Geophysical constraints on the nature of the Highland Boundary Fault Zone in western Scotland

1992 ◽  
Vol 129 (4) ◽  
pp. 411-419 ◽  
Author(s):  
M. C. Dentith ◽  
A. Trench ◽  
B. J. Bluck
Keyword(s):  
Fault Zone ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Thrust Fault ◽  
Gravity Models ◽  
Reverse Fault ◽  
Red Sandstone ◽  
Boundary Fault ◽  
Models Of Gravity ◽  
Surface Geology

AbstractPreviously published models of gravity anomalies across the Highland Boundary Fault in western Scotland interpret this structure as a high-angle reverse fault. These gravity anomalies have been re-interpreted in the light of more extensive gravity data now available, and new density data from the Highland Border Complex. The new data suggest that earlier interpretations have overestimated the fault anomaly and used over-simplified density models. New gravity models of the Highland Boundary Fault Zone are presented which show that the interface between the Dalradian and Highland Border Complex dips to the northwest at an angle of about 20°. We interpret the contact between these two formations as a thrust fault. The interface between the Highland Border Complex and the Lower Old Red Sandstone is shown to be vertical as suggested by surface geology, with the latter rocks a few hundred metres thick.

Forward modeling of gravity data using geostatistically generated subsurface density variations

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. G81-G94 ◽  
Author(s):  
Geoff Phelps
Keyword(s):  
Gravity Data ◽  
Solution Space ◽  
Cumulative Effect ◽  
Normal Fault ◽  
Gravity Anomalies ◽  
Reverse Fault ◽  
Heterogeneous Distribution ◽  
Geostatistical Models ◽  
Air Force Base ◽  
Models Of Gravity

Using geostatistical models of density variations in the subsurface, constrained by geologic data, forward models of gravity anomalies can be generated by discretizing the subsurface and calculating the cumulative effect of each cell (pixel). The results of such stochastically generated forward gravity anomalies can be compared with the observed gravity anomalies to find density models that match the observed data. These models have an advantage over forward gravity anomalies generated using polygonal bodies of homogeneous density because generating numerous realizations explores a larger region of the solution space. The stochastic modeling can be thought of as dividing the forward model into two components: that due to the shape of each geologic unit and that due to the heterogeneous distribution of density within each geologic unit. The modeling demonstrates that the internally heterogeneous distribution of density within each geologic unit can contribute significantly to the resulting calculated forward gravity anomaly. Furthermore, the stochastic models match observed statistical properties of geologic units, the solution space is more broadly explored by producing a suite of successful models, and the likelihood of a particular conceptual geologic model can be compared. The Vaca Fault near Travis Air Force Base, California, can be successfully modeled as a normal or strike-slip fault, with the normal fault model being slightly more probable. It can also be modeled as a reverse fault, although this structural geologic configuration is highly unlikely given the realizations we explored.

Exploring viable geologic interpretations of gravity models using distance-based global sensitivity analysis and kernel methods

Geophysics ◽  
2018 ◽  
Vol 83 (5) ◽  
pp. G79-G92 ◽  
Author(s):  
Geoff Phelps ◽  
Celine Scheidt ◽  
Jef Caers
Keyword(s):  
Sensitivity Analysis ◽  
Gravity Anomaly ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Gravity Models ◽  
Normal Faults ◽  
Reverse Fault ◽  
Density Values ◽  
Generalized Sensitivity

