Regional Extensional Tectonics Research Articles

Quantitative modelling of basin and rheological evolution of the Iberian Basin (Central Spain): implications for lithospheric dynamics of intraplate extension and inversion

Subsidence analysis of Mesozoic sediments in the central part of the Iberian Basin in Central Spain demonstrates that basin evolution was related to three stages of lithospheric stretching, each followed by thermal subsidence. The rifting history was followed by Tertiary inversion. The three extensional phases correlate with rifting stages identified in other parts of the Iberian Basin, on the Betic, Lusitanian and Cantabrian margins of Iberia, and with regional extensional tectonics. The first phase (245-157 Ma), marked by rapid tectonic subsidence in Triassic-Early Jurassic times, yields a lithospheric stretching factor of about 1.10. The second, Oxfordian to earliest Albian (157-112 Ma) in age, is characterised by uplift followed by moderate subsidence. The amount of uplift and subsidence during these phases varies significantly within the basin. The uplift implies subcrustal lithospheric stretching factors ranging from 1.06 to 1.28 assuming a thermal uplift mechanism. The subsequent subsidence corresponds to lithospheric stretching factors in the range from 1.00 to 1.06. The third main phase (95-85 Ma), seen as a Cenomanian-early Senonian acceleration of tectonic subsidence, is modelled with lithospheric stretching factors of about 1.02. The subsidence models are used to reconstruct the paleo-rheological evolution of the area during the Mesozoic, and indicate progressive strengthening of the lithosphere from west to east in the studied area during the Late Jurassic and Cretaceous. For the second phase of rifting and onset of Tertiary basin inversion, strength distributions are not in agreement with localised basin deformation. As an explanation, it is shown that late Variscan faults may have contributed considerably to lithospheric weakening and localisation of extension and inversion.

Tectonic, volcanic, and hydrothermal event sequence on Deception Island (Antarctica)

On the basis of the correlation between seismic tectonostratigraphic units, volcanism, and geochemical distribution patterns of sea-floor sediments on Deception Island (Antarctica), a regional tectonic extension is suggested to explain the sequence of volcanic and hydrothermal events. The syncollapse caldera episode is composed of five tectonostratigraphic units separated by progressively tilted and bent unconformities. Each unit shows a sequence of Quaternary volcanic cones and mound structures that display high contents of Fe, Mn, and Zn. As-rich active fumaroles and hot-spring areas are associated with the postcollapse episode and linked to normal faulting and present volcanism.

Basin genesis associated with strike‐slip faulting in the Basin and Range, southeastern Nevada

The structural nature of two prominent Basin and Range strike‐slip systems, the Lake Mead fault system and the Las Vegas Valley shear zone, are examined using both geological field mapping and geophysical surveys at various scales. The intent of this study is to document deformation directly attributable to strike‐slip faulting with an emphasis on basin genesis. Our field study of the Lake Mead fault system reveals a major left step in the system. The stepover results in the genesis of a 2‐km‐deep pull‐apart, the Overton Arm Basin, consistent with the kinematics and mechanics of strike‐slip faulting. Having established the origin of the Overton Arm basin as a strike‐slip‐related pull‐apart basin, the geometry of poorly exposed strike‐slip systems can be revealed by understanding the related basins along their length. One such system, the Las Vegas Valley shear zone, is shown with the aid of geophysical data to be geographically related to three distinct en echelon basins. When these basins are interpreted as pull‐apart basins, the geometry of the Las Vegas Valley shear zone becomes apparent for the first time. We assert that strike‐slip faulting in southeastern Nevada is a deformational mechanism that contributes locally to the regional extensional tectonics. Such an interpretation modifies extant models that treat strike‐slip faults in rifted terrains as passive lateral boundaries that simply accommodate extension without contributing to the process.

