Mesozoic Rifting Research Articles

Post-orogenic exhumation in the western Pyrenees: evidence for extension driven by pre-orogenic inheritance

We discuss the drivers of the Pyrenean post-orogenic exhumation, including drainage migration, flexural rebound and tectonic reactivation.We provide new low-temperature thermochronological data and inverse thermal modeling from both the hinterland and foreland of the western Pyrenees. Our new thermochronological ages range from 6.6 to 61.4 Ma and reveal a Late Miocene exhumation phase in several massifs. The contrasting thermal histories define a domain of focused exhumation in the western Pyrenees that coincides with the present-day extensional tectonics in a region to the north of the Axial Zone. Based on the inferred cooling rates and paleogradient estimates, we highlight an exhumation phase of c. 1 mm yr –1 between 11 and 9 Ma in the Axial Zone, well above rates expected for a post-orogenic evolution. The thermal evolution inferred from three boreholes of the Aquitaine foreland basin reveals that sediments eroded from the hinterland did not accumulate in the Piedmont region but were transported offshore in the Bay of Biscay. We infer that the significant c. 10 Ma post-orogenic exhumation event must be related to the modern normal faulting regime of the western Pyrenees, associated with contrasting crustal thickness and densities, inherited from the Mesozoic rift evolution of the northern Pyrenees. Supplementary material: Laboratory analytical procedures, radial plot visualization of AFT detrital data and Age/eU relationship of AHe and ZHe samples are available at https://doi.org/10.6084/m9.figshare.c.5212581

The sines contourite depositional system along the SW Portuguese margin: Onset, evolution and conceptual implications

The Sines Contourite Depositional System, located in the Southwest Portuguese Margin, is a central segment of the Iberian Contourite Depositional Complex, built under the influence of the Mediterranean Outflow Water (MOW). This work presents the onset and evolution of this system using multibeam bathymetry, multichannel seismic reflection lines, sediment cores and well data. Six seismic units of Late Miocene through Holocene age have been identified, which defined three evolutionary stages for drift construction: a) an onset and initial stage, from the Latest Miocene to the Late Pliocene (5.33–2.5 Ma), where a sheeted drift was built under a weak flowing MOW; b) a growing stage from the Early to Middle Quaternary (2.5–0.7 Ma), characterized by the deposition of a mounded drift in the south and a sheeted drift in the north, which is correlated with a strengthening of the MOW; and c) a maintenance stage between the Middle Pleistocene and Holocene (0.7 Ma – present), where the modern depositional and erosional contourite features developed associated with three intensification periods of the MOW at 0.7 Ma, 0.4 Ma and 20 ka. The development of the Sines Contourite Depositional System was constrained in the long-term by seafloor paleomorphologies inherited from the Mesozoic rifting phases of the Southwest Portuguese Margin. The NNW-SSE horsts and grabens built during the Mesozoic rifting provided accommodation for drift growth, with limited lateral migration, and locally enhanced the alongslope bottom-current. In short-term, environmental (climate and sea-level) fluctuations modulated changes in bottom-current density and intensity, determining sedimentary cycles in the depositional record, especially during the Quaternary. Our findings emphasize the occurrence of three main plastered morphologies throughout the seismic record, which were compared with other plastered drifts in different continental margins to establish a conceptual model.

Reconstruction of the stress regime in the Jiaolai Basin, East Asian margin, as decoded from fault-slip analysis

The East Asian margin has experienced a complex tectonic history since the Mesozoic and has developed a complex basin-mountain system. The Jiaolai Basin, which is located on the East Asian margin, is a typical Mesozoic rift basin that has undergone multiple tectonic episodes. Here, using field geological surveys, effective stress inversions, and accurate chronological data, we recognize six tectonic episodes that have occurred in the Jiaolai Basin since the Cretaceous: E-W extension (135-121 Ma), which controlled the early deposition of the Laiyang Group; NW-SE extension (120-93 Ma), which controlled the late deposition of the Laiyang Group and the eruption of the volcanic rocks in the Qingshan Group; NE-SW compression (92-86 Ma), which produced folding of the Qingshan Group and its underlying strata and the formation of the unconformity surface between the Wangshi and the Qingshan Groups; N-S extension (85-60 Ma), which controlled the deposition of the Wangshi Group; NW-SE compression (60-55 Ma), which produced the unconformity between the Wangshi Group and the Wutu Group; and NE-SW extension (ca. < 55 Ma), which controlled the deposition of the Wutu Group. Considering the subduction history of the Izanagi plate and the Pacific plate, the activities of these plates may have primarily controlled the temporal variation in the intraplate paleostress regime in the East Asian margin.

