Andrew Meigs

Middle-late Miocene (>10 Ma) formation of the Main Boundary thrust in the western Himalaya

Three independent data sets from northwestern India and Pakistan suggest initial displacement along >1000 km of the Main Boundary thrust prior to 10 Ma, at least 5 m.y. earlier than previously reported. Regionally extensive changes in the depositional characteristics and rates of the foreland-basinfill between 11 and 9.5 Ma are interpreted to reflect new hinterland loading due to the formation of the Main Boundary thrust. Sediment-accumulation rates, sandstone-siltstone ratios, and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 Ma in well-dated stratigraphic sections from Pakistan to Nepal across the Indo-Gangetic foreland basin. In the Himachal Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma conglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred prior to 9 Ma and probably prior to 10 Ma, assuming a gravelprogradationrateof3cm/yr.Threeapatitefission-trackages fromstructuresattheleadingedgeoftheMainBoundarythrustin the Kohat region of northwest Pakistan indicate that rapid cooling below ;105 C1‐2 m.y. earlier. These data indicate that the Main Boundary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of ;10 mm/yr, and has a displacement history that is coeval with late displacement on the Main Central thrust.

Unfolding: An inverse approach to fold kinematics

Preservedfoldshapesusuallyreveallittleabouttheirkinematicevolution.Syntectonic strata preserved in growth synclines in contact with a fold, however, can permit ‘‘unfolding’’: a sequential reconstruction of fold growth backward through time from a geometry observed at present to an initial undeformed state. Such reconstructions can define the kinematics of fold growth. Growth strata associated with anticlinal forelimbs in the Ebro basin exhibit depositional tapering of beds on fold flanks and progressive limb rotation. Unfoldingawell-dateddetachmentfolddefinesitskinematicevolutionandcoevallyvarying rates of shortening, forelimb uplift, and forelimb rotation. Interplay of these rates with sedimentation rates controls onlap and offlap relations.

Crustal wedging triggering recent deformation in the Andean thrust front between 31°S and 33°S: Sierras Pampeanas-Precordillera interaction

[1] We document a new model of crustal structure of the Andean front in Argentina where numerous historic earthquakes destroyed the cities of Mendoza in 1861 (Ms = � 7) and San Juan in 1944 (Mw = 7.0). The Cerro Salinas anticline is formed above the west directed Cerro Salinas thrust. An east facing monocline with an amplitude of about 2 km folds the Cerro Salinas thrust and overlying Neogene succession. This monocline is formed above a blind crustal thrust in the basement. Its dip of 14 west is inferred from fold geometry. This thick-skinned east directed blind thrust and the thin-skinned west directed Cerro Salinas thrust define a tectonic wedge; the wedge tip occurs at a depth of 5.4 km. Growth of the monocline after � 8.5 Ma is revealed on multichannel seismic (MSC) profile 31017 (Repsol-YPF). Rates of Cerro Salinas thrust displacement are of the order of 1 mm/yr, whereas vertical uplift of � 0.45 mm/yr results from the combined displacement on the Cerro Salinas thrust and growth of east facing monocline. The lateral extent of the east directed crustal blind ramp corresponds with the along-strike extent of the Eastern Precordillera. When combined with the low displacement rate, a long earthquake recurrence interval is implied. Smaller magnitude earthquakes, however, indicate that segments of the blind thrust ramps ruptured in historic events. If all the segments of the blind thrust ruptured together the fault area is � 7000 km 2 and could produce a Mw � 7.7 earthquake. The crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region.

