Synsedimentary broken‐foreland tectonics during the Paleogene in the Andes of NW Argentine: new evidence from regional to centimetre‐scale deformation features

Unravelling the spatiotemporal evolution of the Cenozoic Andean (Altiplano‐Puna) plateau has been one of the most intriguing problems of South American geology. Despite a number of investigations, the early deformation and uplift history of this area remained largely enigmatic. This paper analyses the Paleogene tectono‐sedimentary history of the Casa Grande Basin, in the present‐day transition zone between the northern sector of the Puna Plateau and the northern part of the Argentine Eastern Cordillera. Our detailed mapping of synsedimentary structures records the onset of regional contractional deformation during the middle Eocene, revealing reactivation of Cretaceous extensional structures and the development of doubly vergent thrusts. This is in agreement with records from other southern parts of the Puna Plateau and the Eastern Cordillera. These observations indicate the existence of an Eocene broken foreland setting within the region, characterized by low‐lying compressional basins and ranges with spatially disparate sectors of deformation, which was subsequently subjected to regional uplift resulting in the attainment of present‐day elevations during the Neogene.     

Palaeomagnetic data about southern Tibet (Xizang) — I. The Cretaceous formations of the Lhasa block

Summary. A palaeomagnetic study of Middle to late Cretaceous redbeds from Linzhou basin (Lhasa block), north of the Yarlung Zangbo suture zone, gives a stable palaeomagnetic direction of magnetization with a positive fold-test: six sites, 57 samples, D = 333°, I = 38°, k = 78, α₉₅= 8°, pole 64°N, 348°E. We discuss the problem of a possible remagnetization but consider that this direction of magnetization gives a good approximation for the palaeolatitude of the Lhasa block during Middle to late Cretaceous time. Results from more recent Tibetan formations are also presented: late Cretaceous to Palaeocene sediments and volcanics give a lower palaeolatitude of 10° N and but more recent andesites have emplaced about 30°N, close to the present-day latitude. An interpretation is proposed whereby the Lhasa block, which was a part of Asia in the early Cretaceous, has undergone first a southward motion accompanied by an anticlockwise rotation and then, after the Palaeocene, a northward motion under the constraint of the colliding India.     

A definitive model of the geomagnetic field and its secular variation for 1965 — I. Derivation of model and comparison with the IGRF

Summary. Using a very large body of post-1955 data, a spherical harmonic model of the geomagnetic field and its secular variation is derived for 1965.0. This model is compared with the original International Geomagnetic Reference Field (IGRF) and with individual models used, or proposed for use, in producing the IGRF. Positions of the dip-poles, the geomagnetic poles and the eccentric dipole are derived from the model, together with their rates of change, and comparisons are made with other estimates of these positions.     

Modelling the compliance of crustal rock—I. Response of shear-wave splitting to differential stress

We show that seismic shear waves may be used to monitor the in situ stress state of deep inaccessible rocks in the crust. The most widespread manifestation of the stress-related behaviour of seismic waves is the shear-wave splitting (shear-wave birefringence) observed in almost all rocks, where the polarizations of the leading split shear waves are usually subparallel to the direction of the local maximum horizontal stress. It has been recognized that such shear-wave splitting is typically the result of propagation through distributions of stress-aligned fluid-filled microcracks and pores, known as extensive-dilatancy anisotropy or EDA. This paper provides a quantitative basis for the EDA hypothesis. We model the evolution of anisotropic distributions of microcracks in triaxial differential stress, where the driving mechanism is fluid migration along pressure gradients between neighbouring microcracks and pores at different orientations to the stress field. This leads to a non-linear anisotropic poroelasticity (APE) model for the stress-sensitive behaviour of fluid-saturated microcracked rocks. A companion paper shows that APE modelling matches a range of observed phenomena and is a good approximation to the equation of state of a stressed fluid-saturated rock mass.     