Seismic Evidence for Crustal Modification Beneath the Hartford Rift Basin in the Northeastern United States

AbstractExtensive Mesozoic rifting along the eastern North American margin formed a series of basins, including the Hartford basin in southern New England. Nearly contemporaneously, the geographically widespread Central Atlantic Magmatic Province (CAMP) was emplaced. The Hartford basin provides an ideal place to investigate the roles of rifting and magmatism in crustal evolution, as the integration of the dense SEISConn array and other seismic networks provides excellent station coverage. Using full‐wave ambient noise tomography, we constructed a detailed crustal model, revealing a low‐velocity (Vs = 3.3–3.6 km/s) midcrust and a high‐velocity (Vs = 4.0–4.5 km/s) lower crust beneath the Hartford basin. The low‐velocity midcrust may correspond to a layer of radial anisotropy due to extension and crustal thinning during rifting. The high‐velocity crustal root likely represents the remnant of magmatic underplating resulting from the CAMP event. Our findings shed light on crustal modification associated with supercontinental breakup, rifting, extension, and magmatism.

Unravelling the architecture and evolution of the inverted multi-stage North Iberian-Bay of Biscay rift

Current rift models propose polyphase rift evolutions that develop with the same kinematic framework and accommodate continuous and sequential extension, through the stacking of deformation modes. Several rifted margins show, however, evidence for multiple and out of sequence rift events, which developed within different kinematic frameworks, with a complex spatial and temporal evolution. In this work, we address the problem of multi-stage rifting based on the study of the central North Iberian margin, located at the southern Bay of Biscay triangular oceanic domain. This magma-poor rifted margin recorded three major Mesozoic rift events and a subsequent Alpine compressional reactivation, representing a unique setting to study the architecture of multi-stage rift systems and the exerted control on subsequent reactivation. Relying on a dense set of 2D seismic reflection profiles, boreholes, and published velocity models, we map and describe structural domains and major extensional and compressional structures, and we construct depth and thickness maps of syn-rift units. These new maps display the geometry and spatial distribution of major rift basins and bounding structures. Distinctive scenarios resulting from this tectono-stratigraphic approach led us to define three rift systems. A first, diffuse and widespread Triassic system, with classical fault-bounded half-graben basins; a second, narrow, deep and localised Late Jurassic to Barremian transtensional system, including laterally confined pull-apart basins; and a third, widely distributed, Aptian to Cenomanian hyperextended system. Our results show that each rift system controlled the successive extensional events and the subsequent contractional deformation, resulting in a complex 3D template. Deciphering the tectono-stratigraphic architecture of such, multi-stage systems provide key insights on the spatial, temporal, and thermal evolution of divergent plate boundaries. It is likewise indispensable to propose and test kinematic plate deformable models on conjugate rifted margins and to comprehend the implications for their reactivation.

Filling Materials in Brittle Structures as Indicator of Cenozoic Tectonic Events in Southeastern Brazil

