Vargas and Mann 2013 Caldas Tear BullSeismSocAmer

Tearing and Breaking Off of Subducted Slabs as the Result of Collision

of the Panama Arc-Indenter with Northwestern South America

by Carlos A. Vargas and Paul Mann

Abstract We present two regional, lithospheric cross sections that illustrate east-ward- and southeastward-dipping, subducted slabs to depths of 315 km beneath thesurface of Colombia in northwestern South America. These cross-sectional interpre-tations are based on relocated earthquake hypocentral solutions, models supported ongravity and magnetic regional data, and coda-Q (Qc) tomography. The method oftomographic imaging based on spatial inversion of the coda wave has advantagesof providing information on the lateral variations of the anelastic properties and ther-mal structure of the lithospheric system. Mapping of earthquake-defined Benioffzones combined with tomographic imaging reveals the presence of an 240 km longeastwest-striking slab tear, named here the Caldas tear. The proposed Caldas tearseparates a zone of shallow, 2030-dipping, southeastward subduction in the areaof Colombia adjacent to Panama and the Caribbean Sea, which is not associated withsubduction-related volcanism, from an area of steeper, 3040-dipping, slab adjacentto the eastern Pacific Ocean that is associated with an active northsouth chain ofactive arc volcanoes. We propose that the Caldas slab tear separating these two distinctsubducted slabs originally formed as the southern boundary of the Panama indenter,an extinct island arc that began subducting beneath northwestern South America about12 Ma. The area south of the Panama indenter is Miocene oceanic crust of the Nazcaplate, which subducts eastward beneath northwestern South America at normal anglesand melts to form a northsouth-trending active volcanic arc. In addition to the for-mation of the Caldas tear, we propose that impedance of the thicker crustal area of thePanama arc-indenter over the past 12 Ma may have led to down-dip break-off ofpreviously subducted oceanic crust that is marked by an extremely concentratedand active earthquake swarm of intermediate-depth earthquakes beneath east-centralColombia.

Introduction and Tectonic Setting

Hypocentral solutions recorded by the Colombian Na-tional Seismological Network (CNSN) show an 240 kmlong, right-lateral offset of intermediate to deep events withazimuth of 102 (Fig. 1a,b). We infer this discontinuity inearthquakes to be a major slab tear which we have namedthe Caldas tear based on the location in the Caldas departmentof Colombia and the alignment of fault-related surface features(e.g., volcanism, faulting,mineral deposits, geothermal anoma-lies, etc.). Using the distribution of earthquakes >80 km,Ojeda and Havskov (2001) proposed that the discontinuityalong the Caldas tear represented a boundary between two sub-ducted slabs with differing dips and strikes: the northern sub-duction zone, called the Bucaramanga subduction zone, hasa shallower dip (27) and more northeasterly strike, and thesouthern, called the Cauca subduction zone, has a steeperdip (3540) and a more northerly strike (Fig. 1a).

Regional compilations of Global Positioning Systems(GPS) data provide a quantitative tectonic framework forunderstanding the widespread crustal effects of the Panamaarc collision on large areas of northwestern South America(Calais and Mann, 2009; Fig. 1a). GPS vectors in westernColombia show a marked decrease in velocities consistentwith the ongoing collision of the Panama arc with north-western South America along a northsouth-trending suturezone roughly parallel to the international boundary betweenPanama and Colombia (Adamek et al., 1988; Trenkampet al., 2002; Corredor, 2003; Fig. 1a). The eastwest direc-tion of GPS vectors shows that the effects of eastwestshortening and indentation related to the collision of the Pan-ama arc remains relatively constant over a large, V-shaped,fault-bounded area of Colombia due east of the Panama arc-indenter (Fig. 1b). GPS vectors on the Maracaibo block of

2025

Bulletin of the Seismological Society of America, Vol. 103, No. 3, pp. 20252046, June 2013, doi: 10.1785/0120120328

Colombia and Venezuela show a more northerly direction ofplate motions related to northward tectonic escape of theMaracaibo block into the southern Caribbean (Trenkampet al., 2002). In contrast to this fairly uniform GPS velocityfield of deformed crustal rocks produced by the Panama col-lision, underlying, eastward-dipping slabs change abruptlyacross the Caldas tear from dip angles of 3040 betweenlatitudes 3:05:6 N in southern Colombia, to dip angles of

2030 in the area north of 5:6 N (Ojeda and Havskov,2001; Vargas et al., 2007).

Two nests of concentrated intermediate-depth earth-quakes are present beneath Colombia (Fig. 1a). The Bucara-manga earthquake nest (BN) is found at a depth of 160 kmon the down-dip extension of the southern (Bucaramanga)subduction zone and has an estimated volume dimensionof 13 18 12 km (Schneider et al., 1987; Frohlich et al.,

Figure 1. (a) Tectonic map of northwestern South America and Panama showing plate boundaries, neotectonic fault systems, and se-lective distribution of hypocentral solutions of 30;000 earthquakes extracted from the entire catalog of the CNSN (102;000 events) during19932012 with these criteria:mL 0:5; GAP 200; rms 0:5; error in latitude 10:0 km; error in longitude 10:0 km; and error in depth10:0 km. Color scale indicates depth of earthquakes. The north and south profiles symbolize the tomographic sections presented in thisstudy. SMM, Santa Marta massif; CB, Choco block; WC, Western Cordillera; CC, Central Cordillera; EC, Eastern Cordillera; PR, Perija Range;GB, Guajira basin; LB, Llanos foreland basin; MMVB, Middle Magdalena Valley basin; RFZ, Romeral fault zone; SMBF, Santa MartaBucaramanga fault; PF, Palestina fault; CF, Cimitarra fault; MGF, MulatoGetudo fault; HF, Honda fault; SFS, Salinas fault system; GF,Garrapatas fault; LFS, Llanos fault system; IF, Ibague fault; SR, Sandra ridge; BN, Bucaramanga nest; CN, Cauca nest; MN, Murindo nest;PIVC, PaipaIza volcanic complex; RSDV, Romeral and San Diego volcanoes. Yellow stars correspond to (1) the Tauramena earthquake (19January 1995,Mw 6.5); (2) the Armenia earthquake (25 January 1999,Mw 6.2); and (3) the Quetame earthquake (24 May 2008, mL 5:7).Sections AA0 and BB0 correspond to tomographic profiles presented in Figures 5 and 6. (b) Crustal isochron pattern of the Sandra ridge; pink-colored line, Caldas tear zone; arrows, station velocity GPS vectors relative to stable South America (after Calais and Mann, 2009). CHEP andBOGO are GPS stations used as reference to estimate the onset of the Panama-arc and South American plate collision. Other GPS stations inthe Panama-arc collision area are MANZ, RION, BUCM,MONT, and CART. Faded blue arrow enclosing 102 azimuth of the approximately240 km long, right-lateral offset of intermediate to deep events associated with the Caldas tear.

2026 C. A. Vargas and P. Mann

1995). Previous tectonic interpretations of the origin of theBucaramanga nest vary from a zone of two slabs in contact(van der Hilst and Mann, 1994), two slabs overlapping(Taboada et al., 2000), or a single slab undergoing extremebending (Corts and Angelier, 2005) all occurring in theboundary area of the subducted northern (Bucaramanga)and southern (Cauca) subduction zones (Fig. 1a). The Caucaintermediate-depth earthquake nest (CN) is located 400 kmsouthwest of the Bucaramanga nest on the trend of our pro-posed Caldas tear and has been previously interpretedby Corts and Angelier (2005) as a bend in the slab in thisarea (Fig. 1a). There is no clear consensus among seismol-ogists for the tectonic interpretation of the two concentratedColombian intermediate earthquake nests (Frohlich, 2006;Zarifi, 2006).

The Caldas tear defines the northern limit of the activevolcanic front of the northern Andes that has formed as aconsequence of the steeper subduction of oceanic slab ofnormal thickness of the Nazca plate (Fig. 1a). Moreover,associated with active and inactive volcanoes, the eastwestprojected surface trace of the Caldas tear localizes an eastwest alignment of some unusual volcanic rocks includingadakites (Borrero et al., 2009; Fig. 1a). Other volcanic rocksin the vicinity of the eastwest-trending Caldas tear includethe Plio-Pleistocene Paipa-Iza volcanic complex in theEastern Cordillera of Colombia and the Romeral and SanDiego volcanoes (Pardo et al., 2005). The presence of theseeastwest aligned volcanic rocks along with locally elevatedgeothermal gradient values (Vargas et al., 2009) suggests thatthe Caldas tear may penetrate the upper crust as a fault zoneand provide a conduit for the upward rise of magmas andhydrothermal fluids produced by melting of the slabs on ei-ther side of the Caldas tear (Fig. 2). Furthermore, recent,shallow-focus, strong motion events such as the Tauramenaearthquake (19 January 1995; Mw 6.5, h 25 10 km),the Quindio earthquake (25 January 1999; Mw 6.2,h 18:6 km), and the Quetame earthquake (24 May 2008;mL 5.7; h superficial) are all in alignment with the surfacetrace of the Caldas tear.

Previous tomographic studies using both local andregional earthquakes of varying resolution have produceddiffering tectonic interpretations for slabs in this area (vander Hilst and Mann, 1994; Taboada et al., 2000; Vargaset al., 2007). In this paper, we present the results of an in-tegrated geophysics and geologic study that improves the 3Dimaging of the interactions between the eastward-movingPanama indenter and its collisional area in northwesternSouth America.

Data and Methods

The following sections describe data and proceduresused to estimate hypocentral solutions, the attenuation and itsspatial distribution, the simultaneous 2D inversion of gravityand magnetic data, and the correlation of these results withfocal mechanisms, geothermal gradients, geological maps,

and a high-resolution seismic profile, seeking to define thegeometry of the Caldas tear and its geotectonic implicationsin the northwestern corner of South America.

Hypocentral Solutions and Estimationof the Coda-Wave Attenuation

A catalog has been compiled of 102;000 earthquakelocations calculated by the CNSN during the period 19932012 (mL 6:8). Hypocentral solutions were estimated byusing a seismological array of 17 short-period instruments(T 1 s) of the CNSN and complemented by 13 stationsassociated with local volcanic monitoring systems and alsoforeign networks (Panama, Ecuador, and Venezuela). Finalsolutions were calculated with the HYPOCENTER programand the velocity model proposed by Ojeda and Havskov(2001). Then, 9338 waveforms associated with 7645 regionalearthquakes (3:0 mL 6:5; 19932012) were selected forestimating the decay rate of the coda amplitudes (Q1c , codaattenuation). The selected events, on basis of a significantnumber of stations that recorded them (Table 1), have epicen-tral distances to stations ranging between 22.6 and 690.0 kmand depths varying between 0 and 222.0 km. Figure 2 showsall events used for the Q1c plotted on a map of northwesternSouth America along with tectonically significant earth-quake focal mechanisms including the Quindio, Quetama,and Tauramena events aligned along the Caldas tear and in-termediate-depth focal mechanisms from the Bucaramangaand Cauca nests.

