Decrease In Crustal Thickness Research Articles

Outboard Onset of Ross Orogen Magmatism and Subsequent Igneous and Metamorphic Cooling Linked to Slab Rollback during Late-Stage Gondwana Assembly

Changes in magmatism and sedimentation along the late Neoproterozoic-early Paleozoic Ross orogenic belt in Antarctica have been linked to the cessation of convergence along the Mozambique belt during the assembly of East-West Gondwana. However, these interpretations are non-unique and are based, in part, on limited thermochronological data sets spread out along large sectors of the East Antarctic margin. We report new 40Ar/39Ar hornblende, muscovite, and biotite age data for plutonic (n = 13) and metasedimentary (n = 3) samples from the Shackleton–Liv Glacier sector of the Queen Maud Mountains in Antarctica. Cumulative 40Ar/39Ar age data show polymodal age peaks (510 Ma, 491 Ma, 475 Ma) that lag peaks in U-Pb igneous crystallization ages, suggesting igneous and metamorphic cooling following magmatism within the region. The 40Ar/39Ar ages are similar to ages in other sectors of the Ross orogen, but younger than detrital mineral 40Ar/39Ar cooling ages indicative of older magmatism and cooling of unexposed inboard areas along the margin. Detrital zircon trace element abundances suggest that the widespread onset of magmatism in outboard localities of the orogen correlates with a ~560–530 Ma decrease in crustal thickness. The timing of crustal thinning recorded by zircon in magmas overlaps with other evidence for the timing of crustal extension, suggesting that the regional onset of magmatism with subsequent igneous and metamorphic cooling probably reflects slab rollback that coincided with possible global plate motion changes induced during the final assembly of Gondwana.

Moho Changes Beneath the Northeastern Tibetan Plateau Revealed by Multiple Geodetic Datasets

SummaryThe Moho interface depths and variations provide important references to understanding the outward growth and expansion mechanisms of the Tibetan Plateau, which have drawn much attention. Here, we constrain the Moho variations beneath the northeastern Tibetan Plateau (NETP) by using multiple geodetic data and climate models. Hydrological and tectonic gravitational signals are identified from multisource data and removed from the satellite gravity observations. Moreover, the mass variation rate, resulting from vertical Moho changes is calculated. The result shows that the mass variation rate due to the Moho changes is 1.3 ± 0.6 mm/yr (equivalent water height) within the NETP, which corresponds to a Moho uplift rate of 2.7 ± 1.3 mm/yr. Additionally, we calculate the tectonic Earth surface uplift rate from GPS and leveling observations with loading and GIA (glacier isostatic adjustment) influence corrected, which gives an uplift rate of 2.2 ± 0.1 mm/yr and is similar with the Moho uplift rate. Our results indicate that the crust in the NETP is uplifted with a slight decrease in crustal thickness. Combined with the lithospheric structure from seismological tomography studies, we suggest that the crustal uplift of the NETP is mainly caused by isostatic adjustment since the delamination of the thickened lithosphere.

Crustal velocity structure of the Northwest Sub-basin of the South China Sea based on seismic data reprocessing

The deep crustal structure of the Northwest Sub-basin (NWSB) of the South China Sea (SCS) is of great importance for understanding the tectonic nature of the continent-ocean transition (COT) and magmatism in this oceanic basin. The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge (ESR) of the NWSB. In addition to the original data, we added the data of two reprocessed OBS stations, and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile. Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation. The depth of the Moho (23.5–11.8 km) and crustal thickness (20.5–6.5 km) systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB. The COT zone has a width of ∼20 km and the depth of the Moho decreases from 15.0 to 11.0 km, corresponding to a ∼4 km decrease in crustal thickness (6–10 km). A high velocity layer (HVL, 7.2–7.4 km s−1) exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs. Age dating shows that the Double-peak Seamount was formed at ∼23 Ma, after the cessation of the NWSB seafloor spreading (∼32-25 Ma). The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km. We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.

