T44A-01 16:00h
A Synoptic Model of Ridge Transform Fault Seismicity
The size and location of large earthquakes on mid-ocean ridge transform faults (RTFs) can be better understood through a synoptic model that unites the rheology and geology with the seismicity of these faults. In the absence of ocean bottom seismometers on RTFs now is an appropriate time for developing synoptic models to guide up-coming deployments (e.g. NEPTUNE) and direct future investigations of earthquake processes on RTFs. Studies of earthquake focal depths and laboratory friction experiments suggest that the depth extent of oceanic earthquakes is thermally controlled, with the $600^{\circ}$C isotherm as the approximate lower limit. The relatively simple compositional and thermal structure of RTFs, together with the extensive data on high-temperature olivine deformation, allows us to construct a complete rheological model. This includes an aseismic mylonitic zone below the seismogenic layer and likely includes a significant amount of serpentine with velocity and temperature dependent frictional properties. Few large earthquakes occur on RTFs and these events are followed by very few aftershocks. On average, the area ruptured by the largest earthquake scales as the square-root of the area above the 600$^{\circ}$C isotherm. Faulting on RTFs is primarily aseismic, i.e. only $\sim$1/4 of the tectonic offset is accommodated by earthquakes and the remaining 3/4 occurs through steady aseismic creep and slow or silent earthquakes. This aseismic loading drives much of the seismicity and may contribute to the abundance of foreshocks that precede the larger RTF earthquakes. To better understand the physical processes that give rise to the observed seismic characteristics we must integrate our knowledge of the rheological and seismic aspects of RTFs. Our model predicts that RTFs are likely to be weak ($\Delta \sigma < 1$~MPa), but can support high stresses ($\sim$100~MPa) over a narrow depth range in the viscously deforming region beneath the seismogenic zone. We show that when the rheology is combined with the geology and seismicity observed for the Blanco Transform Fault on the Juan de Fuca Ridge, we can start to understand the spatial and size distribution of earthquakes along the fault and identify target locations and problems for future study.
T44A-02 16:15h
Seismotectonics of the Blanco Transform Fault Zone
We present the first detailed, long-term seismotectonic study of an entire oceanic transform fault system. The Blanco Transform Fault Zone (BTFZ) offshore Oregon is seismically active along its entire length and close to a dense broadband seismic network. This unique combination allows analysis of the frequent moderate sized earthquakes providing a seismotectonic framework unprecedented in accuracy and detail. We used regional broadband waveforms to determine the seismic moment tensor of 125 $M_{w}$ $\geq$ 3.8 events for the 1994-1998 period. Combined with 28 Harvard-CMTs, available for larger events since 1976, they represent the most complete earthquake source parameter data set for an oceanic transform fault. We relocated 144 larger earthquakes (M $\geq$ 5) from 1964 to 1998, which removes the bias of the routine locations that are consistently northeast of the BTFZ as defined by high-resolution bathymetry. The 350 km-long BTFZ is the Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Morphologically, the BTFZ consists of several transform segments separated by extensional step-overs. Relocation of the strike-slip and normal faulting events agrees excellently with bathymetry. Source depths are commonly $\leq$6 km near the Gorda and Juan de Fuca ridges; in BTFZ's central part and along eastern Blanco Ridge depths are $\leq$9 km. Most events have strike-slip mechanisms. We found few, generally small events along short transforms west of Gorda Ridge and between the Cascadia and Surveyor depressions. The 150 km-long Blanco Ridge, east of Cascadia Depression, is the longest transform segment and relocations follow bathymetry closely. BTFZ's largest events ($M_{w}$ = 6.5) are located east of $\sim$$128\deg$W where Gorda Ridge-parallel abyssal hill topography is uplifted. West of $128\deg$W, seismicity is lower, events are slightly shallower and large events nucleating in the eastern part possibly rupture into this western part. Seismic moment release along Blanco Ridge accounts for most of the 5.6 cm/yr plate motion rate. Transform motion dominates along BTFZ's 100 km-long western part from Surveyor Depression to Juan de Fuca Ridge. Relocations are more widely spread than in other areas consistent with its complex morphology. Subtle but consistent slip-vector differences suggest three faults strands are possibly active, one along the north wall of West Blanco Depression (WBD), another that runs obliquely through the WBD to the Juan de Fuca Ridge tip, and a third along WBD's south wall. The complex faulting pattern is possibly related to slight plate motion adjustments. We found no normal faulting events near the basins at BTFZ's western end. The average $291\deg$ slip-vector azimuth of all strike-slip events is parallel to the $290\deg$ Pacific-Juan de Fuca motion direction. T-axes of normal faulting events in the Cascadia Depression are rotated $\sim$$45\deg$ relative to all strike-slip and the normal events near the Gorda and Surveyor depressions. A $45\deg$ rotation is generally observed for transform-ridge systems suggesting Cascadia Depression is a short spreading ridge while Gorda and Surveyor depressions are pull-apart basins. Our detailed analysis of the BTFZ suggests oceanic transform fault zones have complexities comparable to their continental counterparts.
