T42B-01 10:20h
0.5-0.6 Ma Onset of Uplift and Transpressive Deformation in the Borrego Badlands, Southern California, due to Initiation of the Coyote Creek Fault
Pleistocene sedimentary rocks between the Clark (CF) and Coyote Creek (CCF) faults in the Borrego Badlands (BB), southern California, record evolution of the central San Jacinto fault zone. The Inspiration Point fault (IPF) is a NE-striking left-lateral fault that cuts all units older than Qal and is bounded by the CF and CCF. The Ocotillo Formation ranges in age from 1.1 to 0.5-0.6 Ma, and consists of: (1) a lower member (200-350 m thick) of conglomerate, sandstone and siltstone preserved throughout the BB, and (2) an upper member (50-190 m) of cobble-boulder conglomerate and pebbly sandstone in the NE BB. Detailed mapping reveals no evidence for growth folding or faulting during deposition of the Ocotillo Fm. The Fonts Point Sandstone (FPS) is a widespread, 2- to 10-m-thick unit of conglomeratic sandstone that overlies the Ocotillo Fm and contains a well developed capping Aridosol up to ca. 2 m thick (Ryter, 1999). The Ocotillo-FPS contact changes from a low-angle unconformity NW of the IPF to a conformable contact in most locations SE of the IPF, north of the Fonts Point escarpment. Subcrop map provides evidence for growth of a NE-trending anticline on the NW side of the IPF shortly prior to FPS deposition; conformable contact SE of the fault suggests early NW-side-up movement associated with initiation and early transpressive slip on the IPF. Magnetostratigraphic study of a continuous section that crosses Beckman Wash in the western BB shows that the contact between the Ocotillo Fm and underlying Borrego Fm is a short-lived hiatus dated at slightly post-1.07 Ma. Polarity reversals correspond to the disconformable base and conformable top of the Jaramillo sub-chron (1.07 and 0.99 Ma, respectively) and the Matuyama-Brunhes boundary (0.78 Ma, 18 +/- 13 m below the 0.758-Ma Bishop Ash). Sedimentation rates slowed from 1.6-2.7 mm/yr during Jaramillo time to 0.31-0.41 mm/yr from 0.99 to 0.758 Ma. SE of the IPF, the 0.74-Ma Thermal Canyon Ash is 70 m below the conformable base of the FPS. Application of sedimentation rates from the Beckman Wash section yields an age of 0.51-0.57 Ma for the base of the FPS. Based on structural and stratigraphic relationships described above, the basal FPS dates initiation of the IPF and related folding in the BB. Presence of a thick Aridosol in the FPS and lack of post-FPS deposits indicates that the beginning of transpressive deformation coincided with the end of deposition and onset of uplift throughout the BB, which produced the modern Fonts Point erosional escarpment. Modern uplift in the BB results from oblique shortening across a restraining bend in the CCF south of Fonts Point. We infer that uplift began when the NW strand of the CCF was initiated at 0.6 +/- 0.1 Ma (Dorsey, 2002), and that the age of basal FPS (0.51-0.57 Ma) provides a refined estimate for the timing of this event. These results are consistent with recent studies that document the end of deposition and onset of deformation in the San Felipe Hills at 0.5-0.6 Ma (Kirby et al., 2004; in prep; Janecke et al., 2004). This study thus supports an emerging model for major reorganization of the San Jacinto fault zone, and possibly initiation of the entire Coyote Creek fault system, at 0.5-0.6 Ma.
T42B-02 10:35h
Critical Assessment of Quantitative Geomorphology in the Footwall of Active Normal Faults, Basin and Range Province, Western U.S.A.
Displacement rates of normal faults in the Basin and Range vary by several orders of magnitude across the province, yet the mountain ranges bounded by these faults demonstrate strikingly similar topography. The observed morphology of normal-fault bounded mountains reflects the balance of relative uplift and erosional processes, raising the question of whether detailed geomorphic studies can yield insight into rates of deformation on timescales important to the dynamics of deformation. Using USGS 30~m Digital Elevation Models we quantify the following geomorphic measures for 19 mountain ranges in northern Nevada: channel concavity ($\theta$), channel steepness index ($k_s$), hypsometric integral, mountain front sinuosity, and the skewness and kurtosis of surface slope distributions. Several measures commonly thought to be indicators of fault activity, in particular mountain front sinuosity and hypsometric integral, show weak correlation with vertical slip rate or no correlation at all. However, channel concavity, $\theta$, channel steepness index, $k_s$, hypsometric integral and the skewness of slope distributions within individual drainage-basins correlate significantly with vertical slip rate, and with one another. No obvious correlation with bedrock lithology or climate is observed. Principal component analysis demonstrates that the first two components account for 70% of the variance in the data. The first principal component is dominated by channel steepness index, concavity and kurtosis of slope distributions.
T42B-03 10:50h
Seismotectonics of an Evolving Intracontinental Plate Boundary in Eastern California
Analysis of seismicity in the northern Mojave block, southern Walker Lane belt and southern Sierra Nevada provides a detailed snapshot of the kinematics of active deformation within a young and probably evolving intra-continental plate boundary in eastern California. Earthquakes were relocated using joint hypocentral inversion, double-difference and cross-correlation techniques. Groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor. The inversion results are synthesized in maps of the seismogenic deformation field. In general, seismogenic deformation east of the Sierra Nevada is characterized by horizontal plane strain and primarily accommodates northwest translation of the Sierra Nevada-Central Valley (Sierran) microplate relative to stable North America. Crustal thinning in the Walker Lane belt is subordinate to NW-directed dextral shear. There is no obvious variation in strain geometry in the transition from the eastern California shear zone to the Walker Lane belt across the Garlock fault, which suggests that this structure is being deformed by distributed NW dextral shear. In contrast, seismogenic deformation in the southeastern Sierra Nevada near lat 36°N is characterized by horizontal extension and locally by oblate flattening; i.e., extension in two perpendicular horizontal directions. The extensional deformation occurs directly east of the "Isabella anomaly", a high-velocity anomaly in the upper mantle interpreted to be lower Sierran lithosphere that detached in late Neogene and descended into the asthenosphere. There is a progressive east-to-west transition from strike-slip faulting in the Walker Lane belt to horizontal extension and vertical crustal thinning within the southern Sierra Nevada, indicating that NW dextral shear extends into the eastern Sierran microplate. The seismotectonics of this region may reflect an early stage in the process whereby the Walker Lane expands westward in discrete steps, progressively narrowing the rigid Sierran microplate and possibly diverting a greater percentage of total Pacific-North American plate motion east of the San Andreas system (Jones et al., 2004). This process may be intimately connected with late Cenozoic detachment and descent of lower Sierran lithosphere.
T42B-04 11:05h
Potential for Blind Thrust(s) Beneath the Marin County - Mt Tamalpais Region
Blind thrust faults pose a significant earthquake hazard, both for the damage they can cause, but also because of the difficulty in locating and characterizing them. In the San Francisco Bay region, the potential impact of blind thrusts has been recognized in the East Bay and Santa Cruz mountains. However, there is still significant uncertainty about even the existence of such structures elsewhere in the region. Here we propose that the Marin County - Mt Tamalpais region is underlain by one or more blind thrust structures. Regional determinations of fault kinematics (WGCEP, 2003), allow the possibility that 2-4 mm/yr of ~N-S shortening could be occurring across the Marin county region. Geologic slip rates along the San Andreas abruptly increase from the San Francisco Peninsula segment ($\sim$ 17 mm/yr) to the San Andreas north of the Golden Gate ($\sim$ 24 mm/yr). This increase on the San Andreas has been attributed to the transfer of slip from the San Gregorio fault in the vicinity of Point Reyes (WGCEP, 2003). However slip rate determinations further south along the San Gregorio indicate only 3-4 mm/yr of slip. We propose that some of the observed slip increase may reflect motion obliquely transferred to the San Andreas from the East Bay fault system along blind structures. To test the potential for such structures, we have combined tectonic interpretation of local geomorphology with crustal kinematics and deformation modeling. One of the primary manifestations of the proposed blind thrusts would be active differential uplift across the region. The topographic edifice of Mt Tamalpais is somewhat enigmatic - relief is locally 4-5 times greater than relief in lithologically similar portions of the peninsula to the south (e.g., Heimsath et al., 1997) - potentially implying a significant difference in rock uplift rates. Analysis of fluvial and hillslope gradients along the Bolinas ridge, north and west of Mt. Tamalpais reveals spatial variations in landscape morphology. Although hillslope gradients remain roughly similar within the uniform lithology of the ridge, channel gradients (normalized for upstream drainage area) exhibit a four-fold increase from north to south. We interpret this increase to reflect spatial variations in erosion rate, driven by differential uplift across the region. Coupled geomorphic analyses and deformational model thus provide a means to place constraints on the spatial extent, geometries and kinematics of this potential earthquake source.
T42B-05 11:20h
Stratigraphic Control on Excess Pore Pressure at the Plate Boundary Fault of Nankai Trough
One of the unresolved issues regarding the subduction front of Nankai Trough is the control over stratigraphic position of the decollement. The Muroto Transect area (ODP Sites 808, 1173, 1174) is atypical of the regional system because hemipelagic strata within the lower Shikoku Basin accumulated above a basement high formed by back-arc spreading and late-stage seamount volcanism. The Ashizuri Transect area (ODP Site 1177) is more characteristic of the system as a whole. Seismic reflection and coring prove that sandy turbidites are common within the lower Shikoku Basin facies, except along the Muroto Transect. Consolidation tests demonstrate that the stratigraphic equivalent of the decollement at Site 1173 is slightly overconsolidated, whereas strata below the decollement horizon are heavily overconsolidated. Samples from Site 1177, in contrast, are slightly to heavily underconsolidated within and immediately below the stratigraphic equivalent of the decollement. Mudstones within the underlying Shikoku Basin turbidite facies show moderate to strong overconsolidation. Evidently, the turbidite facies drains very effectively through a process of compaction-driven fluid flow. There is a 40-m interval of mudstone above the turbidites, however, which acts as an aquitard. Stratigraphic compartments of overpressure build up long before the subducting strata reach the deformation front, especially where the uncemented sand packets pinch out. Tectonic consolidation and up-dip fluid flow beneath the decollement exacerbate the overpressured condition. The unusually high fluid pressures reduce effective stress and provide the optimal location for propagation of the plate boundary fault.
T42B-06 11:35h
Geomorphic and Geophysical Studies in the Atirro-R\'{i}o Sucio Fault System and the Turrialba-Iraz\'{u} Pull-Apart Basin, Central Costa Rica
The northwest-striking Atirro-R\'{i}o Sucio fault system and the associated Turrialba-Iraz\'{u} pull-apart basin in central Costa Rica lie in the region separating the northwestern part of the Panama microplate and the Caribbean Plate. These features reflect part of the regional response to the N-NE directed collision of the Cocos Ridge with the Panama microplate in southeastern Costa Rica. The neotectonic Atirro-R\'{i}o Sucio fault system consists of the dextral-slip Atirro, Turrialba, Tucurrique, Lara faults, central and eastern R\'{i}o Sucio faults, and the predominantly normal Azul, Campano, Chiz, and western R\'{i}o Sucio faults. Left steps in the southeast part of this fault system have formed transpressive zones, a few kilometers in width, in the Rinc\'{o}n de la Esperanza, Mirador and Matama areas. These transpressive zones form part of the complex southeastern end of the Turrialba-Iraz\'{u} pull-apart basin, a 40-km (NW-SE) by 10-15 km (NE-SW) zone formed in a right step between the Atirro fault to the southeast and the R\'{i}o Sucio fault to the northwest. The Azul and R\'{i}o Sucio faults mark the northeastern limit of the pull-apart; the southwestern margin is less well defined. Within this zone, significant Quaternary alluvial and lahar deposits have accumulted in the Turrialba, Juray, and Atirro basins. Magnetic anomalies of 500 nT or more coincide with the Azul, Chiz, and Turrialba faults, suggesting structural discontinuities that coincide with the scarps. The Quaternary Iraz\'{u} volcano formed within the west-central portion of the pull-apart; the Turrialba volcano close to the northeast boundary. In this zone thick volcanic deposits have obscured the geomorphic expression of the pull-apart basin. However, the overall N-S trend of the Iraz\'{u} volcanic complex and a group of monogenetic cones within the basin is compatible with formation under tensional stresses associated with pull-apart development. Historical seismicity in the Atirro-R\'{i}o Sucio fault system and the Turrialba-Iraz\'{u} pull-apart includes both swarms and several earthquakes with magnitude greater than 5.0, notably the Patillos event of December 30 1952 (Ms 5.7) and the Pejibaye sequence of July 1993 (magnitudes 5.5 and 5.3).
