Lineation Data Research Articles

Mapping Fault Distribution Using Lineation and Fault Fracture Density Methods in the Palu Basin and Surrounding Areas

Introduction: Research has been carried out with the title "Mapping Fault Distribution Using Lineation and Fault Fracture Density Methods in the Palu Valley and Surrounding Areas" with the aim of determining the direction of fault distribution and analyzing fault density based on lineation data. Methods: This research uses the lineation method with a total of 40 grids. Data processing uses Global Mapper 18, rockwork 17, PCI Geomatica 2014, and Arcgis 10.8 software. Result and Discussion: The results obtained show that the lineation data from the dominant rose diagram has a relatively east-west direction. Areas that have high fault density are in Lindu District, Palolo District, Nokilalaki District, Gumbasa District, Dolo District, Sigi Biromaru District, and Mantikulore District with fault density values of 5,945 - 18,950/km2. Conclusion: The direction of lineation obtained tectonically is influenced by the Palu-Koro fault which tends relatively north-south, the Graben Palolo A and Graben Palolo B faults which trend relatively southeast-northwest, and the Lore Lindu fault zone. Areas with high density are weak zones that are vulnerable to natural disasters and potential ground movements.

Finding the Mean and 95 Percent Confidence Interval of a Set of Strike-and-Dip or Lineation Data

Quantitative observational data should be reported with appropriate uncertainty estimates. Statistical methods developed by Fisher (1953) to analyze paleomagnetic data are also useful for determining the 95 percent confidence interval associated with site mean strike and dip from three or more observations (at least seven observations preferred) of the orientation of geologic surfaces that are approximately homoclinal or of lineations that are subparallel. The uncertainty in the strike of a surface is a function of the uncertainty and inclination of the dip vector and is greater than the dip uncertainty. For example, surfaces with measured orientations (strike and dip) of 310° 38°NE, 319° 45°NE, and 322° 30°NE at a given site have a site mean orientation of 317 ± 18° and 38 ± 14°NE at a 95 percent confidence interval.

Inferences from shear zone geometry: an example from the Laxfordian shear zone at Upper Badcall, Lewisian Complex, NW Scotland

Abstract This contribution presents a Laxfordian age shear zone near Upper Badcall, NW Scotland, as an example of using field data and theory to assess the kinematics and nature of deformation in a shear zone. The deformation zone includes quartzofeldspathic background gneiss and a dolerite dyke that cuts the gneissic banding at a high angle. A detailed field description of the deformation zone, which is critically discussed in terms of pure and simple shear, is presented. Analysis of gneissic banding and mineral lineation data, together with a consideration of the outcrop pattern, shows that the deformation zone is best described in terms of a simple shear zone with varying finite stretching direction. To analyse this deformation we introduce the concept of local plane strain. Although the deformation of the zone as a whole is 3D, at each point there is a direction in which it does not change its length. This direction is perpendicular to the local shear direction and so varies in orientation across the shear zone. In a reference frame defined at a point, the deformation can thus be understood in terms of conventional simple shear. Details of strain are hence determined according to this conclusion. A stereographic method for the determination of the reorientation of lines is used to calculate shear strain. The shear strain values across the shear zone are then used to restore the sheared dolerite dyke to its undeformed geometry. The success of the restoration provides supports for the strain calculation and also the conclusion of simple shear deformation.

Using basement-hosted clastic dykes as syn-rifting palaeostress indicators: an example from the basal Stoer Group, northwest Scotland