Estimations of the Q1c were done using the SingleBackscattering model proposed by Aki and Chouet (1975).This model assumes that the coda of a local earthquake iscomposed of the sum of secondary S waves produced byheterogeneities distributed randomly and uniformly withinthe lithosphere. The coda is the portion of a seismogram cor-responding to back-scattered S-waves. The estimation of Q1cused the following equation:

P; t 2gjSj2

t2e

tQc

; (1)

where P; t is the time-dependent coda power spectrum, is the angular frequency, is the shear-wave velocity, jSj isthe source spectrum, and g represents the directional scat-tering coefficient. The g term has been defined as 4 timesthe fractional loss of energy by scattering, per unit travel dis-tance of primary waves, and per unit solid angle of the radi-ation direction measured from the direction of primary wavepropagation. Using these assumptions, the geometrical spread-ing is assumed to be proportional to r1, which only appliesto body waves in a uniform medium. The source factor can betreated as a constant value for single frequency. According toequation (1), Q1c values can be obtained as the slope of theleast-squares fit ofLnt2 P; t versus t, for t > t, wheret represents the S-wave travel time (Haskov et al., 1989). The

Tearing and Breaking Off of Subducted Slabs as the Result of Collision of the Panama Arc-Indenter 2027

time-dependent coda power spectrum was calculated using themean squared amplitudes of the coda Aobs; t from band-pass-filtered seismograms around a center frequency.

In order to take into account the deep structure usingcoda waves, 4:64 km=s was assumed and calculated asa weighted average of S-wave speeds in the whole earth vol-ume covered by the scattered waves (Badi et al., 2009). Allrecords were filtered in a chosen frequency band and thenused a coda-wave time window (W) of 20 s, starting from2 t ststart. The average lapse time, defined as tc tstartW=2 ranges between 11.0 and 384.0 s. These large tc valuesensure the sampling of regional structures. Attenuation esti-

mates were performed with short-period records (T 1 s) atseveral frequencies (Table 1). Then we chose estimates in thefrequency band 13 (2 1) Hz because of the high availabil-ity and geographical distribution of observations regardingother frequencies; and also best values of correlation coeffi-cients, the root mean square (rms), and signal-to-noise ratio.In addition, it has been reported that the study region presentsQ1c values in this frequency band with errors 5% (e.g., seeVargas et al. (2004)). In general, errors seen along Q1cestimations are acceptable, for example, the rms of all esti-mations vary between 0.07 and 1.79 ( 0:24, 0:07)and the coefficients of correlation are oscillating between

Figure 2. Epicenter projection of events used during the coda-wave-attenuation (Q1c ) estimation. Colored circles, earthquakes; bluesquares, locations of all seismological stations used in this paper; gray stars are shown with large focal mechanisms, and the most recent andsurficial strong-motion events occurring along the Caldas tear are shown by banded-gray polygon. The main focal mechanisms reported bythe NEIC-USGS (mb 4:0) defining the Bucaramanga nest to the northeast and the Cauca nest to the southwest are shown; pink areasidentified in the epicentral location of these nests are two main geothermal gradient anomalies reported by Vargas et al. (2009).


0:5 and 0:97 ( 0:67, 0:11). Table 2 presents astatistical summary of the main parameters related with theestimation of Q1c values for 30 seismological stations.Figure 3a shows an example of typical waveform used dur-ing this analysis, as well the corresponding record filtered forthe chosen frequency band. The attenuation factor (Q1c ) issuggested as a decay factor for the coda-wave amplitudes.Figure 3b presents histograms for Q1c values and their cor-relation coefficients, as well as distributions for the epicentraldistances, focal depths, and local magnitudes of the eventsanalyzed. Figures 2 and 3b emphasize the presence of attenu-ation contrasts in the region and at least two sources ofevents, one of them surficial and dispersed, and the otherlocated at an intermediate depth (linked to the nests of Buca-ramanga and Cauca).

Tomographic Imaging Using Coda-Wave Attenuation

Mukhopadhyay and Sharma (2010) have proposed thatthe variation of Q1c with tc shows a direct relationship withdepth. These authors interpreted that Q1c values related toscattering processes that penetrate >200 km depth are con-trolled by a crust and a relatively more transparent mantle.These results support the idea thatQ1c estimated with a largetc is representative of a large sampled volume and largesampled depths. A corollary of this hypothesis is that theQ1c value must be near to the intrinsic absorption (Q1i ) con-trolled mainly by the mantle. Following these ideas, Vargaset al. (2004) developed a regional tomographic study usingstations of the CNSN with relative large tc, (up to 180 s) andfound that the Q1c values are near to the Q1i values foralmost all stations, meaning that a large portion of the uppermantle is being sampled. Other studies have suggested adirect relation between the thermal field and anelastic attenu-ation (Faul and Jackson, 2005; Priestley and McKenzie,2006; Yang et al., 2007). The physical meaning of this rela-tionship is not been completely understood, but Karato andJung (1998) proposed that the higher water content in theasthenosphere significantly reduces the seismic-wave veloc-ities through anelastic relaxation and increasing temperature.Convergent margins such as Colombia, which involve largeamounts of sediments and water mobilized during the sub-

duction processes, are a likely site of large contrasts inanelastic attenuation in the subducted lithospheric slabs.

Given the ease to estimate Q1c , we can use this obser-vation for highlighting regional structures related to contrastsin rigidity (e.g., crust or lithospheric plates). One way toregionalize Q1c is based on the work of Malin (1978) who,expanding on the work of Aki (1969) and Aki and Chouet(1975), realized that the first-order scatterers responsible forthe generation of coda waves at any given tc can be locatedon the surface of an ellipsoid with earthquake and stationlocations as foci (Singh and Herrmann, 1983). In the ellip-soidal volume sampled by coda waves at any time t, Pulli(1984) defined the large semi-axis as a1 t=2, and definedthe small semi-axis as a2 a3 a21 r2=41=2, where r isthe sourcereceiver distance of the ellipsoid. The horizontalprojection of this volume is coincident with the elliptical en-velope proposed by several authors as the area occupied bythe scattered energy of the coda-wave record (Mitchell et al.,1997; Mitchell and Cong, 1998; Xie, 2002; Vargas et al.,2004). Following these observations and knowing the valuesof tc, W, and , it is possible to deduce the volumes of theellipsoidal shells where the seismic energy is scattered.HenceQ1c values estimated with large tc correspond to largesampled volumes, and vice versa. Based on these hypotheseswe can perform a generalized inversion for regionalizingQ1c . For the purpose of the inversion, we define a geo-graphic grid around the seismic station that also encloses thehypocenter. We recognize that each measuredQ1c is an aver-age estimate Q1av (or Q1apparent) for the volume as sampled bythe ellipsoidal shell given by