ABSTRACT We have explored ways to integrate alternative geologic interpretations into the modeling of gravity data. These methods are applied to the Vaca Fault east of Fairfield, California, USA, where the structure across the fault is in question, and the Vaca Fault is used as a case study to demonstrate the method. The Vaca Fault is modeled using gravity data collected along a 10 km line perpendicular to the strike of the fault. Of particular interest is how the gravity data might inform on the dip of the Vaca Fault and thickness of the nonmarine section and whether spatial autocorrelation of density internal to the geologic units significantly influences the resulting gravity anomaly. We approach these questions by creating a suite of structural geologic models, which we then populate with geostatistically generated densities and from which the respective synthetic gravity anomalies are calculated. We perform distance-based generalized sensitivity analysis to identify which model inputs most leverage the calculated gravity anomaly. We then use multidimensional scaling to transform the gravity anomalies into a metric space and estimate the posterior probabilities of each structural geologic model using a Bayesian approach. We find that the gravity anomalies are particularly sensitive to zones of autocorrelated density values generated from geostatistical modeling. The structural geologic models most likely to produce gravity anomalies that match the observed data are the moderately dipping normal faults, 45° and 60°, although the probability that the fault dips more steeply, including in a strike slip or reverse fault orientation, is approximately 30%. The probability of a thicker nonmarine unit is 67%, more probable than a thinner nonmarine unit. This suggests that the Vaca Fault dips moderately to the east and truncates a thicker nonmarine unit, but that any further process modeling should include alternatives of the geologic structures.

The gravity field over the Ungava Bay region from satellite altimetry and new land-based data: implications for the geology of the area

1999 ◽  
Vol 36 (1) ◽  
pp. 75-89 ◽  
Author(s):  
Hamid Telmat ◽  
Jean-Claude Mareschal ◽  
Clément Gariépy
Keyword(s):  
Satellite Altimetry ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Gravity Models ◽  
Crustal Thickening ◽  
Seismic Line ◽  
Crustal Thinning ◽  
Gravity Measurements ◽  
George River ◽  
Superior Craton

Gravity data were obtained along two transects on the southern coast of Ungava Bay, which provide continuous gravity coverage between Leaf Bay and George River. The transects and the derived gravity profiles extend from the Superior craton to the Rae Province across the New Quebec Orogen (NQO). Interpretation of the transect along the southwestern coast of Ungava Bay suggests crustal thickening beneath the NQO and crustal thinning beneath the Kuujjuaq Terrane, east of the NQO. Two alternative interpretations are proposed for the transect along the southeastern coast of the bay. The first model shows crustal thickening beneath the George River Shear Zone (GRSZ) and two shallow bodies correlated with the northern extensions of the GRSZ and the De Pas batholith. The second model shows constant crustal thickness and bodies more deeply rooted than in the first model. The gravity models are consistent with the easterly dipping reflections imaged along a Lithoprobe seismic line crossing Ungava Bay and suggest westward thrusting of the Rae Province over the NQO. Because no gravity data have been collected in Ungava Bay, satellite altimetry data have been used as a means to fill the gap in data collected at sea. The satellite-derived gravity data and standard Bouguer gravity data were combined in a composite map for the Ungava Bay region. The new land-based gravity measurements were used to verify and calibrate the satellite data and to ensure that offshore gravity anomalies merge with those determined by the land surveys in a reasonable fashion. Three parallel east-west gravity profiles were extracted: across Ungava Bay (59.9°N), on the southern shore of the bay (58.5°N), and onshore ~200 km south of Ungava Bay (57.1°N). The gravity signature of some major structures, such as the GRSZ, can be identified on each profile.

CHART TO CHECK ELEVATION FACTOR EFFECTS ON GRAVITY ANOMALIES

Geophysics ◽  
1957 ◽  
Vol 22 (3) ◽  
pp. 643-645 ◽  
Author(s):  
L. F. Ivanhoe
Keyword(s):  
Basic Assumption ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Topographic Feature ◽  
Residual Gravity ◽  
Factor Effect ◽  
Surface Geology ◽  
Topographical Relief

The effect of varying surface densities on gravity anomalies is a more common problem in areas of topographical relief than is generally recognized. There is an unjustified tendency to assume that gravity maps are unique and final, even though one basic assumption (density of surface rocks) is inherent in all gravity maps. The use of incorrect elevation factors will produce gravity anomalies over any topographic feature. Both positive and negative gravity anomalies can be produced by either a topographic hill or valley depending on the degree of error in the elevation factor. These “elevation factor anomalies” are especially troublesome on residual gravity maps. The interpretation of gravity data should always include an analysis of the elevation factor effect as well as a study of the surface geology.

