Complete rupture of the Padul Fault represents one of the largest plausible earthquakes in the Sierra Nevada Range, one of the most seismically active regions of Spain. We performed a regional ...assessment of earthquake-triggered slope instabilities in the western part of the range to determine the most likely types of failures from such an earthquake in the region and suggest where such failures have a higher likelihood of occurring. These results are broadly useful for management of regional life-lines and future development. First, a slope-instability inventory of the Sierra Nevada was produced to identify the most common instability types. Subsequently, the Newmark’s sliding rigid-block methodology, implemented in a geographic information system, was used to obtain the distribution of Newmark displacements in the area considering a M w 6.6 earthquake on the Padul Fault. The Newmark displacements were then compared to the distribution of the inventoried slope instabilities to identify the areas where seismicity could reactivate old slope instabilities or generate new ones, and to identify the involved landslide typology. The most likely seismically induced slope instabilities in the Sierra Nevada are rock falls and rock slides. These types of instabilities could be triggered by Newmark displacements of 2 cm or less.
We present the GIS extension CalHypso to automatically extract multiple hypsometric curves from digital elevation models (DEM). This extension is programmed in Visual Basic 6 and uses the ArcObjects ...architecture of ArcGIS. It employs a new and easy methodology to extract the hypsometric curve by using the integer data-model properties of ArcGIS and summations algorithms. This provides an optimal integration within the program environment, allowing the representation and comparison of curves directly within the GIS main program. The CalHypso extension can also calculate the main statistics related to the hypsometric curve by applying polynomial fits. We have tested this tool in several basins of the eastern border of the Sierra-Nevada dome in the Betic Cordillera (SE Spain), showing important differences in the north- and south-slope hypsometric curves. These variations are probably related to the tectonically controlled drainage evolution of the southern border of Sierra Nevada. An eastward decrease in tectonic-driven erosion is also suggested by the hypsometric curves analyzed.
Two orthogonal extensional systems produced the extensional collapse of the Tell and Atlas thrust belts in northern Tunisia during the Late Miocene to Pliocene in a context of NW‐SE plate convergence ...between Africa and Eurasia. The older extensional system shows several low‐angle normal faults (LANFs) and associated high‐angle faults with ENE‐directed transport that produced half‐grabens and hanging‐wall syncline basins during the late Tortonian to Messinian. The direction of extension swinged towards the SE during the Messinian, cutting into, and tilting the previous detachments. Extension was accompanied by the extrusion of 8‐Ma rhyodacites and Messinian basalts, together with the development of mineralized fault breccias. Plio‐Quaternary NW‐SE directed shortening formed inversion arrowhead structures, reverse faults, refolded extensional rollover anticlines and folded the LANFs. ENE‐directed extension is concomitant with the opening of the Tyrrhenian basin. We consequently think that both processes are related and that tearing of the Calabrian slab along the northern Tunisia coast drove the ENE‐directed extension. Meanwhile, the SE‐directed extension that followed was probably related to SE‐directed peeling back of the Tunisian continental lithospheric mantle during NW subduction of the Maghrebian margin. This extension propagated eastwards from the late Tortonian until the Pliocene following the SE migrating subduction front and favored by lateral slab tearing along the Tunisian Atlas dextral Subduction Transfer Edge Propagator boundary. This new hypothesis for the tectonic evolution of northern Tunisia shows for the first time the importance of crustal extension in the denudation of the Tunisian Atlas and Tell foreland thrust belts and its relation to deep mantle tectonic mechanisms.
