to 30–50 °C during sedimentary burial in the Cretaceous. Inland samples experienced lesser amounts of Permo-Triassic exhumation, continued to cool slowly (<1 °C Ma−1) throughout the Jurassic–Cretaceous and did not reach the surface until the Cenozoic. Both fission track and (U–Th)/He ages are offset across faults, highlighting the importance of fault activity throughout the Mesozoic. In combination with previously published results, the new data suggest that the geomorphological evolution of southwestern Norway is closely connected to rift- and post-rift tectonics related to North Sea and North Atlantic rifting. The topographic relief was most likely repeatedly rejuvenated during periods of tectonic activity.

Correspondence: Challenges with dating weathering products to unravel ancient landscapes

K-Ar dating of illite (clay) in weathered bedrock (saprolite) is an exciting but yet incompletely understood new application of the K-Ar dating method that can potentially provide valuable information about the evolution of landforms and continental isostasy. Fredin et al.1 use this approach in an attempt to date the strandflat in coastal western Scandinavia. Based on K-Ar illite ages from three widely separated localities in the North Sea (Utsira High), West Norway (Bømlo), and southern Sweden (Ivö), they suggest a Late Triassic (~210Ma) age for the strandflat. However, when employing such a new methodology, it is particularly important to carefully consider the results together with existing data, and Fredin et al.1 neglect previously published radiometric, stratigraphic, and geomorphic constraints that strongly suggest that the current strandflat erosional level in western Norway is younger than Triassic. The discovery of Late Jurassic (Oxfordian) sediment caught up in a fault zone in Proterozoic bedrock near Bergen north of Bømlo (Fig. 1) revealed that rocks in the strandflat area were at or near the surface at ~160Ma2, opening the possibility that the strandflat may contain Mesozoic elements3. Offshore, the crystalline bedrock surface is seen as a remarkably planar geomorphic feature on seismic data, preserved under Jurassic sediments (offshore part of Fig. 1b). However, this surface is dipping to the west by ~5°, while the strandflat is almost horizontal (onshore part of Fig. 1b; also shown in Fig. 6 in Fredin et al.1), clearly cutting into the Middle Jurassic paleosurface and thus mainly shaped by younger (post-Middle Jurassic) processes. From geometric considerations, it is therefore quite unlikely that the samples from the Utsira High and Bømlo represent the same weathering surface. Fredin et al.1 claim to be able to constrain the age of the strandflat along the west coast of Norway by dating illite in weathered bedrock. However, K-Ar dating of illite to constrain weathering ages is previously untested; all previous studies cited by Fredin et al.1 use K-bearing manganese oxides or alunitegroup sulfates. Hence, such K-Ar illite weathering ages should be interpreted with care and in the framework of independent data, which in this case include low-temperature thermochronology (fission track and (U–Th)/He ages), the offshore stratigraphic record, structural aspects, and the estimated depth of dike intrusions, as briefly summarized below. A significant quantity of fission track and (U–Th)/He data has recently been published from the strandflat area4. Such ages date the cooling of the currently exposed rocks through the partial annealing/retention zone of the respective system, which is 210–140 °C for the zircon (U–Th)/He system, 120–60 °C for the apatite fission track (AFT) system and 70–40 °C for the apatite (U–Th)/He system. All such ages should be older than the age of any preserved in situ weathering products. A regional compilation of AFT ages from Scandinavia5 shows that AFT ages from the entire Norwegian strandflat area are similar to or, more commonly, younger than the ~210Ma illite ages reported by Fredin et al1. Most ages from the strandflat region relatively near their Bømlo locality show early to middle Jurassic (200–160Ma) AFT ages4 (Fig. 1a). These ages roughly indicate that the samples were buried at >2 km depth in the Early Jurassic, assuming a thermal gradient of 30 °C/km. (U–Th)/He zircon data from the same area of ~225Ma4 suggest burial of the present strandflat level to >4 km depth in the Late Triassic. These data are consistent with paleomagnetic analysis of Permian (~250Ma) dikes in the strandflat area north of Bømlo, which suggests that the dikes were emplaced at ambient temperatures between 150–500 °C (5–15 km depth)6. In slowly cooled basement terranes like western Norway, it can be misleading to reconstruct the exhumation history based on fission track ages alone. More precise and detailed cooling paths can be derived from inverse time–temperature modeling. The resulting models, presented by Ksienzyk et al.4, consistently show cooling throughout the Triassic and into the Jurassic, with postJurassic burial and new exhumation for coastal samples (Fig. 2, blue curves). In order to test for potential Late Triassic weathering, we remodeled strandflat samples by imposing constraints to bring them to the surface in the Late Triassic (green box in Fig. 2). With this constraint, most models showed a significantly DOI: 10.1038/s41467-017-01457-9 OPEN

Structural Inheritance and Rapid Rift‐Length Establishment in a Multiphase Rift: The East Greenland Rift System and its Caledonian Orogenic Ancestry

We investigate (i) margin-scale structural inheritance in rifts and (ii) the time scales of rift propagation and rift length establishment, using the East Greenland rift system (EGR) as an example. To investigate the controls of the underlying Caledonian structural grain on the development of the EGR, we juxtapose new age constraints on rift faulting with existing geochronological and structural evidence. Results from K-Ar illite fault dating and syn-rift growth strata in hangingwall basins suggest initial faulting in Mississippian times and episodes of fault activity in Middle-Late Pennsylvanian, Middle Permian, and Middle Jurassic to Early Cretaceous times. Several lines of evidence indicate a close relationship between low-angle late-to-post-Caledonian extensional shear zones (CESZs) and younger rift structure: (i) reorientation of rift fault strike to conform with CESZs, (ii) spatial coincidence of rift-scale transfer zones with CESZs, and (iii) close temporal coincidence between the latest activity (late Devonian) on the preexisting network of CESZs and the earliest rift faulting (latest Devonian to earliest Carboniferous). Lateto post-Caledonian extensional detachments therefore likely acted as a template for the establishment of the EGR. We also conclude that the EGR established its near-full length rapidly, i.e., within 4–20% of rift life. The “constant-length model” for normal fault growth may therefore be applicable at rift scale, but tip propagation, relay breaching, and linkage may dominate border fault systems during rapid lengthening.

Thermal evolution and exhumation history of the Uncompahgre Plateau (northeastern Colorado Plateau), based on apatite fission track and (U-Th)-He thermochronology and zircon U-Pb dating

Over the past two decades, thermochronological studies have greatly increased our knowledge of the Cenozoic evolution of the Colorado Plateau (western United States). There has been particular interest in the southwestern part of the plateau, leading to debate regarding the timing of uplift and fluvial incision along the Colorado River system. We here combine apatite fission track (AFT) and apatite (U-Th)-He (AHe) analyses as well as zircon U-Pb dating to investigate the much less studied northeastern Colorado Plateau, particularly the Uncompahgre Plateau and the Unaweep Canyon, which has a very unusual drainage pattern in two opposite directions. We obtained 12 AFT ages from the Uncompahgre Plateau: 3 from the top of the basement of the plateau reveal Laramide ages (65–63 Ma), and 6 samples from the Unaweep Canyon (35–27 Ma) and 3 from the northeastern plateau margin (33–17 Ma) underwent complete thermal resetting in the late Eocene to Oligocene. Thermal history modeling of top basement samples reveals Late Cretaceous heating to temperatures of at least 90 °C, implying sedimentary burial to ∼3 km, followed by cooling throughout the latest Cretaceous to Eocene. However, AHe ages (38–31 Ma) indicate minor reheating to 40–80 °C for these samples in the late Eocene to Oligocene. Zircons from the La Sal Mountains laccolith gave an Oligocene U-Pb crystallization age of 29.1 ± 0.3 Ma. AFT ages from the laccolith range from 33 to 27 Ma, confirming rapid cooling of this shallow subvolcanic intrusion. This late Eocene to Oligocene magmatism caused thermal resetting of most of the AFT (except top basement) and AHe ages from the Uncompahgre Plateau, even though samples were collected as much as 60 km away from the intrusion. Canyon samples also underwent an increase in cooling rates in the past 5–10 m.y. This Miocene–Pliocene cooling event is interpreted as regional uplift of the Colorado Plateau associated with canyon incision.

Alps to Apennines zircon roller coaster along the Adria microplate margin

We have traced the particle path of high-pressure metasedimentary rocks on Elba Island, Northern Apennines, with the help of a U-Pb-Hf detrital zircon study. One quarter of the analysed zircons are surprisingly young, 41-30 Ma, with a main age peak at ca. 32 Ma, indicating an unexpected early Oligocene maximum deposition age. These Oligocene ages with negative εHf indicate a volcanic source region in the central-southern Alps. Though young by geological means, these zircons record an extraordinary geodynamic history. They originated in a volcanic arc, during the convergence/collision of the the Adria microplate with Europe from ca. 65 to 30 Ma. Thereafter, the Oligocene zircons travelled ca. 400 km southward along the Adria margin and the accretionary prism to present-day Tuscany, where they were subducted to depths of at least 40 km. Shortly thereafter, they were brought to the surface again in the wake of hinge roll back of the Apennine subduction zone and the resulting rapid extensional exhumation. Such a zircon roller coaster requires a microplate that has back-to-back subduction zones with opposing polarities on two sides.