T44A-03 16:30h
Seismic Slip Rate and Effective Seismic Thickness for Oceanic Transform Faults Bounding the Juan de Fuca
The oceanic transform faults which bound the Juan de Fuca plate system: the Revere-Dellwood-Wilson, Sovanco, Nootka, Blanco and Mendocino faults, have deformation rates as predicted by plate models and observed, indirectly using GPS. We examine the degree to which this slip is accommodated seismically. The earthquake rate, derived average slip rate, and the effective vertical seismic thickness have been examined for each fault. The moment release rates are estimated by integrating the moment contribution rate over the magnitude versus frequency of occurrence relation up to a maximum magnitude. Seismicity statistics are related to the rate of slip along a given fault from earthquakes, using the concept of seismic moment. There are significant sources of uncertainty, including: the incompleteness and limited history of the earthquake catalog, the variety of magnitude definitions which can only be related empirically, empirical moment-magnitude relations (and the effect of their stochasticity), uncertainty in fault lengths and the effective seismic thickness, the recurrence relation, the determination of maximum magnitude and how the recurrence relation is truncated at maximum magnitude. Nonetheless, this method has been used successfully to provide estimates of deformation in good agreement with those from plate models, assuming that only the crust is seismic. The similarity of the deformation estimates based on seismicity and on plate models, shows a remarkable consistency in these rates over a significant temporal range- from tens to millions of years. The least constrained parameter is the effective seismic thickness, thus the effect of a 2, 3, 6.5 and 10 km thick zone is investigated for each fault. Many believe that oceanic transform faults are fundamentally different from continental transform faults since it appears that slip cannot be accommodated by observed seismicity alone and hence seismic efficiency is deemed to be low with considerable aseismic slip. However, this finding is tied to the selection of an effective seismic thickness of the order of 10 km, based largely on poorly constrained earthquake depth solutions. The selection of a thin effective seismic layer of about 3 km involving only the lower oceanic crust can consistently explain most of the deformation in the region as being seismically accommodated. The upper mantle is inferred to be aseismic, which is consistent with evidence of its serpentinization beneath these faults. The upper crust has very high porosity and may be sufficiently fractured such that it cannot support significant earthquakes. These geological considerations lend support to the possibility that there is in fact a thin effective seismic layer and that deformation within this layer on these oceanic transform faults can be fully seismically accommodated.