Key Points
Northern Tunisia was extended in the Late Miocene
Extension was driven by deep mantle mechanisms like mantle delamination and slab tearing
Two orthogonal extensional systems with low‐angle detachments and associated listric fans have been differentiated
Cenozoic extension in the Western Mediterranean has been related to the dynamics of back‐arc domains. Although, in most of its orogenic belts extension propagated into the fore‐arc nappe domains. ...Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell, where HP/LT rocks (350°C at 0.8 GPa), were exhumed by the sequential activity of extensional detachments after heating and decompression (410°C–440°C at 0.6–0.3 GPa) in a plate convergent setting. Normal faults thinning the Tunisian Tell detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving Tellian Triassic evaporites in the hanging‐wall forming halokinetic structures in the Mejerda basin late Miocene. The deeper‐detachment bounds metamorphic domes formed by marbles and metapsammites from the Atlas domain. Illite crystallinity on Triassic rocks shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell‐Atlas nappe belt, respectively. New U‐Pb 49.78 ± 1.28 Ma rutile ages from Tellian metabasites, together with existing phlogopite 23–17 Ma K‐Ar ages in Atlas marbles from the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell rocks underthrusted the Kabylian flysch in the early Eocene. Further, early Miocene shortening thrusted the metabasites over lower‐grade sediments, producing HP/LT metamorphism and ductile stretching at the base of the Atlas belt. The exhumation of midcrustal roots of Western Mediterranean nappe belts after tectonic shortening is a common feature related to tearing at the edges of the subduction systems and inboard delamination of their subcontinental lithospheric mantle.
Plain Language Summary
Mountain belts are formed by shortened sedimentary rocks. The Tell cordillera in Northern Tunisia is interpreted as a classic mountain belt developed through protracted shortening from the late Cretaceous until Present, formed by folded and overthrusted rocks, and intruded by salt bodies. However, we show here that conversely, some of the supposed salt bodies are formed by metamorphic rocks that were originally buried at depths of approximately 26 km. Moreover, the remaining salt structures in the Tunisian Tell formed in relation to the late‐stage thinning and collapse of the mountain belt, as they intrude through extensional faults into late Miocene sediments. We characterize the temperature and pressure conditions reached by the metamorphic rocks and obtain a 49 Ma age of an early metamorphic event by radiometric dating of rutile. Metamorphic rocks where also exhumed in other Western Mediterranean mountain belts like the Betics, Rif, Algerian Tell after the main shortening stage. We relate this process to delamination, a deep mantle tectonic mechanism, which strips the nappe belt crustal domain from its underlying mantle root.
Key Points
The Tunisian Tell hosts High‐Pressure Low‐Temperature (HP/LT) domes underlying late Miocene extensional detachments, driven by slab delamination
Metamorphic and rutile U‐Pb data shows that rocks in Northern Tunisia reached HP/LT conditions in the early Eocene and early Miocene
Halokinetic structures in Northern Tunisia are rooted in the Mejerda detachment and related to late Miocene extension
Active tectonics in North Africa are fundamentally driven by NW-SE directed slow convergence between the Nubia and Eurasia plates, leading to a region of thrust and strike-slip faulting. In this ...paper, we analyse the morphometric characteristics of the northern Tunisia sector. We aim to identify unknown active tectonic structures, and to further understand the mechanisms driving the drainage evolution in this region. The interpretation of morphometric data was ground-truthed with field data. Our analysis indicates that recent fluvial captures have been the main factor driving fluvial network reorganization in NE Tunisia. The Medjerda River has increased its drainage area during the Quaternary by capturing adjacent axial valleys to the north and south of its drainage divide. These captures are probably driven by the gradual uplift of adjacent axial valleys by reverse/oblique faults or associated folds like El Alia-Teboursouk and Dkhila faults. Our fieldwork found that these faults cut Holocene sediments thus showing recent fault activity. The growth and stabilization of the axial Medjerda River, contrary to the natural transverse drainage development of mature orogens, might be caused by a combination of crustal shortening structures and mantle dynamic processes. The Medjerda River flows SW-NE from the South Atlassic dextral transfer zone to the regional topographic low defined by the Gulf of Gabes and the Straits of Sicily, which in turn, may be influenced by the underlying Tunisian slab in the region.
•Morphometric relief analysis for the tectonic characterisation of northern Tunisia•Recent fluvial captures are the main factor driving fluvial network reorganization•River captures are caused by crustal shortening structures and mantle dynamics•The Medjerda River has increased its drainage area during the Quaternary•The morphometric analysis supports a sparse distribution of active faulting
We have performed a geomorphic analysis of the Sierra Nevada, the highest range of the Betic Cordillera (SE Spain), with the aim to elucidate its late Miocene to present-day exhumation history. The ...qualitative and quantitative analysis is based on filtered topography, local relief, swath-profile analysis, anomalies on stream orientation, bulk erosion volume, hypsometry, and steepness index (ksn). All these parameters are intimately linked to river incision and development of drainage pattern, having been calculated to assess the role of folding and faulting on the evolution of the Sierra Nevada. Moreover, uplift rates in the core of the Sierra Nevada have been deduced from an extrapolation of the position of Late Tortonian to Pliocene coastline deposits. These data have been compared to apatite (U–Th)/He, fission-track and 10Be cosmogenic data from SE Spain in order to evaluate the consistency among uplift, thermal histories and denudation rates. Our preferred tectonic scenario is one that favors fast exhumation of the western Sierra Nevada in a NW–SE overall compressive setting produced by the convergence between the Nubia and Africa plates. Sub-perpendicular to this compression, a westward 4mm/year extensional hanging-wall displacement promotes uplift and unroofing of the western part of Sierra Nevada.
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•River incision in Sierra Nevada is conditioned by tectonics and local base level.•Western Sierra Nevada concentrates main elevation and higher denudation rates.•Uplift of western Sierra Nevada transforms old longitudinal drainage in water gaps.•We can establish a steady-state between uplift and denudation in the Central Betics.•Extension in the central Betics cannot be understood in terms of thin-shell tectonics.
Active shortening structures in Northern Tunisia have developed by tectonic inversion since the Pliocene, after Late Miocene extensional collapse of the whole region. Restored Plio-Quaternary ...deformation observed on reflection seismic lines indicates deformation rates around 0.6–0.8 mm/yr in the studied segments and larger amounts of shortening to the West of Northern Tunisia (16%) than to the East (7%), which suggests tectonic inversion started earlier to the West and later propagated eastwards, reaching Northeastern Tunisia in the Late Pliocene. This shortening is registered on striated pebbles in Quaternary alluvial terraces and fault-slip data giving two populations of strain ellipsoids with N–S and WNW-ESE maximum shortening. Morphometric analysis in combination with field fault segmentation mapping show that topographic uplift and drainage rejuvenation occurs in relation to 20–30 km long ENE-WSW reverse fault segments and related antiforms that are offset and linked by E-W to WNW-ESE dextral and NE-SW-oriented sinistral faults. The largest fully linked fault system is the Alia-Thibar fault. This 130 km long fault zone shows an helicoidal geometry with five different fault segments, including reverse, dextral, sinistral and oblique faults. Due to the young age of tectonic inversion, after late Miocene extensional collapse of the region, the present relief of Northern Tunisia is characteristic of a young thrust and fold belt, with dominating axial valleys along synforms and an incipient transverse drainage development propagating from West to East.
•NW-SE shortening in Northern Tunisia occurs along 20–30 km long fault segments.•Active reverse faults developed since the Pliocene, mostly by tectonic inversion.•Shortening propagated from W to E since the Pliocene.•ksn and Hi morphometric anomalies mimic active fault segmentation in Northern Tunisia.•The 130 km long Alia-Thibar dextral-reverse fault is the largest linked fault system.
Hypsometry is thought to be sensitive to tectonic uplift rates and lithology differences. In this study we calculated hypsometric integrals (HIs) using as topographic sources two digital elevation ...models of 10 and 90 m of pixel resolution in the Granada basin (SE of Spain). The HI spatial distributions do not show clear spatial patterns and do not correlate with basin parameters as mean elevation or relief amplitude. However, when exploratory spatial data analysis is applied to the data distributions through local indices of spatial autocorrelation, clear hot spots are visible that improve the geologic meaning of the HI. The distributions are robust and independent of the model resolution but are scale influenced. The application of this new method to the Granada basin shows a strong correlation between the main distribution of active normal faults in the basin and the clusters of high or low HI values obtained in our analysis. Clusters with high HI values define the uplifted footwalls of these faults and regions uplifted in relation with rollover anticlines or where epeirogenic uplift has not been counteracted by local extension. Once the method was adjusted in the Granada basin, we tested its applicability in an area of known contractive tectonic activity, central Otago, New Zealand, showing that the meaning of HI values is improved by using the autocorrelation techniques.
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