Clarence R. Allen

Red River and associated faults, Yunnan Province, China: Quaternary geology, slip rates, and seismic hazard

The 900-km-long right-slip Red River fault of southernmost China and northern Vietnam is a profound structural discontinuity that is mechanically associated with the collision of the Indian and Eurasian plates. Although history records no large earthquakes resulting from slippage along at least the principal segment of the fault in China, youthful landforms and disruptions of young sedimentary rocks indicate that it has generated large earthquakes during the Pleistocene and Holocene epochs. The historic quiescence thus must be regarded as being indicative of a current seismic gap, although the recurrence interval between major earthquakes is evidently much longer than for many other major active fault systems. That recent displacement has been primarily right lateral is indicated by consistently displaced drainages, ranging in offset from 9 m to 6 km, and the freshness of the smallest and most recent offsets implies repeated Holocene movements. Although physiographic features typical of active faulting such as scarps and drainage diversions are present throughout, the general absence of sag ponds reflects both the high rate of dissection of the fault by the Red River and its tributaries and the lower degree of activity as compared to highly active faults such as the San Andreas fault of California. In its middle 170 km, the fault zone is made up of two branches. The range-front branch demarcates the northeastern base of the Ailao Mountains and, at least locally, has an appreciable component of dip slip. The mid-valley branch, in large part previously unrecognized, traverses principally deeply dissected Cenozoic valley fill northeast of the range-front fault and has undergone almost pure lateral slip. Lateral postfill offsets along the range-front branch diminish toward the southeast, whereas those along the mid-valley branch diminish northwestward; the net effect is that the total postfill offset across both branches is almost uniform. The Red River and its major tributaries appear to have experienced about 5.5 km of right slip since the beginning of a major episode of incision that continues to the present day. Restoration of this offset provides a remarkable alignment of most large tributaries as well as removing a major kink in the course of the Red River itself. Using maximum credible rates of incision, we estimate an average fault-slip rate of 2 to perhaps 5 mm/yr. At this long-term rate of slip, the smallest offsets observed along the fault (9 m) would occur no more frequently than every 1,800 to 4,500 yr on the average. This is consistent with the historical record of fault dormancy for the past 300 yr. North of the Red River fault, there is a large seismically active region laced with numerous faults of north and northwesterly trends. Several of these faults display clear and even spectacular evidence of youthful normal faulting, and some appear to have left-lateral components as well. These faults, as well as the Red River fault itself, are accommodating regional east-west crustal extension and north-south shortening.

Field study of a highly active fault zone: The Xianshuihe fault of southwestern China

The Xianshuihe fault of western Sichuan Province, China, is one of the worlds most active faults, having produced 4 earthquakes during this century of magnitude ≥7 along a 350-km length of the fault. At least 8 such events have occurred since 1725. In the more limited 150-km-long segment including Luhuo and Daofu, major earthquakes in 1904, 1923, 1973, and 1981 (M = 7, 7½, 7.6, 6.9) were associated with overlapping surficial fault ruptures and with individual left-lateral displacements as large as 3.6 m. Field studies indicate that this high degree of activity is typical of the faults longer-term history. The Holocene left-lateral slip rate on the north-western segment of the fault has been 15 ± 5 mm/yr, decreasing to about 5 mm/yr on its southeastern segment, based on radiometrically dated offset stream-channel and terrace deposits and on offset glacial moraines. Physiographic features of active faulting are fully as diagrammatic as those of Californias San Andreas fault, mainly because of high-altitude preservation and the absence of cultural modification on this eastern margin of the Tibetan Plateau. Detailed en echelon tensional and pushup features resulting from surface ruptures in 1973, 1955, 1923, and 1893 can still be recognized today, and new data have been collected bearing on the offsets and fault-rupture lengths during these and other events. The locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the Xianshuihe fault. Obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events, whereas segmentation control for others remains obscure. Based on the historic record, repeat times estimated from slip rates, and current seismic gaps, two segments are particularly likely sites for M = 7+ earthquakes in the near future: the 65-km-long segment between Daofu and Qianning, and the 135-km-long segment bracketing Kangding. Continuing creep has been documented along some segments of the fault, and this, together with the high degree of activity and other unique attributes, makes the Xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard-evaluation studies.

Geological Criteria for Evaluating Seismicity Address as Retiring President of The Geological Society of America, Miami Beach, Florida, November 1974

This paper argues that the geologic record, and the late Quaternary history in particular, is a far more valuable tool in estimating seismicity and associated seismic hazard than has generally been appreciated. Those parts of the world with the longest historic records of earthquakes — some 2,000 yr for Japan and the Middle East and 3,000 yr for China — are the areas that should give us the greatest pause in using historic records for extrapolations, because earthquakes in these regions show surprisingly large long-term temporal and spatial variations. The very short historic record in North America should, therefore, be used with extreme caution in estimating possible future seismic activity. The geologic history of late Quaternary faulting is the most promising source of statistics on frequencies and locations of large shocks.

Patterns of seismic release in the southern California region

Southern California experiences earthquakes on the San Andreas system of vertical right-lateral predominantly strike-slip faults and on a second system of faults that includes thrusts, oblique-slip, left-lateral, and other faults. Pattern recognition and cluster analysis are used to analyze the catalog of earthquakes with magnitudes ≥5.5 from 1915 to 1994. We use pattern recognition to find a suite of traits that would characterize each of these two systems and distinguish them from each other. Both pattern recognition and cluster analysis show that epochs of seismic release occur in which one or the other system is the predominant form of earthquake activity. For the past 2 decades the second system has been the active one. Small changes in the direction of plate movements could account for this phenomenon. Seismic release on the San Andreas system is preceded by episodes of activity in the Great Basin or in the Gulf of California. Presumably, these episodes would represent extension in the former region and spreading and slip on transform faults in the latter.

Structure of the Lower Blue Glacier, Washington

Structural features studied in lower Blue Glacier are the foliation pattern, an unusual longitudinal septum, ogives, crevasses, and related ice fabrics. A 300-meter icefall separating the major accumulation basins from the ice tongue plays a dominant role in the formation of the principal foliated structures. Three types of ice are involved; coarse-bubbly ice, coarse-clear ice, and fine ice. Most and perhaps all the fine ice represents partly recrystallized insets or infolds of firn. An angular unconformity between stratified firn and well-foliated glacier ice is attributed to a period of high firn limits prior to 1948 and lower firn limits since. Below the firn edge the glacier displays a foliation pattern of two sets of nested arcs, convex down-glacier. These are separated by a narrow zone of strongly foliated, structurally complex ice termed the longitudinal septum. The foliation pattern becomes more irregular toward the terminus because of intersecting folia and discontinuities in strike. The ogives of the Blue Glacier appear on the surface as alternating dark and white bands conformable with the arc-shaped foliation. The dark bands are underlain by well-foliated, heterogeneous material featuring unusually large proportions of fine and coarse-clear ice. The white bands are underlain by relatively massive, uniform coarse-bubbly ice. It is inferred that the transverse foliation pattern originates in a zone of strong compressive flow immediately below the icefall. Transverse inhomogeneities created within the fall may be an important initial factor. Once formed, the foliation passively undergoes deformation within the ice tongue as it flows down the valley. The arc-shaped pattern develops within a short distance below the fall owing to differential flow. A calculation based on borehole data shows that deformation within the glacier during flow is of the correct magnitude to account for the dip of the foliation as observed at the apexes of the nested arcs, assuming that the initial attitude at the base of the icefall is essentially vertical. Complications appearing in the foliation pattern in the lower reaches of the glacier are attributed chiefly to topographic irregularities on the glacier floor, near and below the base of the icefall. It is postulated that the longitudinal septum is formed at the base of the icefall where two ice streams, split by a large rock bastion, reunite. Differences in direction and velocity of flow at the junction result in strong compression and shear which produce the intense foliation of the septum. The .high content of fine ice is attributed to the insetting and infolding of firn within the icefall and in a fosse at its base. The ogives are inferred to be primarily features formed within the icefall but subsequently modified in the zone of compressive flow at its base. The ogive dark bands may represent greatly compressed and partly recrystallized ice breccias which accumulated within crevasses formed at the lip of the icefall. There is no compelling evidence that the Blue Glacier ogives are annual features.