The crustal structure of Beira High, central Mozambique—Combined investigation of wide-angle seismic and potential field data

The timing and geometry of the initial Gondwana break-up between Africa and East Antarctica is still poorly known due to missing information about the continent-ocean boundaries along the rifted margins. In this context, the Beira High off central Mozambique forms a critical geological feature of uncertain crustal fabric. Based on new wide-angle seismic and potential field data across Beira High a P-wave velocity model, supported by amplitude and gravity modelling, provides constraints on the crustal composition of this area. In the Mozambique Basin mainly normal oceanic crust of 5.5–7km thickness with velocities of 6.5–7.0km/s in the lower crust is present. A sharp transition towards Beira High marks the continent-ocean boundary. Here the crust thickens to 23km at maximum. A small velocity-depth gradient and a constant increase in velocity with basal velocities of maximum 7.0km/s are in good agreement with typical velocities of continental crust and continental fragments. The density model indicates the existence of felsic material in greater depths and supports a fabric of stretched, but highly intruded continental crust below Beira High. A gradual decrease in crustal thickness characterizes the transition towards the Mozambican shelf area. Here, in the Zambezi Delta Depression 12km of sediments cover the underlying 7km thick crust. The presence of a high-velocity lower crustal body with velocities of 7.1–7.4km/s indicates underplated, magmatic material in this part of the profile. However, the velocity structure in the shelf area allows no definite interpretation because of the experimental setup. Thus, the crustal nature below the Zambezi Delta and consequently the landward position of the continent-ocean boundary remains unknown. The difference in stretching below the margins of Beira High suggests the presence of different thinning directions and a rift jump during the early rifting stage.

Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion

Oceanic and continental lithosphere distribution within the eastern Mediterranean is not well understood. A gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and lithosphere thinning for the eastern Mediterranean, from which the distribution of oceanic and continental lithosphere, the structure of the ocean–continent transition (OCT) and the location of the continent–ocean boundary (COB) can be determined. The gravity inversion results show thin crust and high continental lithosphere thinning under the Ionian Sea and the Herodotus Basin, consistent with these basins being underlain by oceanic crust. Moho depths from gravity inversion are in agreement with seismic refraction estimates in these basins. Highly thinned continental crust is predicted under the offshore Sirte and Levant basins. The sharp decrease in crustal thickness predicted by gravity inversion off the Libyan and Egyptian coast gives an indication of COB location. Crustal thickness and continental lithosphere thinning determined from gravity inversion have also been used to explore the relationship between the Cretaceous West and Central African Rift System (WCARS: Benue Trough, Chad, Central African Shear Zone (CASZ) and Sudan basins) and the eastern Mediterranean basins; continuity between the Cretaceous WCARS and the eastern Mediterranean basins is not apparent in the gravity inversion results.

High-Temperature Granite Magmatism, Crust–Mantle Interaction and the Mesoproterozoic Intracontinental Evolution of the Musgrave Province, Central Australia

The Musgrave Province lies at the convergence of major structural trends formed during the Proterozoic amalgamation of the North, West and South Australian Cratons prior to c. 1290 Ma. The Musgrave Orogeny, one of three Mesoproterozoic orogenies to affect the province, produced the granites of the Pitjantjatjara Supersuite, which dominate the outcrop. This orogeny was an intracontinental and dominantly extensional event in which ultrahigh-temperature (UHT) conditions persisted from c. 1220 to c. 1120 Ma. The onset of UHT conditions is heralded by a change from low-Yb granites to voluminous Yb-enriched granites, reflecting a rapid decrease in crustal thickness. The Pitjantjatjara granites are ferroan, calc-alkalic to alkali-calcic rocks and include charnockites with an orthopyroxene-bearing primary mineralogy. They were emplaced at temperatures ≥1000°C from c. 1220 to c. 1150 Ma. Their geochemical and Nd and Hf isotopic homogeneity over a scale of >15 000 km 2 reflects a similarly homogeneous source. This source included an old enriched felsic crustal component. However, the bulk source was mafic to intermediate in composition. The long-lived UHT regime, and thermal limits on the amount of crust sustainable below the level of intrusion, indicates a significant (>50%) mantle-derived source component. However, a positive correlation between Mg-number and F suggests that many Pitjantjatjara granites formed through the breakdown of F-rich biotite in a crustal granulite. We suggest that under- and intraplated mafic magmas assimilated the limited available felsic crust into lower crustal MASH (melting, assimilation, storage, homogenization) domains. These partially cooled but were remobilized during subsequent under- and intra-plating events to produce the Pitjantjatjara granites. The duration of UHT conditions is inconsistent with a mantle plume. It reflects an intracontinental lithospheric architecture where the Musgrave Province was rigidly fixed at the nexus of three thick cratonic masses. This ensured that any asthenospheric upwelling was focused beneath the province, providing a constant supply of both heat and mantle-derived magma.

Miocene evolution of the western edge of the Nevadaplano in the central and northern Sierra Nevada: palaeocanyons, magmatism, and structure

The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young ( < 6 Ma); however, recent work has provided evidence that the range is very old (>80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ∼2–5 Myr of subduction-induced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11–10 Ma, records the birth of the ‘future plate boundary’ by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8–7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3–3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial- and mass-wasting deposits, and greater degree of encroachment by Walker Lane-related faults beginning at 10–11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events.

Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography

Axial morphology along the Southeast Indian Ridge (SEIR) systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100°–116°E, west of the Australian‐Antarctic Discordance (AAD). Basalt geochemistry has a consistent Indian–mid‐ocean ridge basalt (MORB) type isotopic signature, so changes in axial topography are attributed to variations in both mantle temperature and melt supply. Wide‐angle seismic refraction lines were shot to four ocean bottom hydrophones within SEIR segments P1, P2, S1, and T, where each segment is characterized by a different morphology. We constructed 2‐D crustal velocity models by jointly inverting hand‐picked P wave refraction (Pg) and Moho reflection (PmP) traveltime data using a top‐down, minimum‐structure methodology. The results show a 1.5 km eastward decrease in crustal thickness across the study area, with segment averages ranging from 6.1 km at P1 to 4.6 km at T. Melt generation models require a ∼30°C decrease in mantle temperature toward the AAD to account for the crustal thickness trend. Significant changes in axial morphology accompany small‐scale variations in crustal thickness, consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature, melt supply, and cooling from hydrothermal circulation. Layer 3 thins by 3.0 km as layer 2 thickens by 1.4 km between segments P1 and T, reflecting the eastward decrease in melt supply and increase in melt lens depth. The trade‐off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material.

Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland

Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0–7.2 km·s − 1 ) in the lower crust of the shelf edge. The thin crust of the continent–ocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments. We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust rotated upwards toward to a 50° dip without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward. With ongoing extension, some high-angle normal faults may have rotated to a sub-horizontal orientation, which would explain the lack of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different amounts of extension of continental crust in the distal margins. The hot rifting model localizes strain much more rapidly, leaving narrow zones of extended continental crust, and it produces a relatively large amount of melt (> 30%) in the final stages of rifting. Continental breakup may occur rapidly in hot lithosphere (< 5 Myr). On the other hand, a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. The cold lithospheric model requires about 23 Myr of extension before continental breakup, and it predicts much less melting in the mantle (13%). The long rift duration, wide zones of thinned continental crust, and small amount of magmatism make the cold rifting model the most applicable to Newfoundland–Iberia rift.

New insights into the origin and distribution of the DUPAL isotope anomaly in the Indian Ocean mantle from MORB of the Southwest Indian Ridge

We report new Sr, Nd, and Pb isotope data for Mid‐Ocean Ridge Basalts (MORB) dredged between 35 and 69°E along a ∼4100 km section of the Southwest Indian Ridge (SWIR), which is one of the slowest spreading ridges of the global mid‐ocean ridge system (full rate of 16 mm yr−1). The SWIR appears isotopically more heterogeneous than other mid‐ocean ridges over similar length scales and comparable sampling density. Isotopic variations are generally independent of ridge segmentation, and the degree of heterogeneity decreases from west to east. This decrease in heterogeneity correlates with the observed increase in axial depth and decrease in crustal thickness, commonly attributed to decreasing mantle potential temperature. Data for the easternmost SWIR in the vicinity of the Indian Triple Junction confirm that these lavas are isotopically distinct from those of the Triple Junction, the Southeast Indian Ridge, and the Central Indian Ridge, reflecting the presence of an isotopic boundary over an along‐axis distance of less than 78 km. Results for lavas from the 39–41°E section extend the isotopic range of MORB to the lowest 206Pb/204Pb values (to 16.58) yet found among oceanic islands and spreading centers worldwide and confirm their extremely anomalously high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb and low 143Nd/144Nd. This extreme Indian signature does not bear any isotopic affinities with recent products of the nearby Marion hot spot. Furthermore, the presence of old subducted marine sediment as a source for the low 206Pb/204Pb component is excluded, as no sediment group has the appropriate parent‐daughter Th/Pb and U/Pb characteristics to yield such compositions, not even when dewatering subduction processes are considered. Incorporation of old subduction‐modified mantle into the MORB source does not yield high enough 207Pb/204Pb and 208Pb/204Pb to explain the features of lavas from the 39–41°E section. The unusual isotopic attributes of this section (low 206Pb/204Pb and 143Nd/144Nd and high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb) are best explained by the presence of stranded lower continental crust embedded in the upper mantle. This component is also inferred to be present in MORB from other spreading centers in the Indian Ocean.

