Temporal and spatial variations in tectonic subsidence in the Iberian Basin (eastern Spain): inferences from automated forward modelling of high-resolution stratigraphy (Permian–Mesozoic)

By subsidence analysis on eighteen surface sections and 6 wells, which cover large part of the Iberian Basin (E Spain) and which are marked by high-resolution stratigraphy of the Permian, Triassic, Jurassic and Cretaceous, we quantify the complex Permian and Mesozoic tectonic subsidence history of the basin. Backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies. The sections and wells have also been forward modelled with a new ‘automated' modelling technique, with unlimited number of stretching phases, in order to quantify variations in timing and magnitude of rifting. It is demonstrated that the tectonic subsidence history in the Iberian Basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence. The number of stretching phases appears to be much larger than found by earlier studies, showing a close match with stretching phases found in other parts of the Iberian Peninsula and allowing a clear correlation with discrete phases in the opening of the Tethys and Atlantic.     

The Palaeoproterozoic Woodlands Formation of eastern Botswana-northwestern South Africa: lithostratigraphy and relationship with Transvaal Basin inversion structures

The Woodlands Formation (uppermost Pretoria Group) of eastern Botswana overlies thick quartzites of the Sengoma Formation (Magaliesberg Formation) and comprises a lower unit of interbedded mudrocks and fine-grained recrystallised quartzitic sandstones, succeeded by chaotic and very coarse-grained inferred slump deposits. Within the adjacent western region of South Africa, interbedded mudrocks and quartzitic sandstones stratigraphically overlying the Magaliesberg Formation are now assigned to the lower Woodlands Formation. Within the entire region, interference folding produced by northeast-southwest (F₁ and F₃) and northwest-southeast (F₂) compression, and concomitant faulting characterised inversion of the Pretoria Group basin. This deformation is of pre-Bushveld age and affected all units in the Pretoria Group, including the uppermost Silverton, Magaliesberg and Woodlands Formations, and intrusive Marico Hypabyssal Suite (pre-Bushveld) mafic sills. The Nietverdiend lobe of the Bushveld Complex, intrusive into this succession, was not similarly deformed. Movement along the major Mannyelanong Fault in the northwest of the study area post-dated Transvaal Basin inversion, after which the “upper Woodlands” chaotic slump deposits were formed. The latter must thus belong to a younger stratigraphical unit and is possibly analogous to apparently syntectonic sedimentary rocks (Otse Group) in the Otse Basin of eastern Botswana.     

True polar wander

A re-evaluation of the existence of true polar wander (TPW) since the Late Cretaceous and a comparison among the various approaches are made using updated paleomagnetic, hotspot and relative motion datasets. Previous attempts to determine the existence of TPW had resulted in different conclusions: comparison of hotspot locations and paleomagnetic poles required significant pole motion, although lithospheric plate displacement analysis yielded insignificant motion. However, these earlier determinations cannot be directly compared to find the reason for the discrepancies, because each used different datasets. For this study the different approaches are applied to a single updated model with three alternative relative motions of East and West Antarctica. Although the results are model-dependent, in general there was not significant motion of the pole relative to the lithosphere (1–5°) since the early Tertiary, but a large motion (10–12°) relative to the hotspot framework. It is unlikely that errors in the determinations could account for this disagreement: the A₉₅ of the plate reconstruction is about 3°, the uncertainty in Antarctica motion is estimated to no larger than 3°, and cumulative errors in the relative plate motions may also amount to 3°. Only if all these errors are present in the maximum estimated amount, and in the same direction, could they account for the 10–12° gap between the two approaches. This conclusion of pole motion relative to the hotspots, but not the lithosphere, may indicate an independent shift of the mesosphere relative to the lithosphere (or “mantle roll” of Hargraves and Duncan).     

Magnitude scale structure from filtered broadbandP-wave records

A series of Hokkaido events, recorded by the FBV Broadband Seismograph System at the KHC Seismic Station, is used to study the structure of the earthquake magnitude scale on the basis of maximum velocity amplitudesA _vmax of teleseismicPwaves in different period bands. Amplitude-periodband (APB) diagrams are constructed for each event. According to the shape of the APB diagrams the events investigated can be divided into three types: (a) events with largestA _vmax values in the intermediate period range (periods ofA _vmax from 2.2 to 23 sec), (b) events with largestA _vmax values in the short-period range (periods ofA _vmax from 1 to 2 sec), (c) events exhibiting anomalous APB diagrams. Type (a) events seem to represent the process of wave generation that prevails for shallow earthquakes. Type (b) events approach to explosive-like generation of seismic waves. The nature of the exceptionally occurring type (c) events must be clarified in further investigations. The influence of the type of earthquake on the magnitude values estimated on the basis of standard class A and B (short-period and intermediate-band) seismograms is demonstrated. It appears that for estimating correct values of earthquake magnitudes complementary information on the process of seismic wave generation in the focus is necessary. At teleseismic distances this information can be obtained from either APB diagrams or amplitude spectra ofP waves recorded, e.g., by broadband velocity sensing instruments.     

