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Where are the missing faults in translated terranes?
Tectonophysics 326 (2000) 23–35 www.elsevier.com/locate/tecto
Where are the missing faults in translated terranes? Paul J. Umhoefer Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, USA
Abstract A central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers, while the faults along which the terranes moved these great distances are not found. Relations long known from geology and from recent studies of modern transform-plate boundaries suggest that preserving strike-slip faults that record 1000-km-scale translations is unlikely, and therefore, in many cases, paleomagnetism may be our only reliable indicator of how far terranes translated. Four reasons are explored here for why large-offset strike-slip faults are likely to be ‘missing’ in translated terranes. (1) Strike-slip faults in continents commonly are partially or wholly destroyed, or reactivated. Only ~400–500 km of over 1000 km of offset on Cretaceous–early Tertiary strike-slip faults in northern British Columbia are accounted for in southern British Columbia. The missing strike-slip faults probably continued south into the hinterland of the Rocky Mountain thrust belt as oblique-thrust faults, much like the modern Alpine–Marlborough fault system of New Zealand. The major strike-slip faults in southeastern British Columbia were obliterated by Eocene extensional faulting, uplift, and erosion. (2) Offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale, many of which are ‘left behind’ and not found in the translated terrane. The greater San Andreas plate boundary has accumulated ~1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern California. There is a narrow ‘terrane’ in the offshore borderland of California that has moved ~1200 km relative to North America, but no single fault has nearly that much offset. (3) Major faults in narrow oceans formed by oblique-divergent plate boundaries have a low potential for preservation. For example, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the oblique rift lie, has a low potential for preservation. In contrast, Baja California is currently a continental terrane that has translated ~300 km relative to North America along the plate boundary in the Gulf of California. Because of the relative buoyancy of continental crust, Baja California is likely to be preserved in any future accretion event. (4) Faults in the margins of oblique-divergent plate boundaries are dominantly normal faults and not strike-slip faults. There is little strike-slip faulting in central Baja California, even though the plate boundary has a rift angle (a) of only 20°. Recent field data and modeling show that at a=0–20° strikeslip faults will dominate the secondary faults. Surprisingly, at a=~20 to ~35°, there are both strike-slip and normal faults, and at a>~35–45°, there are few or no strike-slip faults, even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Missing faults; translated terranes; stike-slip faults; Baja California
1. Introduction Anomalous paleomagnetic data from western North America have been interpreted to indicate that some terranes have translated hundreds to
thousands of kilometers along the continent after they were accreted ( Fig. 1) (Baja and southern California: Hagstrum et al., 1985; Morris et al., 1986; Lund and Bottjer, 1991; Sur-Obispo terrane: McWilliams and Howell, 1982; Salinian terrane:
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Fig. 1. Terrane map of western North America with terranes highlighted for which large translation has been suggested. Baja BC is a composite of many terranes.
translations that are more difficult to challenge. In addition to these challenges to the paleomagnetic data, some geologists have pointed out that the strike-slip faults along which these terranes must have moved have not been found, and thus, this undermines the large-translation interpretation (e.g. Price and Carmichael, 1986). In this paper, I explore these more general points. If the paleomagnetic interpretations for large translations are valid, why are the large strike-slip faults commonly not found? Even if many of the paleomagnetic interpretations for large translation are found to be incorrect, the reasoning presented here may help explain the paucity of large strike-slip faults in the geologic record ( Woodcock, 1986). I discuss a variety of processes by which part or all of the geologic record of large-scale translation by strike-slip faults is destroyed. The processes include well-established processes such as the overprinting of faults and, more recently, understood processes from recent research along the oblique-divergent plate boundary in the Gulf of California.
2. Modern terrane translation Champion et al., 1984; Baja British Columbia– Baja BC: Beck and Noson, 1972; Beck et al., 1981; Irving et al., 1985; Umhoefer, 1987; Cowan et al., 1997). Geologic data have also been cited to suggest large translations (Cowan, 1982; Umhoefer, 1987). The conclusions of these studies are very controversial. In the case of the Salinian terrane, a more detailed paleomagnetic study on indisputable Salinian rocks has shown that it has moved only modest distances since the Late Cretaceous ( Whidden et al., 1998). A more recent study of the Sur-Obispo terrane suggests it may not have moved far (Hagstrum and Murchey, 1996). The anomalous paleomagnetic data from Baja BC and Baja California have been attributed to local tilt of plutons, insufficient cratonic reference poles (Butler et al., 1989; Dickinson and Butler, 1998), and sediment compaction ( Tan and Kodama, 1998). Yet, at least for Baja BC, there are a few paleomagnetic studies ( Wynne et al., 1995; Ward et al., 1998) in well-layered sedimentary and volcanic rocks that indicate large
Terranes translate most commonly by means of simple transform faults or by large strike-slip faults within a more complex zone of distributed deformation along oblique plate boundaries. If we consider that motion of a given plate can be in any direction relative to the opposite plate, then translation dominates in oblique-divergent and oblique-convergent plate boundaries where the angle between the relative motion and the plate boundary is <45°. Woodcock (1986) has shown that active transform and oblique plate boundaries are common. Of the Earth’s active plate boundaries, 35% are within 45° of parallel to the boundary or dominated by transform motion. Active transform faults in continents are fairly well known because of the reasonably well-studied San Andreas and Alpine fault systems. The modern oblique-convergent plate boundary of Sumatra was discussed and modeled by Fitch (1972) and later by Beck (1986) amongst others. In these cases, strike-slip faults form within the arc or
P.J. Umhoefer / Tectonophysics 326 (2000) 23–35
Fig. 2. Four types of oblique plate boundaries and where terranes may form within the plate boundaries. The thick lines are the main plate boundary. (A) is based on Sumatra after Fitch (1972). (B) is based on the present San Andreas fault region based on Namson and Davis (1988). (C ) is based on the early stage of the Gulf of California based on Stock and Hodges (1989). (D) is based on the modern Gulf of California (e.g. Lonsdale, 1989).
forearc of the overriding plate (Fig. 2a). Thus, a translating terrane is formed outboard of the arc. The present San Andreas fault region illustrates a variation of oblique convergence because the relative plate motion is only 5° more northerly than the San Andreas fault (DeMets et al., 1990). A major fault (San Andreas) has transcurrent motion, but it is bounded on either side by belts of thrust faults ( Fig. 2b) (Namson and Davis, 1988). Because of a history of faults jumping in the San Andreas system, there are numerous terranes adjacent to the San Andreas fault that are presently being translated (Fig. 2b). The Gulf of California illustrates two styles of oblique divergence that mirror those in oblique convergence ( Fig. 2c and d). In an earlier stage, the plate boundary had regional strain partitioning
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with a transform fault at the plate boundary and a rift within the continental plate that formed on a recently inactive volcanic arc (Fig. 2c) (e.g. Hausback, 1984; Stock and Hodges, 1989). In this stage, a future terrane was forming between the rift and transform fault. In the modern stage of the Gulf of California, a series of long transform faults and short spreading ridges are linked in a right-stepped fashion to form the plate boundary ( Fig. 2d ) (e.g. Lonsdale, 1989). This new plate boundary formed on the previous rift. A continental terrane (Baja California peninsula and part of southern California) is thus moving along the plate boundary ( Umhoefer and Dorsey, 1997). It is well established that modern fragments of continental crust, or terranes, have translated at least many hundreds of kilometers. Coastal California west of the San Andreas fault and Baja California has moved northward 225–330 km in the Neogene (Crowell, 1979; Gastil et al., 1981; Stanley, 1987; Powell and Weldon, 1992). The eastern part of the south island of New Zealand has translated ~500 km along the oblique convergent plate boundary (Norris et al., 1990; Walcott, 1998). Western Sumatra is moving northward as an active terrane. Modern examples of active terranes that have translated thousands of kilometers are uncommon, but they do exist, as discussed below for the San Andreas fault system.
3. Lack of preservation of major strike-slip faults Large strike-slip fault systems that are now inactive can be preserved in the geologic record (e.g. Great Glen fault system of Scotland, Tintina and other faults of northern British Columbia, Yalakom and related faults of southern British Columbia). However, I maintain that strike-slip fault systems are much less likely to be preserved than thrust or normal fault systems (see also Woodcock, 1986). First, dip-slip systems are commonly wide arrays of faults, while strike-slip systems are commonly narrower in plan view. Thus, dip-slip fault systems are more likely to be preserved and less likely to be wholly overprinted in the geologic record. Thrust fault systems inherently widen because they grow by new faults cutting
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into their own foreland (e.g. Boyer and Elliott, 1982). Normal fault systems extend the land and therefore also inherently widen through time, and gently inclined detachment systems are very wide relative to their offset (e.g. Gibbs, 1984; Wernicke, 1985). Strike-slip fault systems only widen in special circumstances and are more likely to be narrow relative to their length and offset (Sylvester, 1988). Woodcock (1986) also pointed out that seismic reflection data emphasize more gently inclined structures as steep structures are difficult to image. Finally, we commonly simplify plate tectonics into three orthogonal classes of plate boundaries, and this ignores the common oblique boundaries. Four processes contribute to the improbability of preserving large-offset strike-slip fault systems. These are illustrated with well-understood examples from the ancient and modern geologic record. 3.1. Strike-slip faults commonly destroyed The Tintina, Teslin-Kutcho, Finlay, Pinchi, and related faults in north-central British Columbia (Fig. 3) were active in Late Cretaceous to early Tertiary time as a major strike-slip fault system with at least 1000 km of dextral offset (Gabrielse, 1985). Only part of the 1000 km of offset is found in southern British Columbia, yet there is no reason to believe that it died out to the south. Price and Carmichael (1986) presented an interpretation of the Tintina fault that showed that ~300 km of northerly dextral offset can be accounted for by the northeastward motion of the thrust faults of the southern Canadian Rocky Mountain thrust belt and the extension in the southern Omineca belt (Fig. 3). Another ~100 km of offset was transferred to the Fraser fault in southern British Columbia (Price and Carmichael, 1986). It is possible that another ~120 km of offset was transferred south to the Yalakom fault through central British Columbia (Coleman and Parrish, 1991; Umhoefer and Kleinspehn, 1995). This leaves ~500–600 km of offset of the northern British Columbia faults that is not accounted for in the south. Umhoefer (1994) and Irving et al. (1996) pointed out that these missing strike-slip faults may have run through the hinterland of the
Fig. 3. Map of strike-slip faults of British Columbia. Ib is the Intermontane belt and Ob is the Omineca belt, which is also in gray. Note that the strike-slip faults of east-central British Columbia end to the south without explanation. The faults with rectangles are major Eocene normal faults; the thrust faults are major Cretaceous to early Tertiary in age.
Rocky Mountain thrust belt in the southern Omineca and Intermontane belts (Fig. 3). In this model, the strike-slip faults are likely to merge downward in the middle crust with large thrust faults similar to a model for an orogenic float by Oldow et al. (1989, 1990). In this case, two aspects of the geology of those belts would destroy or obscure the faults. First, the southern Intermontane belt includes widespread areas of poor exposure of the Cache Creek terrane or younger rocks that cover older structures. The Cache Creek terrane is dominated by bedded chert and greenstone and argillite that is not readily amenable to finding large offsets on faults. Further, large portions of the southern Intermontane belt are covered by Eocene volcanic and sedimentary rocks that are younger than the faults in question ( Wheeler and McFeely, 1991). Second, in the southern Omineca belt, Eocene
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extension probably destroyed virtually all upper crustal rocks and faults by uplift and erosion or burial beneath Eocene basins. If the large strikeslip faults that extended south from northern British Columbia ran through the Omineca belt, then they were probably confined to the upper crust, as shown by Oldow et al. (1989, 1990). After thrusting stopped at the end of the Paleocene in the southern Omineca belt, a major episode of extension commenced and occurred throughout the Eocene (Parrish et al., 1988). The extension uplifted vast areas of the lower crust of the region, eroded large areas of upper crust into nearby basins, and buried still other areas as the upper plates of the detachment fault systems were covered by basins and volcanic rocks. The result is that when workers produce balanced cross-sections through the southern Omineca belt, they reveal that virtually no upper crust from the thrust belt (where the strike-slip faults probably were) is now exposed at the Earth’s surface (Parrish et al., 1988). In effect, a series of related events make strike-slip faults in the hinterland of an obliqueconvergent thrust belt likely targets for overprinting and destruction. Extension commonly occurs after thrusting ends in the hinterland of a thrust belt, where the belt is the highest and has the most potential energy to extend. It follows, then, that any large-scale strike-slip fault system in the hinterland of an oblique thrust belt will be largely destroyed if subsequent extension occurs because it will produce widespread uplift, erosion, subsidence, and volcanism. 3.2. Larger strike-slip fault systems are anastomosing with complex terrane translation The San Andreas fault system of California and Alpine–Marlborough fault systems of New Zealand provide examples of active, large-scale translation of terranes. We can surmise from these examples that large-scale strike-slip fault systems are unlikely to be preserved in full. Atwater and Stock (1998) recently reanalyzed the motion of the Pacific plate relative to North America off of California. They showed that since 24 Ma, there have been 1020 km of northwestward motion of the Pacific plate relative to North America along
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the greater San Andreas transform plate boundary. Typical of large, strike-slip fault systems, the San Andreas transform system comprises numerous, anastomosing faults (Fig. 4). The transform motion was first accommodated on faults now offshore of California and then jumped inland (Crowell, 1979; Sedlock and Hamilton, 1991; Nicholson et al., 1994). The western faults are not well known in the offshore but lie within the narrow continental shelf off of California (Nicholson et al., 1994). This means that there is a narrow, elongate ‘terrane’ off of California that has moved ~1000 km relative to North America ( Fig. 4). The translation of that terrane has been by means of numerous faults, and importantly, only about 20–25% of that motion is on the largest fault, the San Andreas. There are numerous elongate ‘terranes’ in coastal California that have moved ~300–800 km relative to the North America plate. The anastomosing strike-slip fault geometry and eastward jumping of the San Andreas fault system indicates that no one fault accommodates all, or even the majority, of the translation of the terranes of western California and the offshore borderland. This pattern of faulting also implies that many of the faults related to translation of an individual terrane will not be preserved on the boundary of that particular terrane. As an example, the terrane offshore of California with the most translation has only one major fault bounding it (Santa Lucia, Fig. 4) that has had an offset that was only a small portion of the total terrane translation. In order for a terrane in southern California to include all of the faults responsible for the translation, it would have to move continually along the easternmost fault of the system. In any other scenario, some of the faults involved in the translation will be ‘left behind’ within the North American continent. Because strike-slip fault systems involve common jumping and splaying of faults (e.g. Sylvester, 1988), it is unlikely that the whole system of faults will be preserved in any given terrane that has translated large distances. Fig. 5 illustrates another fault scenario in which the fault along a terrane has less offset than the terrane. In this case, strike-slip faulting jumps to the east (right) and then jumps west. This later jump in
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Fig. 4. Map of major faults of the greater San Andreas fault system. The San Andreas fault is SA. The dashed line to the west is the approximate position of the boundary between oceanic and continental crust. The narrow ‘terrane’ in the offshore borderland of California that has had most or all of the translation of the Pacific plate relative to North America is gray. The Santa Lucia bank fault that bounds the terrane on the east is SL.
faulting leaves behind two terranes, C and D. Thus, fault 4 that bounds terrane B has much less displacement than terrane B itself. The Alpine–Marlborough fault systems in New Zealand offer parallel examples to the San Andreas fault system. Since 6 Ma, the plate boundary within the South Island of New Zealand has been oblique convergent with 70–110 km of convergence and 230 km of dextral translation ( Walcott, 1998). The plate boundary is relatively narrow in the central part of the South Island along the Alpine fault ( Walcott, 1998). In the
northern part of the South Island, the Alpine fault splays out into the Marlborough fault system of large strike-slip faults and a series of thrust faults of the Hikurangi thrust system to the east (Lamb and Bibby, 1989; Walcott, 1998). Here, the plate boundary is at least 150 km wide. Similar to the San Andreas fault system, the outer strike-slip faults of the Marlborough system are actively translating terranes that have moved most of the distance of the Pacific plate relative to Australia (Lamb and Bibby, 1989). Fault slices, or terranes, presently being translated along the Alpine–
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Fig. 5. Model for a hypothetical strike-slip fault system in which faulting jumps progressively east and then jumps west in the last stage. In this case, some faults are left behind and are not connected to the translated terrane. In this manner, terrane B has much more translation than fault 4 that lies adjacent to terrane B. The numbered lines are strike-slip faults in the order of formation, and the lettered areas are the terranes that develop. Thick lines are faults that move in the subsequent stage. The word ‘Continent’ remains fixed. The letter for each terrane is fixed within the terrane and moves when the terrane moves. See text for discussion.
Wairau fault in the northernmost South Island have moved considerably less than the total plate motion. In addition, the plate margin in New Zealand is evolving with major vertical-axis rotations such that many strike-slip faults have become overprinted as oblique-slip or thrust faults (Lamb and Bibby, 1989), illustrating another means of partially destroying strike-slip faults. 3.3. Faults in narrow oceans along obliquedivergent boundaries have a low preservation potential Major faults in oblique-rift oceans have a low potential for preservation. Baja California is currently a continental terrane ( Fig. 6) that has translated ~300 km relative to North America
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since ~6 Ma (Gastil et al., 1981; Lonsdale, 1989) and is likely to be preserved in any future accretion event because of the relative buoyancy of continental crust. In contrast, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the plate boundary lie (Fig. 6), has much less potential for preservation. This means that most of the faults taking up the current motion of Baja California relative to North America are likely to be destroyed by future tectonism. Furthermore, the oblique-divergent plate boundary in the Gulf of California formed within a Miocene volcanic arc (Hausback, 1984) such that the geology on either side of the gulf is similar over vast areas (Carta Geologica, 1980). Thus, older geologic units offer chances for correlation across the plate boundary only in the northern and southern parts of the Baja California peninsula (Gastil, 1991). This emphasizes the importance of the structures on the Baja California peninsula (terrane) for detecting the terrane motion. However, as is discussed next, they are of limited value in detecting the 300 km of translation of the Baja California peninsula. 3.4. Strike-slip faults are under-represented in oblique-divergent plate boundaries Experimental and analytical data on oblique rifting ( Withjack and Jamison, 1986; Fossen and Tikoff, 1993) and field data from the margins of oblique rifts (e.g. Umhoefer and Stone, 1996) show that normal faults predominate over strike-slip faults at all but the most highly oblique plate boundaries. Thus, in the very margins of terranes where the style of faulting is the key to deciphering the type of plate boundary, strike-slip faults are under-represented amongst the secondary faults. The central part of the Baja California peninsula is an almost ideal example of a terrane translating along a highly oblique-divergent plate boundary ( Fig. 6) because it has a relatively simple history ( Umhoefer and Dorsey, 1997). The northern and southern parts of the Baja California peninsula are cut by through-going strike-slip and normal faults, but the central part of the peninsula is not transected by strike-slip faults. That is, at the latitude of central Baja California, plate motion is
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Fig. 6. Bathymetry and faults of the plate boundary in the Gulf of California and domains of Baja California. The bathymetry is shown in contours of 200, 1000, 2000, and 3000 m. The area near Loreto in Fig. 7 is shown in the box in the southern part of the peninsula.
largely confined to the main transform–ridge system in the Gulf of California. Despite the fact that the central domain of Baja California has translated ~300 km since 6 Ma, and that the plate boundary is highly oblique [the
plate boundary has a rift angle (a) between the trend of the boundary and the azimuth of plate motion of only 20°], there are few strike-slip faults in central Baja California. The southern Loreto basin and Loreto fault ( Fig. 7) were active in the
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Fig. 7. (A) simplified geologic map of the region of the Loreto fault and Loreto basin in Baja California Sur, Mexico. The location of the figure is shown on Fig. 6. (B) Typical orientation, type of faults, and extension direction (e) of the SE Loreto fault array. The dashed line is the azimuth of plate motion in the Gulf of California and the gray line the trend of the gulf plate boundary. (C ) Faults with striations from the SE Loreto fault array. (D, E, F ) Plots similar to (B) with the expected orientation and type of faults and extension direction (e) of the margin of oblique divergent plate boundaries with increasing rift angles.
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Pliocene during the highly oblique motion of Baja California relative to North America ( Umhoefer et al., 1994; Dorsey and Umhoefer, 2000). The southern Loreto fault is a dextral-normal fault with average rake of striae in clay gouge of 15° ( Umhoefer and Dorsey, 1997). The strike slip offset on the fault is not well known, but is likely to be in the range of 1–5 km. The Loreto fault is the largest strike-slip fault known from the entire central domain of the Baja California peninsula (Fig. 6). The SE Loreto fault array cuts part of the Loreto basin, and unequivocal evidence shows that it was active during terrane translation in the latest Pliocene, mainly from 2.4 to 2.0 Ma ( Umhoefer and Stone, 1996; Dorsey and Umhoefer, 2000). This fault array is dominated by normal faults with tens to 250 m of offset and has a minor population of dextral-normal faults with up to 500 m of offset ( Umhoefer and Stone, 1996). The SE Loreto fault array has a style of faulting and extension direction relative to plate motion that confirm the first-order results from clay modeling by Withjack and Jamison (1986). If we use the data from the SE Loreto fault array (Fig. 7b and c) ( Umhoefer and Stone, 1996), a larger region of the central domain (Angelier et al., 1981), experiments ( Withjack and Jamison, 1986), and analytical studies (Fossen and Tikoff, 1993), we can summarize the nature of secondary faults on the margins of oblique-divergent plate boundaries as the rift angle (a) increases ( Fig. 7d– f ). At a=0–20°, strike-slip faults will be the majority of the secondary faults. At a=~20 to ~35° both strike-slip and normal faults are present, but normal faults will predominate. At a>~35°, there are few or no strike-slip faults. Secondary strikeslip faults will have the same sense of offset as the plate boundary and be nearly parallel to the rift trend. Normal faults will be oriented about 20– 40° clockwise from the rift trend in transtensional plate boundaries undergoing dextral shear. Unlike the type of faulting, which changes significantly at certain thresholds, the extension direction changes smoothly with increasing rift angle (e in Fig. 7d– f ) (Fossen and Tikoff, 1993). The most important point here is that if the main transform faults in highly oblique-divergent
plate boundaries have a low preservation potential, as argued above, then the structures on the margins on the translating continental terrane are a key to understanding the translation. Recent data suggest that strike-slip faults are only dominant at the most highly oblique boundaries (a=<20°). They further show that for oblique-divergent boundaries where translation is dominant but a=20–45°, normal faults will dominate over strike-slip faults. For these types of oblique-divergent boundaries, correlations of bedrock geology across the former plate boundary or paleomagnetic data will probably be required to understand the amount of translation.
4. Discussion and conclusions The premise of this paper is that it has been difficult to find the major strike-slip faults that produce large-scale terrane translation because we are only beginning to understand how faulting works along oblique and transform plate boundaries and because major strike-slip faults are prone to partial or complete destruction. I have also reiterated some points from Woodcock (1986) that show that strike-slip faults are not as likely to be detected in ancient orogenic belts. Many of our methods of examining mountain belts, especially the practice of using across-strike cross sections and seismic reflection sections, favor the detection of lower-angle, dip-slip faults and not steep strikeslip faults. The evidence presented here that strikeslip fault systems are difficult to preserve does not suggest that geologic evidence is not important to the testing of whether terranes have moved great distances. Cowan et al. (1997) have pointed out numerous geologic methods that can test the validity of far traveled terranes. These are the main arguments presented here that major strike-slip faults are not likely to be preserved. (1) Strike-slip faults in continents are commonly destroyed. Strike-slip fault systems are inherently narrow compared to dip-slip systems. Strike-slip fault systems commonly link downward into thrust or oblique faults and are therefore destroyed when a region is uplifted and eroded. This series of events is especially prone to occur
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because strike-slip faults commonly occur in the hinterland of oblique-convergent margins. (2) Strike-slip fault systems of >1000 km scale will likely be an anastomosing array, and thus only terranes at the margins of the belt will record the full relative plate motion. Strike-slip faults that bound terranes will commonly have much less offset than the total motion of the terrane. (3) Major transform faults in highly oblique-divergent plate boundaries have a low preservation potential because they form in dense oceanic crust in the deep ocean. (4) Oblique convergent and oblique divergent plate boundaries are commonly accommodated by oblique-slip thrust and normal faults such as the examples given here of the Alpine fault of New Zealand and the Loreto fault of Baja California. Plate margins with an obliquity between 20 and 45° are likely to be dominated by dip-slip faults, even though the plate motion is mostly translational. These arguments about the preservation of strike-slip fault systems have important implications for the analysis of translated terranes. If it is unlikely that the complete strike-slip fault system that moved a terrane would be found, how can we evaluate how far terranes have translated? Paleomagnetic data may be the only definitive approach to identify far-traveled terranes (Beck, 1991). However, rigorous paleomagnetic studies that fully meet the methods of paleomagnetism and are conducted in rocks in which paleohorizontal is unambiguously known are essential. The ability to eliminate sediment compaction where sedimentary rocks are studied is necessary. Multiple datasets are necessary to eliminate the possibility of anomalous local results. Where paleomagnetic studies suggest large-scale translation, geologic evidence can suggest compatibility with the style of translation, but probably cannot provide evidence for the magnitude of translation. Despite the conclusions of this paper, evidence from structural geology is a key to demonstrate the feasibility of large-scale translation of terranes. The overall style of deformation should be a wide belt of distributed structures with the range of strike-slip to dip-slip faulting. Major strike-slip faults should be found on the translated terrane and on the plate from which the terrane traveled,
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even if they do not record the complete record of translation. There should be a compatible style of secondary strike-slip faults and related structures. These structures would include oblique-slip thrust or normal faults, fault arrays with mixed dip-slip and oblique-slip faults, and folds compatible with transtensional and transpressional deformation. Many fault arrays may display an en echelon pattern. In ideal cases, the style and orientation of faults may suggest a range of obliquity for the ancient plate margin ( Teyssier et al., 1995; Umhoefer and Dorsey, 1997). Basins formed during major terrane translation will likely be either classic pull-apart basins or strike-slip related basins that are hybrids of classic rifts or foreland basins.
Acknowledgements This research was supported by NSF grants EAR 9304426 and EAR 9526506. I thank Richard Sedlock and Basil Tikoff for reviews of the manuscript and many colleagues too numerous to mention here for discussions of terrane translation, strike-slip faults, and Baja BC. Most importantly, I thank Myrl Beck for his provocative, creative, and pioneering efforts in the problems of terrane translation and Bernie Housen for organizing the session in honor of Myrl.
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ations on the origin of the San Andreas fault. Tectonics 5, 49–64. Beck, M.E., 1991. Case for northward transport of Baja and coastal southern California: Paleomagnetic data, analysis, and alternatives. Geology 19, 506–509. Boyer, S.E., Elliott, D., 1982. Thrust systems. American Association of Petroleum Geologists Bulletin 66, 1196–1230. Butler, R.F., Gehrels, G.E., McClelland, W.C., May, S.R., Klepacki, D., 1989. Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement. Geology 17, 691–694. Carta Geologica, 1980. La Paz: Direccion Geneal de Geografia del Territorio Nacional, San Antonio, Mexico, 1:1,000,000 Scale. Champion, D.E., Howell, D.G., Gromme´, C.S., 1984. Paleomagnetic and geologic data indicating 2500 km of northward displacement for the Salinian and related terranes, California. Journal of Geophysical Research 89, 7736–7752. Coleman, M.E., Parrish, R.R., 1991. Eocene dextral strike-slip and extensional faulting in the Bridge River terrane, southwest British Columbia. Tectonics 10, 1222–1238. Cowan, D.S., 1982. Geological evidence for post-40 m.y. B.P. large-scale northwestward displacement of part of southeastern Alaska. Geology 10, 309–313. Cowan, D.S., Brandon, M.T., Garver, J.I., 1997. Geologic tests of hypotheses for large displacements — a critique illustrated by the Baja British Columbia controversy. American Journal of Science 297, 117–173. Crowell, J.C., 1979. The San Andreas fault system through time. Journal of the Geological Society London 136, 293–302. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophysical Journal International 101, 425–478. Dickinson, W.R., Butler, R.F., 1998. Coastal and Baja California paleomagnetism reconsidered. Geological Society of America Bulletin 110, 1268–1280. Dorsey, R.J., Umhoefer, P.J., 2000. Structural and stratigraphic evolution of the Pliocene Loreto basin, Baja California Sur, Mexico. Geological Society of America Bulletin 112, 177–199. Fitch, T., 1972. Plate convergence, transcurrent faults and internal deformation adjacent to southeast Asia and the western Pacific. Journal of Geophysical Research 77, 4432–4460. Fossen, H., Tikoff, B., 1993. The deformation matrix for simultaneous simple shearing, pure shearing, and volume change, and its application to transpression/transtension tectonics. Journal of Structural Geology 15, 413–422. Gabrielse, H., 1985. Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia. Geological Society of America Bulletin 96, 1–14. Gastil, R.G., Morgan, G.J., Krummenacher, D., 1981. The tectonic history of peninsular California and adjacent Mexico. In: Ernst, W.G. ( Ed.), The Geotectonic Development of California: Rubey 1. Prentice Hall, Engelwood Cliffs, NJ, pp. 284–305.
Gastil, R.G., 1991. Is there a Oaxaca–California megashear? Conflict between paleomagnetic data and other elements of geology. Geology 19, 502–505. Gibbs, A.D., 1984. Structural evolution of extensional basin margins. Journal of the Geological Society of London 141, 609–620. Hagstrum, J.T., McWilliams, M., Howell, D.G., Gromme, S., 1985. Mesozoic paleomagnetism and northward translation of the Baja California peninsula. Geological Society of America Bulletin 96, 1077–1090. Hagstrum, J.T., Murchey, B.L., 1996. Paleomagnetism of Jurassic radiolarian chert above the Coast Range ophiolite at Stanley Mountain, California, and implications for its paleogeogaphic origins. Geological Society of America Bulletin 108, 643–652. Hausback, B.P., 1984. Cenozoic volcanic and tectonic evolution of Baja California, Mexico, Geology of the Baja California Peninsula: Bakersfield, California, Pacific Section, Frizzell, V.A. (Ed.), Society of Economic Paleontologists and Mineralogists 39, 219–236. Irving, E., Woodsworth, G.J., Wynne, P.J., Morrison, A., 1985. Paleomagnetic evidence for displacement from the south of the Coast Plutonic Complex, British Columbia. Canadian Journal of Earth Sciences 22, 584–598. Irving, E., Wynne, P.J., Thorkelson, D.J., Schiarizza, P., 1996. Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma. Journal of Geophysical Research 101, 17901–17916. Lamb, S.H., Bibby, H.M., 1989. The last 25 Ma of rotational deformation in part of the New Zealand plate-boundary zone. Journal of Structural Geology 11, 473–492. Lonsdale, P., 1989. Geology and tectonic history of the Gulf of California. In: Winterer, E.L., Hussong, D.M., Decker, R.W. ( Eds.), The Eastern Pacific Ocean and Hawaii: Boulder, Colorado, The Geology of North America. Geological Society of North America, pp. 499–521. Lund, S.P., Bottjer, D.J., 1991. Paleomagnetic evidence for microplate tectonic development of southern and Baja CaliforniaThe gulf and peninsular provinces of Californias, Dauphin, J.P., Simoneit, B.R.T. (Eds.), American Association of Petroleum Geologists Memoir 47, 231–248. McWilliams, M.O., Howell, D.G., 1982. Exotic terranes of western California. Nature 297, 215–217. Morris, L.K., Lund, S.P., Bottjer, D.J., 1986. Paleolatitude drift history of displaced terranes in southern and Baja California. Nature 297, 844–847. Namson, J.S., Davis, T.L., 1988. Seismically active fold and thrust belt in the San Joaquin Valley, central California. Geological Society of America Bulletin 100, 257–273. Nicholson, C., Sorlien, C.C., Atwater, T., Crowell, J.C., Luyendyk, B.P., 1994. Microplate capture, rotation of the western Transverse Ranges, and initiation of the San Andreas transform as a low-angle fault system. Geology 22, 491–495. Norris, R.J., Koons, P.O., Cooper, A.F., 1990. The obliquelyconvergent plate boundary in the South Island of New Zealand: implications for ancient collision zones. Journal of Structural Geology 12, 715–725.