The Saint-Affrique Permian basin (southern France) : an example of a roll-over controlled alluvial sedimentation during regional extensional tectonics

The Saint-Affrique basin is one of the major Permian continental basins of the southern French Massif Central. Two main sedimentary cycles, separated by an angular unconformity corresponding to an erosion surface, have been distinguished. Cycle 1 is formed by three successive formations, from bottom to top : formation Fl consists of alluvial deposits ; formation F2 is represented by lacustrine to fluvio- lacustrine deposits; formation F3 is represented by fluvial deposits. Palynological analyses indicate a late Autunian to lower Thuringian age for this cycle. Cycle 2 is constituted by fanglomerates (formation F4) and playa deposits (formation F5).Both sedimentological and structural evidence show that the structure of the basin and the depositional patterns were controlled by the development of roll over structures. These roll-overs were associated with north dipping listric normal faults which are responsible for the asymmetrical shape of the basin. The faults which controlled the subsidence of the basin floor formed in response to an extensional reactivation of pre-existing Hercynian thrusts in the basement (“negative tectonic inversion”). Taken together structural and sedimentological evidence indicate that the basin propagated southwards by successive incorporation of previously undeformed parts of the footwall. This mode of propagation can be ascribed to an extensional piggy-back sequence.The model for the development of the Saint-Affrique basin includes 2 extension stages separated by an erosion episode. The first extensional event, which corresponds to Cycle I, deformed the post-Stephanian erosional surface into a smooth hanging wall syn-cline with a large, but shallow deposition area. Well-marked fault activity with local subsidence is observed only during the deposition of formation F3 and denotes an increase in extension rate. The second extensional event is characterized by higher extension and subsidence rates and resulted in deeper individual sub-basins with a southward progression of both margin fault and depocentre.

Late Cenozoic volcanic evolution of the northeastern Aegean region

Abstract Late Cenozoic volcanism occurred in an extensional tectonic regime in the northeastern Aegean Activity commenced in northeastern Greece during the Early Oligocene, and in northwestern Turkey during the Late Oligocene-Early Miocene. The beginning of activity was characterized by calc-alkaline intermediate—acid volcanism; however, alkaline intermediate—basic volcanism became dominant from the Late Miocene until the Pleistocene. This change in nature of volcanism is not related to a change of regional tectonic regime, as previously thought. Instead, we propose that the volcanic evolution reflects an increasing asthenospheric contribution, facilitated by a thinned, extended lithosphere arising from the regional extensional tectonics.

Petrologic, stratigraphic and tectonic significance of Mesozoic volcanic rocks in the Río Wampú area, Eastern Honduras

A thick sequence of dark-red lava flows and clastic rocks with abundant volcanic debris crops out along the Río Wampú of eastern Honduras. Lithologic characteristics of these rocks, field interpretations of the Río Wampú stratigraphy, and limited KAr age determinations on volcanic samples indicate that these redbeds and associated volcanic units are correlative with the Upper Cretaceous Valle de Angeles Group of central Honduras. These strata uncomformably overlie dark clastic units which are tentatively correlated with the Jurassic Agua Fría Formation of the Honduras Group. The lava samples are predominantly microporphyritic andesites and basaltic andesites with abundant fine-grained phenocrysts of plagioclase, clinopyroxene and FeTi oxides. Immobile-element whole-rock and mineral concentrations indicate a high-K tholeiitic composition. Elevated TiO 2 whole-rock and clinopyroxene concentrations found in the basaltic andesites apparently reflect a primary magmatic characteristic. Based on geochemical and modal data, it appears that these rocks were erupted from partial mantle melts which were modified by substantial fractionation of modal phases. The presence of widespread Mesozoic redbeds and associated volcanics in Honduras was previously cited as evidence of regional extensional tectonics. However, structural, geochemical and stratigraphic data collected during this study indicate that the Río Wampú volcanic suite was formed during a compressional (subduction-related) event. Similar high-TiO 2 basaltic andesites and andesites are reported along several active continental margins.