Clastic infills of fractures (here termed clastic veins) in basement rocks immediately underlying sedimentary cover sequences can be used to date fault movements if these demonstrably occurred at the time of infilling or prior to the lithification of the entrained clastic material. This allows reconstruction of the syn-rifting palaeostress system using stress inversion techniques. During Riphean intracontinential rifting of Laurentia, the Torridonian Stoer Group sediments of northwest Scotland were deposited in half-graben basins controlled by faults, e.g. the Coigach and Clachtoll faults. At Clachtoll, northeast–southwest oblique sinistral normal faulting in the underlying basement is associated with extensive development of shear, hybrid and tensile clastic veins filled with Stoer Group sediment, infilled and deformed prior to sediment lithification. Clastic veins initially formed by gravitational infilling of sediment from above, followed by tectonically-driven, forceful hydraulic injection of fluidized sand into new fractures and reactivated pre-existing basement faults. Palaeostress axes, determined from fault lineation data and tensile fracture extension directions in the Clachtoll Fault zone, indicate west-northwest–east-southeast directed extension during rifting. On a regional scale, this implies oblique-dextral extension on the north- to north-northeast-trending Coigach Fault during Stoer Group deposition. Similar orientations, age relationships and kinematics have been obtained from pre-Torridon Group fault arrays developed in the Lewisian basement near Gairloch and Loch Maree. Overall, the faulting patterns reflect a three-dimensional strain (k≠1) formed by east-southeast–west-northwest-directed extension during deposition of the Stoer Group. More speculatitively, asymmetric density patterns of sinistral and dextral faults may indicate that rifting occurred in a regional zone of broadly north–south-oriented dextral transtension.

Geometry and kinematics of vertical-axis rotations in fold and thrust belts

Vertical-axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts. The kinematics of rotating thrust sheets may be best described as a combination of a far-field rotation (or translation in a planar geometry) and a near-field, or local rotation, so that the instantaneous rotation axis lies outside the body of the sheet. This simplifies the thrust-sheet kinematics significantly, as it means that all the points on the sheet have a component of forward motion, and the bounding fault is a thrust over all its area. The consequences of a simple thrust-sheet rotation for the interpretation of fault lineation data are investigated. The lineations recorded in the footwall and the hangingwall are likely to be different, the hangingwall lineations being skewed towards the direction of rotation. The principal mechanism for local thrust-sheet rotation is differential shortening, either on folds or thrusts, or by a combination of these structures. The forward propagation of thrusting towards the foreland can accumulate large (>120°) rotations on the uppermost sheets. These rotational thrust sheets cannot extend indefinitely along strike; instead they terminate against lateral ramps or transfer faults. Sidewall duplexes may form as a consequence of rotation over a lateral ramp. Examples of rotational geometries from the Subbetic Zone of southern Spain are described. On a larger scale, obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike-slip motion. Reliable palaeomagnetic data are not always available for fold and thrust belts, so independent evidence for rotational deformation is of great importance. Fold and thrust geometries, particularly along-strike variability in fold geometries, highly variable axial traces of folds, evidence for near-synchronous refolding and the deviation of regional trends may all be a consequence of thrust-sheet rotation. Detailed mapping, and particularly the identification of the position of footwall and hangingwall cut-offs can also provide strong evidence for rotation.

Unidirectional slip vectors in the western Betic Cordillera: implications for the formation of the Gibraltar arc

We have investigated the kinematics of deformation in a sector of the Gibraltar arc in SW Spain, by synthesizing lineation data from major faults, the variation in structural trends, and palaeomagnetically determined vertical-axis rotations. The kinematic data show that transport directions throughout formation of the arc were consistently WNW, regardless of structural trend, which varies between 050 and 350 in the study area. The data suggest that this part of the Gibraltar arc can be explained simply by oblique convergence with a curved indenter. Taken together with other published data it appears likely that the variation in thrust transport direction within the Betic Cordillera as a whole may be considerably less than that in the Moroccan part of the arc. Unidirectional convergence, combined with the presence of palaeomagnetic rotations, implies either eYcient overprinting of old, rotated lineations, and/or a convergence rate that is much greater than the rotation rate. Also required is a low degree of partitioning in the thrust wedge. The latter probably increases east of the study area, where the structural trend becomes more oblique to the transport direction, and major strike-slip faults occur.

Structural analysis in mineral exploration using a Geographic Information Systems ‐adapted stereographic‐projection plotting program

Geographic Information Systems (GIS) have proved to be a useful tool in the appraisal of mineral prospectivity at regional scales. The results, however, are generally not totally applicable on the local scale (e.g. district or local mine/exploration scales) owing to the increasing importance of the third dimension. At these scales, it is more appropriate to use smaller scale parameters, such as structural data, to aid the identification of targets for exploration or for discerning controls on known mineralisation. In GIS, however, these smaller scale datasets (e.g. bedding, foliation, lineation data) are underutilised in their representation and analysis, primarily due to the lack of suitable tools within a GIS to perform routine structural analysis using conventional methods of stereographic projection. Small‐scale interactive analysis, query and display of structural data are now possible with the advent of recent developments in the integration of structural methods with a GIS. This has culminated in the creation of a GIS‐adapted stereographic‐projection plotting program (GIS‐Stereoplot) that incorporates the functionality of conventional stereoplotting applications with the tools that are available within a GIS enabling the integration of structural data with other thematic map features as an aid to mineral exploration.

Palaeomagnetic analysis of plunging fold structures: Errors and a simple fold test

The conventional corrections for bedding dip in palaeomagnetic studies involve either untilting about strike or about some inclined axis—the choice is usually governed by the perceived fold hinge orientation. While it has been recognised that untilting bedding about strike can be erroneous if the beds lie within plunging fold structures, there are several types of fold which have plunging hinges, but whose limbs have rotated about horizontal axes. Examples are interference structures and forced folds; restoration about inclined axes may be incorrect in these cases. The angular errors imposed upon palaeomagnetic lineation data via the wrong choice of rotation axis during unfolding are calculated here and presented for lineations in any orientation which could be associated with an upright, symmetrical fold. This extends to palaeomagnetic data previous analyses which were relevant to bedding-parallel lineations. This numerical analysis highlights the influence of various parameters which describe fold geometry and relative lineation orientation upon the angular error imparted to lineation data by the wrong unfolding method. The effect of each parameter is described, and the interaction of the parameters in producing the final error is discussed. Structural and palaeomagnetic data are cited from two field examples of fold structures which illustrate the alternative kinematic histories. Both are from thin-skinned thrust belts, but the data show that one is a true plunging fold, formed by rotation about its inclined hinge, whereas the other is an interference structure produced by rotation of the limbs about non-parallel horizontal axes. Since the angle between the palaeomagnetic lineations and the inclined fold hinge is equal on both limbs in the former type of structure, but varies from limb to limb in the latter, a simple test can be defined which uses palaeomagnetic lineation data to identify rotation axes and hence fold type. This test can use pre- or syn-folding lineations and could be useful in areas of non-coaxial folding.

Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns

We have conducted a comparative study of the tectonic morphology of young seafloor using SeaMARC II side scan sonar surveys of the intermediate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR) (8°30′–10°N), and the super fast spreading EPR (18°–19°S). We find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments (i.e., with proximity to large‐ and short‐offset discontinuities). We also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer. For example, regions of super fast spreading are characterized by the largest numbers of short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates. Rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from the displacement and length distributions of faults, and strain estimates of ∼4% are obtained for each area. Assuming that fault spacing reflects fracture depth extent where faults initiate, we infer a brittle layer thickness of ∼1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long‐term reduced magma supply along the eastern third of the Ecuador Rift. Evidence for local increased brittle extension is also found within 15 km of transform faults. Discordant zones left by overlapping spreading centers (OSCs) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be useful indicators of plate motion. Within the EPR 8°30′–10°N area, fault trends record a recent change in Pacific‐Cocos plate motion (3°–6° at ∼1 m.y.) consistent with magnetic anomaly and fault lineation data from elsewhere along the northern EPR. Within the Ecuador Rift, fault azimuths scatter within 3° of predicted trends and are consistent with constant spreading about one pole for the past 1.5 m.y.

Displacement localization and palaeo-seismicity of the Rencurel Thrust Zone, French Sub-Alpine Chains