Pre-Carboniferous history of the Midland Valley of Scotland

1984 ◽  
Vol 75 (2) ◽  
pp. 275-295 ◽  
Author(s):  
B. J. Bluck
Keyword(s):  
Early Carboniferous ◽  
Strike Slip ◽  
Late Devonian ◽  
Reverse Fault ◽  
Red Sandstone ◽  
Northern Margin ◽  
Boundary Fault ◽  
Thrust Faulting ◽  
Marginal Basin ◽  
History Of

ABSTRACTThe pre-Carboniferous Midland Valley of Scotland comprises three tectonic elements: an arc, a proximal fore-arc basin and a marginal basin. These tectonic elements have been juxtaposed by strike-slip and thrust faulting, both of which have effected a 300% reduction in the width of the orogenic belt.Rocks which span Arenig to Late Devonian or Early Carboniferous times and which are found S of the Highland Boundary fault have no clasts of certain Dalradian provenance despite substantial uplift of the Dalradian block at this time. This, combined with other evidence, suggests the Midland Valley to have been remote from this rapidly uplifting terrane. The Dalradian block, eroded down by c. 410 Ma was thrust southeastwards in Late Devonian–Early Carboniferous times. However, this thrust movement was minor, yielding little sediment, but it caused Dalradian rocks to cover the northern margin of the Midland Valley where (1) the source for part of the Old Red Sandstone rocks existed and (2) the faults along which the Midland Valley block was transported to dock against the Dalradian block are thought to be present. The existing Highland Boundary fault is therefore seen as a late Old Red Sandstone reverse fault which covered more significant older structures.

Record of fluid flow history through fractured conglomerates, Lower Old Red Sandstone of central Scotland

2004 ◽  
Vol 40 (2) ◽  
pp. 145-157 ◽  
Author(s):  
Martin Baron ◽  
John Parnell
Keyword(s):  
Fluid Flow ◽  
Low Temperature ◽  
Fluid Inclusion ◽  
Fault Zone ◽  
Middle Devonian ◽  
Multiple Populations ◽  
Red Sandstone ◽  
Boundary Fault ◽  
Moderate Salinity ◽  
Oriented Parallel

SynopsisExtensively fractured quartzite clasts in Lower Old Red Sandstone conglomerates adjacent to the Highland Boundary Fault Zone in central Scotland contain multiple populations of healed microfractures delimited by fluid inclusion planes. Microthermometric analysis indicates that the healing of microfractures oriented perpendicular to the trend of the Highland Boundary Fault Zone, which probably formed during Acadian overthrusting towards the SE along the fault zone in Middle Devonian times, involved moderate temperature (Th 102 to 238°C), low to moderate salinity (3 to 17 wt% NaCl eq.) aqueous fluids. The clasts also contain a second set of fluid inclusion planes oriented parallel to the Highland Boundary Fault Zone, which formed during extension associated with the intrusion of late Carboniferous quartz dolerite dykes. The healing of this set of microfractures involved low temperature (Th 58 to 155°C), low to moderate salinity (1 to 12 wt% NaCl eq.) aqueous fluids.

scholarly journals Glaciotectonic Processes  Associated with the Central  Alpine Fault:  A Gravity Study of the  Central West Coast, South  Island, New Zealand

2021 ◽  
Author(s):  
◽  
Richard Davy
Keyword(s):  
West Coast ◽  
Gravity Data ◽  
Hanging Wall ◽  
Gravity Models ◽  
Reverse Fault ◽  
Flood Plains ◽  
Alpine Fault ◽  
The Pacific ◽  
River Valleys ◽  
Fox River

