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Slip Rate Studies Along the Sierra Madre-Cucamonga Fault System Using Geomorphic and 10 Be Cosmogenic Surface Exposure Age Constraints.

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Presentation on theme: "Slip Rate Studies Along the Sierra Madre-Cucamonga Fault System Using Geomorphic and 10 Be Cosmogenic Surface Exposure Age Constraints."— Presentation transcript:

1 Slip Rate Studies Along the Sierra Madre-Cucamonga Fault System Using Geomorphic and 10 Be Cosmogenic Surface Exposure Age Constraints

2 Active Faults in the Los Angeles Region

3 Geodetic vs Geologic Rates Geodetic (GPS) shortening rates Geodetic (GPS) shortening rates ~6 mm/yr between JPL and USC (Argus et al., 1999) ~6 mm/yr between JPL and USC (Argus et al., 1999) How much slip on Sierra Madre fault system? How much slip on Sierra Madre fault system? What is the magnitude of strain, and how is strain partitioned across the L.A. basin? What is the magnitude of strain, and how is strain partitioned across the L.A. basin? Two competing models Two competing models Crustal escape model (Walls et al., 1998) Crustal escape model (Walls et al., 1998) Crustal thickening model (Argus et al., 1999; Bawden 2001) Crustal thickening model (Argus et al., 1999; Bawden 2001)

4 Previous Slip Rate Studies

5 Escape Tectonics vs Crustal Thickening > 50% of N-S contraction is accommodated by E-W extension > 50% of N-S contraction is accommodated by E-W extension N-S contraction rates 7-9 mm/yr N-S contraction rates 7-9 mm/yr E-W extension rates ~6 mm/yr E-W extension rates ~6 mm/yr Does not consider viscoelastic effects of the SAF and SJF Does not consider viscoelastic effects of the SAF and SJF Walls et al. (1998) Argus et al. (1999) N-S contraction is almost entirely accommodated by crustal thickening N-S contraction is almost entirely accommodated by crustal thickening N-S contraction rates 5.8 ± 1.9 mm/yr N-S contraction rates 5.8 ± 1.9 mm/yr E-W extension 0-2 mm/yr E-W extension 0-2 mm/yr Removes viscoelastic effects of the SAF and SJF Removes viscoelastic effects of the SAF and SJF

6 Two Study Areas

7 Tectonic Geomorphic Analysis and Mapping Fault scarps, fluvial terraces and fan surfaces were identified and mapped using aerial photography and digital topography. Fault scarps, fluvial terraces and fan surfaces were identified and mapped using aerial photography and digital topography.

8 Measuring Uplift of Geomorphic Surfaces Construct topographic profiles across surfaces and fault scarps Construct topographic profiles across surfaces and fault scarps Total station surveys Total station surveys 5-20 ft contour maps 5-20 ft contour maps Measure vertical separation (uplift) of surfaces Measure vertical separation (uplift) of surfaces

9 Uplift, Shortening, and Dip-Slip Rates

10 Formation of Uplifted and Abandoned Fluvial Terrace Surfaces

11 Cosmogenic Nuclide Formation Cosmic rays consisting of neutrons, protons and muons strike and penetrate rocks at the earths surface. Cosmic rays consisting of neutrons, protons and muons strike and penetrate rocks at the earths surface. The cosmic rays interact The cosmic rays interact with Si and O in quartz to produce 10 Be and 26 Al nuclides. Cosmogenic Surface Exposure Ages

12 Western Sierra Madre Fault System

13 Pacoima Wash Terraces Abandoned terraces Abandoned terraces Q1-Q6 Q1-Q rupture 1971 rupture Three older fault scarps Three older fault scarps Qt4 surface Qt4 surface Moderately preserved Moderately preserved Fine-course gravel with discontinuous overbank deposits and cm soil development [seems thin] Fine-course gravel with discontinuous overbank deposits and cm soil development [seems thin] Agricultural modification Agricultural modification

14 Qt4 Surface Profile Total vertical separation of Qt4 surface across 3 fault scarps = 27 ± 2 m Total vertical separation of Qt4 surface across 3 fault scarps = 27 ± 2 m