Tectonic consequences of Martian dichotomy modification by lower-crustal flow and erosion

The Martian hemispheric dichotomy shows compressional features a few hundred kilometers south of the dichotomy boundary, and contemporaneous extensional features along the boundary. Two processes that may have contributed to the observed deformation are lateral lower-crustal flow and erosion. Crustal flow causes a decrease in crustal thickness in the south, and an increase in the north. The result is compressional surface stresses in the highlands and extensional stresses around the dichotomy boundary. The magnitude, location, and timing of the predicted stresses are consistent with the observed tectonic features and strongly suggest that moderate lateral flow occurred. For the nominal heat flux and rheology, sufficient lateral flow is generated for a mean crustal thickness of 70 km. Erosion generates extension in the south and compression in the north, opposite to what is observed. Either erosion only resulted in local, as opposed to regional, distribution of highland material, or the rigidity of the lithosphere was sufficiently high at the time of erosion that significant stresses did not develop.

Geothermal Investigations and Hydrocarbon Potentialities of the Northwestern Desert, Egypt

bottom hole temperature measurements at 124 wells distributed in the Northwestern Desert and the Nile Delta of Egypt were used for obtaining the geothermal gradient values at each well. Heat flow values over the studied area were found to range between 43-110 Wm -2 . There is a regional decrease in geothermal gradient values northward, which was interpreted as due to thinning of the con- tinental crust toward the Mediterranean Sea. This trend apparently correlates with decrease in crustal thickness from about 33 km at the southern part of the studied area, to 24 km beneath the Mediterranean coast. These geothermal gradient values were used for constructing the upper and lower limit of the expected hydrocarbon window in the area. The thickness of the hydrocarbon windows reaches up to 3 km. Maps representing the depth of these isotherms and the isopach map for the hydrocarbon window were correlated with known sedimentary basins in the Paleozoic. Fairly good correlation was found especially for thickness of hydrocarbon window values > 2 km with the dis- tribution of these basins. Geothermal trend analysis reveals the pres- ence of the following main trends: N45o-65oE (Syrian arc, Pelusium and Siwa-Alexandria), N25oW (Red Sea and Gulf of Suez) and N80oE (Tethyan Sea). These lineaments appear to be caused primarily by basement structures and overlying sediments. They affect the hydro- carbon oil window of the area. Parts of the study area which are not explored yet but show thick hydrocarbon window (2.5-3 km) are con- sidered as target areas for future exploration programs.

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.

Early Paleozoic transform-margin structure beneath the Mississippi coastal plain, southeast United States

A geophysical transect across the central Gulf of Mexico coastal plain shows that the early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the crystalline crust decreases abruptly across the margin over a distance of ∼50 km, from 35 km beneath the Black Warrior foreland basin to 10 km beneath the Ouachita fold-and-thrust belt. This abrupt decrease in crustal thickness is similar to modern transform margins, but very different from most rifted margins, which display much more gradual transitions in crustal thickness. The geophysical data indicate an absence of synrift intrusive and volcanic rocks, underplated mafic rocks at the base of the crust, and abnormally thick oceanic crust adjacent to the margin. The lack of these features is also characteristic of modern transform margins. Combined with transects across the margin farther west, the data confirm previous suggestions that the central Gulf of Mexico coastal plain overlies an ∼800-km-long transform segment of the late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas to southeast Alabama.