VTOTALQav

Xj

VBlock-jQj

; (2)

where VBlock-j is the fraction of volume (block) sampled bythe ellipsoidal shell with the true attenuation coefficient Q1j(or Q1true). Assuming a constant S-wave velocity of propaga-tion, the volume traveled by a ray that leaves the hypocentermoves outward to the ellipsoidal shell as defined by theobservation time of the coda and is scattered to the receiver,can be determined. Equation (2) can be written as

Table 1Estimated Values of Coda-Wave Attenuation (Q1c ) at Various Frequencies

Q1c 103 Coefficient of Correlation rms Signal/Noise

FrequencyWaveformsAnalyzed Min MeanStd Max Min MeanStd Max Min MeanStd Max Min MeanStd Max

2 9338 1.7 7.12.7 47.6 0.97 0.670.11 0.5 0.07 0.240.07 1.79 2.0 16.247.5 985.58 5421 0.8 1.80.7 20.4 0.96 0.600.08 0.5 0.18 0.320.06 2.67 2.0 11.025.6 842.612 4441 0.6 1.20.4 1.2 0.94 0.590.08 0.5 0.15 0.350.06 1.89 2.0 9.721.1 600.916 3741 0.5 0.90.3 9.4 0.95 0.580.07 0.5 0.17 0.350.09 4.13 2.0 9.018.1 315.7

Also presented are extreme values, averages, standard deviations, and quality parameters. Q1c values at 2 Hz were used to estimate tomograms because ofthe high availability of observations regarding other frequencies, best values of correlation coefficients, rms, and signal-to-noise ratio. A power law equationfor all Q1c observations suggested a high-frequency dependence of the attenuation in this region: Q1c f 13:2 0:6 103f0:970:06.


1Qav 1

Q1

VBlock-1VTOTAL

1Q1

VBlock-1VTOTAL

1Qn

VBlock-nVTOTAL

; (3)

where the ratio VBlock-j=VTOTAL is the volume fractionassociated with the total scattered-wave travel path spentin the jth block. If the process is repeated for each stationhypocentral pair, the entire region is sampled. Equation (3) isof the form

a1x1 aixi anxn y; (4)

where

y

1

Qav

xi

1

Qi

ai

VBlock-iVTOTAL

:

Then, a least-squares estimation of the xi is given by thecompact matrix equation AX Y where A is a (k n)coefficient matrix, X is a (n 1) vector, Y is a (k 1) vector,and k is the number of stationhypocenter pairs. A linearinversion of the matrix equation was formulated as an iter-atively damped least-squares technique (Levenberg, 1944;Marquardt, 1963). The damping factor (), which addsto the diagonal parameters of the matrix, was computed

-1 -0.9 -0.8 -0.7 -0.6 -0.50200400600800

1000120014001600

Freq

uenc

y

=-0.67 =0.11

Coefficient of correlation0 100 200 300 400 500 600 700 800 9000

50010001500200025003000350040004500

Freq

uenc

y

=155.0 =96.0

Epicentral distance

0 50 100 150 200 2500

5001000150020002500300035004000

Freq

uenc

y

=87.7 =65.6

Depth (km)0 1 2 3 4 5 6 7

0

500

1000

1500

2000

2500

3000

3500

Frequ

ency

=3.1 =0.7

(a)(a)

(b)(b)

0 10 20 30 40 500

5001000

15002000

25003000

35004000

Fre

quen

cy

Figure 3. (a) Example of waveform used for estimating the Qc values. Upper trace represents the original record of an earthquakerecorded by a short-period seismological station of the CNSN. Middle trace represents the filtered record in frequency band 13(2 1) Hz. Lower trace represents the decay envelope of the coda wave in a window of 20 s, starting from 2 t s. Qc value was obtainedas the slope of the least-squares fit of Lnt2 P; t versus t (dashed line with arrowheads), for t > t, where t represents the S-wavetravel time (Haskov et al., 1989). (b) Histograms for Q1c values and their correlation coefficients, as well as distributions for the epicentraldistances, focal depths, and local magnitudes of all events analyzed.


Table2

Seismological

Stations

oftheCNSN

Usedin

thisStudy

Station

Longitude

()

Latitu

de()

Altitude

(masl)