15 View of Qt4 Surface

16 Sampling of Qt4 Surface Processed 5 individual subsurface samples Processed 5 individual subsurface samples Surveyed site for Surveyed site for suitable samples Collected 3 initial samples in depth profile Collected 3 initial samples in depth profile Subsurface depths: Subsurface depths: PW-1 = 1.09 m PW-1 = 1.09 m PW-2 = 0.70 m PW-2 = 0.70 m PW-3 = 0.33 m PW-3 = 0.33 m

17 Qt4 Model Surface ages Depth Corrected Ages Sample ID 10 Be model age PW-133,239 ± 1564 PW-231,196 ± 1287 PW-331,024 ± 1073 PW-B69,825 ± 3055 PW-C64,720 ± 2143

18 Results - Qt4 Surface Total uplift = 27 ± 2 m Total uplift = 27 ± 2 m Measured from topographic profile Measured from topographic profile 10 Be Model surface age = 31,561 ± 729 yr 10 Be Model surface age = 31,561 ± 729 yr Weighted mean age corrected for depth/latitude/altitude Weighted mean age corrected for depth/latitude/altitude Assumes zero erosion and zero inheritance Assumes zero erosion and zero inheritance Uplift rate = 0.9 ± 0.1 mm/yr Uplift rate = 0.9 ± 0.1 mm/yr Horizontal Shortening rate = 0.9 ± 0.3 mm/yr Horizontal Shortening rate = 0.9 ± 0.3 mm/yr Dip Slip rate = 1.2 ± 0.4 mm/yr Dip Slip rate = 1.2 ± 0.4 mm/yr Using estimated fault dip of 45 ± 10° Using estimated fault dip of 45 ± 10° Oblique slip rate = 1.2 ± 0.4 mm/yr Oblique slip rate = 1.2 ± 0.4 mm/yr Using estimated 45 ± 10 ° rake from a 1:1 ratio of left oblique motion during the 1971 San Fernando earthquake Using estimated 45 ± 10 ° rake from a 1:1 ratio of left oblique motion during the 1971 San Fernando earthquake

19 Lopez Canyon Surface

20 Abandoned alluvial surface Abandoned alluvial surface Two fault scarps Two fault scarps Scarps 4 and 5 Scarps 4 and 5 Limited preservation of remnants (along ridges) in highly dissected surface Limited preservation of remnants (along ridges) in highly dissected surface

21 Surveyed Topographic Profiles Uplift across scarp 5 = 8.5 ± 0.5 m Uplift across scarp 4 = 7.0 ± 0.5 m Cumulative offset across both scarps = 15.5 ± 0.7 m

22 Lopez Canyon Surface Ages Processed 4 surface samples Processed 4 surface samples Lat./Alt. Corrected Ages Sample ID 10 Be model age LC-129,029 ± 884 LC-233,443 ± 1009 LC-333,181 ± 943 LC-424,273 ± 851

23 Results - Lopez Canyon Surface Total offset = 15.5 ± 0.7 m Total offset = 15.5 ± 0.7 m Measured from topographic profiles. Measured from topographic profiles. 10 Be Model surface age = 29,540 ± 458 yr 10 Be Model surface age = 29,540 ± 458 yr Weighted mean age corrected for latitude/altitude Weighted mean age corrected for latitude/altitude Assumes zero erosion and zero inheritance. Assumes zero erosion and zero inheritance. Uplift rate = 0.5 ± 0.1 mm/yr Uplift rate = 0.5 ± 0.1 mm/yr Horizontal Shortening rate = 0.6 ± 0.2 mm/yr Horizontal Shortening rate = 0.6 ± 0.2 mm/yr Dip Slip rate = 0.8 ± 0.3 mm/yr Dip Slip rate = 0.8 ± 0.3 mm/yr Using measured fault dip in exposure of 40 ± 10° Using measured fault dip in exposure of 40 ± 10°

24 Wilson Canyon Fan Surface

25 Abandoned fan surface Abandoned fan surface Uplifted from modern valley floor Uplifted from modern valley floor Remnants of surface preserved along ridge lines Remnants of surface preserved along ridge lines

