Nicky White

Origin of the Betic‐Rif mountain belt

In recent years, the origin of the Betic-Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present-day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic-Rif orocline during the early to middle Miocene.

Normal faulting in the upper continental crust: observations from regions of active extension

Abstract Observations of present-day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins. The most obvious manifestation of active extension is normal faulting in earthquakes. Earthquake foci are mostly confined to the upper (seismogenic) continental crust, whose thickness imposes a scale on the observed deformation. Large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer. Large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30–60°: dips significantly gentler than 20° have not been observed in fault plane solutions of large earthquakes. Large seismographic normal faults are approximately planar in cross-section and cut through the base of the upper seismogenic layer, rotating about a horizontal axis as they move. A reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound (the ‘domino model’). Such faults are rarely continuous for more than 15–20 km, but commonly change strike or step in an en echelon fashion. This segmentation may occur on a scale controlled by the thickness of the seismogenic layer, and is an important influence on sedimentation and drainage. These observations are common to large seismogenic normal faults from a variety of tectonic settings, and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension, which can vary widely in nature and magnitude. Small earthquakes, which move faults that are small compared with the seismogenic layer thickness, show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults. In some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology. Faults in the sedimentary cover may be strongly curved in cross-section, requiring their hangingwalls to deform internally. Estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation. Estimates made in this way are often very non-unique and prone to large errors. We postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis.

Measuring the pulse of a plume with the sedimentary record

Magmatic underplating associated with mantle plume activity is an important mechanism for driving regional surface uplift and denudation of large portions of the continents,. Such uplift occurs rapidly because substantial volumes of basaltic melt are added to the crust over geologically short periods of time (1–10 Myr), and can lead to large amounts of clastic sediment being shed into surrounding basins. An intensively studied example of this process occurred in the North Sea basin during the Palaeogene period, where discrete pulses of deposition were triggered when sands were remobilized downslope from the shelf by turbidity currents and debris flows as a result of episodic changes of relative sea level. Here we correlate the timing of these sediment pulses with the timing of surface uplift inferred to have been caused by episodic magmatic underplating on the continental shelf of northwestern Europe. This magmatism was related to activity of the Iceland plume, suggesting that individual pulses of sedimentation provide a potentially sensitive measure of plume activity, and so may be used to resolve time-dependent fluctuations in mantle plume activity predicted by theoretical studies of mantle convection.

The relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls

Abstract We derive an analytical expression that relates the shape of a fault in cross-section to the shape of the bedding horizons in its hanging wall block. The expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout. Although this paper concentrates on normal faults, the expression is equally valid and applicable to thrust faults. The direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry. There is no reason to believe that the simple shear occurs on vertical planes, as is commonly assumed in graphical approaches to this problem, and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts. Similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account. For a planar normal fault such compaction may result in the development of a hanging wall syncline.

Sedimentary basin inversion caused by igneous underplating: Northwest European continental shelf

A considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the British Isles underwent permanent exhumation during the Tertiary. The most dramatic indicator of this process is the present-day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of Scotland. Any explanation of this observation must take into account the fact that the entire region has very small,-long-wavelength, free-air gravity anomalies. This important constraint implies either that the crust has been thickened or that low-density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the North Atlantic. Tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to Alpine mountain building. However, to remove ∼3 km of sedi- mentary rock from a basin ∼100 km wide requires >15 km of shortening. Minor Tertiary compression is observed all over the continental shelf, but nowhere is it sufficient to account for the required amount of uplift and erosion. In addition, exhumation dramatically increases from south to north, whereas the observed compression decreases markedly in the same direction. At the beginning of the Tertiary, rifting associated with the initiation of the Iceland plume generated substantial volumes of melt. Inversion of rare-earth- element concentrations of MgOrich igneous rocks suggests that a minimum of ∼5 km of melt was produced beneath at least part of the continental shelf. We infer that much of this melt remains trapped within the lithosphere, presumably close to the Moho, which acted as a density filter. Such underplating will generate rapid uplift.

Relations between normal-fault geometry, tilting and vertical motions in extensional terrains: an example from the southern Gulf of Suez

Abstract Earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern Gulf of Suez are approximately planar, with dips of 30–40°, from the surface to around 10 km depth. These faults, and the blocks they bound, appear to rotate about a horizontal axis as they move, causing tilting. This tilting is seen both in young vertical movements of the coastline, such as raised beaches and marshlands, and in the distribution of Middle Miocene marine rocks, which are uplifted to elevations of 400–500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens. The absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw-tooth topography on a regional subsidence caused by crustal and lithospheric thinning. The observations required for this simple model are: the present day fault dip, the amount of tilting and the width of the rotating blocks. The virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents.