P.J. Umhoefer / Tectonophysics 326 (2000) 23–35 Oldow, J.S., Bally, A.W., Ave´ Lallemant, H.G., Leeman, W.P., 1989. Phanerozoic evolution of the North America Cordillera: United States and Canada. In: Bally, A.W., Palmer, A.R. ( Eds.), The Geology of North America — An Overview: The Geology of North America Series, A., 139–232. Oldow, J.S., Bally, A.W., Ave´ Lallemant, H.G., 1990. Transpression, orogenic float, and lithospheric balance. Geology 18, 991–994. Parrish, R.R., Carr, S.D., Parkinson, D.L., 1988. Eocene extensional tectonics and geochronology of the southern Omineca belt, British Columbia and Washington. Tectonics 7, 181–212. Powell, R.E., Weldon II, R.J., 1992. Evolution of the San Andreas fault. Annual Review of Earth and Planetary Sciences 20, 431–468. Price, R.A., Carmichael, D.M., 1986. Geometric test for Late Cretaceous–Paleogene intracontinental transform faulting in the Canadian Cordillera. Geology 14, 468–471. Sedlock, R.L., Hamilton, D.H., 1991. Late Cenozoic tectonic evolution of southwestern California. Journal of Geophysical Research 96, 2325–2351. Stanley, R.G., 1987. New estimates of displacement along the San Andreas fault in central California based on paleobathymetry and paleogeography. Geology 15, 171–174. Stock, J.M., Hodges, K.V., 1989. Pre-Pliocene extension around the Gulf of California and the transfer of Baja California to the Pacific plate. Tectonics 8, 99–115. Sylvester, A.G., 1988. Strike-slip faults. Geological Society of America Bulletin 100, 1666–1703. Tan, X., Kodama, K.P., 1998. Compaction-corrected inclinations from southern California Cretaceous marine sedimentary rocks indicate no paleolatitudinal offset of the Peninsular Ranges terrane. Journal of Geophysical Research 103, 27169–27192. Teyssier, C., Tikoff, B., Markley, M., 1995. Oblique plate motion and continental tectonics. Geology 23, 447–450. Umhoefer, P.J., 1987. Northward translation of Baja British Columbia along the Late Cretaceous to Paleocene margin of western North America. Tectonics 6, 377–394. Umhoefer, P.J., 1994. Discrepancy between northward translation from paleomagnetic and fault-offset data sets in the
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NW Cordillera. Geological Society America Abstracts with Program 26, A459. Umhoefer, P.J., Dorsey, R.J., Renne, P., 1994. Tectonics of the Pliocene Loreto basin, Baja California Sur, Mexico, and evolution of the Gulf of California. Geology 22, 649–652. Umhoefer, P.J., Kleinspehn, K.L., 1995. Mesoscale and regional kinematics of the northwestern Yalakom fault system: Major Paleogene dextral faulting in British Columbia, Canada. Tectonics 14, 78–94. Umhoefer, P.J., Stone, K.A., 1996. Description and kinematics of the SE Loreto basin fault array, Baja California Sur, Mexico: a positive field test of oblique-rift models. Journal of Structural Geology 18, 595–614. Umhoefer, P.J., Dorsey, R.J., 1997. Translation of terranes: A perspective from the central domain of the Baja California peninsula, Mexico. Geology 25, 1007–1010. Walcott, R.I., 1998. Modes of oblique compression: Late Cenozoic tectonics of the South Island of New Zealand. Reviews of Geophysics 36, 1–26. Ward, P.D., Hurtado, J.M., Kirschvink, J.L., Verosub, K.L., 1998. Measurements of the Cretaceous Paleolatitude of Vancouver Island: Consistent with the Baja–British Columbia hypothesis. Science 277, 1642–1645. Wernicke, B., 1985. Uniform-sense normal simple shear of continental lithosphere. Canadian Journal of Earth Sciences 22, 108–125. Wheeler, J.O., McFeely, P., 1991. Tectonic assemblage map of the Canadian Cordillera and adjacent parts of the United States of America. In: Geological Survey of Canada, Map, 1712 A, Scale 1:2,000,000. Whidden, K.J., Lund, S.P., Champion, D., Bottjer, D.J., Howell, D.G., 1998. Paleomagnetic evidence that the central block of Salinia (California) is not a far-traveled terrane. Tectonics 17, 329–343. Withjack, M.O., Jamison, W.R., 1986. Deformation produced by oblique rifting. Tectonophysics 126, 99–124. Woodcock, N.H., 1986. The role of strike-slip fault systems at plate boundaries. Philosophical Transactions of the Royal Society London A 317, 13–29. Wynne, P.J., Irving, E., Maxson, J., Kleinspehn, K., 1995. Paleomagnetism of the Upper Cretaceous strata of Mt. Tatlow: Evidence for 3000 km of northward displacement of the eastern Coast Belt, British Columbia. Journal of Geophysical Research 100, 6073–6091.
Where are the missing faults in translated terranes? Paul J. Umhoefer Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, USA
Abstract A central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers, while the faults along which the terranes moved these great distances are not found. Relations long known from geology and from recent studies of modern transform-plate boundaries suggest that preserving strike-slip faults that record 1000-km-scale translations is unlikely, and therefore, in many cases, paleomagnetism may be our only reliable indicator of how far terranes translated. Four reasons are explored here for why large-offset strike-slip faults are likely to be ‘missing’ in translated terranes. (1) Strike-slip faults in continents commonly are partially or wholly destroyed, or reactivated. Only ~400–500 km of over 1000 km of offset on Cretaceous–early Tertiary strike-slip faults in northern British Columbia are accounted for in southern British Columbia. The missing strike-slip faults probably continued south into the hinterland of the Rocky Mountain thrust belt as oblique-thrust faults, much like the modern Alpine–Marlborough fault system of New Zealand. The major strike-slip faults in southeastern British Columbia were obliterated by Eocene extensional faulting, uplift, and erosion. (2) Offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale, many of which are ‘left behind’ and not found in the translated terrane. The greater San Andreas plate boundary has accumulated ~1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern California. There is a narrow ‘terrane’ in the offshore borderland of California that has moved ~1200 km relative to North America, but no single fault has nearly that much offset. (3) Major faults in narrow oceans formed by oblique-divergent plate boundaries have a low potential for preservation. For example, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the oblique rift lie, has a low potential for preservation. In contrast, Baja California is currently a continental terrane that has translated ~300 km relative to North America along the plate boundary in the Gulf of California. Because of the relative buoyancy of continental crust, Baja California is likely to be preserved in any future accretion event. (4) Faults in the margins of oblique-divergent plate boundaries are dominantly normal faults and not strike-slip faults. There is little strike-slip faulting in central Baja California, even though the plate boundary has a rift angle (a) of only 20°. Recent field data and modeling show that at a=0–20° strikeslip faults will dominate the secondary faults. Surprisingly, at a=~20 to ~35°, there are both strike-slip and normal faults, and at a>~35–45°, there are few or no strike-slip faults, even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Missing faults; translated terranes; stike-slip faults; Baja California
1. Introduction Anomalous paleomagnetic data from western North America have been interpreted to indicate that some terranes have translated hundreds to
thousands of kilometers along the continent after they were accreted ( Fig. 1) (Baja and southern California: Hagstrum et al., 1985; Morris et al., 1986; Lund and Bottjer, 1991; Sur-Obispo terrane: McWilliams and Howell, 1982; Salinian terrane:
0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 4 0- 1 9 51 ( 0 0 ) 0 01 4 4 -X
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Fig. 1. Terrane map of western North America with terranes highlighted for which large translation has been suggested. Baja BC is a composite of many terranes.
translations that are more difficult to challenge. In addition to these challenges to the paleomagnetic data, some geologists have pointed out that the strike-slip faults along which these terranes must have moved have not been found, and thus, this undermines the large-translation interpretation (e.g. Price and Carmichael, 1986). In this paper, I explore these more general points. If the paleomagnetic interpretations for large translations are valid, why are the large strike-slip faults commonly not found? Even if many of the paleomagnetic interpretations for large translation are found to be incorrect, the reasoning presented here may help explain the paucity of large strike-slip faults in the geologic record ( Woodcock, 1986). I discuss a variety of processes by which part or all of the geologic record of large-scale translation by strike-slip faults is destroyed. The processes include well-established processes such as the overprinting of faults and, more recently, understood processes from recent research along the oblique-divergent plate boundary in the Gulf of California.