T44A-04 16:45h
Deformation of Oceanic Lithosphere Near Slow-spreading Ridge Discontinuities
Transform and non-transform discontinuities that offset slow spreading mid-ocean ridges involve complex thermal and mechanical interactions. Effects of the truncated ridge are noticeable in the contrast between seafloor topography at inside corners and outside corners, along-axis variations in rift valley depth and crustal accretion, and distribution of earthquakes. At inside corners of ridge-discontinuity intersections, and along traces of discontinuities in older oceanic lithosphere, oceanic core complexes or mega-mullion structures are a rather common tectonic feature. In an attempt to understand deformation of oceanic lithosphere near ridge offsets, and conditions that may favor oceanic core complex formation, a three-dimensional thermo-mechanical model has been developed. The numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside corners than was possible in previous 3-D models. The initial suite of results presented here focuses on deformation when axial properties do not vary along-strike or with time. This shows the extent to which plate boundary geometry alone can influence deformation. We find that non-transform discontinuities are represented by a wide, oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments. This contrasts with predicted deformation near transform discontinuities, where initial orientation is maintained in time. The boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside corners near the ridge-transform intersection. Ridge-normal tensile stresses concentrate in line with the ridge tip in the older plate opposite from the discontinuity and high amplitudes are absent in the inside corners during the magmatic accretionary phase simulated by our models.
T44A-05 17:00h
Rapid Changes in Transform Fault Length and the Dynamic Evolution of the Southwest Indian Ridge Since 26Ma.
The dynamic evolution of transform faults and non-transform discontinuities (NTDs) over the past 26Ma along the Southwest Indian Ridge (SWIR) between the Gallieni and Melville Transforms (52\deg E to 46\deg E) provides fundamental insight into the growth and development of transform faults along the SWIR and other oceanic ridges in general. Between 54\deg 45'E and 58\deg 45'E along the SWIR and up to 30Ma (280km) from the ridge axis, bathymetric and magnetic data reveal constant full-spreading rates within the area, but the average half-spreading rates since 26Ma vary from highly ($\sim$2:1) asymmetric to symmetric. This extreme asymmetric spreading led to major changes in the offset lengths of NTDs and transform faults over time. In particular, since 26Ma a $>$50km long transform fault at 56\deg 30'E shrank to become an 8km offset NTD today, while at the same time the Atlantis II transform grew by over 70km to reach its current length of 199km. The data suggest that the longest transforms (the Gallieni, Atlantis II and Melville transforms) in the region are lengthening, while the intervening spreading-parallel offsets are shrinking. Between these $\sim$450km spaced large offset transform faults, the SWIR is therefore rotating to become increasingly orthogonal to plate-spreading direction. We infer that these changes in plate boundary geometry are in response to a $\sim$$40\deg$ CW change in plate-spreading direction at 40Ma and a lesser $10\deg$ CCW change at 19.5Ma.
T44A-06 17:15h
Flexural uplift of a Lithospheric Slab near the Vema Transform (Central Atlantic): Timing and Mechanisms
The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 11° N in the Atlantic. It rises abruptly about 137 km from the axis of the Mid Atlantic Ridge (MAR) in ~10 My old crust and runs continuously up to ~ 25 My old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 My; thus, the uplift of the VTR must have ended roughly 10 My. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere. Multibeam data show that the MAR-parallel seafloor fabric south of the VTR shifts its orientation by 5° to 10° clockwise in ~ 11-12 My old crust, indicating a change at that time of the orientation of the MAR axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 My ago by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the entire VTR rose vertically as a single block within the active transform offset. Erosion has gradually removed material from the top of the VTR and has modified its slopes. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the extent of the uplift depends on the thickness of the brittle layer, consistent with the observed topography of the VTR. Spreading half rate of the crust south of the transform decreased from 17.2 mm/yr between 26 and 19 My ago to ~16.9 mm/yr between 19 and ~10 My ago, to ~13.6 mm/yr from 10 My ago to present. The slowing down of spreading occurred close in time to the change in ridge/transform geometry, suggesting that the two events are related. The events outlined by this study illustrate transform-related vertical tectonics in slow-spreading ridges.
T44A-07 17:30h
Small scale mantle heterogeneities sampled through oceanic basalts from ridge-transform intersections
Studies of MORB have shown that the oceanic upper mantle is heterogeneuos. The nature, distribution and scale of these heterogeneities are still a matter of debate. Short-wavelength chemical variability has been observed along mid-ocean ridges. Melt inclusions show isotopic and chemical variabilty within a single basalt sample and even within a single olivine grain. The distance along axis affected by the ridge-transform intersection (RTI) cold edge effect is proportional to slip rate and offset length. Numerical calculations show a strong decrease of crustal production as a ridge approaches a transform. When the amount of melt produced is small, the capability of melt to aggregate and to mix in magma chambers on the way to the surface is low; therefore, the chemical and isotopic signatures of preaggregated melts can be detected. These predictions make long-offset RTI's potential areas to detect chemical variability in the mantle through the study of MORB. Basalts sampled in the equatorial Atlantic along a ridge segment south of the Romanche RTI (ERRS), show an increase in trace element variability within a single dredge haul, with wavelengths comparable to that observed in suites of melt inclusions. This variability implies that melt transport beneath the ERRS is inefficient at mixing melts and suggests melt transport in channelized systems that limit interaction between melt and solid. The correlation between trace element and isotopic enrichement suggests that most of the chemical variability is probably due to source heterogeneity. Melt transport preserves correlations present in the source, suggesting channels nucleation in the deepest part of the molten region just within melting heterogeneities. Correlations will be probably destroyed if channel networks cross source heterogeneities. Volatile-rich low degree partial melts, generated at depth from enriched peridotite or from pyroxenite lumps, react with and metasomatize the surrounding peridotite, favouring nucleation of high porosity dissolution channels.
T44A-08 17:45h
The Traill \O \ - V\o ring Igneous Complex Along the East Jan Mayen Fracture Zone in the North Atlantic
Geological Survey of Norway aeromagnetic data, acquired in the Norwegian Sea of the NE Atlantic, have been merged with reprocessed adjacent data sets. Reconstruction to Chron 22 (c. 47.7 Ma) of the Norwegian-Greenland Sea aeromagnetic data reveals a c. 50 km wide continuous anomaly "belt" extending obliquely from the East Greenland to the conjugate mid-Norway margin, i.e. crossing Chrons 24 b and a, Chron 23, as well as the spreading axis. This anomaly is interpreted to represent an igneous complex, here referred to as the Traill \O-V\o ring igneous complex. The central portion of the complex coincided with the East Jan Mayen Fracture Zone, suggesting that the fracture zone was a center of magmatic activity at an early stage of formation. On the Greenland margin the complex can be linked up with the NE-trending initial magmatic lineament (IML) extending between Traill \O \ and Kangerlussuaq. The IML has been suggested to relate to a failed attempt of direct linkage between the Reykjanes and Mohns Ridges. Several sub-volcanic complexes on mainland Greenland form an integrated part of this igneous complex, including the magmatic complex near Kangerlussuaq. While various intrusions in East Greenland remain somewhat poorly dated, magmatic rocks in the Kangerlussuaq area are better constrained and are dominated by a c. 50 Ma event. The magnetic response to the Traill \O - V\o ring igneous complex has earlier been interpreted to represent spreading anomalies 24A and 24B along the V\o ring margin. This interpretation in turn introduced the need to invoke an abandoned spreading ridge and the Gleipne Fracture Zone in this area. The new data reveal intense magmatic activity NE and SW of the East Jan Mayen Fracture Zone. In addition to the existence of the Traill \O -V\o ring igneous complex, the new compilation strongly suggests that previously interpreted oceanic fracture zones (Gleipne, Surt, Bivrost, Jenegga and Vester\aa len) do not exist; these were artefacts of poor navigation and wide line spacing of the vintage datasets. Consequently, opening of the Norwegian-Greenland Sea between the Jan Mayen and Senja-Greenland fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies or jumps in spreading axis. Because the mentioned fracture zones do not exist, there can be no spatial relationship between transfer zones and fracture zones on the Lofoten margin, and nor on the NE Greenland margin where they have been projected.
<a href='http://www.ngu.no' >http://www.ngu.no