Ten-million-year history of a thrust sheet

The final, deformed state of a fold-and-thrust belt may be reached by an infinite number of kinematic paths. Two end-member kinematic paths are due to continuous or discontinuous rates of deformation. We have used a new magnetostratigraphic section from the Spanish Pyrenees to calibrate the emplacement history, over ≈10 m.y., of a major thrust sheet (the Sierras Marginales thrust sheet) and the deformation of both its hanging wall and footwall. Six time windows from before 36.5 Ma until after 24.7 Ma were recognized on the basis of structural and stratigraphic relationships between syntectonic strata and major structures. Footwall deformation of the Sierras Marginales thrust sheet occurred continuously on a detachment within the foreland-basin sequence (4.5 km net shortening; shortening rates steadily increased from 0.14 to 1.5 mm/yr with time). Although the detachments at the base of the foreland and the detachment at the base of the Sierras Marginales thrust sheet were active coevally, the rate of displacement on the Sierras Marginales detachment decayed with time and shows considerable variability. Emplacement of the thrust sheet may be divided into three distinct periods: a rapid 13.8 km translation from 37.0 to 36.5 Ma (27.6 mm/yr), a gradual climb of the toe of the thrust sheet up a 4.3-km-long ramp across the foreland-basin succession from 36.5 to 32.0 Ma (0.95 mm/yr), and a final 8.9 km translation from 32.0 to 29.5 Ma (3.56 mm/yr). Internal deformation of the thrust sheet occurred only after it reached its present position at some time before 29.5 Ma. Shortening rates steadily decreased from 0.6 mm/yr between 29.5 and 27.8 Ma to 0.26 mm/yr from 27.8 until after 24.7 Ma. Only ≈1 km of shortening accumulated during each of the two periods; the last shortening localized on the most northerly thrust in the study area. Folding and subsequent faulting above the detachment beneath the foreland suggest that slip was transmitted to its tip point continuously throughout the deformation. In contrast, translation followed by internal deformation on the hinterland side of the toe of the Sierras Marginales thrust sheet indicates a successive deactivation of the southern parts of the detachment with time. Rate of deformation on both detachments was discontinuous and shows substantial variability about the mean. In general, the spatial and temporal pattern of deformation was distributed and continuous. In detail, however, shortening was spatially and temporally discontinuous above each detachment, and structures related to each one display distinctly different deformational patterns, rates, and styles.

Active tectonics and the LiDAR revolution

A revolution in remote sensing, light detection and ranging (LiDAR) laser altimetry swath mapping, reveals the details of topographic features at such high resolution that they have transformed our understanding of tectonic forcing of the shape of the Earth’s surface. Meter-scale DEMs (digital elevation models) capture fault offsets, fault zone structure, off-fault deformation, and landscape properties at microgeomorphic scale, highlighting that the surface faithfully records the complexity and sensitivity of deformation in detail.

Using Vertical Rock Uplift Patterns to Constrain the Three-Dimensional Fault Configuration in the Los Angeles Basin

Comparison of geologic uplift patterns with results of three-dimensional mechanical models provides constraints on the fault geometry compiled by the South- ern California Earthquake Center community fault model in the northern Los Angeles basin, California. The modeled uplift matches well the geologic pattern of uplift as- sociated with the Santa Fe Springs and Coyote Hills segments of the Puente Hills thrust fault but does not match structures to the west of the San Gabriel River. To better match the geologic patterns in this area, alternative fault configurations were tested. The best match to geologic uplift is attained with a model incorporating (1) a steep blind thrust fault at the location of the Los Angeles segment of the Puente Hills thrust system (following interpretations of the Las Cienegas fault geometry at this location), (2) removal of an inferred linking fault between the Raymond and Holly- wood faults, and (3) lateral continuation of the Lower Elysian Park fault, a blind low- angle detachment at >10 km depth, along strike to the northwest. These geometric revisions alter the connectivity of northern Los Angeles basin faults and significantly improve the match of model uplift pattern to geologic data. Model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area. The preferred model alters slip rates by >0:2 mm=yr for the Upper Elysian Park, Hollywood, Lower Elysian Park, Raymond, Sierra Madre West, and Verdugo faults. Additionally, the preferred model alters the surface area of several faults in the northern Los Angeles basin, such as the Puente Hills thrust and the Lower Elysian Park fault, which may have important implications for seismic hazard assess- ment in the northern Los Angeles basin.

Shortening rate and Holocene surface rupture on the Riasi fault system in the Kashmir Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge

New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The >60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6−7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14−7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6−7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11−12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%−50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal decollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal decollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw > 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir.