Paul Andriessen

Meso‐Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology

Apatite fission track (AFT) thermochronology of Precambrian and Paleozoic basement samples from southern Norway reveals a post-Paleozoic exhumation history, related to offshore Mesozoic and Cenozoic extensional basin development. The data indicate two major phases of rapid exhumation. A first Mesozoic phase started in the Triassic (∼220 Ma) in the east and south of the study area and migrated to the west where Jurassic (∼160 Ma) ages of exhumation predominate. A second event is indicated by thermal history modeling of AFT ages and track length distributions. It is inferred to be Neogene in age, initiated at about 30 Ma, and it produced a domal pattern of AFT isochrons which follow present-day topographic elevation. Youngest AFT ages (∼100 Ma) are encountered at sealevel in the inner fjords near the areas of highest topography; ages increase radially outward to the mountain peaks and the coastlines. Forward modeling of age-elevation patterns suggests that Mesozoic geothermal gradients were 10–15°C/km higher than the present value of 20°C/km. During the Triassic and Jurassic, a total of 1.3–3.5 km of overburden was removed from the study area, assuming a 30°C/km geothermal gradient for that period. We attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift, as evidenced by thick continental clastic sequences deposited in Triassic-Jurassic half grabens in the North Sea basins. We propose that 1.5–2.5 km of Neogene exhumation were a result of late stage domal uplift. This is supported by basinward dipping pre-Neogene strata in the basins surrounding southern Norway and the infill of a 1- to 2-km-thick Neogene sediment wedge containing various internal unconformities. Domal uplift probably started in the Late Oligocene, may have been amplified in the Pliocene, and was overprinted by Plio-Pleistocene glacial erosion. Maximum Neogene tectonic uplift is estimated at approximately 1–1.5 km, radially decreasing outward to a value <500 m near the shoreline. Neogene domal uplift is coincident with Oligocene and Pliocene plate reorganizations in the North Atlantic; similar Neogene domes are found around the Norwegian-Greenland Sea (i.e., Svalbard and the Barents Sea, northern Norway, east Greenland), suggesting a regional tectonic cause. The onset of Neogene uplift postdates major volcanism and continental breakup by ∼25 m.y. and predates Plio-Pleistocene glaciations. Its origin is possibly a combination of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses.

Lithospheric folding in Iberia

Integration of stress indicator data, gravity data, crustal kinematics data, and analysis of topography and recent vertical motions demonstrates the occurrence of consistently oriented spatial patterns of large-scale Alpine to recent intraplate deformation in Iberia. The inferred upper crustal and lithospheric deformation patterns and the timing of the associated expressions at or near the surface support the existence of a close coupling with plate boundary processes operating at the margins of Iberia. Patterns of lithosphere and upper crustal folds are oriented perpendicular to the main axis of present-day intraplate compression in Iberia inferred from structural analysis of stress indicator data and focal mechanism solutions. These findings suggest the presence of lithospheric folds, with wavelengths compatible with theoretical predictions of folding wavelengths of Variscan lithosphere. Stress-induced intraplate deformation set up by plate interactions is compatible with indications for the absence of present-day deep mantle-lithosphere interactions inferred from seismic tomography.

Thermo-mechanical controls on the mode of continental collision in the SE Carpathians (Romania)

Abstract The Carpathians orogenic system, with its along-arc variations in topography developed in the aftermath of continental collision, is associated with unusual foredeep basins, large-scale strain and seismicity concentration and high-velocity mantle bodies. The East Carpathians continental collision was non-cylindrical, leading to large-scale variations in thrust nappe kinematics, orogenic uplift patterns and foredeep subsidence, controlled by the mechanics and geometry of the lower plate. Thermo-mechanical modelling demonstrates that in this low-rate convergence regime, the subducted lithosphere had enough time to interact with the mantle to advance towards a thermal resettlement. This is favored by the low degree of metamorphism, mechanical weakness of the lower plate and the lack of active surface processes at the contact with and in the upper plate. In contrast, low-buoyant, thick lower crust and active surface processes keep the continuity of the slab intact and promote the development of typical foredeep basins. The model explains in a self-consistent manner the unusual geometry of the Vrancea seismogenic slab in the bend zone of the Romanian Carpathians. The model is also consistent with the presence of two high-velocity bodies inferred from seismic tomography studies and explains the depth zonation of seismicity in the Vrancea area. Differences between the northern part of East Carpathians and the southeastern bend of the Carpathians arc are largely controlled by lateral variations in crustal structure, topography emplacement and surface processes along the arc. Mechanical heterogeneity of the Carpathians subduction leads to the development of two end member modes of collision, allowing a study of these states and their transition. Lithospheric configuration and tectonic topography appear to be prime factors controlling variations in slab behavior. In the SE Carpathians, at the terminal phase of continental convergence, slab delamination, roll-back and depocenter migration appear to play a more limited role at shallow and lithospheric levels.