<p>The rugged topographic relief of the central West Coast reflects ongoing interplay between active tectonic and climatic processes. Major geomorphological features have formed in response to convergence between the Pacific and Australian continental plates, and the principal locus of this collision is the transpressive Alpine Fault. This thesis describes a gravity study of glaciotectonic structures in the footwall of the central Alpine Fault and the processes responsible for their formation. During this study 361 new gravity observations were collected in the Wanganui, Whataroa, Waiho, and Fox river  flood plains on the western (footwall) side of the Alpine Fault. When combined with existing gravity observations, the available database comprises 932 measurements over the four catchments. These gravity data are used to produce detailed gravity maps and 2-3/4D gravity models of the subsurface structure below the  flood plains. Models reveal extensive glacial erosion focused within the  flood plains, with individual glacial channels reaching depths of ~ 800 m. Based on fault-perpendicular models, it is proposed that the South Westland Fault is a transition between a thrust-driven monocline structure in South Westland and the steeply dipping Hohonu reverse fault in North Westland. Using gravity data, dextral off sets on the Alpine Fault since the Last Glacial Maximum have been determined by examining the structure and geomorphology of deeply incised glacial erosional channels. By studying how the lower reaches of the Wanganui, Whataroa, and Fox rivers have been translated with respect to their channels on the eastern (hanging wall) side of the Alpine Fault, horizontal fault displacements have been determined in three of the four catchments. Fault offsets of 383 ± 388 m, 372 ± 88 m, and 450 ± 99 m are estimated for the Wanganui, Whataroa, and Fox River valleys respectively. A range of possible channel formation ages are used to estimate dextral strike-slip movement rates, with the preferred formation age of 19 ± 1 ka yielding rates of 20.2 ± 24.0 mm/yr, 19.6 ± 6.0 mm/yr and 23.7 ± 8.5 mm/yr for the Wanganui, Whataroa, and Fox river valleys respectively.</p>

scholarly journals Accuracy assessment of test flights using the Turnkey Airborne Gravity System over Alabama in 2008

2013 ◽  
Vol 3 (2) ◽  
pp. 136-142 ◽  
Author(s):  
Y.M. Wang ◽  
S. Preaux ◽  
T. Diehl ◽  
V. Childers ◽  
D. Roman ◽  
...  
Keyword(s):  
Gravity Data ◽  
Gravity Anomalies ◽  
Gravity Models ◽  
Surface Gravity ◽  
Airborne Gravity ◽  
National Geodetic Survey ◽  
The Mean ◽  
The Impact ◽  
Gravity System ◽  
Better Than

AbstractThe National Geodetic Survey (NGS) performed a few test flights using Micro-g’s Turnkey Airborne Gravity System (TAGS) at altitude of 1700, 6300 and 11000 meters over Alabama in 2008. The cross-track spacing was 10 km for the two lower flights and 5 km for the highest flight. The test flights not only provided important information regarding the precision and accuracy of the TAGS but also revealed the impact of flight altitudes and track spacing on the collected gravity data. The gravity anomalies at three altitudes were modeled using 3-dimensional Fourier series, then compared at the three altitudes. The agreement was excellent - the gravity anomalies agree with each other from 1.4 to 3.3 mGal RMS at the three altitudes. When the bias was removed, the agreement was improved to better than 1.1 mGal. On the ground (h =0), the three gravity models agree from 1.9 to 3.8 mGal RMS. After removing the mean, the agreement improved to better than 1.7 mGal. Similar results were obtained in comparison with recent surface gravity which was of sub-mGal accuracy. The overall agreement between the downward continued airborne gravity and the surface gravity was better than 1.7 mGal after removing the mean values.As expected, the flight altitude had a direct impact on accuracy of the values of gravity downward continued to the Earth’s surface. The comparisons with terrestrial gravity show that gravity collected at 11000 m is having an accuracy of ±3 mGal on the ground. This accuracy is slightly worse than the other two altitudes most probably due to smaller signal/noise ratios and larger downward continuation effects. The RMS values of differences between the downward continued airborne gravity at altitude 1700 and 6300 meters and the surface gravity are 2.0 and 1.6 mGals, respectively. Based on these comparisons, airborne gravity data collected at altitudes below 6300 meters should result in accuracy better than ±2 mGals on the ground. Note, however, that the test area is flat and the accuracy of airborne gravity would likely be worse in more rugged mountainous regions.

