Geophysics Unit of Menlo Park-GUMP

GMEG - Geophysics Unit of Menlo Park, CA- (GUMP)

 

Geophysics Unit of Menlo Park, CA (GUMP)

U.S. Geological Survey - Western Region - Geology and Geophysics

SAN BERNARDINO BASIN, SOUTHERN CALIF.


GUMP is participating in a project with the USGS Earthquake Hazards Team, the USGS Water Resources Division, and the Southern California Areal Mapping Project (SCAMP) to decipher the position and geometry of the Rialto-Colton fault in the San Bernardino basin, CA. This information will be used by the Water Resources Division in their hydrologic model of the San Bernardino basin. The following report is extracted from our 1999 American Geophysical Union poster: Structural Model of the San Bernardino Strike-Slip Basin, Southern California, from Regional Gravity Data.

The San Bernardino basin is a strike-slip basin centered over a right bend on the northern end of the San Jacinto fault zone. Slip is transferred from the San Jacinto fault zone to the San Andreas fault across the basin. There are few known surface expressions of strike-slip faults in the San Bernardino basin; what mappable traces do exist occur along the young San Jacinto fault north of the Santa Ana River wash (Fife et al., 1976; Morton, 1978; Wesnousky et al., 1991). The Quaternary alluvial fill from sources in the surrounding mountain ranges has obscurred evidence of other faults in the basin. Isostatic gravity mapping, gravity modeling, geologic data, and aeromagnetic data interpretations indicate that slip on the San Jacinto fault was initially transferred to surrounding faults by stepping both in a right-lateral and left-lateral sense. More recent movement on the fault has been accommodated by a right-bending fault geometry. A reconstruction of the magnetic data along known and inferred faults provides a mechanism for interpretation of fault movements in the San Bernardino basin.

One of the biggest hurdles in understanding the history of movement along the San Andreas fault system is understanding which fault strands have been active at what times and how slip is transferred between fault strands. The faults between San Gorgonio Pass and the Mojave Desert comprise one of the more complex knots in the San Andreas fault system.

The San Jacinto fault is a major element of this complex system. It has experienced from 25-28 km of right slip in the past 1.2-5 Ma (Powell, 1993; Matti and Morton, 1993). It does not continue through the San Gabriel Mountains and there is no agreement as to how its slip has been accommodated. Many conjecture that this slip has been taken up partly by thrusting and left-lateral slip in the San Gabriel Mountains and partly by transfer to the Mojave Desert segment of the San Andreas fault (Powell, 1993; Matti, 1993). Transfer of slip to the Mojave Desert segment of the San Andreas fault has created the San Bernardino strike-slip basin.

Index Map

Click on image to see fault index map (64 K).


What is the method of slip transfer?

Fault slip on the San Jacinto fault was initially transferred to surrounding faults by stepping both in a right-lateral and left-lateral sense. Subsequent slip was accommodated by a right bend developed in the fault.


What is the supporting data?

A. Isostatic gravity data.

Gravity Map

Click image to see large format (120 K).

Figure 2: Isostatic gravity map of the San Bernardino basin. New gravity data define more precisely the western edge of the San Bernardino Basin. The steep gradient associated with the San Jacinto fault outlines a stepping geometry along the western edge of the basin (black line). In addition, distinct, steep gradients splay off of the San Jacinto gradient up to the San Gabriel Mountain front (blue and magenta lines). We interpret these steep gradients as faults that offset the basement surface.

 

B. 2-D gravity modeling.

Figure 3: Simple 2-D model based on gravity, well log, and density data. Model line is shown in figure 2. Basin fill density = -0.5 g/cc.

The Rialto-Colton fault (blue line in figure 2) had been inferred by previous workers on the basis of water-table topography and ground water modeling, as have many of the faults in the San Bernardino basin (Dutcher and Garrett, 1963; Woolfenden and Kadhim, 1997). 2-D modeling confirms that the Rialto-Colton is a major, basement-cutting fault. The 2-D model was based on measured and assumed density contrasts between basin fill and basement rocks, well log data, and the gravity profile across the fault. It suggests that as much as 600 m of vertical offset has occurred on a sloping to near-vertical fault plane.

