Abstract Archaeointensity and rock-magnetic studies were undertaken on 49 baked clay artefacts from four archaeological sites (Ter, Junnar, Nalasopara and Kanheri) in Maharashtra, India. Rock-magnetic properties, including bulk magnetic susceptibility, magnetic remanence and thermomagnetic analysis, indicate the presence of a low-coercivity magnetite in fine (superparamagnetic, single domain) grain-sizes. The ratio of anhysteretic remanent magnetization to saturation isothermal remanent magnetization, the reversible high-temperature susceptibility curves and the 3-axes isothermal remanent magnetization tests also indicate that the artefacts dominantly possess fine-grained magnetic particles, carrying a stable thermoremanent magnetization (TRM). Archaeointensity was estimated using Coe's modified Thellier method corresponding to the linear behaviour of natural remanent magnetization loss and TRM gained plots, which were evaluated with ThellierTool4.0 software. Cooling rate and anisotropy of the TRM corrections were applied and the corrected intensities were used to calculate a mean archaeointensity value for each one of the four sites. The new archaeointensity values were plotted along the existing Indian archaeointensity values derived only from archaeological artefacts, and were compared with the SHA.DIF.14k and ARCH10k.1 global models’ predictions. The present study aims to improve the overall understanding of Indian geomagnetic field variation in the past by providing new high-quality archaeointensity results. However, still more archaeointensity values are required to develop a reliable secular variation curve for India.

Abstract The complex tectonic evolution of northwestern South America is recorded by a variety of deformed, onland basins ranging in age from Paleozoic to recent. We integrate the Mesozoic to recent structural geology and basinal history of the 12,000 km 2 , intermontane Cesar-Rancheria Basin (CRB) in northern Colombia to reconstruct the tectonic history of the basin and tectonic controls on its sedimentation, subsidence history, and petroleum systems. Methods and data used for this study include (1) interpretations of crustal structure using gravity and magnetic modeling; (2) subsurface mapping of key horizons using 3500 km (2175 mi) of two-dimensional (2-D) seismic reflection data tied to 16 wells; (3) construction of serial structural cross sections, which are balanced to the level of Late Cretaceous sedimentary rocks; and (4) construction of burial history graphs and development of a 2-D basin model showing predicted oil and gas windows. Basement-involved southeast-dipping reverse faults expose Jurassic to early Cretaceous rocks in the CRB and Paleozoic rocks in the Perija range (PR) east of the CRB. About 10% of shortening across the CRB is a consequence of motion along these faults driven by discrete interactions between the Caribbean and South America plates. Two major periods of NW-SE crustal, interplate shortening are identified: (1) an early–middle Eocene, west-to-east shortening event that produced east-dipping Cretaceous and Paleocene strata beneath a major unconformity that increases in erosional hiatus from east to west across the study area; this shortening and overlying unconformity is related to the collision of the Great Arc of the Caribbean with the northwestern continental margin of the South American plate; and (2) a late Miocene–Pliocene, west-to-east shortening event with major exhumation of the eastern CRB where faulting of recent sedimentary deposits are related to the collision between the Panama Arc and northwestern South America. The west-to-east migration of deformation during the Cenozoic controlled by the west-to-east, diachronous collision between the Great Arc of the Caribbean and northern South America also led the present-day distribution of preserved depocenters with a major thickness of the Paleocene-early Eocene section to the east of the study area, and a major Miocene depocenter to the west.

The Archean granites of the Vredefort impact structure show a high intensity of natural remanent magnetization (NRM) and a random dispersion of directions of high-coercivity components on the centimeter scale. It has been suggested that this anomalous remanence is carried by rod-shaped single-domain (SD) magnetites along planar deformation features (PDFs) in shocked quartz produced as a consequence of the impact event. To determine the carriers of this NRM, we conducted surface magnetic field observations using scanning magneto-impedance (MI) magnetic microscopy during stepwise alternating field (AF) demagnetization over a 1-mm-thick slice of Vredefort granite. We found that the stable component after demagnetization gives rise to just three strong magnetic anomalies. Progressive thinning of the scanned section and micro-Raman spectroscopy revealed that the source of these magnetic anomalies, the highly coercive remanence-carrying mineral, is an assemblage of relatively coarse-grained (1–200 μm) magnetite in biotite, not single-domain magnetite embedded along PDF lamella.