The filling materials in brittle structures can provide useful information about the Cenozoic evolution developed over proterozoic terrains. When these materials are affected by faults, they record deformation phases that can be determined chronologically and, in the occurrence of lateritic materials, it is possible to infer the paleoenvironmental conditions during the mineral formation. This work aimed to identify crystalline phases of brittle structure filling materials and to propose evolutionary interpretations for Cenozoic tectonic reactivation based on literature data. The study area is located in the Southern part of the Espírito Santo State, near the Brazilian Southeastern Continental Margin, where proterozoic geological structures have been reactivated since the mesozoic rift phase, up to the Holocene. The mineral assemblage found in the filling materials includes primary minerals such as quartz, muscovite, microcline, rutile, titanite, and bannisterite; and the weathering minerals such as kaolinite, illite, hematite, goethite, hydrobiotite, lithiophorite and, birnessite. The mineralogical association found in the filling materials denotes the action of fluid phases with mineral precipitation at the brittle discontinuities during the weathering processes that occurred during the Cenozoic, probably between the Miocene and the Pleistocene. The faults, which striations are marked on the filling materials, originated after (in the case of the manganese oxides) or during (in the case of the illite) the mineral formation, indicating that the maximum age of these faults is in the Miocene. The origin of the brittle structures that affected the filling materials studied here is linked to the uplifting of the Continental Brazilian Margin, when ancient geological structures were reactivated as normal faults due to the local action of an extensional regime.

Basement reservoir plumbing: fracture aperture, length and topology analysis of the Lewisian Complex, NW Scotland

Upfaulted ridges of Neoarchean crystalline basement rocks formed in the Faeroe-Shetland basin as a consequence of Mesozoic rift processes and are an active target for oil exploration. We carried out a comprehensive fault and fracture attribute study on the extensive exposures of geologically equivalent crystalline basement rocks onshore in NW Scotland (Lewisian Gneiss Complex) as an analogue for the offshore oil and gas reservoirs of the uplifted Rona Ridge basement high. Our analysis shows a power-law distribution for fracture sizes (aperture and length), with random to clustered spacing and high connectivity indices. Regional variations between the Scottish mainland and the Outer Hebrides are recognized that compare directly with variations observed along the Rona Ridge in the Faeroe-Shetland basin. Here we develop a model for the scaling properties of the fracture systems in which variations in the aperture attributes are a function of the depth of erosion beneath the top basement unconformity. More generally, the combination of size, spatial and connectivity attributes we found in these basement highs demonstrates that they can form highly effective, well-plumbed reservoir systems in their own right. Supplementary material: Additional methods and results are available at: https://doi.org/10.6084/m9.figshare.c.5017139 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

A reconstruction of Iberia accounting for Western Tethys–North Atlantic kinematics since the late-Permian–Triassic

Abstract. The western European kinematic evolution results from the opening of the western Neotethys and the Atlantic oceans since the late Paleozoic and the Mesozoic. Geological evidence shows that the Iberian domain recorded the propagation of these two oceanic systems well and is therefore a key to significantly advancing our understanding of the regional plate reconstructions. The late-Permian–Triassic Iberian rift basins have accommodated extension, but this tectonic stage is often neglected in most plate kinematic models, leading to the overestimation of the movements between Iberia and Europe during the subsequent Mesozoic (Early Cretaceous) rift phase. By compiling existing seismic profiles and geological constraints along the North Atlantic margins, including well data over Iberia, as well as recently published kinematic and paleogeographic reconstructions, we propose a coherent kinematic model of Iberia that accounts for both the Neotethyan and Atlantic evolutions. Our model shows that the Europe–Iberia plate boundary was a domain of distributed and oblique extension made of two rift systems in the Pyrenees and in the Iberian intra-continental basins. It differs from standard models that consider left-lateral strike-slip movement localized only in the northern Pyrenees in introducing a significant strike-slip movement south of the Ebro block. At a larger scale it emphasizes the role played by the late-Permian–Triassic rift and magmatism, as well as strike-slip faulting in the evolution of the western Neotethys Ocean and their control on the development of the Atlantic rift.