2. Modern terrane translation Champion et al., 1984; Baja British Columbia– Baja BC: Beck and Noson, 1972; Beck et al., 1981; Irving et al., 1985; Umhoefer, 1987; Cowan et al., 1997). Geologic data have also been cited to suggest large translations (Cowan, 1982; Umhoefer, 1987). The conclusions of these studies are very controversial. In the case of the Salinian terrane, a more detailed paleomagnetic study on indisputable Salinian rocks has shown that it has moved only modest distances since the Late Cretaceous ( Whidden et al., 1998). A more recent study of the Sur-Obispo terrane suggests it may not have moved far (Hagstrum and Murchey, 1996). The anomalous paleomagnetic data from Baja BC and Baja California have been attributed to local tilt of plutons, insufficient cratonic reference poles (Butler et al., 1989; Dickinson and Butler, 1998), and sediment compaction ( Tan and Kodama, 1998). Yet, at least for Baja BC, there are a few paleomagnetic studies ( Wynne et al., 1995; Ward et al., 1998) in well-layered sedimentary and volcanic rocks that indicate large
Terranes translate most commonly by means of simple transform faults or by large strike-slip faults within a more complex zone of distributed deformation along oblique plate boundaries. If we consider that motion of a given plate can be in any direction relative to the opposite plate, then translation dominates in oblique-divergent and oblique-convergent plate boundaries where the angle between the relative motion and the plate boundary is <45°. Woodcock (1986) has shown that active transform and oblique plate boundaries are common. Of the Earth’s active plate boundaries, 35% are within 45° of parallel to the boundary or dominated by transform motion. Active transform faults in continents are fairly well known because of the reasonably well-studied San Andreas and Alpine fault systems. The modern oblique-convergent plate boundary of Sumatra was discussed and modeled by Fitch (1972) and later by Beck (1986) amongst others. In these cases, strike-slip faults form within the arc or
P.J. Umhoefer / Tectonophysics 326 (2000) 23–35
Fig. 2. Four types of oblique plate boundaries and where terranes may form within the plate boundaries. The thick lines are the main plate boundary. (A) is based on Sumatra after Fitch (1972). (B) is based on the present San Andreas fault region based on Namson and Davis (1988). (C ) is based on the early stage of the Gulf of California based on Stock and Hodges (1989). (D) is based on the modern Gulf of California (e.g. Lonsdale, 1989).
forearc of the overriding plate (Fig. 2a). Thus, a translating terrane is formed outboard of the arc. The present San Andreas fault region illustrates a variation of oblique convergence because the relative plate motion is only 5° more northerly than the San Andreas fault (DeMets et al., 1990). A major fault (San Andreas) has transcurrent motion, but it is bounded on either side by belts of thrust faults ( Fig. 2b) (Namson and Davis, 1988). Because of a history of faults jumping in the San Andreas system, there are numerous terranes adjacent to the San Andreas fault that are presently being translated (Fig. 2b). The Gulf of California illustrates two styles of oblique divergence that mirror those in oblique convergence ( Fig. 2c and d). In an earlier stage, the plate boundary had regional strain partitioning
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with a transform fault at the plate boundary and a rift within the continental plate that formed on a recently inactive volcanic arc (Fig. 2c) (e.g. Hausback, 1984; Stock and Hodges, 1989). In this stage, a future terrane was forming between the rift and transform fault. In the modern stage of the Gulf of California, a series of long transform faults and short spreading ridges are linked in a right-stepped fashion to form the plate boundary ( Fig. 2d ) (e.g. Lonsdale, 1989). This new plate boundary formed on the previous rift. A continental terrane (Baja California peninsula and part of southern California) is thus moving along the plate boundary ( Umhoefer and Dorsey, 1997). It is well established that modern fragments of continental crust, or terranes, have translated at least many hundreds of kilometers. Coastal California west of the San Andreas fault and Baja California has moved northward 225–330 km in the Neogene (Crowell, 1979; Gastil et al., 1981; Stanley, 1987; Powell and Weldon, 1992). The eastern part of the south island of New Zealand has translated ~500 km along the oblique convergent plate boundary (Norris et al., 1990; Walcott, 1998). Western Sumatra is moving northward as an active terrane. Modern examples of active terranes that have translated thousands of kilometers are uncommon, but they do exist, as discussed below for the San Andreas fault system.
3. Lack of preservation of major strike-slip faults Large strike-slip fault systems that are now inactive can be preserved in the geologic record (e.g. Great Glen fault system of Scotland, Tintina and other faults of northern British Columbia, Yalakom and related faults of southern British Columbia). However, I maintain that strike-slip fault systems are much less likely to be preserved than thrust or normal fault systems (see also Woodcock, 1986). First, dip-slip systems are commonly wide arrays of faults, while strike-slip systems are commonly narrower in plan view. Thus, dip-slip fault systems are more likely to be preserved and less likely to be wholly overprinted in the geologic record. Thrust fault systems inherently widen because they grow by new faults cutting
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into their own foreland (e.g. Boyer and Elliott, 1982). Normal fault systems extend the land and therefore also inherently widen through time, and gently inclined detachment systems are very wide relative to their offset (e.g. Gibbs, 1984; Wernicke, 1985). Strike-slip fault systems only widen in special circumstances and are more likely to be narrow relative to their length and offset (Sylvester, 1988). Woodcock (1986) also pointed out that seismic reflection data emphasize more gently inclined structures as steep structures are difficult to image. Finally, we commonly simplify plate tectonics into three orthogonal classes of plate boundaries, and this ignores the common oblique boundaries. Four processes contribute to the improbability of preserving large-offset strike-slip fault systems. These are illustrated with well-understood examples from the ancient and modern geologic record. 3.1. Strike-slip faults commonly destroyed The Tintina, Teslin-Kutcho, Finlay, Pinchi, and related faults in north-central British Columbia (Fig. 3) were active in Late Cretaceous to early Tertiary time as a major strike-slip fault system with at least 1000 km of dextral offset (Gabrielse, 1985). Only part of the 1000 km of offset is found in southern British Columbia, yet there is no reason to believe that it died out to the south. Price and Carmichael (1986) presented an interpretation of the Tintina fault that showed that ~300 km of northerly dextral offset can be accounted for by the northeastward motion of the thrust faults of the southern Canadian Rocky Mountain thrust belt and the extension in the southern Omineca belt (Fig. 3). Another ~100 km of offset was transferred to the Fraser fault in southern British Columbia (Price and Carmichael, 1986). It is possible that another ~120 km of offset was transferred south to the Yalakom fault through central British Columbia (Coleman and Parrish, 1991; Umhoefer and Kleinspehn, 1995). This leaves ~500–600 km of offset of the northern British Columbia faults that is not accounted for in the south. Umhoefer (1994) and Irving et al. (1996) pointed out that these missing strike-slip faults may have run through the hinterland of the
Fig. 3. Map of strike-slip faults of British Columbia. Ib is the Intermontane belt and Ob is the Omineca belt, which is also in gray. Note that the strike-slip faults of east-central British Columbia end to the south without explanation. The faults with rectangles are major Eocene normal faults; the thrust faults are major Cretaceous to early Tertiary in age.