Morphotectonic evolution of rifted continental margins: Inferences from a coupled tectonic‐surface processes model and fission track thermochronology

We use a numerical model to study the topographic evolution and erosional history of rifted continental margins. The model combines a kinematic description of lithospheric extension (“tectonic model”) with a surface processes model that includes short-range hillslope and long-range fluvial transport. The tectonic model predicts the evolution of the lithospheric temperature and strength (elastic thickness) distribution as well as the tectonic uplift through time. This is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound. Modeled denudation histories across the margin are used to predict apatite fission track age and length patterns. Modeling results indicate that, depending on the adopted parameters, an uplifted rift flank should degrade by erosion within 50–100 m.y., without significant retreat of the topographic elevation maximum. The development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift, but results from the existence of a high elevation interior plateau, erosional base-level lowering as a consequence of rifting and regional isostatic response to erosion of the margin. However, apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth. Such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks. Modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief (i.e., initiation of rifting). Fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift. The timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data. The model is applied to the Saudi Arabian Red Sea margin and the southeastern Australian highlands, where it is constrained by present-day topography and apatite fission track data, as well as seismic and gravity data. For the Saudi Arabian Red Sea margin, synrift regional plateau uplift with a magnitude of approximately 1 km is inferred, possibly as a result of asthenospheric upwelling. Flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift. The relatively high elevation of the southeastern Australian highlands, their steep front and the relatively high amounts of erosion suggest that, apart from Mesozoic synrift flexural uplift, Tertiary rejuvenation of topography has occurred, possibly as a result of renewed lithospheric thinning and underplating. The low elevation of the Australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift.

Denudation history of the Malawi and Rukwa Rift flanks (East African Rift System) from apatite fission track thermochronology

Abstract Thirty apatite fission track ages and 22 track length measurements are presented from samples of basement rocks flanking the Malawi and Rukwa Rifts (East African Rift System) in order to elucidate the thermotectonic history of the rift flanks. The apatite fission track ages fall in the range 30 ± 15 to 296 ± 10 Ma. The relatively short (11.0–13.2 μm) mean track lengths and wide (1.3–2.3 μm) track length distributions suggest a protracted cooling history for the region, spanning Permian (Karoo) to Recent times. Thermal history reconstruction by inverse model calculations of the track length distribution suggests repeated phases of rapid cooling and denudation of the rift flanks at 250-200 Ma, ∼150 Ma and ≤40–50 Ma. These appear to be linked to the different rifting events in the area and can be correlated with deposition of the different sedimentary units within the basins. Erosion and isostatic rebound have modified the tectonically induced topography around the rifts: the elevation of the footwall flanks is augmented by flexural isostatic rebound, whereas the topography of the hanging wall flanks has been lowered by erosion. The footwall escarpments of the Malawi and Rukwa rifts are erosional features. The highly elevated plateaus flanking the Western Rift represent an erosional surface traditionally referred to as the “Gondwana surface”. The apatite fission track results of this study suggest that initial exhumation of the “Gondwana surface” to temperatures around 60–70°C took place during Karoo times, but that sub-aerial exposure of the surface did not take place until at least the Early Tertiary.

Late Miocene to present exhumation in the Ligurian Alps (southwest Alps) with evidence for accelerated denudation during the Messinian salinity crisis

Detrital apatite (U-Th)/He and fission-track thermochronometry are applied to constrain the exhumation history of the Ligurian Alps (southwest Alps). Exhumation of the Ligurian Alps is recorded from ca. 11 Ma and continues until present. More than 4 km of exhumation is recorded at an average rate of 0.4 ± 0.2 mm/yr. Exhumation is accommodated along a north-dipping thrust at the foot of the Ligurian margin. Combined (U-Th)/He and fission-track thermal modeling show a phase of accelerated denudation between 7 and 5 Ma, associated with the base-level drop during the Messinian salinity crisis.

Pattern and timing of the post-Caledonian denudation of northern Scandinavia constrained by apatite fission-track thermochronology

Abstract Apatite fission-track thermochronology has been used to study the post-Caledonian denudation history of northern Scandinavia. Post-orogenic denudation progressively shifted from the interior of the continent towards the North Atlantic margin. The present-day area of maximum elevation in the Northern Scandes mountain range has experienced continuous denudation at least since Jurassic time. In Jurassic-Cretaceous time, the area north and east of this region experienced either no denudation at all or some denudation followed by a transient thermal event with a peak temperature in late Cretaceous time. Final denudation of the area to the east of the Northern Scandes probably started in late Cretaceous-Paleogene time and possibly accelerated in Neogene time. The denudation history of northern Scandinavia can be explained by scarp retreat of an uplifted rift flank. The pattern and timing of denudation of the Northern Scandes is different from that of the Southern Scandes, which experienced domal-style, late-stage postrift uplift in Neogene time. Geomorphological observations, offshore data from the Atlantic and Barents Sea margins, and scarce stratigraphical information from the mainland are in general agreement with the new thermochronological data.