Neogene overflow of Northern Component Water at the Greenland‐Scotland Ridge

In the North Atlantic Ocean, flow of North Atlantic Deep Water (NADW), and of its ancient counterpart Northern Component Water (NCW), across the Greenland-Scotland Ridge (GSR) is thought to have played an important role in ocean circulation. Over the last 60 Ma, the Iceland Plume has dynamically supported an area which encompasses the GSR. Consequently, bathymetry of the GSR has varied with time due to a combination of lithospheric plate cooling and fluctuations in the temperature and buoyancy within the underlying convecting mantle. Here, we reassess the importance of plate cooling and convective control on this northern gateway for NCW flow during the Neogene period, following Wright and Miller (1996). To tackle the problem, benthic foraminiferal isotope data sets have been assembled to examine δ13C gradients between the three major deep water masses (i.e., Northern Component Water, Southern Ocean Water, and Pacific Ocean Water). Composite records are reported on an astronomical timescale, and a nonparametric curve-fitting technique is used to produce regional estimates of δ13C for each water mass. Confidence bands were calculated, and error propagation techniques used to estimate %NCW and its uncertainty. Despite obvious reservations about using long-term variations of δ13C from disparate analyses and settings, and despite considerable uncertainties in our understanding of ancient oceanic transport pathways, the variation of NCW through time is consistent with independent estimates of the temporal variation of dynamical support associated with the Iceland Plume. Prior to 12 Ma, δ13C patterns overlap and %NCW cannot be isolated. Significant long-period variations are evident, which are consistent with previously published work. From 12 Ma, when lithospheric cooling probably caused the GSR to submerge completely, long-period δ13C patterns diverge significantly and allow reasonable %NCW estimates to be made. Our most robust result is a dramatic increase in NCW overflow between 6 and 2 Ma when dynamical support generated by the Iceland Plume was weakest. Between 6 and 12 Ma a series of variations in NCW overflow have been resolved.

V‐shaped ridges around Iceland: Implications for spatial and temporal patterns of mantle convection

[1] V-shaped lineations in the bathymetry and in the free-air gravity field surrounding Iceland result from crustal thickness variations caused by temporal variations in melt production rate at the Mid-Atlantic Ridge. We have studied the record of V-shaped ridges in the basins surrounding Iceland by plotting the shortwavelength component of the gravity field in terms of age versus distance from Iceland. The V-shaped ridge gravity signal is obscured by crustal segmentation and by sediment more than 1–2 km thick. The best V-shaped ridge record is found in the unsegmented part of the Irminger Basin, where Oligocene-Recent Vshaped ridges occur with a primary periodicity of 5–6 Myr and a secondary periodicity of 2–3 Myr. Vshaped ridge records from the Iceland Basin and from east of the Kolbeinsey Ridge to the north of Iceland correlate with the record from the Irminger Basin but are less complete. A record of uplift of the GreenlandIceland-Faroes Ridge based on paleoceanographic data is correlated with the gravity record of V-shaped ridges. There is less decisive evidence for V-shaped ridges in crust of Eocene age. The observation that Vshaped ridges propagate up to 1000 km from Iceland is compatible with a model in which the Iceland Plume head spreads out from the plume stalk below a depth of � 100 km, as suggested by geochemical arguments and studies of mantle rheology. Time-dependent flow in the plume head probably results from time-dependent flow up the plume stalk from deep below Iceland. These pulses may have triggered jumps in location of the spreading axis observed in the Icelandic geological record.

Present and past influence of the Iceland Plume on sedimentation

Abstract The Cenozoic development of the North Atlantic province has been dramatically influenced by the behaviour of the Iceland Plume, whose striking dominance is manifest by long-wavelength free-air gravity anomalies and by oceanic bathymetric anomalies. Here, we use these anomalies to estimate the amplitude and wavelength of present-day dynamic uplift associated with this plume. Maximum dynamic support in the North Atlantic is 1.5–2 km at Iceland itself. Most of Greenland is currently experiencing dynamic support of 0.5–1 km, whereas the NW European shelf is generally supported by <0.5 km. The proto-Iceland Plume had an equally dramatic effect on the Early Cenozoic palaeogeography of the North Atlantic margins, as we illustrate with a study of plume-related uplift, denudation and sedimentation on the continental shelf encompassing Britain and Ireland. We infer that during Paleocene time a hot subvertical sheet of asthenosphere welled up beneath an axis running from the Faroes through the Irish Sea towards Lundy, generating a welt of magmatic underplating of the crust which is known to exist beneath this axis. Transient and permanent uplift associated with this magmatic injection caused regional denudation, and consequently large amounts of clastic sediment have been shed into surrounding basins during Cenozoic time. Mass balance calculations indicate agreement between the volume of denuded material and the volume of Cenozoic sediments deposited offshore in the northern North Sea Basin and the Rockall Trough. The volume of material denuded from Britain and Ireland is probably insufficient to account for the sediment in the Faroe-Shetland Basin and an excess of sediment has been supplied to the Porcupine Basin. We emphasize the value of combining observations from both oceanic and continental realms to elucidate the evolution of the Iceland Plume through space and time.

Generating melt during lithospheric extension: Pure shear vs. simple shear

The uniform stretching model has been used to calculate the volume and composition of melt generated by adiabatic decompression during extension of the continental lithosphere. The consequences of this approach for melt generation by litbospheric simple shear were investigated. The results show that, given an initially planar detachment fault, it is extremely difficult to generate melt from the asthenosphere, under either normal or elevated asthenospheric potential temperatures. Our conclusion is independent of the initial dip of the detachment fault and also holds when the initial dip in the lithospheric mantle is double that in the crust. In the North Sea, lithospheric simple shear fails to account for the existence and location of magmatism. These conclusions may also apply, more generally, to passive margin development.