We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites
Swarm
, ...CryoSat-2, CHAMP, SAC-C and Ørsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF
B
z
and
B
y
at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in
Swarm
observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.
Mineral systems can be thought of as a combination of several critical elements, including the whole‐lithosphere architecture, favorable geodynamic/tectonic events, and fertility. Because they are ...driven by processes across various scales, exploration benefits from a scale‐integrated approach. There are open questions regarding the source of ore‐forming fluids, the depth of genesis, and their transportation through the upper crust to discrete emplacement locations. In this study, we investigate an Au–Cu metal belt located at the margin of an Archean‐Paleoproterozoic microcontinent. We explore the geophysical signatures by analyzing three‐dimensional models of the electrical resistivity and shear‐wave velocity throughout the lithosphere. Directly beneath the metal belt, narrow, vertical, finger‐like low‐resistivity features are imaged within the resistive upper‐middle crust and are connected to a large low‐resistivity zone in the lower crust. A broad low‐resistivity zone is imaged in the lithospheric mantle, which is well aligned with a zone of low shear‐wave velocity, examined with a correlation analysis. In the upper‐middle crust, the resistivity signatures give evidence for ancient pathways of fluids, constrained by a structure along a tectonic boundary. In the lower lithosphere, the resistivity and velocity signatures are interpreted to represent a fossil fluid source region. We propose that these signatures were caused by a combination of factors related to refertilization and metasomatism of the lithospheric mantle by long‐lived subduction at the craton margin, possibly including iron enrichment, F‐rich phlogopite, and metallic sulfides. The whole‐lithosphere architecture controls the genesis, evolution, and transport of ore‐forming fluids and thus the development of the mineral system.
Plain Language Summary
The whole‐lithosphere structure of mineral systems, the link between deep and shallow regions, and the nature, origin, and depth of the source fluids that form mineral deposits are open questions. In this study, we investigate a gold and copper metal belt that is located at the margin of an ancient microcontinent and craton with a history of long‐lived subduction. We explore the region by examining three‐dimensional geophysical images of both the electrical resistivity structure and the shear‐wave velocity structure throughout the lithosphere. Narrow, vertical, fingers of low resistivity in the crust give evidence for ancient pathways of fluids beneath the metal belt. Low velocity and low resistivity signatures in the lower lithosphere are interpreted to represent a fossil fluid source region. We suggest that the geophysical signatures observed were caused by a combination of factors related to mantle metasomatism caused by long‐lived subduction and magmatism. The possible causes include iron enrichment in a more fertile mantle, the presence of F‐rich phlogopite in the lithospheric mantle, and metallic sulfides in the lower lithosphere, including at the base of the crust. The whole‐lithosphere structure and favorable geodynamic/tectonic events control the evolution of ore‐forming fluids that create metal/mineral deposits.
Key Points
Vertical, finger‐like low‐resistivity zones in the upper crust beneath an Au‐Cu metal belt give evidence for ancient pathways of fluids
Low velocity and low resistivity in the lower lithosphere are interpreted to represent a fossil source region for ore‐forming fluids
Signatures may be due to a combination of factors: Metallic sulfides, phlogopite, and iron enrichment by refertilization and metasomatism
We present electrical resistivity models, derived from magnetotelluric data, of the crust beneath the Bulnay region, Mongolia. They reveal that the lower crust contains a pattern of discrete zones ...(width of ~25 km) of low resistivity (<30 Ωm). Such features may be an effect of unaccounted‐for electrical anisotropy. However, when anisotropy is considered in the modeling, the features remain. We investigate an alternative explanation, based on a conceptual model of fluid localization and stagnation by thermally activated compaction, and demonstrate it is compatible with the observed low‐resistivity zones. The model explains the location, shape, and size of the zones, with plausible values of the activation energy for lower crustal creep (270–360 kJ/mol), and a viscous compaction length on the order of 10 km. The results imply tectonic deformation and compaction processes, rather than lithological‐structural heterogeneity, control the regional lower crustal fluid flow.
Plain Language Summary
We collected magnetotelluric data in the Bulnay region, Mongolia, which is a compressive intracontinental region, by measuring electric and magnetic fields at the surface. Using these data, we generated high‐resolution electrical resistivity models. The models image the lower crust and show that it contains discrete zones of low resistivity that have a distinct pattern. Other studies have shown that such a pattern may be an effect of ignoring electrically anisotropy. But when anisotropy is considered in the modeling the features remain nearly the same. Because of this, we investigate whether an alternative explanation can cause these features. We find that a conceptual model of fluid localization and stagnation by hydromechanical compaction is compatible with the observed pattern of the low‐resistivity zones. In fact, it can explain their location, shape, and size. In addition, we use the conceptual model to determine which viscous rheology is consistent with the data. Finally, we find that estimates for hydraulic and rheological properties of the region are consistent with this explanation. This conceptual model has implications for fluid flow in the lower crust, showing that it is controlled by tectonic deformation and compaction processes, rather than lithological or structural features.
Key Points
Electrical resistivity models across a compressive intracontinental region image a pattern of low‐resistivity zones in the lower crust
The pattern is consistent with hydrodynamic stagnation of crustal fluids due to thermally activated compaction
The results demonstrate that compaction processes, rather than lithological structure, control the regional lower crustal fluid flow
This study presents results of mapping three-dimensional (3-D) variations of the electrical conductivity in depths ranging from 400 to 1200 km using 6 years of magnetic data from the
Swarm
and ...CryoSat-2 satellites as well as from ground observatories. The approach involves the 3-D inversion of matrix Q-responses (transfer functions) that relate spherical harmonic coefficients of external (inducing) and internal (induced) origin of the magnetic potential. Transfer functions were estimated from geomagnetic field variations at periods ranging from 2 to 40 days. We study the effect of different combinations of input data sets on the transfer functions. We also present a new global 1-D conductivity profile based on a joint analysis of satellite tidal signals and global magnetospheric Q-responses.
This paper presents a methodology to sample equivalence domain (ED) in nonlinear partial differential equation (PDE)-constrained inverse problems. For this purpose, we first applied state-of-the-art ...stochastic optimization algorithm called Covariance Matrix Adaptation Evolution Strategy (CMAES) to identify low-misfit regions of the model space. These regions were then randomly sampled to create an ensemble of equivalent models and quantify uncertainty. CMAES is aimed at exploring model space globally and is robust on very ill-conditioned problems. We show that the number of iterations required to converge grows at a moderate rate with respect to number of unknowns and the algorithm is embarrassingly parallel. We formulated the problem by using the generalized Gaussian distribution. This enabled us to seamlessly use arbitrary norms for residual and regularization terms. We show that various regularization norms facilitate studying different classes of equivalent solutions. We further show how performance of the standard Metropolis–Hastings Markov chain Monte Carlo algorithm can be substantially improved by using information CMAES provides. This methodology was tested by using individual and joint inversions of magneotelluric, controlled-source electromagnetic (EM) and global EM induction data.
SUMMARY
We present a novel frequency‐domain inverse solution to recover the 3‐D electrical conductivity distribution in the mantle. The solution is based on analysis of local C‐responses. It exploits ...an iterative gradient‐type method—limited‐memory quasi‐Newton method—for minimizing the penalty function consisting of data misfit and regularization terms. The integral equation code is used as a forward engine to calculate responses and data misfit gradients during inversion. An adjoint approach is implemented to compute misfit gradients efficiently. Further improvements in computational load come from parallelizing the scheme with respect to frequencies, and from setting the most time‐consuming part of the forward calculations—calculation of Green’s tensors—apart from the inversion loop. Convergence, performance, and accuracy of our 3‐D inverse solution are demonstrated with a synthetic numerical example. A companion paper applies the strategy set forth here to real data.
Crustal architecture strongly influences the development and emplacement of mineral zones. In this study, we image the crustal structure beneath a metallogenic belt and its surroundings in the ...Bayankhongor area of central Mongolia. In this region, an ophiolite belt marks the location of an ancient suture zone, which is presently associated with a reactivated fault system. Nearby, metamorphic and volcanic belts host important mineralization zones and constitute a significant metallogenic belt that includes sources of copper and gold. However, the crustal structure of these features, and their relationships, are poorly studied. We analyze magnetotelluric data acquired across this region and generate three-dimensional electrical resistivity models of the crustal structure, which is found to be locally highly heterogeneous. Because the upper crust (< 25 km) is found to be generally highly resistive (> 1000 Ωm), low-resistivity (< 50 Ωm) features are conspicuous. Anomalous low-resistivity zones are congruent with the suture zone, and ophiolite belt, which is revealed to be a major crustal-scale feature. Furthermore, broadening low-resistivity zones located down-dip from the suture zone suggest that the narrow deformation zone observed at the surface transforms to a wide area in the deeper crust. Other low-resistivity anomalies are spatially associated with the surface expressions of known mineralization zones; thus, their links to deeper crustal structures are imaged. Considering the available evidence, we determine that, in both cases, the low resistivity can be explained by hydrothermal alteration along fossil fluid pathways. This illustrates the pivotal role that crustal fluids play in diverse geological processes, and highlights their inherent link in a unified system, which has implications for models of mineral genesis and emplacement. The results demonstrate that the crustal architecture—including the major crustal boundary—acts as a first‐order control on the location of the metallogenic belt.