Evolution of the South-Iberian paleomargin: From hyperextension to continental subduction

Three crustal-scale sequentially restored cross-sections along the Central External Betic Cordillera enable us to pinpoint the role of extreme crustal thinning, tectonic inheritance and salt tectonics during the evolution of the South-Iberian paleomargin from Mesozoic rifting to Cenozoic continental subduction. This interpretation puts forward the existence of a former highly extended Mesozoic rift system, namely the Subbetic Basin. The basin comprises three main domains that coincide with the classical zoning of the External Betics: (i) The Intermediate units in the northern basin margin represent primary thick minibasins separated by salt walls; (ii) the External Subbetic units attest an inflated salt body in a central trough; and (iii) the Median and Internal Subbetic units in the southern basin margin are analogous to a group of primary minibasins with intercalations of submarine volcanic rocks overlying hyperextended crust. Shortening led to the extrusion of a salt canopy and the development of secondary minibasins. Furthermore, the tectonic inversion of the Subbetic Basin created two major thrusts sheets detached at the Upper Triassic evaporites, accommodating a shortening of ≈100–145 km at early Eocene-middle Miocene time. Compression reshaped the hyperextended domain into a subduction trench, and the former transfer faults into tear faults. Ongoing shortening led to the subduction of the South-Iberian paleomargin beneath the Alborán Domain lithosphere. Our results provide a new perspective on the inversion of segmented rift basins partly floored by mantle rocks and call for a major reconsideration on the tectonic architecture and evolution of the Betic-Rif Cordillera.

Exhumation and Surface Evolution of the Western High Atlas and Surrounding Regions as Constrained by Low‐Temperature Thermochronology

AbstractThe Moroccan High Atlas, the Anti‐Atlas belts, and the Siroua volcanic massif form an orographic system locally risen to elevations over 4 km. Topographic growth of this system occurred by Cenozoic transpressive inversion of Mesozoic rift structures concomitant with volcanic activity and related mantle processes. In order to constrain the topographic growth, we analyze the regional‐scale exhumation pattern, and we compare it to physiographic features. We contribute 35 new mean (U‐Th‐Sm)/He and 24 new fission track ages on apatite from the High Atlas and Anti‐Atlas, which all together range from 196 to 5 Ma. In the Anti‐Atlas, we find that the rate and amount of Cenozoic exhumation are &lt;0.05 km/Ma and ~2 km, respectively. In the High Atlas, our new data together with previous ones show that the axial regions feature the highest topography, relief, channel steepness, and orographic precipitations together with the maximum rate and amount of Cenozoic exhumation, in the range of 0.2–0.3 km/Ma and 4–6 km, respectively. This region is bounded by high‐angle oblique‐slip faults, which display a Neogene vertical motion of a few kilometers. Moreover, the most deeply incised valley in the axial region of the High Atlas shows stepwise cooling and exhumation with a final acceleration after 6 Ma. We conclude that the location in the same region of high river channel steepness, topography, orographic precipitations, and rates and amount of exhumation, together with large vertical offsets along oblique‐slip faults, suggests that crustal deformation may exert a major control on the topographic growth of Atlas orographic swell.

Aeromagnetic patterns in Southern Uruguay: Precambrian-Mesozoic dyke swarms and Mesozoic rifting structural and tectonic evolution

New high-resolution airborne magnetic data of Uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms, main faults and other magnetic bodies, which compose the Uruguayan Shield. We combined geophysical analyses (vertical derivatives, upward continuation, Euler deconvolution), structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the Uruguayan Shield and contribute to a better understanding of its tectonic evolution.The magnetic maps revealed several outstanding features in the Uruguayan Shield. The Paleoproterozoic dyke swarm is larger, denser, more widespread and complex than originally thought, suggesting a possible plume origin. In addition, a new Mesozoic dyke swarm, as complex as the previous one, was identified crosscutting the Paleoproterozoic dyke swarm and the Neoproterozoic orogenic structures. Moreover, this swarm is connected to volcanic calderas in the Merín basin, and shows displacements along Neoproterozoic shear zones, in the magnetic maps, revealing its brittle reactivation during Mesozoic times.The new observations clarify how Proterozoic basement structures controlled the development of the Mesozoic rift. Paleoproterozoic dyke swarms were reactivated as normal faults and Neoproterozoic structures hindered the rift growth, deflecting the deformation in transcurrent movements. Meanwhile, the Mesozoic dyke swarm was developed in a perpendicular direction to the Neoproterozoic structures. Moreover, these findings contradict the current rift model for Uruguay and rise a new model in which the Mesozoic rift developed as two rift basins connected by a central transfer zone, generated by the reactivation of Dom Feliciano Belt structures, between the Sierra Ballena and Sarandí del Yí Shear Zones.

Reservoir and sealing properties of the Newark rift basin formations: Implications for carbon sequestration

The Newark Basin is one of the major Mesozoic rift basins along the U.S. Atlantic coast evaluated for carbon dioxide (CO2) storage potential. Its geologic setting offers an opportunity to assess both the traditional reservoir targets, e.g., fluvial sandstones, and less traditional options for CO2 storage, e.g., mafic intrusions and lavas. Select samples from the basal, predominantly fluvial, Stockton Formation are characterized by relatively high porosity (8%–18%) and air permeability (0.1–50 mD), but borehole hydraulic tests suggest negligible transmissivity even in the high-porosity intervals, emphasizing the importance of scale in evaluating reservoir properties of heterogeneous formations. A stratigraphic hole drilled by TriCarb Consortium for Carbon Sequestration in the northern basin also intersected numerous sandstone layers in the predominantly lacustrine Passaic Formation, characterized by core porosity and permeability up to 18% and 2000 mD. However, those layers are shallow (predominantly above 1 km in this part of the basin) and lack prominent caprock layers above. The mudstones in all three of the major sedimentary formations (Stockton, Lockatong, and Passaic) are characterized by a high CO2 sealing capacity — evaluated critical CO2 column heights exceed several kilometers. The igneous options are represented by basalt lavas, with porous flow tops and massive flow interiors, and a crystalline but often densely fractured Palisade Sill. The Newark Basin basalts may be too shallow for sequestration over most of the basin's area, but many other basalt flows exist in similar rift basins. Abundant fractures in sedimentary and igneous rocks are predominantly closed and/or sealed by mineralization, but stress indicators suggest high horizontal compressional stresses and strong potential for reactivation. Overall, the basin potential for CO2 storage appears low, but select formation properties are promising and could be investigated in the Newark Basin or other Mesozoic rift basins with similar fill but a different structural architecture.

The Catalan magnetic anomaly: Its significance for the crustal structure of the Gulf of Lion passive margin and relationship to the Catalan transfer zone

Abstract The western part of the Gulf of Lion (GoL) passive margin, at the foot of the Pyrenees mountain chain, hosts the Catalan magnetic anomaly (CMA), a significant NW-SE-trending linear magnetic anomaly 50 km long with no associated gravimetric signal. The CMA is located on the continental slope near the NW-SE-trending Catalan transfer zone (CTZ), and it extends from the shelfbreak to the abyssal plain across thinned to hyper-thinned continental domain. This study interprets an updated geophysical dataset to better understand the Pyrenees–GoL transition and proposes a new interpretation of the CMA's source. Gravimetric and magnetic modelling of the lithosphere along two perpendicular cross-sections indicates that the CMA is generated by a magnetic body (80 × 20 × 12 km), called here the Catalan body, within the thinned continental domain. We interpret it as a mass of mafic material underplating and intruding the continental crust along a crustal fault belonging to the CTZ system. From the new map of Moho depths, a wide NW-SE-trending zone is highlighted on the western extremity of the GoL margin at the foot of the eastern Pyrenees Chain. It is characterized by (i) a heterogeneous crustal structure and mantle rise underneath the Tertiary Catalan sedimentary basin in the continental shelf area, (ii) a necking zone in a more proximal location compared to the eastern GoL margin, and (iii) the presence of magmatism (Catalan body) under and in the thinned continental crust, in the necking zone domain. These characteristics may be induced by the transtensional motion along the CTZ during the Tertiary opening of the Liguro-Provencal Basin, contemporary with the extension of the GoL margin. However, the role of inherited structures from the Hercynian orogeny, Mesozoic rifting and Pyrenean orogeny cannot be excluded when explaining the crustal heterogeneity, transfer zones and Moho morphology in this region.

