Extinct Spreading Ridge Research Articles

Gravity anomalies over extinct spreading centers: A test of gravity models of active centers

Gravity fields over active mid‐ocean ridges display long wavelength free‐air gravity highs with superimposed short wavelength anomalies over the spreading axis. These axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30–70 mGal and wavelengths of 40–60 km but decrease in amplitude and wavelength with increasing spreading rate until, at fast spreading centers, the anomalies are positive (10–20 mGal) with wavelengths of about 20–30 km. Similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo‐spreading rate dependency, suggesting that a large component of the anomaly is not dynamically maintained. Calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here, even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows. The amplitudes of the residual lows at slow paleo‐spreading rates are 25–45 mGal with half widths of about 20–45 km, and decrease with increasing spreading rate to about 5 mGal amplitude and 10–15 km half‐width over fast paleo‐spreading ridges. The amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers. Most previously proposed models of mid‐ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid‐ocean ridges. This research provides a further constraint on geometric models of accretion, as these models predict vastly different gravity fields. Results indicate that a frozen‐in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis. This conclusion is consistent with the presence of a low‐density gabbroic root situated at shallow mantle depths beneath the ridge axis. This root tends to narrow with increasing spreading rate, producing the required residual gravity field variation.

Seismogenesis in Eastern Canada

Abstract The pattern of seismicity in eastern Canada depends on the presence of weak zones from previous major tectonic orogenies and how these weak zones are reactivated by local and regional stress fields and geophysical processes. Within the Canadian Shield, away from seismotectonic trends, there is a low level of seismicity and earthquakes tend to be small, less than M5. However, along seismically active trends, earthquakes as large as M7 have occurred. The seismotectonic features fall into four main categories: positive (uplift) continental basement linears; grabens formed by old plate separation; passive rifted margins offshore; and extinct spreading ridges. Two of the positive seismotectonic trends are the Boothia Uplift-Bell Arch that transects the northeastern part of the craton and northeastern Baffin Island, where the effects of postglacial rebound on the upper crustal stress field are the most pronounced. The St. Lawrence Valley (and interconnecting grabens) is a seismically active graben system that contains the most seismically active region (the Charlevoix zone) in eastern Canada. The extinct spreading ridge along the Labrador Sea and the Mesozoic rifted margin along Baffin Bay and Labrador Sea contain clusters of moderate seismicity. There are diffuse zones of moderate seismicity over some geological provinces (e.g. Central Metasedimentary Belt in western Quebec) apart from major tectonic features, a confined seismic zone (within an intrusion) in the Miramichi region and seismicity at the intersection of faults in northern Ontario. In the Nahanni region, which is situated near the boundary between the northeast Cordillera and the Interior Platform, the commencement of a noteworthy earthquake sequence with magnitude up to Ms 6.9 indicates considerable stress-strain build-up over a large area. There is an anticline in the epicentral area that is bounded by thrust faults and mountain ranges. In order to enhance our understanding of causative factors of current seismicity, it is necessary to determine in greater detail the tectonic forces and geophysical processes that are reactivating pre-weakened faults along the seismotectonic trends and over broad, diffuse seismogenic regions. Some of these factors are the rate of stress build-up, stress concentration at the intersection of faults and between mountain ranges, residual stress, the role of pore fluids, individual block movement, whether this movement is due to postglacial rebound or to other underlying viscoelastic phenomena and the rate of sediment deposition along the continental slope. Paleoseismicity is useful not only for the reconstruction of old large earthquakes but also for providing insight as to why surface fault offsets have not been observed in regions where large earthquakes (and associated high rate of microseisrnicity) have occurred within the past several hundred years.

A possible explanation of gravity anomalies over mid‐ocean ridges

Gravity fields over the crests of slow spreading mid‐ocean ridges are characterized by small amplitude free air gravity anomaly lows (30–70 mGal) with wavelengths of 40–60 km, flanked by smaller gravity highs. In general, the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until, at fast spreading ridges (> 5 cm/yr), free air gravity highs (10–20 mGal) are observed. Previous explanations of these anomalies, which involve uncompensated ridges, thermally expanded ridges and elastic plate compensation models, are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest. In particular, the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid‐ocean ridge basalts, which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more. In addition, similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers. Geometrical models of mid‐ocean ridges derived from detailed structural and petrologic studies of the Bay of Islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates. Modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly: the ridge topography, the crustal level magma chamber, the configuration of the isotherms near the ridge crest, and a low‐density, partially gabbroic cumulate root extending vertically into the mantle beneath the ridge. Systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers. Major contributions are found to come from the topography and from the lowdensity root, which broadens with slower spreading rates, resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate. At extinct spreading centers, contributions from the magma chamber, isotherms, and topography gradually diminish with time once the ridge ceases to spread, whereas the low‐density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field. Seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the Bay of Islands ophiolite and the gravity analysis presented here.

Evidence for two fossil spreading ridges in the southeast Pacific

Two different sets of extinct spreading ridge segments (Pacific-Farallon and Pacific-Nazca) are identified from bathymetric and magnetic data in the southeast Pacific. The older set is composed of several topographically subdued segments of a fossil ridge (Pacific-Farallon) that trends northwest, parallel to and about 175 to 550 km distant from the younger side of anomaly 7 (26 m.y. B.P.) The younger set trends north-northeast subparallel to the present East Pacific Rise (EPR), at an angle of about 45° to the older fossil ridge. The younger ridge forms the Galapagos Rise and includes a set of west-northwest–trending fracture zones. This was the original site of the EPR before it jumped westward to its present position. Oligocene magnetic anomalies (7–12) were generated at the older northwest-trending ridge rather than from the younger northeast-trending Galapagos Rise. The change in ridge orientation from the older Pacific-Farallon to the younger Pacific-Nazca direction is likely to be associated with the birth of the Cocos-Nacza spreading ridge system, about 20 to 25 m.y. ago.

Plate Convergence and Accretion in Taiwan-Luzon Region

During the middle and late Miocene the northern continuation of the Luzon volcanic arc approached the Asian continental margin through subduction of oceanic crust on its western side. Collision deformation began in late Miocene time because middle Miocene marine fossils are present in the slate belt of Taiwan. South Taiwan, the crust-crust contact between the Philippine and Asia plates occurs at the large negative free-air gravity anomaly over the deformed accretionary wedge of the Manila Trench. The deformed sediment wedge has been uplifted to shallower depths toward Taiwan and forms part of the southernmost peninsula of Taiwan. The pattern of gravity anomalies suggests that the Luzon arc has underthrust the Central Range of Taiwan, to at least 40 km west of the longitud nal valley. Thus the west edge of the Philippine plate at Taiwan appears to be chisel shaped in cross section. Recent transcurrent faulting along the longitudinal valley is being superimposed on this collisional suture. The Ilan plain is a landward extension of the Okinawa Trough and forms the southwestern end of that extensional back-arc structure of the Ryukyu Island arc. The crust-crust contact between the Ryukyu arc and the Philippine plate occurs at the negative free-air anomaly of the accretionary wedge. On the west this contact abuts the northward end of the longitudinal valley fault. We speculate that the Luzon arc convergence zone in the region of Taiwan has been sealing southward and that concomitantly a new plate boundary may be developing on the east side of Luzon and progres ing northward. Magnetic anomalies trending N70°E in the China basin west of Luzon are associated with the crest of an extinct spreading ridge which formed the deep part of the China basin by separating older but shallower crust. Magnetic anomalies in the westernmost Philippine basin suggest the existence of north-trending anomalies. The consistency of the skewness, the small amplitude factor, and to a lesser extent the bathymetric cross sections in the westernmost Philippine basin compared with those of the Central Basin Ridge region on the southeast support the view that the Gagua Ridge may be an extinct spreading center and that the adjacent topography was formed by seafloor-spreading.

Origin of the West Philippine Basin

Several alternative modes of development for the West Philippine Basin have been suggested during the past few years. The results of Deep Sea Drilling Project leg 31 indicate that none of these speculations is correct and that the basin probably formed by complex pulses of extension during the early Tertiary. A striking lineament which trends northwest across the basin, variously called the Central Basin Fault or Philippine Ridge, had previously been identified as an extinct spreading ridge of Mesozoic to early Tertiary age. On the contrary, this feature is probably a strike slip fault, possibly with a left-lateral displacement.