The lithospheric folding model applied to the Bighorn uplift during the Laramide orogeny

ABSTRACT The Bighorn uplift, Wyoming, developed in the Rocky Mountain foreland during the 75–55 Ma Laramide orogeny. It is one of many crystalline-cored uplifts that resulted from low-amplitude, large-wavelength folding of Phanerozoic strata and the basement nonconformity (Great Unconformity) across Wyoming and eastward into the High Plains region, where arch-like structures exist in the subsurface. Results of broadband and passive-active seismic studies by the Bighorn EarthScope project illuminated the deeper crustal structure. The seismic data show that there is substantial Moho relief beneath the surface exposure of the basement arch, with a greater Moho depth west of the Bighorn uplift and shallower Moho depth east of the uplift. A comparable amount of Moho relief is observed for the Wind River uplift, west of the Bighorn range, from a Consortium for Continental Reflection Profiling (COCORP) profile and teleseismic receiver function analysis of EarthScope Transportable Array seismic data. The amplitude and spacing of crystalline-cored uplifts, together with geological and geophysical data, are here examined within the framework of a lithospheric folding model. Lithospheric folding is the concept of low-amplitude, large-wavelength (150–600 km) folds affecting the entire lithosphere; these folds develop in response to an end load that induces a buckling instability. The buckling instability focuses initial fold development, with faults developing subsequently as shortening progresses. Scaled physical models and numerical models that undergo layer-parallel shortening induced by end loads determine that the wavelength of major uplifts in the upper crust occurs at approximately one third the wavelength of folds in the upper mantle for strong lithospheres. This distinction arises because surface uplifts occur where there is distinct curvature upon the Moho, and the vergence of surface uplifts can be synthetic or antithetic to the Moho curvature. In the case of the Bighorn uplift, the surface uplift is antithetic to the Moho curvature, which is likely a consequence of structural inheritance and the influence of a preexisting Proterozoic suture upon the surface uplift. The lithospheric folding model accommodates most of the geological observations and geophysical data for the Bighorn uplift. An alternative model, involving a crustal detachment at the orogen scale, is inconsistent with the absence of subhorizontal seismic reflectors that would arise from a throughgoing, low-angle detachment fault and other regional constraints. We conclude that the Bighorn uplift—and possibly other Laramide arch-like structures—is best understood as a product of lithospheric folding associated with a horizontal end load imposed upon the continental margin to the west.

The 1815 Tambora eruption: Its significance to the understanding of large-explosion caldera formations

Volcanic calderas, plentiful on the Earth and the moon, have been of much interest to volcanologists because of their large dimensions and extensive volumes of ejecta. Here, we consider the dynamics of caldera-forming by major explosive eruptions, examining how the breakdown of the earth's surface is caused by violent igneous activity. This leads to the definition of “typical explosion caldera”, which is a prototype of several newly-formed calderas in the historical timescale. There are three examples of such calderas: Tambora (Sumbawa), Krakatau (Sunda Straits), and Novarupta (Alaska). Tam- bora Caldera is the best example of a well-documented, recently formed typical explosion caldera, with no significant subsequent eruptions occurring after its formation. The subsurface structure of Tambora Caldera is discussed and compared to the 1883 eruption of Krakatau, the second largest eruption in historical times. Then, contrasting with the typically basaltic “collapse-type” calderas, a “Tambora-caldera type” is defined as a large “explosion-type” caldera, that may reach up to 10 km in diameter. The Tambora- type caldera concept is useful to qualify and understand the structure and components of other major calderas in the world. Fully developed larger explosion calderas such as Aso and Aira Calderas in Kyushu, Japan are discussed and explained as composite calderas based on geophysical data. Those calderas have repeatedly ejected massive pyroclastic products causing their original structures to grow wider than 10 km.

Leveling airborne geophysical data using a unidirectional variational model

Airborne geophysical data leveling is an indispensable step to the conventional data processing. Traditional data leveling methods mainly explore the leveling error properties in the time and frequency domain. A new technique is proposed to level airborne geophysical data in view of the image space properties of leveling error, including directional distribution property and amplitude variety property. This work applied unidirectional variational model on entire survey data based on the gradient difference between the leveling errors in flight line direction and the tie-line direction. Then spatially adaptive multi-scale model is introduced to iteratively decompose the leveling errors which effectively avoid the difficulty on the parameter selection. Considering the anomaly data with large amplitude may hide the real data level, a leveling preprocessing method is given to construct a smooth field based on the gradient data. The leveling method can automatically extract the leveling errors of the entire survey area simultaneously without the participation of staff members or tie-line control. We have applied the method to the airborne electromagnetic, magnetic data, and apparent conductivity data collected by Ontario Geological Survey to confirm its validity and robustness by comparing the results with the published data.

The unified system for storing and accessing geophysical data. Traditions and new approaches

The features of the development of data collection systems within the Information Processing Center (IPC) of the Geophysical Survey of the Russian Academy of Sciences (GS RAS) are presenting. Historical information is given, including technical details related to the systematization of the archive and the evolution of geophysical data formats. The historical, territorial features, as well as the experience of deploying such information systems within the Federal Research Center of the Unified State Social Service of the Russian Academy of Sciences and abroad are taken into account. A new concept of building an information system is proposed, which takes into account the requirements for scalability, reproducibility at various objects and the use of standard software. A lot of work was done to form a homogeneous archive of waveforms and an inventory of metadata for seismic stations, which resulted in the possibility of including the GS RAS in the international centers for processing geophysical information based on FDSN. Creation of a distributed collection and processing system using a cloud service allows abstracting from the territorial features of collecting and storing geophysical information, which increases the performance of the data access service and the degree of technical readiness of key system nodes.

‘They call ‘im Crowie’: An investigation of the Aboriginal significance attributed to a wrecked River Murray barge in South Australia

The Indigenous intangible heritage related to wrecked vessels has been poorly studied and documented. This article provides a counter to dominant maritime archaeology discourses via the investigation of the Aboriginal significance attributed to a wrecked and submerged River Murray barge (Crowie) in South Australia. There are numerous layers of Aboriginal significance that may be attributed to Crowie including the relationship of the community with their ‘underwater country’, Indigenous contributions to the riverboat industry, and the use of Aboriginal terms in vessel-naming practices. Geophysical data from multibeam and sidescan sonar surveys allowed for confirmation of the proposed location of the wreck and through comparison with historical descriptions and photographs provided evidence to substantiate the assignation of the wreck as the Crowie barge.

India Indenting Eurasia: A Brief Review and New Data from the Yongping Basin on the SE Tibetan Plateau

Successive indentations of Eurasia by India have led to the Tibet-Himalaya E–W orthogonal collision belt and the SE Tibetan Plateau N–S oblique collision belt along the frontal and eastern edges of the indenter, respectively. The belts exhibit distinctive lithospheric structures and tectonic evolutions. A comprehensive compilation of available geological and geophysical data reveals two sudden tectonic transitions in the early Eocene and the earliest Miocene, respectively, of the tectonic evolution of the orthogonal belt. Synthesizing geological and geochronological data helps us to suggest a NEE–SWW trending, ~450 km-long, ~250 km-wide magmatic zone in SE Tibet, which separates the oblique collision belt (eastern and SE Tibet) into three segments of distinctive seismic structures including the mantle and crust anisotropies. The newly identified Yongping basin is located in the central part of the magmatic zone. Geochronological and thermochronological data demonstrate that (1) this basin and the magmatic zone started to form at ~48 Ma likely due to NNW–SSE lithosphere stretching according to the spatial coincidence of the concentrated mantle-sourced igneous rocks on the surface with the seismic anomalies at depth; and (2) its fills was shortened in the E–W direction since ~23 Ma. These two dates correspond to the onset of the first and second tectonic transitions of the orthogonal collision belt. As such, both the orthogonal and oblique belts share a single time framework of their tectonic evolution. By synthesizing geological and geophysical data of both collision belts, the indenting process can be divided into three stages separated by two tectonic transitions. Continent–continent collision as a piston took place exclusively during the second stage. During the other two stages, the India lithosphere underthrust beneath Eurasia.