Deep Origin of the Dome‐Shaped Hyblean Plateau, Southeastern Sicily: A New Tectono‐Magmatic Model

AbstractSoutheastern Sicily is characterized by a prominent topography known as the Hyblean Plateau. It is commonly considered as a flexural bulge formed by the northwest plunging of the Hyblean‐Malta platform beneath the Sicilian Fold and Thrust Belt. However its noncylindrical shape and multiphase uplift history correlated to the Late Miocene and Plio‐Pleistocene volcanism outline a possible magmatic control on the Hyblean Plateau topographic expression. We argue, using 2‐D flexural models, that the flexural bending alone is unlikely to explain the high anomalous elevation of the Hyblean Plateau. Moreover, topographic analysis show that the Hyblean Plateau has a dome shape composed of radially tilted preserved surfaces defining individual blocks. The block edges seem to be related to deep‐seated inherited faults. Using an analogue modeling approach, we show that magmatic intrusions beneath the thick Meso‐Cenozoic sedimentary cover can reproduce the first‐order morphology of the Hyblean Plateau when a structural inheritance is imposed. Accordingly, we propose a tectono‐magmatic model that favors mafic sill intrusions as the major uplift source for the topographic anomaly. However, to explain the strong positive Bouguer anomaly, a deeper high‐density intracrustal body is needed. Moho uprising related to Mesozoic rifting or mafic lower crust underplating/intraplating since the Late Cretaceous can be evoked. The small wavelength of the topography and the deep intracrustal mass anomaly makes the lithospheric elastic stresses sufficiently high to maintain a nonisostatic state over the geological time scales.

The Cenozoic magmatism of East Africa: Part II – Rifting of the mobile belt

Despite the foundational role East Africa has played in the advancement of our understanding of continental rifting, there remains substantial ambiguity as to how magmatism has evolved during progressive rift development. This contribution is a comparative study that explores the temporal development of magmatism in both rifts, and within the interposing Turkana Depression. Notwithstanding the independent evolution of the Main Ethiopian Rift and Kenya Rift prior to their Quaternary linkage, magmatic events within them show remarkable parallelism. Following an initial pulse of basaltic magmatism ca. 20 Ma, more evolved compositions (flood phonolites in Kenya, rhyolites in Ethiopia) dominated the landscape until ca. 12 Ma. From ca. 12 Ma to 9 Ma, a renewed phase of widespread basaltic volcanism (Mid-Miocene Resurgence Phase) impacted the entire region from the Afar margin to Kenya, though activity in the south slightly predates equivalent basaltic events to the north. Following this widespread basaltic event, silicic magmatism again dominated the now nascent rifts until a renewed phase of basaltic activity commenced ca. 4 to 1.6 Ma termed the ‘Stratoid Phase’. Following the termination of the Stratoid Phase, the modern expression of volcanism in both rifts has been dominated by central silicic volcanoes. Magmatic activity within the Turkana Depression broadly parallels that in both rifts – basaltic events are evident at ca. 20 Ma, 12 Ma, and 4 Ma, however the silicic interludes that are evident within the rifts, are generally absent. Moreover, from 4 Ma to present, there is a progressive decrease in magmatic activity from an initial fissural basalt, to a series of large shield volcanoes, to modern belts of cinder cones. The results of this synthesis suggest widespread pulsed basaltic events precede silicic flare-ups throughout the East African Rift System, and the lack of such flare-ups in Turkana may relate to the Mesozoic rifting that modified the continental lithosphere in that region.

Crustal structure of Mesozoic rifting in the northeastern Gulf of Mexico from integration of seismic and potential fields data