Waveforms

Analyzed

Q1

c103

t cCoefficientsof

Correlatio

nEpicentralDistances

Min

Mean

Std

Max

Min

Mean

Std

Max

Min

Mean

Std

Max

Min

Mean

Std

Max

ANIL

75.40

4.49

2300

248

2.9

6.8

1.6

17.2

12.1

89.3

51.5

231

0.94

0.670.11

0.50

26.7

166.3

90.2

413.6

BAR

73.18

6.58

1864

754

1.7

6.1

2.5

32.3

11.8

71.219.2

227

0.94

0.670.11

0.50

26.6

142.0

33.7

414.6

BCIP

79.84

9.17

615

5.5

6.9

2.0

12.2

104.7

126.4

15.7

146

0.86

0.690.10

0.59

200.7

238.7

27.5

272.8

BET

75.44

2.68

540

662.9

6.6

3.0

14.9

16.7

68.156.3

246

0.91

0.680.11

0.50

46.7

136.8

98.5

448.0

BRI

72.79

7.72

1427

462.9

6.0

3.5

47.6

13.1

81.636.7

150

0.97

0.660.13

0.50

24.7

159.6

65.8

280.2

CHI

73.73

4.63

3140

724

2.7

6.8

3.0

20.0

1189.453.3

263

0.97

0.690.11

0.50

24.5

173.5

93.8

477.6

CLIM

77.89

0.94

4232

172.5

7.0

5.0

12.0

31.7

78.935.5

132

0.91

0.690.11

0.53

73.0

155.5

62.1

247.6

COD

73.44

9.94

108

104

3.4

7.3

3.1

18.2

19.3

47.348.7

180

0.95

0.700.12

0.50

33.8

100.2

85.3

332.2

CPA

S77.25

1.22

2620

85.1

8.8

4.6

15.9

16.6

14.110.2

38.4

0.91

0.720.12

0.56

29.1

42.217.8

84.7

CRU

76.95

1.57

2761

330

2.5

6.7

2.9

16.1

17.6

79.260.2

339

0.95

0.690.11

0.50

30.8

156.0

105.3

610.4

CTA

B74.2

5.01

3500

25.6

8.1

8.2

14.5

132.4

133

0.85

134

0.86

0.700.22

0.55

249.2

250.2

1.48

251.3

CTA

U74.04

5.20

3868

54.3

8.0

6.7

28.6

52.8

96.231.2

119.1

0.9

0.730.12

0.6

109.9

185.9

54.5

225.9

CUM

77.83

0.94

3420

234

3.0

7.0

3.2

22.7

12.9

82.757.9

384

0.97

0.690.12

0.50

22.6

162.3

101.3

690.0

GCAL

77.42

1.21

2353

87.9

10.31.8

13.5

14.6

22.821.12

69.4

0.91

0.830.05

0.74

25.6

57.437.0

139.0

GCUF

77.35

1.23

3800

563.7

7.4

2.7

18.2

22.0

52.340.1

152

0.93

0.700.11

0.50

38.5

108.1

70.5

283.5

GUA

72.63

2.54

217

113.6

6.2

2.9

10.2

20.3

174.4

85.5

227

0.86

0.670.11

0.51

53.0

322.7

149.7

415.3

HEL

75.53

6.19

2815

216

3.0

6.1

2.1

19.6

21.0

110.2

40.9

190

0.96

0.650.10

0.50

36.8

210.0

71.5

350.2

MAL

77.34

4.01

75391

2.4

6.2

0.0

15.9

14.9

65.137.5

323

0.95

0.670.11

0.50

43.6

131.4

65.7

582.4

MARA

75.95

2.84

2207

783.0

5.9

2.4

18.2

17.9

87.665.2

259

0.92

0.650.11

0.51

31.3

170.8

114.1

469.9

NOR

74.87

5.57

536

596

2.5

5.9

2.1

20.4

16.3

113.4

35.5

297

0.94

0.660.11

0.50

28.5

215.8

62.1

537.4

OCA

73.32

8.24

1264

3304

2.0

6.0

2.3

21.7

16.4

102.8

23.1

287

0.96

0.660.11

0.50

28.7

197.4

40.4

520.3

OTA

V78.45

0.24

3492

264.3

7.6

2.1

13.9

23.6

70.225.7

147

0.88

0.730.12

0.53

58.8

140.3

44.9

274.1

PCON

76.4

2.33

4294

120

2.0

6.6

3.2

16.9

12.5

72.452.8

289

0.96

0.670.11

0.50

39.4

144.1

92.4

522.6

PRA

74.89

3.71

468

313

2.5

6.1

2.7

19.6

17.6

101.1

61.3

259

0.95

0.670.11

0.50

30.8

194.5

107.4

470.1

ROSC

74.33

4.86

3020

149

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33.3

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88.247.6

204

0.95

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31.3

171.8

83.3

373.6

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75.35

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4743

205

2.5

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12.7

113.7

118.1

48.8

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0.50

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73.08

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114.5

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The

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7645

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ating9338

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automatically for each iteration (Hoerl and Kennard, 1970;Hoerl et al., 1975). According to this technique, the solutionand resolution matrixes can be found for the followingequations:

X ATA 2I1ATY; (5)and

R ATA 2I1ATA: (6)

Similar procedures for the Q1c imaging have been usedin previous works in order to establish a deterministic char-acterization of the heterogeneity in the lithosphere as analternative technique for traditional tomographic measure-ments (ODoherty et al., 1997; Lacruz et al., 2009).

Resolution and Reliability

A spatial inversion of attenuation of 32 32 8blocks with block dimensions of 60 km latitude50 km longitude 40 km thickness was designed inorder to detect relevant structures in the region. We qualifiedthe tomographic inversion by means of three approaches:(a) hit count of ellipsoidal shells; (b) solving controlled tests;and (c) mapping the diagonal elements of the resolution ma-trix (RDE) by using equation (6). The hit count is a veryrough quality estimation that only tells about summing upthe number of ellipsoids that contribute to the solution ofa block. Based on this discretized volume, we mapped thehit count with the available data in eight layers (0, 45, 90,135, 180, 225, 270, and 315 km; Fig. 4a). Although a largepart of northwestern South America (including Colombia,western Venezuela, eastern Panama, and northern Ecuador)is covered by ellipsoidal shells (over 500 crossings), it is innorthern Colombia and northwestern Venezuela (71 W to76 W; 5 N to 10 N; 0180 km depth) where the largestnumber of shells run through the blocks (based on more than5000 hits per block). This approach emphasizes the impor-tance of the Bucaramanga nest data in the estimation of to-mographic images.