26 Southwest View of Wilson Canyon Fan Surface Remnant

27 Topographic Profile Uplift across fault scarp is 63.5 ± 5 m Uplift across fault scarp is 63.5 ± 5 m Hospital fault ‘Wilson Canyon’ fault

28 Wilson Canyon Fan Surface Ages Processed 6 surface samples Processed 6 surface samples Lat./Alt. Corrected Ages Sample ID 10 Be model age PW-1352,071 ± 1435 PW-1461,489 ± 1534 PW-1549,265 ± 1705 PW-1672,255 ± 1955 PW-1741,688 ± 1082 PW-1864,584 ± 2122

29 Wilson Canyon Fan Surface Ages Older sample ages reflect significant soil development but still may underestimate age of surface (?) Older sample ages reflect significant soil development but still may underestimate age of surface (?) Lat./Alt. Corrected Ages Sample ID 10 Be model age PW ± 1534 PW ± 1955 PW ± 2122 W* Mean65,245 ± 1049

30 Results - Wilson Canyon Fan Total Uplift = 63.5 ± 5 m Total Uplift = 63.5 ± 5 m 10 Be Model surface age = 65,345 ± 1049 yr 10 Be Model surface age = 65,345 ± 1049 yr Weighted mean age corrected for latitude/altitude Weighted mean age corrected for latitude/altitude Assumes zero erosion and zero inheritance. Assumes zero erosion and zero inheritance. Uplift rate = 1.0 ± 0.1 mm/yr Uplift rate = 1.0 ± 0.1 mm/yr Horizontal Shortening rate = 1.2 ± 0.7 mm/yr Horizontal Shortening rate = 1.2 ± 0.7 mm/yr Dip Slip rate = 1.5 ± 0.9 mm/yr Dip Slip rate = 1.5 ± 0.9 mm/yr Using estimated fault dip of 40 ± 20° Using estimated fault dip of 40 ± 20°

31 Cumulative Shortening Rate 1.2 ± 0.7 mm/yr 0.6 ± 0.2 mm/yr +0.9 ± 0.3 mm/yr 2.7 ± 0.8 mm/yr Check math 2.7 or 2.6 mm/yr?

32 Cucamonga Fault Zone

33 Day Canyon Fan Study Site Modified from Matti and Morton (1987)

34 Oblique Aerial Photograph of Day Canyon Fan Surface

35 Topographic Profile Analysis Three profiles across strand C Three profiles across strand C One profile across strand A and B One profile across strand A and B All profiles were constructed from total station surveys All profiles were constructed from total station surveys

36 Topographic Profile Analysis Total uplift across profile A (Qyf 1b )=5 ± 0.5 m Total uplift across profile A (Qyf 1b )=5 ± 0.5 m

37 Topographic Profile Analysis Total uplift across profile B (Qyf 1a )=7 ± 0.5 m Total uplift across profile B (Qyf 1a )=7 ± 0.5 m

38 Topographic Profile Analysis Total uplift across profile C (Qyf 1a )=12 ± 0.5 m Total uplift across profile C (Qyf 1a )=12 ± 0.5 m

39 Topographic Profile Analysis Total offset across profile D (Qyf 1a )= 20 ± 0.5 m Total offset across profile D (Qyf 1a )= 20 ± 0.5 m Total offset across fan surface Qyf 1a = 34 ± 0.7 Total offset across fan surface Qyf 1a = 34 ± 0.7

40 Sampling of Day Canyon Fan Surfaces Fan surface Qyf 1b Fan surface Qyf 1b 5 south of strand C 5 south of strand C Fan surface Qyf 1a Fan surface Qyf 1a 6 south of strand C 6 south of strand C 3 between A/B and C 3 between A/B and C 4 north of strand A/B 4 north of strand A/B 3 depth profile samples south of strand C 3 depth profile samples south of strand C Active wash Active wash 3 between A/B and C 3 between A/B and C