Geophysical consequences of the Cordillera–Craton thermal transition in southwestern Canada

There is a pronounced increase in heat flow and lithosphere temperatures across the transition from the stable North American Craton to the southeastern Canadian Cordillera. The heat flow increases from 40–60 mW m −2 in the Craton to 80–100 mW m −2 in the Cordillera. There are numerous reliable heat flow data in the Cordillera but measurements in the Western Canada Sedimentary Basin overlying the adjacent Craton are in petroleum exploration wells with inherent low accuracy and are affected by hydrological effects. However, the deep thermal boundary is well defined based upon contrasts in several other temperature-sensitive geophysical parameters. The boundary is located 100 km west of the mountain front in the region of the Rocky Mountain Trench, and it must occur over a distance of less than 200 km. Temperatures in the deep crust and upper mantle are first computed from the heat flow and radioactive heat generation data. These temperatures are then compared to those estimated from the temperature dependence of uppermost mantle seismic velocity, Pn, and from xenoliths in kimberlites for the Craton, and in Tertiary volcanics for the Cordillera. Pn velocities decrease from about 8.2 km s −1 in the exposed Craton to 7.8 km s −1 in the Cordillera. The temperatures just below the Moho are 400–500°C for the Craton and 900–1000°C beneath the Cordillera based upon all three constraints. Two temperature-sensitive changes across the thermal boundary are examined. There is a westward decrease in crustal thickness from 40–50 km for the Craton to 32–34 km for the Cordillera with no significant change in average elevation nor in gravity. Isostatic balance is maintained by thermal expansion and density reduction in the high-temperature Cordillera lithosphere. The average temperature to a depth of 150 km is about 400°C higher for the Cordillera compared to the Craton. There also is a pronounced westward decrease in lithosphere strength and thus deformation style. In the Craton, the crust and upper mantle are very strong to at least 100 km depth. As a result, the foreland belt deep crust and upper mantle have remained undeformed during late Mesozoic to early Tertiary tectonics. Shortening occurred primarily in overlying sedimentary thrust sheets, mediated by high pore fluid pressures. In the Cordillera hinterland, the high temperatures result in the rheological lithosphere with significant strength being limited to the upper 10–15 km of the crust. Mesozoic–Cenozoic shortening deformation and subsequent extension included this whole thin lithosphere.

A model for the structure, composition and evolution of the Kenya rift

The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5–10 km in the south and 35–40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle P n velocities are low, being 7.5–7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40–45 km and 60–65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45–50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle P n velocity of 7.5–7.7 km/s and crustal thinning of 5–10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P n velocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal P n velocities of 8.0–8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15–20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45–50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20–30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5–10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35–40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

Thin Crust On the Flanks of the Slow-Spreading Southwest Indian Ridge

SUMMARY The Southwest Indian Ridge marks an end-member of mid-ocean ridge spreading rates, with a current full rate of only 13-15 mm a-'. Most thermal models of mid-ocean ridges suggest a decrease in crustal thickness at such slow spreading rates, because conductive heat loss from the upwelling asthenospheric mantle decreases the volume of melt generated by decompression. Seismic measurements of the thickness of crust formed at very slow-spreading ridges are sparse. We have reanalysed data from a twoship, split spread seismic refraction experiment conducted in 1962 on the southern flank of the Southwest Indian Ridge (Francis & Raitt 1967). We used synthetic seismograms to model amplitude variations, which were carefully recorded by the original investigators. 1- and 2-D modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust, with an unusually high velocity of -6.0 km s--' near the top of the crust, increasing to -7.0 km s-' just above the Moho, and a crustal thickness of 5 km. Converted shear waves yield a Poisson's ratio of 0.30+0.01 in the crust, which is intermediate between values for gabbros and serpentinized upper mantle. The crustal thickness is consistent with a passive mantle upwelling model of melt generation at mid-ocean ridges. The unusual velocity structure may indicate the presence of gabbroic rocks near the seabed, unroofed by extension.