Correlation of nonmarine Cretaceous strata of Africa and the Middle East

Nonmarine Cretaceous rocks of mainly Early to mid-Cretaceous age are found widely scattered throughout the African continent, including Madagascar and the Middle East. Correlation of these rocks between regions has been attempted in the northern part of Africa, but the less frequent outcrops south of the Sahara are poorly understood and correlations are very tentative. It is premature to attempt a continent-wide correlation scheme, but inter-regional correlations are presented to understand better the nonmarine Cretaceous throughout Africa and the Middle East. The Saharan region is dominated by nonmarine “Continentale Intercalaire” and “Nubian Sandstone” clastics of Early to mid-Cretaceous age, and in Egypt, locally Late Cretaceous. The rocks labelled “Nubian Sandstone” can be correlated into the Arabian Peninsular, where it was deposited around the Arabian craton prior to inundation by Tethyan transgressions. The opening of the South Atlantic invoked tectonic stresses forming pull-apart basins in West Africa, and exploited the Pan-African shear zone cutting central Africa forming basins from Nigeria to southern Sudan. Intracratonic basins in northern Sudan and Egypt resulted from tectonic stresses associated with the opening of the Red Sea and the northward convergence of Africa into the European continent. Nonmarine Cretaceous deposits in central and inland south-eastern Africa outcrop sporadically and are associated with regional extensional tectonics related to the separation of Madagascar. Coastal basins of Mozambique and South Africa containing marginal nonmarine Cretaceous facies appear to be associated with the separation of Madagascar and the Falkland Plateau from Africa. The ages of nonmarine Cretaceous strata in Africa and the Middle East are often imprecisely known, although recent palynostratigraphic results have improved precision. Vertebrate and megaplant fossils are found throughout the continent, but their biostratigraphic value is rather general. Nonmarine sedimentation in Africa and the Middle East is predominantly pre-Cenomanian, with some Upper Cretaceous rocks occurring in Egypt, Somalia, Yemen, Nigeria and Madagascar during regressive episodes.

Low-Pressure Granulites from the Ketilidian Mobile Belt of Southern Greenland

Early Proterozoic cordierite-garnet–orthopyroxene–K–feldspar metasedimentary gneisses are developed in the flat—lying complex (FLC) of the southern part of the Ketilidian mobile belt (1·8–1·7GA), Greenland. These granulites contain low—pressure assemblages and are developed in both regional metamorphic rocks and in some thermal aureoles of contemporaneous rapakivi granite plutons. Thermobarometry shows that both the regional and contact metamorphism took place at 2–4 kb and 650–800°C. The granulites were developed in an extensional tectonic regime, and appartently record the culmination of a single thermotectonic event. There is a close temporal association between the peak of high—grade metamorphism and the emplacement of synorogenic rapakivi granites, and melts from the metasedimentary pile probably contributed largely to the granites. The low pressures and high temperatures for regional metamorphism require geothermal gradients in excess of 60°C/km, and are consistent with the presence of regional extensional tectonics, synorogenic magmatism, and underplating of mafic magma in this area during the Ketilidian.

Vertical surface displacements at Yellowstone Caldera, Wyoming, 1976–1986

Between first‐order leveling surveys in 1976 and 1986, the east central floor of Yellowstone caldera rose 15.4 ± 1.4 mm/yr and the west central floor rose 17.0 ± 2.0 mm/yr with respect to points on the south caldera rim. The same pattern has prevailed since an initial survey in 1923, except that the western caldera rose 30% faster during 1976–1986 than during 1923–1976. The 1976–1986 uplift extends beyond the northwest caldera rim near the adjacent Hebgen Lake fault zone, site of a magnitude 7.3 earthquake in August 1959. Marginally significant displacements in the eastern caldera suggest that uplift continued during 1983–1984, stopped during 1984–1985, and reversed to subsidence during 1985–1986; thus long‐term uplift may accumulate episodically. Historical uplift is especially noteworthy because regional crustal extension and thermal contraction during cooling would cause the caldera floor to subside if other processes were not acting to warp it upward. The pattern and dimensions of the uplift indicate a source in the upper crust beneath Yellowstone caldera, probably a reservoir of rhyolite magma within a cooling granitic pluton. A simple point source model would suggest a source 10–15 km deep, but the reservoir is probably lens‐shaped and closer to the surface. Uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir. Subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids. Magmatic inflation, volatile accumulation, and crustal extension may culminate eventually in renewed eruptive activity, but there is no evidence of an imminent volcanic hazard.

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Geological, chronological, and structural studies of the Long Valley‐Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km3 prior to its climatic caldera‐forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km3 by additional magma since mid‐1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non‐double‐couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low‐viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500‐year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000‐year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.