To incorporate the second approach, we evaluated theefficiency of the method described above solving the 3D di-rect problem. The ellipsoidal shell volume associated with allfoci (pairs of earthquake and station) were used to relate theQ1c values in two controlled test boards. The first one wasfor appraising large domains of attenuation, for example, azone with flat subduction in the north, and the other zonewith normal subduction in the south (Fig. 4b). The secondtest board evaluated the ability of the method to detect smallanomalies by use of a typical chessboard (Fig. 4c). In tests,we assigned two values of Q1true that represent attenuationcontrasts (1=70 and 1=200) into the 3D grid. Then we esti-mated the Q1av values (or theoretical Q1c values) for all el-lipsoids (each one related to foci [earthquakestation]) byestimating the weighted average of Q1true involved in the vol-ume of each ellipsoid.

For the 3D inverse problem, we estimated the fractionof volumes associated with each Q1av in order to establishequation (3). Using all foci related to the events selected inthis study, we assembled the compact matrix of equation (4)and then we inverted the Q1av values using equation (5).Finally, a spatial interpolation of the Q1av values was donebased on the Kriging method (Oliver and Webster, 1990)and presented on Mercator projection. Figure 4d showsthe results of the inversion for the synthetic experiment basedon two domains of contrasting attenuation (Fig. 4b). Thisexperiment is comparable to a slab-tearing model for whichtwo zones with different angles of subduction, are related todifferent attenuations. This hypothetical model linked a flatsubduction zone in the north (lower attenuation) and a nor-mal subduction zone in the south (higher attenuation). Ingeneral, the available data may allow detection of large struc-tures with significant contrasts of attenuation as much as270 km depth. On the other hand, Figure 4e presentsthe inversion for a chessboard based on two areas of contrast-ing attenuation (Fig. 4c). This experiment suggests thatthe available data may allow detection of smaller bodies(e.g., 100 km 100 km 60 km) with significant contrastsof attenuation, mainly in Colombia, and as much as 180 kmdepth.

After several trials of accurate resolution and sta-bility, the spatial inversion of attenuation with real datawas performed with the same grid (32 32 8 blocks).Figure 4f,g shows results of the tomographic estimationand their maps of the RDE at different depths. Because eachRDE shows the amount of independence in the solution ofone model parameter (RDE oscillates between 0 and 1), thelarger value of the RDE for one model parameter suggests amore independent solution for this parameter. 3D inversionpresents higher RDE values (e.g., >0:4) limited by the avail-ability and geographical concentration of Q1c values, indi-cating that the method is useful for areas for which a largestacking of attenuation observations is present. From theavailable earthquake data, the tomographic solution of theattenuation efficiently images large areas of the crust andupper mantle of northwestern South America includingColombia, eastern Panama, and western Venezuela withsampling depths reaching >315 km (Fig. 4f). Because theellipsoids related to deeper hypocentral solutions can sampleprofounder volumes, the 3D inversion may detect the thermalinflux from the mantle adequately.

In order to infer the geometry of the Caldas tear and itsrelationships with the adjacent Nazca and Caribbean plates,we made two regional cross sections: (1) a northern sec-tion (AA0, Fig. 1a) from the Caribbean plate to the Llanosforeland basin of eastern Colombia and crossing the inter-mediate-depth earthquakes of the Bucaramanga nest; (2) asouthern section designed for imaging the corridor betweenthe Nazca plate and the Llanos basin and crossing the inter-mediate-depth earthquakes of the Cauca nest (BB0, Fig. 1a).As discussed subsequently, it is essential to incorporate all


available geophysical data for proper interpretation of thetomograms along these sections.

Integrating Earthquake Data with RegionalSeismic-Reflection Lines

Hypocentral solutions of the CNSN (rms < 0:3 s;GAP < 200; stations 6; error in latitude 10:0 km; error in

longitude10:0 km; error in depth5:0 km)were plotted onthe tomographic profiles along two 60 km wide corridors(Figs. 5 and 6). Because seismicity in a corridor parallel tothe northern section is sparse, we have included an interpreta-tion of theTrans-Andeanmegaregional seismic-reflection linethat extends from the Caribbean coast to the Eastern Cordil-lera of Colombia (Vargas et al., 2010) and to the northern to-mographic section. This 383 km long reflection line is a 20 s

Figure 4. Resolution, reliability, and results of the spatial inversion of attenuation based on a geometry of 32 32 8 blocks withdimensions of 60 km latitude 50 km longitude 40 km thickness. Coda-wave tomograms were estimated with 9338 Q1c ob-servations associated with 7645 regional earthquakes (mL 6:7; 19932012) in the frequency band 13 (2 1) Hz. (a) Hit count of ellip-soidal shells, suggesting that the 3D inversion of Q1c may solve large areas of Colombia, eastern Panama, western Venezuela, and northernEcuador. (b) Synthetic model that represents two large domains of attenuation (e.g., a zone with flat subduction in the north and normalsubduction in the south, limited by a slab tear). The contrasts of attenuation incorporated into the model to evaluate the effectiveness of themethod wereQ1c 1=200 andQ1c 1=70. (c) Chessboard with smaller and regular distribution of attenuation contrasts. As in the previouscase, the contrasts of attenuation incorporated into the model were Q1c 1=200 and Qc1 1=70. (d) 3D inversion of the synthetic modelpresented in (b) suggesting that the distribution of the available data may allow detection of large structures with significant contrasts ofattenuation as much as 270 km depth. (e) 3D inversion of the chessboard model presented in (c) suggesting that the distribution of theavailable data may permit detection of smaller bodies (e.g., 100 km 100 km 60 km) with significant contrasts of attenuation, mainly inColombia, and as much as 180 km depth. (f) Results of the tomographic inversion with the available data. (g) Maps of the RDE at differentdepths. Higher RDE values (e.g., 0:5) indicate zones efficiently solved. However, these higher RDE values were limited by the geographicalconcentration of Q1c values, indicating that the method is useful for areas where a large stacking of attenuation observations is present.Tomographic solution of the attenuation efficiently images large areas of the crust and upper mantle of northwestern South America includingColombia, eastern Panama, and western Venezuela with sampling depths reaching>315 km. High-attenuation anomalies suggest that Buca-ramanga and Cauca seismic nests may be related to asthenospheric emplacements. (Continued)