T42B-07 11:50h
Intracontinental Strain Localization due to Heterogeneous Strength I: Evidence From a Recently Discovered, S-Directed Cenozoic Thrust Belt at the SW end of the Altyn Tagh Fault
Upper crustal deformation in many active orogens appears to be concentrated along first-order fault systems that are thousands of kilometers long and hundreds of kilometers wide parallel and perpendicular to strike, respectively. But what controls the formation and evolution of these intracontinental "microplate" boundaries? To address this question, we are investigating the evolution of the SW termination of the active, left-slip Altyn Tagh fault, the largest intracontinental strike-slip system in Asia. Although the fault presently terminates into E-W striking, N-directed thrust belts at both its NE and SW ends, this structural geometry cannot account for 475$\pm$70 km of displacement recently documented along the central section of the fault. To address this problem, we have conducted 1:100,000-scale structural mapping in westernmost Tibet, at the SW terminus of the Altyn Tagh fault, to test the idea that early to middle Cenozoic slip along the Altyn Tagh fault was absorbed at its SW end by a S-directed thrust belt prior to formation of the presently active N-directed thrust system. Our mapping indicates that western Tibet has been shortened by a regionally extensive system of Late Cretaceous or younger, N-dipping, S-directed thrusts. This S-directed thrust belt shows systematically shallowing structural levels from north to south. For example, in the north, the steeply north-dipping Tianshuihai thrust places a sequence of greenschist facies Proterozoic metasediments over Carboniferous strata in its footwall. About 45 km to the south, these Carboniferous strata are in turn thrust southwards over Cretaceous strata that are deformed by S-directed thrusts and associated folds within the Loqzung range. The Aksai Chin anticline is the southernmost structure we have mapped within the S-directed thrust belt. This E-W trending fold has a N-S interlimb width of 12 km and deforms both Carboniferous rocks in its core and unconformably overlying Cretaceous strata on its limbs. Cretaceous beds dip 25-45\deg N on the N limb but are vertical to 10\deg overturned on the south limb. Shortening due to folding alone is at least 35% but could be as high as 65% within the Aksai Chin anticline, and ongoing work aims to address total shortening across the thrust belt. The S-directed thrust belt coincides with a late Paleozoic to early Mesozoic subduction-accretion complex, while the zone of most intense shortening within the belt coincides with a Mesozoic suture along the N edge of the Qiangtang block. Thus, heterogeneous crustal strength inherited from Paleozoic and Mesozoic assembly of Asia has likely controlled the location and geometry of first-order fault systems produced during Cenozoic orogeny.
T42B-08 12:05h
Intracontinental Strain Localization due to Heterogeneous Strength II: Evidence From Suture Reactivation in the Loqzung Range, Western Tibet
The active, left-slip Altyn Tagh fault is a first-order structure within the Indo-Asian collision zone. Reconstruction of mid-Paleozoic and early Mesozoic arcs displaced by 475$\pm$70 km along this fault predicts that early Tertiary left-slip along the Altyn Tagh fault was absorbed by a S-directed thrust belt at the SW tip of the fault, producing the marked asymmetry in the N-S width of the Tibetan Plateau seen today. To test this prediction, we have conducted 1:100,000-scale structural mapping in the Loqzung range, which lies in western Tibet between the Karakorum range to the south and the western Kunlun Shan range to the north. Our mapping supports four conclusions about Mesozoic assembly and post-Cretaceous deformation of the NW-SE striking western Loqzung Range. First, the range coincides with a major tectonic boundary: the Taaxi-Qiaoertianshan-Hongshanhu (TQH) suture between the Qiangtang block to the south and the Songpan-Ganzi subduction-accretion complex to the north. In the Loqzung range, the TQH suture juxtaposes Jurassic melange and late Paleozoic diamictites of Gondwanian affinity to the south against Carboniferous strata to the north that have previously been reported to show Cathaysian affinity. The location of this suture has formerly been disputed. Second, the range shows evidence of both S- and N-directed post-Cretaceous deformation, although the S-directed deformation predominates. Along the north edge of the range, a S-directed thrust places Carboniferous deposits over Cretaceous strata that are deformed by S-vergent folds. Post-Cretaceous, shortening has also reactivated the S-dipping TQH suture, thrusting Jurassic melange that lies to the south of the Loqzung range northwards over folded Cretaceous rocks that lie within the range to the north. Ongoing work aims to establish the timing relations between the N- and S-directed phases of deformation. Third, the structural style of post-Cretaceous deformation within the Loqzung range varies along strike and appears to be controlled by underlying basement structure. Specifically, the structural style in the SE portion of the range is characterized by tightly folded Cretaceous deposits whereas to the NW these units are deformed by broad, upright folds. Although regional fold axes generally trend parallel to the TQH suture, in detail their orientations vary along strike, trending N70W in the SE and N50W in the NW. We estimate that post-Cretaceous shortening varies along strike from roughly 45 to 20 percent from the SE to the NW. Thrust-displacement along the reactivated TQH suture also appears to vary along strike, placing Jurassic melange over younger Cretaceous units to the SE and older Cretaceous units to the NW. Fourth, the western Loqzung range is bounded at its southeast end by the E-W striking, active, left-slip Ghoza Co fault. This fault separates older, less deformed Cretaceous deposits to the north from younger, more intensively deformed Cretaceous units to the south. Our field observations suggest that the location, geometry, and structural style of post-Cretaceous deformation in western Tibet was controlled by the TQH suture. Heterogeneous crustal strength thus appears to have been important in influencing Cenozoic deformation in the region.