The palaco-seismicity and history of strain accumulation within the Rencurel Thrust Zone, French Sub-Alpine Chains, has been investigated by examining fracture-filling cements. Two generations of fracture-filling cement with distinct petrographic characteristics, cation geochemistries and 13C and 18O stable isotopic compositions have been distinguished within the Rencurel Thrust Zone. The hanging-wall rocks contain fracture-filling calcite cements, whereas ferroan calcite fills fractures within the Central Gouge Zone. Calcitecemented faults include frontal, oblique and lateral ramps, as well as faults dipping in the movement direction, whereas, ferroan calcite-cemented faults include only frontal ramps and faults dipping in the movement direction. Fragments of calcite cement occur as clasts within the Central Gouge Zone, indicating that the calcite cements formed prior to precipitation of ferroan calcite in the Central Gouge Zone. Lineation data indicate that precipitation of both generations of cement occurred during a single phase of thrusting. The lack of ferroan calcite cements in the hanging-wall rocks suggests that the hanging-wall of the Rencurel Thrust was not fractured during displacements within the Central Gouge Zone. This contrasts with seismogenic faults where rocks surrounding major faults are fractured during fault slip, and it is inferred that the exposed portion of the Rencurel Thrust may have experienced aseismic fault displacements. The aseismic fault slip may have been the consequence of the shallow burial depths (<3 km) experienced by the exposed portion of the Rencurel Thrust during faulting.

Strain and vorticity patterns in ideally ductile transpression zones

The prevalent model for ductile shear zones assumes that they develop by progressive simple shearing, resulting in a monoclinic fabric in which the vorticity vector is parallel to the shear zone and perpendicular to the lineation. But some ductile shear zones exhibit an amount of coaxial flattening, or a fabric pattern which appear to be incompatible with the assumptions of plane strain and progressive simple shear. In certain sections of the Archean Larder Lake—Cadillac deformation zone (LCDZ), for example, vorticity indicators (asymmetric pressure wings, Z-folds, SC fabrics), best seen on horizontal surfaces, indicate dextral transcurrent motion, whereas stretching lineations have variable but steep plunges. In the Proterozoic Mylonite Zone (MZ) of south-west Sweden, vorticity indicators combined with foliation and lineation data suggest a continuous change from reverse dip-slip motion close to the footwall to sinistral transcurrent motion adjacent to the hangingwall of the zone. Such departures from the ideal progressive simple shear zone pattern may in fact be common. Rather than invoke two stages of deformation, we explore the possibility that these patterns could be the result of ductile transpression. Ductile transpression between relatively rigid walls implies an extrusion of material out of the shear zone. When the material cannot slip freely along the boundaries of the zone, the extrusion strain is by necessity heterogeneous. In order to explore these heterogeneous strain distributions, we have developed a continuum mechanics model in which the ‘transpressed’ rock is a linear viscous material squeezed upward between two parallel, rigid, vertical walls. Transpression is further generalized by modelling oblique (i.e. with a dip-slip component) relative displacements of the walls. Models, which can vary in their obliquity and their ‘press’/‘trans’ ratio, are examined for their distributions of K-values, strain rate intensity, ‘lineation’ (direction of maximum principal strain rate), ‘foliation’ (plane perpendicular to the direction of minimum principal strain rate) and vorticity. To quantify the expected petrographic effect of the vorticity when the strain path has triclinic symmetry, we introduce a sectional kinematic vorticity number, W s k. The model predicts ‘foliations’ and ‘lineations’ which vary in orientation and intensity across the zone. In some model zones, the vorticity vector can be nearly parallel to the ‘foliation’ and perpendicular to the ‘lineation’, as expected in progressive simple shear, but it can also be locally nearly parallel to the ‘lineation’, as in the LCDZ. Commonly, however, the vorticity vector is not parallel to any of the principal directions of instantaneous strain, and the deformation has triclinic symmetry. The pattern of foliations and lineations in the MZ can readily be matched to that in an oblique transpression model zone.

Late Cenozoic tectonic development of the Southeast Asian continental margin in the Banda Sea area

The late Cenozoic tectonic development of the Southeast Asian continental margin around the Banda Sea is complicated by interaction with external geological elements. Southeast Asian internal elements are the Banda Volcanic Arc and previous older arcs. External elements are the Australian continental crust and the Indian and Pacific oceanic crusts. These external elements are now trapped behind the Banda Volcanic Arc. Three main geological events are responsible for the present configuration of the Banda Sea and adjacent areas: 1. (1) collision between the Banda Volcanic Arc and Australian continental crust; 2. (2) emplacement of the Banda Sea oceanic crust; and 3. (3) emplacement of microcontinents now present in and around the Banda Sea. The geology of Timor is fundamental to tectonic interpretations of the collision between the Banda Volcanic Arc and Australia. Differences exist in the tectonic interpretation of Timor and include either overthrusting, upthrusting or mélange formation. The present paper follows geological data contained in geological maps published by the Geological Survey of Indonesia in which overthrust structures are clearly shown. Paleomagnetic and paleontologic data indicate that the overthrust units were from southern paleolatitudes. Depth and magnetic lineation data support the interpretation that the Banda Sea is underlain by old oceanic crust. Similar orientation of magnetic lineations in the Banda Sea and the Argo Abyssal Plain suggests that they have the same origin. Geological data from northern Banda microcontinents, dredged samples from the Banda/Lucipara ridges and comprehensive geological development of the northern Banda Arc support the interpretation that the microcontinents were translated left-laterally westward from Irian Jaya.

Crustal domains in the northern Kaapvaal craton as defined by magnetic lineaments

The northern part of the Kaapvaal craton and Limpopo belt can be subdivided into domains on the basis of aeromagnetic lineament patterns. These subdivisions reflect portions of the lithosphere with different tectonic histories and many of these domains correlate with known metamorphic-tectonic provinces. The dykes causing the lineation pattern intruded intermittently over a very long period of time, and, within a domain, the trend of the intruded dykes is determined not only by the stress field operative at the time of intrusion, but also by an inherited fracture pattern in that domain. The event that introduced the fracture pattern has to be older than 2456 ± 78 Ma and younger than 2603 ± 129 Ma. The magnetic lineation data show that a 15° anticlockwise rotation of the central zone of the Limpopo belt with respect to the southern marginal zone will restore parallelism in the fracture pattern of the two domains. The presence of the greenstone belts has no effect on the dyke pattern, and domain boundaries cut across these structures.

The magnetic fabric of the Late Permian Dundee Ignimbrite, Dundee, N.S.W.

The Dundee Ignimbrite has a distinct magnetic fabric, defined by susceptibility anisotropy. Magnetic foliation data for the ignimbrite mass at Dundee define a basinal structure. The magnetic lineation data which are argued to be flow lineations generally have a NNE?SSW orientation. This suggests that the source vent(s) for the ignimbrite is external to the mass.

Relationship Between Structural Development, Carbonate Reservoir Development, and Basement Faulting in Southern Scott and Northwestern Finney Counties, Kansas: ABSTRACT

Hydrocarbon production from carbonate reservoirs in the Hugoton embayment, southwest Kansas, is mainly from structurally positive features with well-developed porosity and permeability. In the study area, these reservoirs tend to be associated with oolite development on shallow shelflike areas and accumulations of oolites along relatively rapid changes in paleorelief. However, not all positive structures of similar depositional environments displayed productive reservoir characteristics. In an attempt to understand reservoir development and the timing of the apparent tectonic events in this area, structural and magnetic lineation data (defined by airborne magnetic data) were correlated.

Hydrocarbon Production Potential of Mississippian Rocks in Cowley County, Kansas: ABSTRACT

Petroleum geologists are continually searching for new and better techniques of exploring for hydrocarbons. This study uses one such technique, discriminant function analysis, to predict trends of potential hydrocarbon production of Mississippian rocks in Cowley County, Kansas, Discriminant function analysis is a method of studying differences between two or more groups, using a set of discriminating variables. The two groups used for this study were (1) wells producing from the Mississippian and (2) wells not producing from the Mississippian. Variables used were thought to be potentially significant to hydrocarbon accumulation in Mississippian rocks within the study area. These variables included structural tops and thicknesses, surface elevation, presence of tripolitic Mississippian chert, trend surface residuals, and four types of satellite lineation data. A total of 564 wells, each with 21 distinct parameters, was used in performing the two-phased analysis. The two phases, consisting of a principal components analysis and the discriminant function analysis, were accomplished through use of computer programs and resulted in a single discriminant score value for each well. These values were used to generate a discriminant score surface map. Statistical analysis of the map resulted in 80.6% of the producing wells being correctly classified within producing areas. Alternately, 75.9% of the nonproducing wells were correctly classified. These favorable results suggest that this technique could be successful in establishing trends of production and potential production while simultaneously delineating areas of nonproduction. End_of_Article - Last_Page 314------------