Syn-rift thermal structure and post-rift evolution of the Oslo Rift (southeast Norway): New constraints from fission track thermochronology

Abstract The Permo-Carboniferous (240–305 Ma) Oslo Graben in southeast Norway is a classical magmatic continental rift. We present fission track (FT) data on apatites, zircons and sphenes from the rift and surrounding areas in order to clarify its syn- and post-rift thermal evolution. Zircons and sphenes within the rift record FT ages of between ∼ 270 and 180 Ma (i.e., syn-post-rifting). In contrast, the Precambrian basement surrounding the graben yields zircon FT/sphene FT ages of ∼ 500–650 Ma, indicating that heating associated with rifting was focused inside the rift as a result of advective heat flow. Syn-rift to early post-rift temperatures reached > 240°C at the present erosion level in the graben. Syn-rift advective heating ( ∼ 260–270 Ma) was probably a result of large-scale batholith intrusion; post-rift ( ∼ 180–220 Ma) heating seems to have occurred by hydrothermal circulation of high-temperature (100–300°C) fluids. The FT data, organic geochemical indicators and fluid inclusions suggest that hydrothermal circulation followed a highly complex pattern controlled by the tectonic and volcanic structure of the rift. Apatite FT ages and confined track length distributions reveal the post-rift exhumation history of the area. Apatite FT ages are continuous within and outside the graben and decrease from Triassic (200–240 Ma) in the southeast to Jurassic ( ∼ 160 Ma) in the northwest, indicating that post-rift exhumation took place on a much larger scale than Permian rifting. Modelling of apatite FT ages and track lengths suggests cooling/denudation events in the Triassic, Jurassic and Neogene, accounting for a total post-Permian erosion of 3–4 km. Triassic-Jurassic denudation coincides with the migration of rifting from the Oslo-Skagerrak area to the North Sea/Danish Basin, as revealed by offshore seismic data, and caused destruction of the hydrothermal regime. Neogene uplift and erosion of the area is also supported by seismic evidence. The inferred timing and amount of regional post-rift erosion suggests that post-rift subsidence and sedimentation within the Oslo Graben was minimal.

The use of palaeo-thermo-barometers and coupled thermal, fluid flow and pore-fluid pressure modelling for hydrocarbon and reservoir prediction in fold and thrust belts

Abstract Basin modelling tools are now more efficient to reconstruct palinspastic structural cross sections and compute the history of temperature, pore-fluid pressure and fluid flow circulations in complex structural settings. In many cases and especially in areas where limited erosion occurred, the use of well logs, bottom hole temperatures (BHT) and palaeo-thermometers such as vitrinite reflectance (Ro) and Rock-Eval (Tmax) data is usually sufficient to calibrate the heat flow and geothermal gradients across a section. However, in the foothills domains erosion is a dominant process, challenging the reconstruction of reservoir rocks palaeo-burial and the corresponding calibration of their past thermal evolution. Often it is not possible to derive a single solution for palaeo-burial and palaeo-thermal gradient estimates in the foothills, if based solely on maturity ranks of the organic matter. Alternative methods are then required to narrow down the error bars in palaeo-burial estimates, and to secure more realistic predictions of hydrocarbon generation. Apatite fission tracks (AFT) can provide access to time–temperature paths and absolute ages for the crossing of the 120 °C isotherm and timing of the unroofing. Hydrocarbon-bearing fluid inclusions, when developing contemporaneously with aqueous inclusions, can provide a direct access to the pore-fluid temperature and pressure of cemented fractures or reservoir at the time of cementation and hydrocarbon trapping, on line with the tectonic evolution. Further attempts are also currently made to use calcite twins for constraining reservoir burial and palaeo-stress conditions during the main deformational episodes. Ultimately, the use of magnetic properties and petrographical measurements can also document the impact of tectonic stresses during the evolution of the layer parallel shortening (LPS). The methodology integrating these complementary constraints will be illustrated using reference case studies from Albania, sub-Andean basins in Colombia and Venezuela, segments of the North American Cordillera in Mexico and in the Canadian Rockies, as well as from the Middle East.

Calibration and comparison of etching techniques for apatite fission-track thermochronology

Abstract Understanding time–temperature histories using apatite fission-track thermochronology involves sample preparation, analysis and then thermal modelling using an appropriate annealing algorithm. A subtle point in this sequence is ascertaining that the sample preparation utilized is compatible with the methodology used in obtaining the data for constructing the annealing data set. This issue is important if one wishes to utilize the relatively new multikinetic annealing algorithm of Ketcham et al. that is implemented in their AFTSolve and HeFTy models which is based on a different etching recipe than those previously used. A preliminary calibration step involves comparing published etch pit diameters for a suite of samples with those analysed by an operator. Results show that the operator can reliably reproduce the calibration data set. We then report a laboratory experiment using samples from Finland and Spain that compares the results obtained using two different etching methodologies (7% nitric acid with qualitative etching conditions and 5.5 M nitric acid at constant conditions). The two raw data sets yield variable results. Comparing the two etching methodologies reveals the influence of this procedure on the kinetic parameter Dpar.