Rocky Mountain thrust belt in the southern Omineca and Intermontane belts (Fig. 3). In this model, the strike-slip faults are likely to merge downward in the middle crust with large thrust faults similar to a model for an orogenic float by Oldow et al. (1989, 1990). In this case, two aspects of the geology of those belts would destroy or obscure the faults. First, the southern Intermontane belt includes widespread areas of poor exposure of the Cache Creek terrane or younger rocks that cover older structures. The Cache Creek terrane is dominated by bedded chert and greenstone and argillite that is not readily amenable to finding large offsets on faults. Further, large portions of the southern Intermontane belt are covered by Eocene volcanic and sedimentary rocks that are younger than the faults in question ( Wheeler and McFeely, 1991). Second, in the southern Omineca belt, Eocene
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extension probably destroyed virtually all upper crustal rocks and faults by uplift and erosion or burial beneath Eocene basins. If the large strikeslip faults that extended south from northern British Columbia ran through the Omineca belt, then they were probably confined to the upper crust, as shown by Oldow et al. (1989, 1990). After thrusting stopped at the end of the Paleocene in the southern Omineca belt, a major episode of extension commenced and occurred throughout the Eocene (Parrish et al., 1988). The extension uplifted vast areas of the lower crust of the region, eroded large areas of upper crust into nearby basins, and buried still other areas as the upper plates of the detachment fault systems were covered by basins and volcanic rocks. The result is that when workers produce balanced cross-sections through the southern Omineca belt, they reveal that virtually no upper crust from the thrust belt (where the strike-slip faults probably were) is now exposed at the Earth’s surface (Parrish et al., 1988). In effect, a series of related events make strike-slip faults in the hinterland of an obliqueconvergent thrust belt likely targets for overprinting and destruction. Extension commonly occurs after thrusting ends in the hinterland of a thrust belt, where the belt is the highest and has the most potential energy to extend. It follows, then, that any large-scale strike-slip fault system in the hinterland of an oblique thrust belt will be largely destroyed if subsequent extension occurs because it will produce widespread uplift, erosion, subsidence, and volcanism. 3.2. Larger strike-slip fault systems are anastomosing with complex terrane translation The San Andreas fault system of California and Alpine–Marlborough fault systems of New Zealand provide examples of active, large-scale translation of terranes. We can surmise from these examples that large-scale strike-slip fault systems are unlikely to be preserved in full. Atwater and Stock (1998) recently reanalyzed the motion of the Pacific plate relative to North America off of California. They showed that since 24 Ma, there have been 1020 km of northwestward motion of the Pacific plate relative to North America along
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the greater San Andreas transform plate boundary. Typical of large, strike-slip fault systems, the San Andreas transform system comprises numerous, anastomosing faults (Fig. 4). The transform motion was first accommodated on faults now offshore of California and then jumped inland (Crowell, 1979; Sedlock and Hamilton, 1991; Nicholson et al., 1994). The western faults are not well known in the offshore but lie within the narrow continental shelf off of California (Nicholson et al., 1994). This means that there is a narrow, elongate ‘terrane’ off of California that has moved ~1000 km relative to North America ( Fig. 4). The translation of that terrane has been by means of numerous faults, and importantly, only about 20–25% of that motion is on the largest fault, the San Andreas. There are numerous elongate ‘terranes’ in coastal California that have moved ~300–800 km relative to the North America plate. The anastomosing strike-slip fault geometry and eastward jumping of the San Andreas fault system indicates that no one fault accommodates all, or even the majority, of the translation of the terranes of western California and the offshore borderland. This pattern of faulting also implies that many of the faults related to translation of an individual terrane will not be preserved on the boundary of that particular terrane. As an example, the terrane offshore of California with the most translation has only one major fault bounding it (Santa Lucia, Fig. 4) that has had an offset that was only a small portion of the total terrane translation. In order for a terrane in southern California to include all of the faults responsible for the translation, it would have to move continually along the easternmost fault of the system. In any other scenario, some of the faults involved in the translation will be ‘left behind’ within the North American continent. Because strike-slip fault systems involve common jumping and splaying of faults (e.g. Sylvester, 1988), it is unlikely that the whole system of faults will be preserved in any given terrane that has translated large distances. Fig. 5 illustrates another fault scenario in which the fault along a terrane has less offset than the terrane. In this case, strike-slip faulting jumps to the east (right) and then jumps west. This later jump in
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Fig. 4. Map of major faults of the greater San Andreas fault system. The San Andreas fault is SA. The dashed line to the west is the approximate position of the boundary between oceanic and continental crust. The narrow ‘terrane’ in the offshore borderland of California that has had most or all of the translation of the Pacific plate relative to North America is gray. The Santa Lucia bank fault that bounds the terrane on the east is SL.
faulting leaves behind two terranes, C and D. Thus, fault 4 that bounds terrane B has much less displacement than terrane B itself. The Alpine–Marlborough fault systems in New Zealand offer parallel examples to the San Andreas fault system. Since 6 Ma, the plate boundary within the South Island of New Zealand has been oblique convergent with 70–110 km of convergence and 230 km of dextral translation ( Walcott, 1998). The plate boundary is relatively narrow in the central part of the South Island along the Alpine fault ( Walcott, 1998). In the
northern part of the South Island, the Alpine fault splays out into the Marlborough fault system of large strike-slip faults and a series of thrust faults of the Hikurangi thrust system to the east (Lamb and Bibby, 1989; Walcott, 1998). Here, the plate boundary is at least 150 km wide. Similar to the San Andreas fault system, the outer strike-slip faults of the Marlborough system are actively translating terranes that have moved most of the distance of the Pacific plate relative to Australia (Lamb and Bibby, 1989). Fault slices, or terranes, presently being translated along the Alpine–
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Fig. 5. Model for a hypothetical strike-slip fault system in which faulting jumps progressively east and then jumps west in the last stage. In this case, some faults are left behind and are not connected to the translated terrane. In this manner, terrane B has much more translation than fault 4 that lies adjacent to terrane B. The numbered lines are strike-slip faults in the order of formation, and the lettered areas are the terranes that develop. Thick lines are faults that move in the subsequent stage. The word ‘Continent’ remains fixed. The letter for each terrane is fixed within the terrane and moves when the terrane moves. See text for discussion.
Wairau fault in the northernmost South Island have moved considerably less than the total plate motion. In addition, the plate margin in New Zealand is evolving with major vertical-axis rotations such that many strike-slip faults have become overprinted as oblique-slip or thrust faults (Lamb and Bibby, 1989), illustrating another means of partially destroying strike-slip faults. 3.3. Faults in narrow oceans along obliquedivergent boundaries have a low preservation potential Major faults in oblique-rift oceans have a low potential for preservation. Baja California is currently a continental terrane ( Fig. 6) that has translated ~300 km relative to North America
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since ~6 Ma (Gastil et al., 1981; Lonsdale, 1989) and is likely to be preserved in any future accretion event because of the relative buoyancy of continental crust. In contrast, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the plate boundary lie (Fig. 6), has much less potential for preservation. This means that most of the faults taking up the current motion of Baja California relative to North America are likely to be destroyed by future tectonism. Furthermore, the oblique-divergent plate boundary in the Gulf of California formed within a Miocene volcanic arc (Hausback, 1984) such that the geology on either side of the gulf is similar over vast areas (Carta Geologica, 1980). Thus, older geologic units offer chances for correlation across the plate boundary only in the northern and southern parts of the Baja California peninsula (Gastil, 1991). This emphasizes the importance of the structures on the Baja California peninsula (terrane) for detecting the terrane motion. However, as is discussed next, they are of limited value in detecting the 300 km of translation of the Baja California peninsula. 3.4. Strike-slip faults are under-represented in oblique-divergent plate boundaries Experimental and analytical data on oblique rifting ( Withjack and Jamison, 1986; Fossen and Tikoff, 1993) and field data from the margins of oblique rifts (e.g. Umhoefer and Stone, 1996) show that normal faults predominate over strike-slip faults at all but the most highly oblique plate boundaries. Thus, in the very margins of terranes where the style of faulting is the key to deciphering the type of plate boundary, strike-slip faults are under-represented amongst the secondary faults. The central part of the Baja California peninsula is an almost ideal example of a terrane translating along a highly oblique-divergent plate boundary ( Fig. 6) because it has a relatively simple history ( Umhoefer and Dorsey, 1997). The northern and southern parts of the Baja California peninsula are cut by through-going strike-slip and normal faults, but the central part of the peninsula is not transected by strike-slip faults. That is, at the latitude of central Baja California, plate motion is
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P.J. Umhoefer / Tectonophysics 326 (2000) 23–35
Fig. 6. Bathymetry and faults of the plate boundary in the Gulf of California and domains of Baja California. The bathymetry is shown in contours of 200, 1000, 2000, and 3000 m. The area near Loreto in Fig. 7 is shown in the box in the southern part of the peninsula.