Deformation of Pebbles in Lower Old Red Sandstone Conglomerates Adjacent to the Highland Boundary Fault

1964 ◽  
Vol 101 (3) ◽  
pp. 228-248 ◽  
Author(s):  
Donald M. Ramsay
Keyword(s):  
Principal Stress ◽  
Direct Relationship ◽  
Vertical Direction ◽  
Reverse Fault ◽  
Principal Stresses ◽  
Red Sandstone ◽  
Boundary Fault ◽  
Experimental Deformation ◽  
Southern Side ◽  
Wrench Fault

AbstractThe deformation produced by the pre-Upper Old Red Sandstone movements on the Highland Boundary Fault in a zone close to the fault was sufficiently intense to rupture the pebbles in conglomerates. The patterns of pebble fracturing are similar in type and orientation over a wide area and are comparable with the patterns obtained in the experimental deformation of brittle materials. In experimental deformation a direct relationship has been established between the fractures and the causal stresses and so from a statistical orientation of the fractures in the pebbles the directions of the principal stresses can be deduced.The principal compression in the vicinity of the Highland Boundary Fault acted N.W.–S.E. varying in places to N.N.W.–S.S.E., normal to the trend of the fault, while the least principal stress acted parallel to the strike of the fault.The orientation of the least principal stress parallel to the direction of the fault in the conglomerates arises through increased stress in a vertical direction due to the nature of the downbuckling of the rocks on the southern side of the fault.The orientation of the maximum compression confirms the Highland Boundary Fault as a reverse fault of late Caledonian age rather than a Proto-Armorican wrench fault produced by N.N.E.-S.S.W. compression.

scholarly journals LOCAL EVALUATION OF EARTH GRAVITATIONAL MODELS, CASE STUDY: IRAN

2017 ◽  
Vol 43 (1) ◽  
pp. 1-13 ◽  
Author(s):  
Ismael FOROUGHI ◽  
Yosra AFRASTEH ◽  
Sabah RAMOUZ ◽  
Abdolreza SAFARI
Keyword(s):  
Gravity Data ◽  
Low Frequency ◽  
Gravity Anomalies ◽  
Gravity Models ◽  
Data Sets ◽  
Observation Data ◽  
Satellite Gravity ◽  
Global Gravity ◽  
Marine Gravity ◽  
Global Gravity Models

Global gravity models are being developed according to new data sets available from satellite gravity missions and terrestrial/marine gravity data which are provided by different countries. Some countries do not provide all their available data and the global gravity models have many vague computational methods. Therefore, the models need to be evaluated locally before using. It is generally understood that the accuracy of global gravity models is enough for local (civil, mining, construction, etc.) projects, however, our results in Iran show that the differences between synthesized values and observation data reach up to ∼300 mGal for gravity anomalies and ∼2 m for geoid heights. Even by applying the residual topographical correction to synthetized gravity anomalies, the differences are still notable. The accuracy of global gravity models for predicting marine gravity anomalies is also investigated in Persian Gulf and the results show differences of ∼140 mGal in coastal areas. The results of evaluating selected global gravity models in Iran indicate that the EIGEN-6C4 achieves the lowest RMS for estimating the geoid heights. EGM08 predicts the closest results to terrestrial gravity anomalies. DIR-R5 GOCE satellite-only model estimates the low-frequency part of gravity field more accurately. The best prediction of marine gravity anomalies is also achieved by EGM08.

Sign in / Sign up

Export Citation Format

Share Document