We have investigated the crustal structure of a 400 km wide zone of thinned continental crust in the northeastern Gulf of Mexico (GOM) using gravity and magnetic modeling along two deeply penetrated seismic transects. Using this approach, we identify two zones of prominent, southward-dipping reflectors associated with 7–10 km thick, dense, and highly magnetic material. Previous workers have interpreted the zones as either coarse clastic redbeds of Mesozoic age that are tilted within half-grabens or seaward-dipping reflectors of magmatic origin. Both seismic reflection lines reveal a 10 km thick and 67 km wide northern zone of high density near the Florida coastline beneath the Apalachicola rift (AR). The southern zone of high density occurs 70 km to the south in the deepwater central GOM along the northern flank of the marginal rift, a 48 km wide, southeast-trending structure of inferred Late Jurassic age that is filled by 3 km of low-density and low-magnetic susceptibility sediments including complexly deformed salt deposits. We propose that these two subparallel rifts and their associated magmatic belts formed in the following sequence: (1) AR formed during Triassic-early Jurassic (210–163 Ma) phase 1 of diffuse continental stretching and was partially infilled on its northern edge by southward-dipping volcanic flows; and (2) the similarly southward-dipping southern magmatic zone formed adjacent to the marginal rift during the early phase 2 of late Jurassic (161–153 Ma) rifting of the GOM continental extension; this southern area of SDR formation immediately preceded the formation of the adjacent oceanic crust that separated the rift-related evaporates into the northern and southern GOM. Our integrated approach combining 2D seismic, gravity, and magnetic data sets results in a more confident delineation of these deep crustal features than from seismic data alone.

Using a Paleosurface to Constrain Low‐Temperature Thermochronological Data: Tectonic Evolution of the Cuevas Range, Central Andes

AbstractDispersion of low‐temperature thermochronologic data from nine samples collected on a deformed paleosurface preserved on the Cuevas range (Central Andes) can be exploited to unravel complex thermal histories. The nine samples yielded data that have both intersample and intrasample dispersions; the data set includes apatite fission‐track ages (180–110 Ma), mean track lengths (11–13 μm), apatite helium (10–250 Ma), and zircon helium ages (180–348 Ma). We ran inverse thermal history models for each sample that reveal spatial variations of the Miocene reheating along the paleosurface. Next, we ran a multiple‐sample joint model to infer a common form for thermal history for all samples. Our results suggest that initial exhumation during the Famatinian orogeny was followed by a residence between ~2.5 and 7.0 km depth during the Paleozoic and the Triassic. The onset of Mesozoic rifting was responsible for an increase of the geothermal gradient and extensive horst exhumation, which brought the basement of the Cuevas range close to the surface (~1–2 km) in the Late Jurassic. Between the Late Cretaceous and the Paleocene, the combination of low relief, a humid climate, and low erosion rates (0.006–0.030 km/Ma) facilitated the development of the Cuevas paleosurface. During the Miocene, this paleosurface experienced differential reheating with a high geothermal gradient (&gt;25 °C/km) due to the sedimentary cover and local magmatic heat sources. During the Andean orogeny, in the Pliocene, the Cuevas paleosurface was deformed, exhumed, and uplifted.

Crustal Structure of Eastern North Carolina: Piedmont and Coastal Plain

Abstract We present the results from an onshore seismic refraction and wide‐angle reflection profile, conducted in 2015, across the coastal plain and eastern Piedmont provinces of North Carolina. We use forward modeling to create 1D synthetic seismogram models and then invert first break picks to create 2D P‐ and S‐wave velocity models. The crustal thickness is 38 km beneath the Piedmont and central coastal plain, but it thins to 32 km at the coastline. The average thickness of the upper crust is 11 km with an average P‐wave velocity (VP) of 6.0 km/s and S‐wave velocity (VS) of 3.5 km/s. A prominent seismic low‐velocity zone (LVZ) (VP&lt;6.0 and VS&lt;3.6 km/s) exists between the depths of 6 and 11 km, beneath the western third of the seismic profile. The middle crust varies greatly in thickness, increasing from 3 km in the west (eastern Piedmont) to 13 km in the east (coastal plain), with seismic velocities of 6.5 km/s for VP and 3.8 km/s for VS. The lower crust thins significantly toward the rifted Atlantic margin, decreasing from 24 km thick in the west (Piedmont) to 8 km at the coastline, with velocities of approximately 6.9 km/s for VP and 3.9 km/s for VS. We estimate the composition of the crust by comparing the measured values of VP and Poisson’s ratio with laboratory measurements. The upper and middle crusts are in agreement with a felsic composition, while the lower crustal composition is predominately felsic to intermediate. The LVZ in the upper crust is associated with thin layers of the mylonitic rocks involved in the top and the bottom of thrusting, and the top of the lower crust could be the master detachment fault during the thin‐skinned Alleghanian orogeny. The eastward thinning of the lower crust is consistent with crustal extension during the Mesozoic rifting of the Atlantic margin.