2-D Model

Click image to see large format (160 K).


 

C. Geologic data.

Geologic Map

Click on image to see large format (172 K).

Figure 4: Simplified geologic map of the San Bernardino basin. There are no known surface expressions of any strike-slip faults in the San Bernardino basin (Morton and Matti, 1993). The current trace of the San Jacinto fault (dotted line) is inferred from earthquake data (Hill et al., 1990).

A trench dug in 1995 at the inferred location of the Rialto-Colton fault near the foot of the San Gabriel Mountains revealed a fault striking N45 to 55 W and dipping to the NE at 53 degrees (Treiman, 1999, personal communication). The most recent sediment disturbed by this fault was Holocene in age and showed evidence for reverse faulting.

Geologic maps of the San Gabriel Mountains show strike-slip faults that extend from the mountain front up to the South Fork Lytle Creek fault (Morton and Matti, 1987). Two of the these faults line up with fault locations inferred from the gravity map (blue and magenta lines). In addition to displaying right-lateral displacement of ~2 km on a mylonite zone, the Day Canyon fault (which aligns with the Rialto-Colton fault) coincides with an apparent offset of fault traces in the Cucamonga fault zone of approximately the same magnitude.


 

D. Aeromagnetic data.

Figure 5: An enhanced aeromagnetic map defines subtle, linear magnetic zones within basement rocks beneath the San Bernardino basin and surrounding areas. The Rialto-Colton fault crosses one such feature; the magnetic anomaly is offset in a right-lateral sense in four distinct breaks at a bend in the fault. We interpret this as a left-step in the Rialto-Colton fault. Another linear anomaly crosses the present path of the San Jacinto fault at an oblique angle. This anomaly shows a minimum of 4 km of right-lateral offset. Aeromagnetic Map

Click image to see large format (244 K).

Figure 6: The aeromagnetic map of the San Gabriel Mountains confirms the offsets in the mylonite zone mapped by Morton and Matti (1987). It also shows possible right-lateral and left-lateral offsets connected with the Lytle Creek fault, the and the San Antonio Canyon fault. Aeromagnetic Map

Click image to see large format (120 K).


What is the tectonic reconstruction?

Reconstruction

Click on image to see reconstructed maps.

The magnetic and gravity data are in concurrence with existing geologic data as far as the relative amount of offset for each fault, so the following reconstruction is based on all sets of data. Relative magnetic highs are shown for reference. The Rialto-Colton fault is on strike with the southern extension of the San Jacinto fault and parallel with the San Andreas fault across the San Bernardino basin. In addition, there is no cross-fault continuation for the Rialto-Colton fault after restoration of 25 km slip on the San Jacinto fault. We therefore propose that the Rialto-Colton fault was an early strand of the San Jacinto fault zone.


What are the problems with this model,
and how could they be solved?


Further directions for this research include:

  1. Further analysis of magnetic data to resolve discrepancies with the geologic data.
  2. Analysis of seismic reflection data across the Rialto-Colton fault that has been collected by the USGS earthquake hazards team.
  3. Further 2-D and 3-D basin analysis using the most recent gravity and rock density data gathered in November.
  4. Analysis of fault movement progression for documented principals of how a junction such as this might develop with time; i.e. does the proposed progression minimize space problems, or can we predict how a similar junction might develop?
  5. Find possible analogues in other parts of the San Andreas System.

 

References and Further Reading

Anderson, M.L., Jachens, R., and Woolfenden, L., 1999, Structural model of the San Bernardino strike-slip basin, southern California, from regional gravity data: Eos, Transactions, American Geophysical Union, 1999 Fall Meeting Suppl., v. 80, no. 46, p. F1002.

Dutcher, L.C., and Garrett, A.A., 1963, Geologic and hydrologic features of the San Bernardino area, California-with special reference to underflow across the San Jacinto fault: U. S. Geological Survey Water-Supply Paper 1419, 114 p.

Fife, D.L., Rodgers, D.A., Chase, G.W., Chapman, R.H., and Sprotte, E.C., 1976, Geologic hazards in southwestern San Bernardino County, California: California Division of Mines and Geology Special Report 113, 40 p.

Hill, D.P., Eaton, J.P., and Jones, L.M., 1990, Seismicity, 1980-86: in Wallace, R.E., ed., The San Andreas Fault System, U. S. Geological Survey Professional Paper 1515, p. 114-151.

Matti, J.C., and Morton, D.M., 1993, Paleogeographic evolution of the San Andreas fault in southern California: A reconstruction based on a new cross-fault correlation: in Powell, R.E., Weldon, R.J. II, and Matti, J.C., eds., The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution, Geological Society of America Memoir 178, p. 107-159.

Morton, D.M., 1976, Geologic map of the Cucamonga fault zone between San Antonio Canyon and Cajon Creek, southern California: U.S. Geological Survey Open-File Report 76-726, scale 1:24,000.

Morton, D.M., 1975, Synopsis of the geology of the eastern San Gabriel Mountains, southern California: in Crowell, J.C., ed., San Andreas fault in southern California: California Division of Mines and Geology Special Report 118, p. 170-176.

Morton, D.M., 1978, Geologic map of the San Bernardino South 7.5' quadrangle, California: U.S. Geological Survey Open-File Report 78-20, scale 1:24,000.

Morton, D.M., and Matti, J.C., 1993, Extension and contraction within an evolving divergent strike-slip fault complex: The San Andreas and San Jacinto fault zones at their convergence in southern California: in Powell, R E., Weldon, R.J. II, and Matti, J.C., eds., The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution, Geological Society of America Memoir 178, p. 217-230.

Morton, D.M., and Matti, J.C., 1990, Geologic map of the Cucamonga Peak 7.5' Quadrangle, California: U.S. Geological Survey Open-File Report 90-694, scale 1:24,000.

Morton, D.M., and Matti, J.C., 1990, Geologic map of the Devore 7.5' Quadrangle, California: U.S. Geological Survey Open-File Report 90-695, scale 1:24,000.

Morton, D. M., and Matti, J. C., 1987, The Cucamonga fault zone: Geologic setting and Quaternary history: U. S. Geological Survey Professional Paper 1339, p. 179-203.

Powell, R.E., 1993, Balanced palinspastic reconstruction of pre-late Cenozoic paleogeography, southern California: geologic and kinematic constraints on evolution of the San Andreas fault system: in Powell, R.E., Weldon, R.J. II, and Matti, J.C., eds., The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution, Geological Society of America Memoir 178, p. 1-106.

Wesnousky, S.G., Prentice, C.S., and Sieh, K.E., 1991, An offset Holocene stream channel and the rate of slip along the northern reach of the San Jacinto fault zone, San Bernardino Valley, California: Geological Society of America Bulletin, v. 103, p. 700-709.

Woolfenden, L.R., and Kadhim, D., 1997, Geohydrology and water chemistry in the Rialto-Colton basin, San Bernardino County, California: U. S. Geological Survey Water-Resources Investigations Report 97-4012.

Information on the most recent geologic mapping in the San Bernardino basin area can be obtained at the Southern California Areal Mapping Project (SCAMP) web site.


By Megan Anderson

Further information can be obtained by contacting the following co-investigators:

Robert Jachens (650-329-5300; jachens@usgs.gov)

Linda Woolfenden (916-278-3014; lrwoolfe@usgs.gov)


Geology and Geophysics || GUMP || USGS Geophysics Hub

 

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://geomaps.wr.usgs.gov/gump/people/jachens/rialto/rialto.html
Page Contact Information: G & G Webmasters
Page Last Modified: 13-Dec-2016@16:37