SAN ANDREAS FAULT ZONE IN SAN GORGONIO PASS, SOUTHERN CALIFORNIA

Unusual features of the San Andreas fault in the San Gorgonio Pass area of Southern California are the absence of rift topography, absence of lateral stream offsets, an abrupt change in trend of the fault trace, seismic evidence for the predominance of thrusting over strike-slip faulting, and a lack of great earthquakes in historic time. Rocks of the San Gorgonio igneous-metamorphic complex crop out over most of the map area and constitute a metamorphic terrane of intermediate to basic composition that has been intimately intruded and partially reconstituted by Mesozoic(?) quartz monzonite. Flaser gneiss, greenschist, and piedmontite-bearing gneiss are distinctive rock types in the otherwise rather uniform migmatitic gneisses. Most of the younger rocks of the pass area are sedimentary and reflect a history of recurrent deformation and deposition; they range in age from upper Miocene(?) to Quaternary, and nearly all are of alluvial-fan or flood-plain origin. Quaternary alluvial fans that once buried a former rugged topography are now being dissected along the north side of the pass, leaving numerous surfaces of low relief and associated stream terraces. Warping of these surfaces suggests Quaternary arching of the mountain range along a north-south axis. Within San Gorgonio Pass, the San Andreas fault curves abruptly southward from its normal southeast trend and butts into the eastward-trending Banning fault at 45°. The Banning fault is a major tectonic feature that delineates the north side of the pass and forms the southern limit of the Transverse Ranges in this region. Thrust and reverse movements of at least 5000 feet have taken place on this fault in Quaternary time, and, although there is little evidence of Recent lateral displacements, late Pliocene and Pleistocene right-hand strike-slip movements totaling at least 5 miles are suggested. The Mill Creek and Mission Creek faults are major branches of the San Andreas fault that diverge northward; both evince considerable late Cenozoic vertical displacement, but possible lateral movements are unknown. This study neither proves nor disproves the existence of lateral displacements amounting to perhaps hundreds of miles along the San Andreas fault zone as a whole. But if large lateral displacements have taken place, they must have been followed by deformation and disruption of the fault traces then existent, because lateral movements of even 1 mile are difficult to reconcile with the complex surface geometry of faults within the pass area. Faults previously considered branches of the San Andreas, particularly the Mission Creek and San Jacinto faults, may have absorbed much of the lateral strain, and the Banning fault may represent an ancestral San Andreas fault—now deformed into a Transverse Range fault. The deformation and disruption of former breaks appear to represent a pattern that is typical of the entire eastern half of the Transverse Ranges, where elements of San Andreas and Transverse Range structure have been vying for control; evidently one set has alternated with the other in attaining temporary dominance. Southeast of San Gorgonio Pass it is not clear which, if any, fault trace deserves the name of San Andreas, and it is suggested that the entire area between the Elsinore fault on the west and the east side of the Salton depression on the east be termed the San Andreas fault zone.