The magnitude of the palaeomagnetic field: a new non-thermal, non-detrital method using Sundried bricks

Summary. A new method for determining the magnitude of the palaeomagnetic field is described. The material used is adobe Sun-dried mud brick, and some of the magnetic properties of these bricks are described. The magnetization acquired by the mud in the higher coercive force region seems to be caused by shear strain (squeezing) alone. This magnetization is called an SRM. The method is tested on a modern adobe brick from Lima, and it is shown to be accurate and repeatable. The method may also be used to determine values of D and I , if the adobe brick is found in situ . Three results from old adobe bricks — two from Peru and one from Egypt — are presented and are compared with the world-wide average of published ancient VDM's.     

Disaggregating the historical demographic sources of regional foreign-born and native-born population growth in the United States: a new method with applications

This paper analyzes the demographic processes that contribute to population growth and redistribution in a multiregional system using a new method. The method incorporates a historical perspective that can be used to trace dynamic population processes as they evolve over time. It uses an open multiregional projection model framework in identifying the contributions to regional growth made by each of the principal demographic components of change: fertility, mortality, immigration, emigration, in-migration, and out migration. At the same time, the method recognizes the importance of disaggregating the native-born and foreign-born populations. Available public data and indirect estimation techniques are used to develop the data inputs for the projection model, with which the regional population changes for each 5-year period between 1950 and 1990 were reconstructed. Regional growth rates for the foreign-born and native-born populations are partitioned into the separate demographic components of change, and the projection model identifies the separate contributions to regional growth made by each population. This allows a direct comparison of the impact of immigration with those of corresponding native-born contributions effected through internal migration and natural increases. Finally, the application of the method allows the identification of the contribution that 'recent' (post-1965) immigrant cohorts have made to the composition of the youngest age groups in each region, and also to simulate the impacts of zero immigration scenarios on regional growth.     

Patterns of basin fill in Triassic turbidites of the Nanpanjiang basin: implications for regional tectonics and impacts on carbonate‐platform evolution

The Nanpanjiang Basin occurs in a key position for resolving controversies of basin tectonics and patterns of plate assembly at the junction between south China and Southeast Asian plates. Paleocurrent measurements indicate that siliciclastic turbidites in the basin were sourced by the Precambrian Jiangnan uplift to the northeast, the Precambrian Yunkai uplift to the southeast and the Triassic Songma suture to the south. Detrital zircon geochronology reveals Archean (2500 Ma), Paleoproterozoic (1800–1900 Ma), Neoproterozoic (900–1000 Ma) and Paleozoic (420–460 Ma) ages consistent with derivation from the Jiangnan and Yunkai uplifts. A large Permian‐Triassic peak of 250 Ma is present in the southern basin and attenuates northward suggesting derivation from an arc developed along the Songma suture. Sandstone QFL compositions average 65/12/23% and plot in the recycled orogen field except for a few samples in the southern basin that fall in the dissected arc field. The compositions are consistent with derivation from Precambrian basement that includes orogenic complexes. In the southern basin, Middle Triassic turbidites contain greater lithics and feldspars and Lower Triassic turbidites have volcaniclastic composition consistent with derivation from a southerly arc. Our preferred interpretation is evolution from remnant basin to a large peripheral foreland with southward subduction and convergence with Indochina along the Songma suture. The previously proposed Dian‐Qiong zone is not a suture as its map location places it within carbonate platforms bounded by identical stratigraphy. The Nan‐Uttaradit zone is too distant to have provided voluminous siliciclastic flux to the basin. The Nanpanjiang Basin provides an example of the evolution of an exceptionally large foreland with far‐field rejuvenation of Precambrian uplifts and carbonate platforms that were significantly influenced by siliciclastic flux. The timing and pattern of turbidite basin fill impacted platform evolution by enabling margin progradation in areas proximal to siliciclastic sources, whereas platforms distant from sources were driven to aggradation and extreme relief with large‐scale gravitational sector collapse.