Structural evolution and the partitioning of deformation during basin growth and inversion: A case study from the Mizen Basin Celtic Sea, offshore Ireland

AbstractThe Celtic Sea basins lie on the continental shelf between Ireland and northwest France and consist of a series of ENE–WSW trending elongate basins that extend from St George’s Channel Basin in the east to the Fastnet Basin in the west. The basins, which contain Triassic to Neogene stratigraphic sequences, evolved through a complex geological history that includes multiple Mesozoic rift stages and later Cenozoic inversion. The Mizen Basin represents the NW termination of the Celtic Sea basins and consists of two NE–SW‐trending half‐grabens developed as a result of the reactivation of Palaeozoic (Caledonian, Lower Carboniferous and Variscan) faults. The faults bounding the Mizen Basin were active as normal faults from Early Triassic to Late Cretaceous times. Most of the fault displacement took place during Berriasian to Hauterivian (Early Cretaceous) times, with a NW–SE direction of extension. A later phase of Aptian to Cenomanian (Early to Late Cretaceous) N–S‐oriented extension gave rise to E–W‐striking minor normal faults and reactivation of the pre‐existing basin bounding faults that propagated upwards as left‐stepping arrays of segmented normal faults. In common with most of the Celtic Sea basins, the Mizen Basin experienced a period of major erosion, attributed to tectonic uplift, during the Paleocene. Approximately N–S Alpine regional compression‐causing basin inversion is dated as Middle Eocene to Miocene by a well‐preserved syn‐inversion stratigraphy. Reverse reactivation of the basin bounding faults was broadly synchronous with the formation of a set of near‐orthogonal NW–SE dextral strike‐slip faults so that compression was partitioned onto two fault sets, the geometrical configuration of which is partly inherited from Palaeozoic basement structure. The segmented character of the fault forming the southern boundary of the Mizen Basin was preserved during Alpine inversion so that Cenozoic reverse displacement distribution on syn‐inversion horizons mirrors the earlier extensional displacements. Segmentation of normal faults therefore controls the geometry and location of inversion structures, including inversion anticlines and the back rotation of earlier relay ramps.

What is the structure of the forearc region in the Central Andes of northern Chile? An approach from field data and 2-D reflection seismic data

The tectonic evolution of the Andean forearc in the Central Andes is understood by the superimposition of deformational events associated with different tectonic regimes, which control the main morpho-tectonic units of northern Chile. Varied interpretations have opened a great discussion about the deep structure of the Andean forearc and its implications in the evolution of the Central Andes. These ideas are commonly accompanied by the lack of regional tectonic models that explain the geometry and kinematics of first-order structures. The main limitations are due to the unknown origin of the structural styles in the substrate and their relationships with those exposed in the field. In order to solve this problem, we present a new structural model of the forearc region of northern Chile, specifically along the Central Depression (22°-23°S) supported by field observations and the structural interpretations of 2D seismic data. Four major tectono-sequences have been recognized related to (a) a Paleozoic-Triassic basement rocks, (b) a Jurassic-Lower Cretaceous synrift deposits, (c) Upper Cretaceous-Paleocene volcano synorogenic deposits and (d) a Late Cenozoic covered gravel. The first-order structural styles identified correspond to inverted half-graben basement structures related to the Jurassic Tarapacá Basin. The hanging wall blocks of the master faults are usually composed of large inversion anticlines along which, Jurassic and Lower Cretaceous siliciclastic deposits were expelled. Other basement contractional structures consist of short-cut and by-pass faults that modified the previous geometry of extensional systems. The second-order thin-skinned fold and thrust exposed in the region accommodated the shortening experienced by the Mesozoic synrift deposits during the basin inversion. The Upper Cretaceous-Paleocene deposits usually unconformably overlie the folded Mesozoic synrift deposits. This structural feature is first-order evidence of the positive reactivation of the Mesozoic rift basins and therefore is a regional marker of the onset of the Andean uplift in the region.