Figure 4. Continued.


record and 200-fold, and shows the subduction geometry ofnorthern (Bucaramanga) slab dipping at a shallow angleto the southeast beneath northwestern Colombia (Fig. 7).Although reflectors are difficult to distinguish in deeper areasof the line, the overall distribution of deep reflectors dips east-ward in same amount as the subducted slab on the gravity andmagnetic model in Figure 5c. Deep reflections are concen-trated within what we interpret as the lower crust whereasthe uppermantle appears more transparent (Tittgemeyer et al.,1999). Prominent reflections in the upper crust can be corre-lated with major sedimentary basins such as the SinuSan Ja-cinto, Lower Magdalena, Middle Magdalena, and EasternCordillera, as well as major faults such as the Romeral faultzone (RFZ). This major fault separates oceanic crustal rocks inthewestern terranes of Colombia and continental basement ineastern Colombia (Cediel et al., 2003). In general, seismicityeast of the Romeral is more concentrated in the older and

more anisotropic continental crust of northwestern SouthAmerica.

Gravity and Magnetic Modeling

Coincident gravity andmagnetic models were completedfor this study based on regional information (Maus et al.,2007; Sandwell and Smith, 2009; National HydrocarbonsAgency of Colombia, 2010; Figs. 5b and 6b). The gravityand magnetic data was merged with the 90 m elevation topo-graphic information available from NASA (Jarvis et al.,2008), with corrections from the International Gravity Stand-ardization Net 1971 (IGSN71), the World Geodetic System1984 (WGS-84), the International Geomagnetic ReferenceField (IGRF) and the Observed Magnetic Intensity (Hinzeet al., 2005; Maus et al., 2005). The final database allowedus to estimate free air and magnetic anomalies. We then cal-culated Bouguer anomalies using densities of 2:67 g=cm3 for

Figure 5. Section crossing the northern Panama-arc indenter and its down-dip Bucaramanga nest (Fig. 1a, AA0). (a) Geologic and geo-thermal observations; (b) gravity and magnetic data; (c) interpreted tomographic section. Green dots with vertical bars that represent verticalerrors, hypocentral solutions in a 60 km wide corridor. Plotted events have the following selection criteria: rms < 0:3 s; GAP 200; stations6; error in latitude 10:0 km; error in longitude 10:0 km; error in depth 5:0 km. Some representative focal mechanisms (beach balls)have been also plotted.


land and 1:03 g=cm3 for marine water. In order to estimategravity and magnetic model responses (Talwani et al.,1959; Talwani and Heirtzler, 1964; Geosoft, 2010) and com-paringwith the observed data, we used values of density,mag-netization, and magnetic susceptibility shown on Table 3. Inaddition, the gravity and magnetic models were constrainedwith the seismological and seismic data, as well as geologictransects compiled with superficial cartography and represen-tative seismic lines (Section AA0, Figs. 1a and 5; and SectionBB0, Figs. 1a and 6; Lopez, 2004).

Because of restrictions on the data use of the Trans-Andean megaregional seismic-reflection line, it was notpossible to estimate refraction travel-time tomography forcorrelating with the profiles presented in this paper. However,in order to interpret the thermal and tectonic structure in thisregion, we used the velocity anomalies reported by Vargaset al. (2007) and van der Hilst and Mann (1994) that showsimilar distribution anomalies. In general, trends of high

velocities match with slabs suggested by the gravity andmagnetic models.

Other Geophysical Information for Constrainingthe Interpretation

Seismic attenuation has been used for examining theacoustic contrast between the upper mantle and the litho-sphere because it is believed that this seismic factor is aphysical parameter closely related to the thermal state ofthe volume sampled by the waves (Faul and Jackson, 2005;Priestley and McKenzie, 2006; Yang et al., 2007). Therefore,in order to evaluate the empirical superficial response of thelithospheric thermal field, we plotted geothermal anomaliesreported from oil wells in Colombia (Vargas et al., 2009)onto the two sections (Figs. 5a and 6a). Furthermore, topo-graphic response and focal mechanisms compiled from NEICare plotted on the two profiles.

Figure 6. Coda-wave-attenuation section crossing the southern part of the Panama indenter and the Cauca nest (Fig. 1b, BB0). (a) Geo-logic and geothermal observations; (b) gravity and magnetic data; (c) interpreted section. Green dots with vertical bars that represent verticalerrors, hypocentral solutions in a 60 km wide corridor. Plotted events have the following selection criteria: rms < 0:3 s; GAP 200; stations6; error in latitude 10:0 km; error in longitude 10:0 km; error in depth 5:0 km.


To support our interpretation, we have presented a totalof eight variables along the profiles: hypocenter solutions,focal mechanisms, coda-wave attenuation, gravity, magneticand geothermal anomalies, and geologic and topographicdata derived from seismic-reflection profiles. The two sec-

tions flank the intersection of the Panama arc-indenter andthe Caldas tear to the north (Section AA0, Figs. 1a and 5)and south (Section BB0, Figs. 1a and 6). The profiles are ex-tended to the west from the Caribbean Sea and Pacific Oceanto the Llanos foreland basin in the east. Both images cross

Figure 7. (a) Deep seismic profile nearly parallels the tomographic section AA0 (see inset map). Seismic image was assembled with threesegments of the Trans-Andean Seismic Line acquired by the National Hydrocarbons Agency of Colombia (ANH; Vargas et al., 2010). (b) Aninterpretation of the seismic line. Black lines, suggested sedimentary basins and deeper reflections; blue lines, faults and main tectonicfeatures (e.g., the Romeral fault zone in bold line); yellow line, suggested detachment surface associated with the Caribbean plate subduction.Red dots with vertical bars that represent vertical errors, hypocenter solutions in a 60 km wide corridor. Plotted events have the followingselection criteria: rms < 0:3 s; GAP 200; stations 6; error in latitude 10:0 km; error in longitude 10:0 km; error in depth 5:0 km.Depthtime relation has been estimated by using several oil wells in the area (Vargas et al., 2010).