AGUA BLANCA FAULT—A MAJOR TRANSVERSE STRUCTURE OF NORTHERN BAJA CALIFORNIA, MEXICO

Agua Blanca fault is a major right-handed strike-slip fault at least 80 miles in length that cuts transversely across the peninsula of Baja California about 70 miles south of the international border. Its trend is anomalous in being more nearly parallel to the Transverse Ranges of southern California than to the San Andres fault system that elsewhere dominates the tectonic grain of the peninsula. Geographic features delineating the fault trace are, from east to west: Paso San Matias, Valle de La Trinidad, Canon de Dolores, Valle de Agua Blanca, Valle de Santo Tomas, Bahia Soledad (south branch of fault), Punta Banda (north branch). Farther west, both branches of the fault control submarine topography, and possibly the fault system is continuous with the northwest-trending San Clemente fault off the southern California coast. Physiographic expression of the Agua Blanca fault is remarkably similar to that of the San Andreas. Typical features are Recent scarps, offset streams, shutterridges, fault sags and saddles, side-hill ridges, and fault-controlled valleys. Most of these features are particularly well exhibited in Valle de Agua Blanca, which is designated as the type locality. Rocks cut by the fault are mainly Cretaceous plutonic rocks of the southern California batholith and Lower Cretaceous (Albian) metavolcanic rocks. Along the Pacific Coast, the fault cuts Upper Cretaceous (Maestrichtian) post-batholithic sedimentary rocks that are otherwise surprisingly little deformed as compared to rocks of similar age in most of California. Agua Blanca fault shows a history of right-lateral displacement throughout its length: Recent stream offsets occur from Valle de La Trinidad nearly to the Pacific Ocean; distinctive Quaternary(?) fan gravels in Valle de Agua Blanca are offset laterally 3 miles from their most likely source area across the fault; in the same area, a fault slice of distinctive antiperthitic granodiorite is best explained by 7 miles of lateral displacement, and a nearby slice of quartz diorite may indicate displacement as great as 14 miles. In general, evidence of both Recent activity and amount of total displacement appear to increase westward. The fault coincides in gross aspect with a broad east-west zone of seismic activity in a region elsewhere characterized by relative quiescence, but no large historical earthquakes can be positively correlated with this fault. Despite its orientation athwart the regional tectonic grain, Agua Blanca fault does not appear to represent a deep-seated structural feature analogous to those of the Transverse Ranges. Instead, it is probably one of several paths by which the San Andreas fault tends to break around the “knot” caused by the great bend of the San Andreas in southern California.

Comparisons Between the North Anatolian Fault of Turkey and the San Andreas Fault of California

The North Anatolian fault of Turkey is remarkably similar to the San Andreas fault of California in its style of displacement, high seismicity, physiographic expression, neotectonic history, presence of creep, and problems of seismic-hazard evaluation. On the other hand, significant contrasts between the two faults exist in their space-time patterns of seismicity and in their plate-tectonic relationships. Workers in the two areas have much to learn from one another.

Magnetic stratigraphy and a test for block rotation of sedimentary rocks within the San Andreas fault zone, Mecca Hills, southeastern California

A 500-m section of the Palm Spring Formation in the southern Mecca Hills, located within the San Andreas fault zone in southeastern California, has been paleomagnetically sampled to determine possible tectonic rotation in this area and to establish time-stratigraphic control. This work was partly stimulated by the fact that 80 km farther south, previous studies demonstrated 35° of postdepositional rotation in the Palm Spring Formation of the Vallecito-Fish Creek basin east of the Elsinore fault. Several lines of evidence suggest that hematite is the main magnetic carrier of the Mecca Hills samples. Large anhedral hematite grains observed in magnetic extracts and a positive fold test imply a detrital origin of the remanence. The polarity reversal patterns, together with earlier vertebrate paleontologic studies, restrict the time span for deposition of this unit to the middle-late Matuyama chron (2.0–0.75 myr ago), thus of uppermost Pliocene and early Pleistocene age. Characteristic directions of best least-squares fit for 73 samples suggest little or no overall rotation, despite the severe late Quaternary tectonic activity demonstrated by the intense deformation of these strata.

Robert E. Wallace

On 8 January 2007, Robert E. (Bob) Wallace passed away peacefully and comfortably in Reno, Nevada. His sister, Harriet Wallace, of Savoy, Illinois, and his son, Alan, of Reno, survive him. Trudy, his wife and best friend of 60 years, died in 2005. Bob wished to be cremated with no memorial gathering. He was 90 years old. Bob was an authority on tectonics, earthquake geology, paleoseismology, engineering geology, mineral resources, and geomorphology. He was a patient teacher—a man of great modesty, rigor, and humor. He authored more than 160 technical articles, books, geologic maps, and reports on the western United States, Alaska, China, the Soviet Union, Central America, the Philippines, and Turkey. He served on scores of councils and advisory bodies to governments in California and at the national level, as well as to foreign countries and scientific societies. One of his most significant professional contributions, however, was to bring together geologists, seismologists, and earthquake engineers to try to understand the behavior and damage potential of earthquakes. ▴ Robert E. Wallace. Bobs success as a geologist was just one reflection of an immensely fertile and talented intellect. He was also a master birdwatcher, astronomer, and amateur radio operator; his excellent watercolor landscape paintings won numerous awards. Each year, an original painting graced the front of Bob and Trudys Christmas cards, the cards themselves worth keeping for the artwork alone. He also was …