Crustal structure and tectonics of the southeastern Canadian Cordillera

The crustal structure of the Omineca (OB) and Foreland (FB) belts of the southeastern Canadian Cordillera are interpreted from the inversion of seismic refraction/reflection travel times and amplitudes, and modeling of the Bouguer gravity data from a 350‐km east‐west wide‐angle profile. The main features of the resultant velocity and density models include (1) a low average crustal velocity of 6.2 km/s, (2) variable upper crustal velocities (5.6–6.3 km/s) within the FB and Purcell Anticlinorium as compared to farther west in the OB (6.1–6.2 km/s), (3) a midcrustal (∼20 km depth) 0.4–0.5 km/s velocity increase within the OB, (4) eastward crustal thickening from 35 to 42 km over 80 km distance beneath the OB‐FB boundary, (5) the Slocan Lake fault (SLF) dipping east at 15–20° to at least 35 km depth, (6) decreased lower crustal velocities (6.7–6.4 km/s) across the SLF from the OB into the FB, (7) increased uppermost mantle velocities (7.9–8.0 km/s) from the OB into the FB, and (8) an average crustal density increase of 40 kg/m3 from the OB into the FB. Lower velocities, higher conductances, and increased densities within the upper crust (0–10 km depth) of the Purcell Anticlinorium appear to be associated with Middle and Upper Proterozoic rift‐related metasediments and gabbroic dikes (35–55% by volume) in contrast to the mainly Mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the OB. Elevated crustal temperatures within the OB (a high heat flow province) are responsible for low‐velocity gradients to 20 km depth. A more mafic lower crust beneath the OB (inferred from higher velocities) and the westward decrease in crustal thickness are interpreted as resulting primarily from Middle and/or Late Proterozoic extension and rifting of the craton. Penetration of the SLF into the lower crust may be controlled by lateral strength contrasts at the edge of the craton. Farther west, a midcrustal strength contrast (a velocity boundary at 20–25 km depth) may have acted as a regional detachment zone during compressional and extensional tectonic episodes.

Segmentation and rift propagation at the Valu Fa Rridge, Lau Basin: Evidence from gravity data

Gravity and bathymetry data from a detailed ship‐based survey of a 40‐km length of the Valu Fa ridge allow an analysis of the fine‐scale segmentation of this young, back arc spreading center, where the primary morphological segmentation takes the form of small, overlapping spreading centers (OSC) separating ridge segments typically 30 km long. The gravity survey covers one complete segment and the overlap at its northern end and part of the overlap at its southern end. The mantle Bouguer anomaly shows a small (&lt;5 mGal) negative anomaly associated with the ridge axis along much of its length. However, the northern OSC is characterized by a larger (&gt;10 mGal), subcircular negative anomaly. The pattern of anomalies implies that the northern OSC is the site of particularly vigorous magmatism, with either increased crustal thickness or density reduction in the mantle (or both) accounting for the observed gravity low. This interpretation confirms the results of seismic analyses of this spreading center, which also imply that the OSC is the site of enhanced magmatism. The observations appear to contrast with widely accepted models of segmentation at mature, ocean basin spreading centers, where morphological offsets are believed to coincide with a relatively weak supply of magma. On a longer (130 km) gravity profile crossing the ridge, the mantle Bouguer anomaly shows a large (∼50 mGal), longer‐ wavelength (∼100 km) positive anomaly centered on the spreading axis. This can be modeled by an increase in crustal density and/or a decrease in crustal thickness over a 40‐ to 50‐km‐wide area centered on the axis. This is consistent with recent models for the evolution of the basin, involving recent southward propagation of seafloor spreading through preexisting, but stretched, thinned, and intruded island arc lithosphere.

Numerical simulation of high- and low-frequencyLg-wave propagation

SUMMARY Lg waves are sometimes extinguished when traversing geological structures such as mountain ranges or grabens. Previous investigations have applied numerical, full-waveform simulations at relatively low frequencies (on the order of 1 Hz) to investigate the relationship of different structural models and the amplitude anomalies of the Lg waves. These studies often fail to link specific known structures to the amplitude anomalies. However, the observed waveforms also often contain data of significantly higher frequency than the full waveform simulations. We compare such low-frequency simulations computed with the boundary-integral-equation (BIE) technique with synthetic seismograms generated by dynamic ray tracing (DRT). Beginning with a plane-layered model of the crust in central France, we show that the general characteristics of the Lg wave train over distances ranging from 100 km to around 500 km can be modelled by computing a relatively small number of rays reflected between the free surface and the Moho. Matching the details of the waveforms is more difficult, but can be improved by the addition of reflections between other interfaces in the model. Considering a one-layer model of the crust with a step-like change in Moho depth that leads to a rapid decrease in crustal thickness, we show again that the BIE and DRT solutions are very similar in character. Significantly, both solutions predict that the Lg wave will not be extinguished on traversing the Moho step. Finally, we consider a more complete and detailed model of the Pyrenees. Though it is more difficult to match exactly the ray-and integral-equation solutions, the general trends continue to be the same, and the Lg waves are still predicted to cross the tectonic structures associated with the mountains. Therefore, it appears that observed amplitude changes must be caused by something other than the general structure of the Pyrenees, such as scattering by smaller scale features within the lower crust.