New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr(-1), though their characteristics are commonly found at rates up to approximately 20 mm yr(-1). The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change.

[1] We argue that key features of thrust earthquake triggering, inhibition, and clustering can be explained by Coulomb stress changes, which we illustrate by a suite of representative models and by detailed examples. Whereas slip on surface-cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress on some nearby zones, particularly above the source fault. Blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks. Short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults. We calculate that during a progressive thrust sequence in central California the 1983 M w = 6.7 Coalinga earthquake brought the subsequent 1983 M w = 6.0 Nuñez and 1985 M w = 6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to Coulomb failure. The idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events, in agreement with findings of prior investigations. Subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust-faulting events on the periphery of the seismic rupture and its downdip extension. These features are evident in aftershocks of the 1957 M w = 9.1 Aleutian and other large subduction earthquakes. We further examine stress changes on the rupture surface imparted by the 1960 M w = 9.5 and 1995 M w = 8.1 Chile earthquakes, for which detailed slip models are available. Calculated Coulomb stress increases of 2-20 bars correspond closely to sites of aftershocks and postseismic slip, whereas aftershocks are absent where the stress drops by more than 10 bars. We also argue that slip on major strike-slip systems modulates the stress acting on nearby thrust and strike-slip faults. We calculate that the 1857 M w = 7.9 Fort Tejon earthquake on the San Andreas fault and subsequent interseismic slip brought the Coalinga fault $1 bar closer to failure but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 M w = 7.3 Kern County shock but inhibited slip on the left-lateral Garlock fault, which has not ruptured since 1857. We thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales.INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7230 Seismology: Seismicity and seismotectonics; 7223 Seismology: Seismic hazard assessment and prediction; 8164 Tectonophysics: Stresses-crust and lithosphere; 8123 Tectonophysics: Dynamics, seismotectonics; KEYWORDS: earthquake triggering, thrust and subduction earthquakes, stress transfer, fault interaction, blind thrust, San Andreas Fault Citation: Lin, J., and R. S. Stein (2004), Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearb...

Abstract. In a study of geological and geophysical data from the Mid-Atlantic Ridge, we have identified 17 large, domed edifices (megamullions) that have surfaces corrugated by distinctive mullion structure and that are developed within inside-corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and limited sampling has recovered gabbros and serpentinites, suggesting that they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphic core complexes in scale and structure, and they may originate by similar processes. The megamullions are interpreted to be rotated footwall blocks of low-angle detachment faults, and they provide the best evidence to date for the common development and longevity (-1-2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment fault probably occurs when a spreading segment experiences a lengthy phase of relatively amagmatic extension. During these periods it is easier to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift-valley lithosphere; slip on the detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serpentinization. At the segment center, minor, episodic magmatism may continue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism becomes robust enough to reach the segment end, weaken the axial lithosphere, and promote inward fault jumps there. This mechanism may be generally important in controlling the longevity of normal faults at segment ends and thus in accounting for variable and intermittent development of inside-corner highs.

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Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1-2 Myr along the northern continent-ocean boundary (COB). A southward ridge jump of 20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from 23.6 to 21.5 Ma. The terminal age of seafloor spreading is 15 Ma in the East Subbasin and 16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from 20 to 80 km/Myr, but mostly decreased with time except for the period between 26.0 Ma and the ridge jump (23.6 Ma), within which the rate was the fastest at 70 km/ Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and

Abstract. First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patems occur at these discontinuites and to constrain the processes that control the patterns. The results show that along their older, inside-comer sides, both firstand semond-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-comer sides of discontinuities has more normal thickness, regular fault patems, and common volcanic forms. These patterns are consistent with tectonic th'mning of crust at inside comers by lowangle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hang'rag wall of the detachment, is stripped from the inside-comer footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spread'rag at <25-30 mm/yr, that it may be intermittent at intermediate rates of 2540 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift 'into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection proffi'mg over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies 'indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmafic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can Ix: exhumed very rapidly, perhaps in as littic as 0.5 m.y. Togc•cr, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.

Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identifi ed on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%-50% of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.Keywords: mid-ocean ridge, detachment fault, megamullion, oceanic core complex, oceanic magmatism, numerical modeling. INTRODUCTIONThere is ongoing debate about the role of magmatism during normal faulting at mid-ocean ridges (MORs), particularly for very long-lived (~1-2 m.y.) normal faults, which we here term detachment faults. Plate separation at MORs is accomplished by both magmatic intrusion and slip on normal faults. Intrusion of dikes can occur when the difference between maximum and minimum stresses is lower than that required for fault slip, and intrusion further reduces the stress difference (Rubin, 1995). This implies that major normal faults should form only when there is little or no magma supply. Some numerical models also suggest that detachments can form during amagmatic extension if the lithosphere is thin and cohesion decreases with fault displacement (Lavier et al., 2000). These ideas are consistent with the observation that most detachment faults occur at the ends of spreading segments where melt supply is usually low compared to segment centers (e.g., Lin et al., 1990;Tucholke et al., 1998). Furthermore, large tracts of unusually smooth seafl oor, interpreted as successive fault surfaces, have been observed on the ultraslow-spreading Southwest Indian Ridge, where magmatism is extremely limited (Cannat et al., 2006).In contrast, some recent observations suggest that magmatism may be important during the evolution of detachment faults. Because of their long-lived slip, these faults can exhume tens of kilometers of lower ocean crust and upper mantle. To maintain isostatic equilibrium, the footwalls typically roll over (Buck, 1988) and form megamullions characterized by domed shapes and by large (to hundreds of meters), enigmatic, fault-surface corrugations as well as smaller mullions and striations that parallel fault-slip direction (Tucholke et al., 1998) (Fig. 1A). Three megamullions have been drilled, two on the Mi...

Stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases. The 1999 Hector Mine earthquake in southern California (magnitude 7.1) occurred only 20 km from, and 7 years after, the 1992 Landers earthquake (magnitude 7.3). This suggests that the Hector Mine earthquake was triggered in some fashion by the earlier event. But uncertainties in the slip distribution and rock friction properties associated with the Landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the Hector Mine hypocentre-with estimates varying from -1.4 bar (ref. 6) to +0.5 bar (ref. 7). More importantly, coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events. Here we present the results of a three-dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the Landers earthquake. Using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the Landers earthquake, our calculations suggest that lower-crustal or upper-mantle flow can lead to postseismic stress increases of up to 1-2 bar at the location of the Hector Mine hypocentre during this time period, contributing to the eventual occurrence of the 1999 Hector Mine earthquake. These results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard.