Deciphering the Late Paleozoic–Cenozoic Tectonic History of the Inner Central Andes Forearc: An Update From the Salar de Punta Negra Basin of Northern Chile

We integrated new and existing geological, geochronological, thermochronological, and two-dimensional (2D) seismic data from the Salar de Punta Negra Basin to define the Late Paleozoic–Cenozoic tectonic evolution of the inner Andean forearc of northern Chile more precisely. Our results indicate that this region experienced early Late Paleozoic–Mesozoic crustal extension, creating several basement half-graben structures bounded by east- and west-dipping master faults. These extensional basins were filled by Upper Permian to Jurassic volcanic and sedimentary (continental and marine) syn-rift deposits. The genesis of these structures is related to the early breakup of the western Gondwana continent and the development of the large Tarapacá Basin in northern Chile and southern Perú. Subsequently, Late Cretaceous to Paleocene contraction occurred, which led to the tectonic inversion of the pre-existing rift system and the uplift of the Paleozoic–Mesozoic syn-rift deposits. Seismic data show that Upper Cretaceous and Paleocene synorogenic deposits accumulated along and over inversion anticlines, recording the initial contraction and marking the change from an extensional to a contractional tectonic setting. During the final episodes of basin inversion, crustal shortening was accommodated by the Eocene to recent basement reverse faulting accompanied by the rapid exhumation of basement pre-rift blocks, which served as the principal sources for the sediments that filled the pre-Andean basins during the Late Cenozoic. Finally, the exhumed basement pre-rift blocks and the reverse faults compartmentalized the contractional intermontane basins, which constitute the main low topographic relief of the inner forearc of northern Chile.

POTENTIAL ICE CRYSTAL MARKS FROM PENNSYLVANIAN–PERMIAN EQUATORIAL RED-BEDS OF NORTHWEST COLORADO, U.S.A.

ABSTRACT The Pennsylvanian–Permian Maroon Formation of northwest Colorado is an up to 4,600 m thick succession of mainly siliciclastic continental red-beds deposited in equatorial intermontane basins of the Ancestral Rocky Mountains. Sedimentary surfaces of fluvio-lacustrine to eolian siltstones and fine-grained sandstones from various stratigraphic levels within the Maroon Formation preserve cm-sized straight to gently curved sediment-filled acicular structures referred to five morphological groups: single, branched, stellate, rosette, and bunched. Depositional environment, shape, and size of the structures are most similar to ice crystal marks that result from freezing of water-saturated fine-grained substrate at the sediment-air interface. They differ from other syngenetically produced crystals and crystal pseudomorphs in sedimentary rocks mainly by crystal shape and environmental conditions. The potential ice crystal marks of the Maroon Formation are notable for the fidelity and morphological diversity of the crystal casts and could be a key for the understanding of similar but hitherto often only called enigmatic structures of the sedimentary rock record. The ice crystal mark occurrences in the Maroon Formation suggest that night frost affected lower elevation equatorial areas during the climax of the Late Paleozoic Ice Age and may stimulate research on evolutionary adaptations of early terrestrial biota to overcome significant air temperature fluctuations.

Transition from slab roll-back to slab break-off in the central Apennines, Italy: Constraints from the stratigraphic and thermochronologic record

Stratigraphic and thermochronologic data are used to study the processes that shaped the topography of the central Apennines of Italy. These are part of a major, active mountain belt in the center of the Mediterranean area, where several subduction zones control a complex topography. The Apennines were shaped by contraction at the front of the accretionary wedge overlying the subducting Adria microplate followed by extension at the wedge rear in response to eastward slab roll-back. In the central Apennines, intermontane extensional basins on the western flank rise eastward toward the summit. We contribute with new data consisting of 28 (U-Th-Sm)/He and 10 fission track ages on apatites to resolve a complex pattern of thermal histories in time and space, which we interpret as reflecting the transitional state of the orogen, undergoing a two-phase evolution related to initial slab retreat, followed by slab detachment. Along the Tyrrhenian coast, we document cooling from depths ≥3−4 km occurring between 8 and 5 Ma and related to the opening of marine extensional basins. Post−5 Ma, a broader region of the central Apennines exhibits cooling from variable depths, between <2 km in most areas and ≥3−4 km in the northeast, and with different onset times: at ca. 4 Ma in the west, at ca. 2.5 Ma in the center and northeast, and at ca. 1 Ma in the southeast. Between 5 and 2.5 Ma, exhumation is associated with modest topographic growth during the late stages of thrusting. Since 2.5 Ma, exhumation has concurred with the opening of intermontane basins in the west and in the east, with regional topographic growth and erosion, that we interpret to be associated with the locally detaching slab.

STUDY OF THE DEPTH STRUCTURE OF SEISMICALLY ACTIVE ZONES BY THE METHOD OF MAGNETOTELLURIC SOUNDING

The paper presents the results of the analysis of data from magnetotelluric soundings performed in 2003-2020. A comparison of geoelectric models for the Chui and Kochkor basins of the Northern Tien Shan is shown. The main objective of the study is to identify patterns in the distribution of geoelectric inhomogeneities in the deep structure of the Bishkek geodynamic test site. Particular attention in geoelectric models was paid to the study of key objects of transition zones from mountain ranges to intermontane basins. The nature of crustal electrical conductivity anomalies is considered.

STUDY OF THE DEPTH STRUCTURE OF SEISMICALLY ACTIVE ZONES BY THE METHOD OF MAGNETOTELLURIC SOUNDING

The paper presents the results of the analysis of data from magnetotelluric soundings performed in 2003-2020. A comparison of geoelectric models for the Chui and Kochkor basins of the Northern Tien Shan is shown. The main objective of the study is to identify patterns in the distribution of geoelectric inhomogeneities in the deep structure of the Bishkek geodynamic test site. Particular attention in geoelectric models was paid to the study of key objects of transition zones from mountain ranges to intermontane basins. The nature of crustal electrical conductivity anomalies is considered.

Distribution and niche of Suillus aurihymenius in Asia

We defined the regularities spatial distribution of Suillus aurihymenius based on the occurrence data, as well as bioclimatic characteristics in order to control species status in the natural environment. Suillus aurihymenius is a rare species known only from our collections from the Republic of Altai and Republic of Tuva. Realised niche of S. aurihymenius on the western border of its currently known range in larch forests in the floodplains of rivers of the steppe of southeast Altai. Conservation of natural habitats of S. aurihymenius in the steppes of intermontane basins in the southeast of Altai will contribute to its preservation in Russia.