41 Sample Ages for Qyf 1b Surface (East) Samples collected south of fault strand C Samples collected south of fault strand C Lat./Alt. Corrected Ages I.D. 10Be Model Age DC-127,233 ± 883 DC-226,411 ± 906 DC-328,578 ± 958 DC-423,457 ± 956 DC-522,111 ± 760

42 Sample Ages of Qyf 1a Surface (West) Weighted mean model surface age = 33,395 ± 332 years Excluding samples that plot outside the yellow box Excluding samples that plot outside the yellow box

43 Results - Day Canyon Fan Total uplift = 34 ± 0.7 m (across 3 scarps) Total uplift = 34 ± 0.7 m (across 3 scarps) 10 Be Model surface age = 33,395 ± 332 yr 10 Be Model surface age = 33,395 ± 332 yr Weighted mean age corrected for depth/latitude/altitude Weighted mean age corrected for depth/latitude/altitude Assumes zero erosion and zero inheritance Assumes zero erosion and zero inheritance Uplift rate = 1.1 ± 0.1 mm/yr Uplift rate = 1.1 ± 0.1 mm/yr Horizontal Shortening rate = 1.6 ± 0.3 mm/yr Horizontal Shortening rate = 1.6 ± 0.3 mm/yr Dip Slip rate = 1.9 ± 0.35 mm/yr Dip Slip rate = 1.9 ± 0.35 mm/yr Using measured fault dip of 32.5 ± 5° from Matti et al. (1982) Using measured fault dip of 32.5 ± 5° from Matti et al. (1982)

44 Comparison With Previous Study (Morton and Matti, 1987 or Dolan et al., 200???) Geomorphic and soil chronologic study Geomorphic and soil chronologic study 36 m of uplift (vs our 34 m) surface Qyf 1a 36 m of uplift (vs our 34 m) surface Qyf 1a Surface age of ~13 ka using soil comparisons with radiometrically dated soil at Cajon Pass Surface age of ~13 ka using soil comparisons with radiometrically dated soil at Cajon Pass Slip rate ~ mm/yr Slip rate ~ mm/yr

45 Sources of strain accommodated by the Cucamonga fault zone. Regional north-south compression Regional north-south compression Slip transfer from the San Jacinto fault to San Andreas fault. Slip transfer from the San Jacinto fault to San Andreas fault. ????

46 Implications for a Lower Slip Rate 1) Strain distributed between the San Jacinto and San Andreas fault is almost entirely accommodated by interactions between smaller crustal blocks. 1) Strain distributed between the San Jacinto and San Andreas fault is almost entirely accommodated by interactions between smaller crustal blocks. 2) Only a small portion of the geodetically observed contraction is accommodated by the Cucamonga fault. 2) Only a small portion of the geodetically observed contraction is accommodated by the Cucamonga fault.

47 Slip Rates vs. Geodetic Rates Based on geologic slip rates, the Sierra Madre-Cucamonga fault zone (SMCFZ) accommodates between 10% - 50% of the estimated geodetic contraction rate of 4.4 ± 0.8 mm/yr Based on geologic slip rates, the Sierra Madre-Cucamonga fault zone (SMCFZ) accommodates between 10% - 50% of the estimated geodetic contraction rate of 4.4 ± 0.8 mm/yr Where is the additional slip accommodated? Where is the additional slip accommodated? Active structures located within the L.A. basin Active structures located within the L.A. basin Distributed deformation Distributed deformation Additional fault strands located within the SMCFZ. Additional fault strands located within the SMCFZ.

48 Active Faults in the Los Angeles Region

49 Conclusions Western Sierra Madre fault zone accommodates 2.7 ± 0.8 mm/yr of horizontal shortening Western Sierra Madre fault zone accommodates 2.7 ± 0.8 mm/yr of horizontal shortening Excluding the Wilson Canyon fault Excluding the Wilson Canyon fault Horiz. shortening rate = 1.5 ± 0.4 mm/yr Horiz. shortening rate = 1.5 ± 0.4 mm/yr Cucamonga fault accommodates 1.6 ± 0.3 mm/yr of horizontal shortening Cucamonga fault accommodates 1.6 ± 0.3 mm/yr of horizontal shortening


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