•Electrical resistivity models image crust and mantle below intraplate volcanic zone.•Ancient magma pathways imaged in the crust possibly due to metasomatic alteration.•In the mantle, a potential ...zone of melt is detected that can be the magma source.•Models consistent with direct magma ascent from mantle and no crustal storage.•Cause of volcanism is attributed to a broad mantle upwelling and thermal anomaly.
The structure of continental intraplate volcanic systems — which occur far from tectonic boundaries, unlike the majority of Earth's volcanism — is enigmatic and not fully understood, as are the underlying mechanisms responsible, due in part to a lack of high-resolution geophysical data. Central Mongolia contains Quaternary–Neogene aged alkaline basalt flows and volcanic cones, thousands of kilometres from active tectonic margins, in addition to an abundance of geochemical and petrological data — making this a natural laboratory to study intraplate volcanism. Using a recently collected, high-resolution, multi-scale, magnetotelluric dataset acquired across central Mongolia, we generate and analyze electrical resistivity models of the structure beneath the Tariat and Chuluut volcanic zones with the goal of imaging the volcanic system from surface to mantle source. The models reveal narrow, subvertical, lower resistivity anomalies in the middle-upper crust that are conspicuously located beneath surface expressions of volcanism. The lower crust (depths of 25–50 km) is characterized by the widespread distribution of isolated low-resistivity zones. A local low-resistivity zone is imaged in the mantle (depths of 60–90 km) above a broad, homogenous, doming low-resistivity feature. Considering the available evidence, we propose that the low-resistivity anomalies in the middle-upper crust are the remnant signatures of past transient magma pathways (or collection of pathways), caused by metasomatic alteration during the ascent of hot magmatic fluids. The lower crustal anomalies are interpreted to be domains of saline fluids in a thermally perturbed lower crust. In the mantle, the low-resistivity structure is explained by a broad mantle upwelling and thermal anomaly with a local zone of low-percent partial melt — the source for intraplate volcanism. The geophysical images are consistent with geochemical and petrological evidence from erupted lavas that indicates a single common mantle source region, limited crustal contamination, and rapid direct ascent, making crustal magma storage unlikely. Thus the geophysical models show remarkable and unique translithospheric images of a continental intraplate volcanic system, from surface to mantle source, with the results relevant to other continental regions.
A conventional magnetotelluric (MT) survey layout implies measurements of horizontal electric and magnetic fields at every site with subsequent estimation and interpretation of impedance tensors
Z
or ...dependent responses, such as apparent resistivities and phases. In this work, we assess advantages and disadvantages of complementing or substituting conventional MT with inter-site transfer functions such as inter-site impedance tensor,
Q
, horizontal magnetic,
M
, and horizontal electric,
T
, tensors. Our analysis is based on a 3-D inversion of synthetic responses calculated for a 3-D model which consists of two buried adjacent (resistive and conductive) blocks and thin resistor above them. The (regularized) 3-D inversion is performed using scalable 3-D MT inverse solver with forward modelling engine based on a contracting integral equation approach. The inversion exploits gradient-type (quasi-Newton) optimization algorithm and invokes adjoint sources approach to compute misfits’ gradients. From our model study, we conclude that: (1) 3-D inversion of either
Z
or
Q
tensors recovers the “true” structures equally well. This, in particular, raises the question whether we need magnetic field measurements at every survey site in the course of 3-D MT studies; (2) recovery of true structures is slightly worse if
T
tensor is inverted, and significantly worse if
M
tensor is inverted; (3) simultaneous inversion of
Z
and
M
(or
Z
and
T
) does not improve the recovery of true structures compared to individual inversion of
Z
or
Q
; (4) location of reference site, which is required for calculating inter-site
Q
,
T
and
M
tensors, has also marginal effect on the inversion results.
We present a new model of the radial (1-D) conductivity structure of Earth's mantle. This model is derived from more than 10 yr of magnetic measurements from the satellites Ørsted, CHAMP, SAC-C and ...the Swarm trio as well as the global network of geomagnetic observatories. After removal of core and crustal field as predicted by a recent field model, we fit the magnetic data with spherical harmonic coefficients describing ring current activity and associated induction effects and estimate global C-responses at periods between 1.5 and 150 d. The C-responses are corrected for 3-D effects due to induction in the oceans and inverted for a 1-D model of mantle conductivity using both probabilistic and deterministic methods. Very similar results are obtained, consisting of a highly resistive upper mantle, an increase in conductivity in and beneath the transition zone and a conductive lower mantle. Analysis of the Hessian of the cost function reveals that the data are most sensitive to structures at depths between 800 and 1200 km, in agreement with the results obtained from the probabilistic approach. Preliminary interpretation of the inverted conductivity structure based on laboratory-based conductivity profiles shows that the recovered structure in the lower mantle either requires higher temperatures or the presence of material of high conductivity related to ponding of carbonate melts below the transition zone.
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