Post-Miocene Cayman Trough evolution: A speculative model

Research Article| December 01, 1983 Post-Miocene Cayman Trough evolution: A speculative model Troy L. Holcombe; Troy L. Holcombe 1Marine Geology Branch, Seafloor Geosciences Division, Naval Ocean Research and Development Activity, NSTL Station, Mississippi 39529 Search for other works by this author on: GSW Google Scholar George F. Sharman George F. Sharman 2Department of Oceanography, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar Author and Article Information Troy L. Holcombe 1Marine Geology Branch, Seafloor Geosciences Division, Naval Ocean Research and Development Activity, NSTL Station, Mississippi 39529 George F. Sharman 2Department of Oceanography, Texas A&M University, College Station, Texas 77843 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1983) 11 (12): 714–717. https://doi.org/10.1130/0091-7613(1983)11<714:PCTEAS>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Troy L. Holcombe, George F. Sharman; Post-Miocene Cayman Trough evolution: A speculative model. Geology 1983;; 11 (12): 714–717. doi: https://doi.org/10.1130/0091-7613(1983)11<714:PCTEAS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Cayman Trough exhibits cross-trough lineations, which we interpret as crustal fabric formed parallel to the spreading center at the time of crustal generation. Alignment and variations of the spreading fabric are roughly symmetrically distributed about the mid-Cayman spreading center, as are variations in the cross-trough width of the Cayman Trough. These suggest to us a simple, eight-stage model of stepwise-varying spreading directions and rates. For this model it is assumed that (1) tectonic spreading fabric is perpendicular to spreading direction, (2) cross-trough extension both widens the trough and lengthens the spreading center, and (3) cross-trough compression narrows the trough but does not shorten the spreading center. Combining the spreading-fabric lineation data with these assumptions, we predict a cross-trough width variation that is essentially that of the Cayman. Similar variations in spreading direction should be evidenced elsewhere on the plate boundary if the variations we observe do, in fact, record the history of changes in relative motion between the North American and Caribbean plates. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geoid anomalies over the north-east Pacific fracture zones from satellite altimeter data

Summary. Seasat altimeter profiles have been used to recover geoid height anomalies over the north-east Pacific fracture zones. Geoid anomalies observed over fracture zones show a step-like signal, of 1–4m amplitude, downward from the younger, shallower side to the older, deeper side of the fracture zone. Assuming that fracture zones are in isostatic equilibrium at the base of the lithosphere, the geoid offset occurs because the portions of lithosphere on each side of the fracture have different thickness and density structures as predicted by thermal models of lithospheric cooling. Along the Mendocino Fracture Zone, whose age offset is of the order of 20–30Myr, the observed geoid anomalies present a clear change from east to west, both in shape and in amplitude: in the youngest part of Mendocino (around 130° W longitude), there is a marked step in the geoid (3–4m amplitude), while west of 140° W, the older part of Mendocino, the geoid offset does not exceed 1.5 m, the signal becoming more and more distorted. The (geoid offset/age offset) relation estimated at different locations along Mendocino presents a progressive decrease from east to west, e.g. from the young part to the old part of the fracture zone. For the Pioneer Fracture Zone, the geoid offset is barely visible, never exceeding 40 cm. Using the known bathymetry and crustal structure inferred from seismic refraction data at 128° 30°W longitude as well as ages inferred from magnetic lineation data, we have computed the theoretical geoid height anomaly at this location: assuming (local) isostatic compensation at the base of the lithosphere, we have deduced the subcrustal density structure from lithospheric cooling models, taking account of the age/thickness relationship. Comparison between computed and observed geoid anomaly at 128° 30°W longitude is very satisfactory, although a very slight overcompensation at depth needs to be assumed in order to give a better agreement. Over the western, older part of Mendocino, observed geoid anomalies approach isostatic anomalies. Since subcrustal lateral density contrasts are smaller than in the young part of the fracture zone, their isostatic shape and smaller amplitude imply smaller crustal discontinuities: this suggests that the crustal thickness discontinuity across a fracture zone may decrease with age.