Geodynamic interpretation of the earthquake distribution in the kermadec subduction zone

Summary The morphology of the Wadati-Benioff zone in the Kermadec region, based on the distribution of 1100 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Pacific plate. Two cycles of the recently active subduction in the Tonga-Kermadec island arc were found.     

To the problem of noise reduction in sea-level records used in vertical crustal movement detection

The aim of this investigation is to develop a simple technique that would allow us to use the sea-level records for detecting contemporary vertical crustal movements of duration from several months to several years. The choice of auxiliary data needed for any such analysis is restricted to the regularly available meteorological data to make this approach possible in routine search for precursory movements in earthquake-prone areas. A linear mathematical model is designed to evaluate the effect of atmospheric temperature and pressure variations, river discharge, long periodic tides and Chandlerian motion. Spectra of the residual sea-level variations are also shown. It is concluded that local episodic crustal movements of a magnitude larger than some 10 cm may be detectable by this approach. If finer resolution is needed then it it necessary to also account for steric level, wind, and sea-current variations, for which data are largely non-existent.     

Mesozoic/cenozoic calcite compensation depth and the global distribution of calcareous sediments

From late Jurassic to early Cenozoic time the calcite compensation depth (CCD) in the world oceans was shallow (above 4000 m). About 40 m.y. ago it dropped to about 4500 m, sharply in the Indian and Pacific oceans and gradually in the Atlantic. In the early Miocene it began to rise again, reaching a very shallow peak 10–15 m.y. ago, then descended to its present deep position. In the Pacific, fluctuations during the last 40 m.y. appear to result mainly from changes in bottom-water structure related to the progressive glaciation of the Antarctic and Arctic regions. Analogous explanations hold for CCD fluctuations during this period in the Indian and Atlantic oceans. The very shallow CCD prior to about 50 m.y., on the other hand, cannot be explained in this manner but must be attributed either to a low input of calcium in the oceans from deeply weathered continents or, more probably, to large changes in the distribution of carbonate deposition between shallow and deep seas.     

Tensor structure and the least squares

It is shown that for linear parametric adjustment models all the least-squares equations can be obtained from a commutative diagram, where the observation and parameter spaces are regarded as covariant. Their contravariant counterparts are defined through the metric property of the covariance matrix of the observations.     

Recent crustal movements in the Sierra Nevada—Walker lane region of California—Nevada: Part i, rate and style of deformation

This review of geological, seismological, geochronological and paleobotanical data is made to compare historic and geologic rates and styles of deformation of the Sierra Nevada and western Basin and Range Provinces. The main uplift of this region began about 17 m.y. ago, with slow uplift of the central Sierra Nevada summit region at rates estimated at about 0.012 mm/yr and of western Basin and Range Province at about 0.01 mm/yr. Many Mesozoic faults of the Foothills fault system were reactivated with normal slip in mid-Tertiary time and have continued to be active with slow slip rates. Sparse data indicate acceleration of rates of uplift and faulting during the Late Cenozoic. The Basin and Range faulting appears to have extended westward during this period with a reduction in width of the Sierra Nevada.The eastern boundary zone of the Sierra Nevada has an irregular en-echelon pattern of normal and right-oblique faults. The area between the Sierra Nevada and the Walker Lane is a complex zone of irregular patterns of hörst and graben blocks and conjugate normal-to right- and left-slip faults of NW and NE trend, respectively. The Walker Lane has at least five main strands near Walker Lake, with total right-slip separation estimated at 48 km. The NE-trending left-slip faults are much shorter than the Walker Lane fault zone and have maximum separations of no more than a few kilometers. Examples include the 1948 and 1966 fault zone northeast of Truckee, California, the Olinghouse fault (Part III) and possibly the almost 200-km-long Carson Lineament.Historic geologic evidence of faulting, seismologic evidence for focal mechanisms, geodetic measurements and strain measurements confirm continued regional uplift and tilting of the Sierra Nevada, with minor internal local faulting and deformation, smaller uplift of the western Basin and Range Province, conjugate focal mechanisms for faults of diverse orientations and types, and a NS to NE—SW compression axis (σ₁) and an EW to NW—SE extension axis (σ₃).