largely confined to the main transform–ridge system in the Gulf of California. Despite the fact that the central domain of Baja California has translated ~300 km since 6 Ma, and that the plate boundary is highly oblique [the
plate boundary has a rift angle (a) between the trend of the boundary and the azimuth of plate motion of only 20°], there are few strike-slip faults in central Baja California. The southern Loreto basin and Loreto fault ( Fig. 7) were active in the
P.J. Umhoefer / Tectonophysics 326 (2000) 23–35
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Fig. 7. (A) simplified geologic map of the region of the Loreto fault and Loreto basin in Baja California Sur, Mexico. The location of the figure is shown on Fig. 6. (B) Typical orientation, type of faults, and extension direction (e) of the SE Loreto fault array. The dashed line is the azimuth of plate motion in the Gulf of California and the gray line the trend of the gulf plate boundary. (C ) Faults with striations from the SE Loreto fault array. (D, E, F ) Plots similar to (B) with the expected orientation and type of faults and extension direction (e) of the margin of oblique divergent plate boundaries with increasing rift angles.
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P.J. Umhoefer / Tectonophysics 326 (2000) 23–35
Pliocene during the highly oblique motion of Baja California relative to North America ( Umhoefer et al., 1994; Dorsey and Umhoefer, 2000). The southern Loreto fault is a dextral-normal fault with average rake of striae in clay gouge of 15° ( Umhoefer and Dorsey, 1997). The strike slip offset on the fault is not well known, but is likely to be in the range of 1–5 km. The Loreto fault is the largest strike-slip fault known from the entire central domain of the Baja California peninsula (Fig. 6). The SE Loreto fault array cuts part of the Loreto basin, and unequivocal evidence shows that it was active during terrane translation in the latest Pliocene, mainly from 2.4 to 2.0 Ma ( Umhoefer and Stone, 1996; Dorsey and Umhoefer, 2000). This fault array is dominated by normal faults with tens to 250 m of offset and has a minor population of dextral-normal faults with up to 500 m of offset ( Umhoefer and Stone, 1996). The SE Loreto fault array has a style of faulting and extension direction relative to plate motion that confirm the first-order results from clay modeling by Withjack and Jamison (1986). If we use the data from the SE Loreto fault array (Fig. 7b and c) ( Umhoefer and Stone, 1996), a larger region of the central domain (Angelier et al., 1981), experiments ( Withjack and Jamison, 1986), and analytical studies (Fossen and Tikoff, 1993), we can summarize the nature of secondary faults on the margins of oblique-divergent plate boundaries as the rift angle (a) increases ( Fig. 7d– f ). At a=0–20°, strike-slip faults will be the majority of the secondary faults. At a=~20 to ~35° both strike-slip and normal faults are present, but normal faults will predominate. At a>~35°, there are few or no strike-slip faults. Secondary strikeslip faults will have the same sense of offset as the plate boundary and be nearly parallel to the rift trend. Normal faults will be oriented about 20– 40° clockwise from the rift trend in transtensional plate boundaries undergoing dextral shear. Unlike the type of faulting, which changes significantly at certain thresholds, the extension direction changes smoothly with increasing rift angle (e in Fig. 7d– f ) (Fossen and Tikoff, 1993). The most important point here is that if the main transform faults in highly oblique-divergent
plate boundaries have a low preservation potential, as argued above, then the structures on the margins on the translating continental terrane are a key to understanding the translation. Recent data suggest that strike-slip faults are only dominant at the most highly oblique boundaries (a=<20°). They further show that for oblique-divergent boundaries where translation is dominant but a=20–45°, normal faults will dominate over strike-slip faults. For these types of oblique-divergent boundaries, correlations of bedrock geology across the former plate boundary or paleomagnetic data will probably be required to understand the amount of translation.
4. Discussion and conclusions The premise of this paper is that it has been difficult to find the major strike-slip faults that produce large-scale terrane translation because we are only beginning to understand how faulting works along oblique and transform plate boundaries and because major strike-slip faults are prone to partial or complete destruction. I have also reiterated some points from Woodcock (1986) that show that strike-slip faults are not as likely to be detected in ancient orogenic belts. Many of our methods of examining mountain belts, especially the practice of using across-strike cross sections and seismic reflection sections, favor the detection of lower-angle, dip-slip faults and not steep strikeslip faults. The evidence presented here that strikeslip fault systems are difficult to preserve does not suggest that geologic evidence is not important to the testing of whether terranes have moved great distances. Cowan et al. (1997) have pointed out numerous geologic methods that can test the validity of far traveled terranes. These are the main arguments presented here that major strike-slip faults are not likely to be preserved. (1) Strike-slip faults in continents are commonly destroyed. Strike-slip fault systems are inherently narrow compared to dip-slip systems. Strike-slip fault systems commonly link downward into thrust or oblique faults and are therefore destroyed when a region is uplifted and eroded. This series of events is especially prone to occur
P.J. Umhoefer / Tectonophysics 326 (2000) 23–35
because strike-slip faults commonly occur in the hinterland of oblique-convergent margins. (2) Strike-slip fault systems of >1000 km scale will likely be an anastomosing array, and thus only terranes at the margins of the belt will record the full relative plate motion. Strike-slip faults that bound terranes will commonly have much less offset than the total motion of the terrane. (3) Major transform faults in highly oblique-divergent plate boundaries have a low preservation potential because they form in dense oceanic crust in the deep ocean. (4) Oblique convergent and oblique divergent plate boundaries are commonly accommodated by oblique-slip thrust and normal faults such as the examples given here of the Alpine fault of New Zealand and the Loreto fault of Baja California. Plate margins with an obliquity between 20 and 45° are likely to be dominated by dip-slip faults, even though the plate motion is mostly translational. These arguments about the preservation of strike-slip fault systems have important implications for the analysis of translated terranes. If it is unlikely that the complete strike-slip fault system that moved a terrane would be found, how can we evaluate how far terranes have translated? Paleomagnetic data may be the only definitive approach to identify far-traveled terranes (Beck, 1991). However, rigorous paleomagnetic studies that fully meet the methods of paleomagnetism and are conducted in rocks in which paleohorizontal is unambiguously known are essential. The ability to eliminate sediment compaction where sedimentary rocks are studied is necessary. Multiple datasets are necessary to eliminate the possibility of anomalous local results. Where paleomagnetic studies suggest large-scale translation, geologic evidence can suggest compatibility with the style of translation, but probably cannot provide evidence for the magnitude of translation. Despite the conclusions of this paper, evidence from structural geology is a key to demonstrate the feasibility of large-scale translation of terranes. The overall style of deformation should be a wide belt of distributed structures with the range of strike-slip to dip-slip faulting. Major strike-slip faults should be found on the translated terrane and on the plate from which the terrane traveled,
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even if they do not record the complete record of translation. There should be a compatible style of secondary strike-slip faults and related structures. These structures would include oblique-slip thrust or normal faults, fault arrays with mixed dip-slip and oblique-slip faults, and folds compatible with transtensional and transpressional deformation. Many fault arrays may display an en echelon pattern. In ideal cases, the style and orientation of faults may suggest a range of obliquity for the ancient plate margin ( Teyssier et al., 1995; Umhoefer and Dorsey, 1997). Basins formed during major terrane translation will likely be either classic pull-apart basins or strike-slip related basins that are hybrids of classic rifts or foreland basins.
Acknowledgements This research was supported by NSF grants EAR 9304426 and EAR 9526506. I thank Richard Sedlock and Basil Tikoff for reviews of the manuscript and many colleagues too numerous to mention here for discussions of terrane translation, strike-slip faults, and Baja BC. Most importantly, I thank Myrl Beck for his provocative, creative, and pioneering efforts in the problems of terrane translation and Bernie Housen for organizing the session in honor of Myrl.
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