Development Of Extensional Structures Research Articles

Assessing landscape response to tectonics in the Jalisco block and adjacent areas (west-central Mexico) using topographic analysis

To elucidate how landscapes respond to either tectonic and climatic changes through time, geomorphologists use the analysis of river and basin morphometry. Many studies have evaluated how rivers, and consequently hillslopes, respond to changes in the rate of rock uplift in compressional settings. The case of extensional settings, however, has received less attention. We examine the case of landscape response to tectonics in west-central Mexico to identify zones where there is notable increase in the tectonic activity. The study of west-central Mexico is challenging for the morphometric analysis since there is an interplay between the development of extensional structures and pulses of volcanism, mainly from Eocene to Oligocene and Pliocene to Pleistocene, that have buried large parts of the landscape. For our purpose we use the normalized channel steepness index (ksn) to evaluate how this index correlates with other morphometric parameters such as the hillslope gradient, hillslope convexity and concavity, terrain rugosity and first order stream gradient (Fosg). We use the landform units map as the spatial unit of analysis and from this, we estimate the morphometric properties of the landscape. The results of the morphometric analysis indicate that the topography of Eocene to Oligocene volcanic landscapes are well explained (~70%) by the ksn and in less degree for Pliocene to Pleistocene landscape where ksn accounts for ~55–66% of landscape morphometry. Our results also indicate that the ksn is the best suited among all morphometric variables used in this study and confirm that the landscape of west-central Mexico is dominated by river incision. Landscape incision is particularly high on the northern part of the Jalisco block, mainly along the Ameca river and its tributaries such as Atenguillo river, and north of the Jalisco block, along the Santiago river and on tributaries incising the Sierra Madre Occidental. Finally, the morphometric analysis indicates that even for the case where the volcanic rocks and sediments are filling up large parts of the landscape, river incision resulting from the tectonic activity is capable to cut into lavas, propagating the change imposed by the tectonic activity.

Structural context of the paleogeography of the Cenomanian-Turonian anoxic event in the eastern Atlas basins of the Maghreb

The regional Cenomanian-Turonian (C/T) structural framework of surface and subsurface coupled with subsidence data inferred from palegeographic and organic-rich C/T isopachus maps, generated in this work, from several basins, wells and outcrops of Tunisia and eastern Algeria were investigated and provide constraints on the structural and sedimentary evolution of this part of the southern Tethyan margin during the C/T times. This period is characterized by the development of extensional structures with tilted blocks and grabens, bounded listric normal faults. Subsidence during C/T time led to the accumulation of a thick sequence of laminated black shales and marls (up to 80 m thick in northwestern Tunisia and about 100 m thick in the Aures-Kef trough in northeastern Algeria). Sharp variations in thickness and facies towards the north and northeast indicate that basin subsidence was driven by regional tectonic movements along NW-SE, NE-SW and east-west trending normal faults issued from Tethyan rifting of the Upper Jurassic-Lower Cretaceous. The reduction of emerged areas since the Cenomanian inferred from the generated palaeogeographic map is demonstrated. The overall evolution is also maintained by halokinesis of Triassic salt creating subsident rim-synclines, where organic-rich facies prevail.

An overview of the Late Cretaceous–Eocene positive inversions and Oligo-Miocene subsidence events in the foreland of the Tunisian Atlas: Structural style and implications for the tectonic agenda of the Maghrebian Atlas system

Structural interpretation of surface and subsurface data in the Eastern foreland basins of Tunisia, allows us to recognize positive inversion structures, i.e. related to compressional events, expressed and recorded in the Paleogene sedimentary pile of the Atlas domain. These episodes are followed by a period of relative tectonic quiescence during Oligocene–Early Miocene with development of extensional structures with slightly tilted panels, grabens and locally the development of listric normal faults branched at depth on “décollement” levels. Comparison of the seismic interpretations and field data collected along the South Atlas Front as well as in the Atlas belt, allows us to propose a tectonic scenario at the scale of Eastern Tunisia Atlas. In particular, we emphasise the role of the so-called “Atlas event” (Middle–Late Eocene), which was initially defined in Algeria but poorly outlined in Tunisia. We will show that the tectonic agenda defined in Eastern Tunisia is consistent with the one proposed elsewhere in the Maghreb allowing us to propose a unified view of the geodynamic evolution of the whole Atlas system during this period. The negative inversion recorded during Oligocene–Early Miocene along with the occurrence of synsedimentary normal faults are related to an increase of the subsidence rate in the frame of continuous shortening coeval to basin formation in the front of the Atlas. The whole Oligo-Miocene evolution results from two different mechanisms: flexuration in the front of the Atlas and the onset of the extension of the Pantelleria–Linosa–Malta rift system of central Mediterranean, which attained a climax stage later during Pliocene–Quaternary.

Fluid flow at fault intersections in an active oblique collision zone, Southern Alps, New Zealand

Hydrothermal fluid flow in the core of the active dextral oblique Southern Alps collision zone is controlled by steeply dipping faults. Fluids rise from middle crustal depths along faults and can transport gold to near-surface regions. Fluid mobility is pronounced in and adjacent to north-striking faults oblique to the northeast striking orogen, especially where these faults intersect orogen-parallel oblique dextral faults. Fluid flow is enhanced by development of extensional structures where relatively rigid bodies of rock are rotated by progressive deformation of the orogen. Gold mineralization occurs in these extensional sites and in small extensional fractures associated with sinistral faults along the margins of rotating rigid bodies. Fluid flow persists when early structures are overprinted by strike-slip faults. Mineralizing fluids are distinct chemically from the topographically driven meteoric waters that dominate hydrothermal systems on the flanks of the orogen.

Diapiric features in the southeastern Mediterranean Sea: possible indication of extension in a zone of incipient continental collision

Several types of diapiric structures were studied in the zone between the Nile deep-sea fan and the Eratosthenes seamount in the SE Mediterranean Sea. The study discovered four stages in the structural evolution of the diapiric features, starting as a domal piercement structure, and developing into initial basin as seawater dissolution starts to effect the diapiric salt. Subsequently V-shaped, and then mature, U-shaped basins formed as products of dissolution, sedimentary collapse and sediment deposition. The structures—both protrusions and depressions, are aligned along discrete lineaments, most of them trending NE-SW and a few-NW-SE. The NE-SW lineaments are probably a series of normal faults and the NW-SE ones seem to be strike-slip faults. The development of extensional structures on the underthrust edge of the African Plate, as it approaches the subduction zone, are commonplace in the Mediterranean and elsewhere, and the occurrence of normal faults in this area is compatible with the intensive subsidence of the Eratosthenes Seamount in the Plio-Pleistocene. The lithostatic load of the Nile deep-sea fan could have contributed to the subsidence and thus enhanced the diapirism in the SE Mediterranean.