Table 3Physical Properties Expressed in SI for the Materials Used in Gravity and Magnetic Models

Unit Density (kg=m3) Susceptibility Magnetization (A=m)

Caribbean and Nazca plates 3:30 103 1:26 1011:38 100 1:00 1031:45 100

Lower continental crust 2:78 103 1:26 105 1:26 1051:90 100

Upper continental crust 2:31 103 3:21 1024:90 102 5:51 1011:00 100

Mantle 3:15 103

Accreted sediments and oceanic crust 2:07 1032:35 103 1:26 101 1:00 1038:01 101

Oceanic sediments 1:68 1032:00 103 1:26 105 1:00 1034:50 101

Water 1:03 103


the northern Andes, the Bucaramanga and Cauca seismicnests of intermediate depth earthquakes, and major faultsincluding the Romeral, the Llanos, the Santa MartaBucaramanga, the Garrapatas, and the Ibague faults.

Results and Discussion

Northern Transect Crossing the Caribbean Plateand the Bucaramanga Nest

Our northern transect shows the presence of an oceaniccrust with thicknesses greater than 20 km related to the Car-ibbean oceanic plateau with a shallow subduction angle of

of events. Although lineal regression of the temporal series ofevents is of low confidence, this unidirectional, westwarddisplacement of events may be caused by down-dip andsouthwestward propagation of tearing of the subductedCaribbean slab.

Southern Tomographic Transect Crossing the NazcaPlate and the Cauca Nest

The Nazca oceanic slab has been modeled with an1522 km deep crustal thickness and a constant dip angleof 3040 to a depth >150 km beneath the active volcanicline (Figs. 1a and 6). The volcanic line is underlain byhigh-attenuation anomalies indicative of a normal meltingrange for the subducted oceanic slab (Figs. 2, 4c, 6). High-attenuation anomalies and the presence of shallow to inter-mediate seismicity around the Romeral fault zone suggestthis major strike-slip provides another major upward conduitfor the release of upper mantle heat. A large low-attenuationanomaly corresponds to the low geothermal gradient ob-served between the Colombian trench along the Pacific mar-gin and the Central Cordillera. The low geothermal gradientcoincides with thick volcanic and sedimentary material ac-creted to western Colombia, mainly in the Western Cordilleraand the Baudo Range. The accretion of this area may have

accompanied a proposed westward jump in subduction fromthe Romeral fault zone in the Central Cordillera to thepresent Colombia trench (Cediel et al., 2003). An additionalattenuation anomaly observed on tomographic data indicatesa prominent high thermal inflow from the mantle that islocated beneath the forebulge of the Llanos basin and is ap-proximately coincident with the largest geothermal anomalymeasured in this basin.

Seismicity of the Cauca nest is highly dispersed in depth(70150 km) and its time evolution seems more complexthan the Bucaramanga nest (Fig. 9). Even with the low con-fidence of the lineal regression, seismicity from the CNSCcatalog shows an eastward displacement of events. The Caucanest exhibits earthquakes with focal mechanisms ranging frompure gravitational collapse to strike-slip in the north andreverse with strike-slip component in the south (Fig. 2).

Nature of the Caldas Tear

In addition to the hypocenter solutions that initiallyrevealed the 240 km long offset of intermediate to deepseismicity of the Caldas tear, we found that differentialdisplacements between the southern of the Caldas tear in theSouth American plate and the Panama arc-indenter, based onGPS observations, suggest a trend of decreasing displacement

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014-77

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ngitu

de()

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 20143.5

4

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5.5

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ude(

)

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014-200

-150

-100

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0

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th(km

)

Time (year)

y(x) = a (x - b)a = 0.021514b = 5553.5R = 0.40882

y(x) = a x + ba = 0.0085141b = -12.525R = 0.12753

y(x) = a x + ba = 1.0488b = -2211.1R = 0.19102

Figure 9. Temporal evolution of the hypocentral parameters of the Cauca nest. Earthquake information provided by the CNSN from 1993to 2012. The Cauca nest events were selected around the point 76.3 W, 4.5 N. Parameters of selection were Radii 0:5; 0 rms 0:3 s;h 60 km; GAP 200; (a) error in longitude 10:0 km; (b) error in latitude 10:0 km; (c) error in depth 10:0 km. Error bars have beenassociated with each event. Dashed polygon, the linear trend of trend of occurrences estimated by least-squares means.


toward the east (Trenkamp et al., 2002; e.g., BOGO versusMZAL, RION, BUCM,MONT, and CART GPS stations). As-suming the BOGO station as the reference point south of theCaldas tear, and the CHEP station as the reference point on thePanama indenter, we estimate 24 mm=year of active right-lateral displacement across the Caldas tear. The hypothesisof lateral homogeneity of the crust, constant displacement rate,and a seismic offset along Caldas tear of 240 km, wouldsuggest an 10:0 Ma initiation of the Panama arcColombiacollision (240 km=24 mm=year). Geologic field observa-tions in the Panama-arc (Coates et al., 2004) suggest that theage for the initiation of the Panama arc collision with northernSouth America occurred between 12.8 and 7.1 Ma, which isconsistent with our estimated 10:0 Ma initiation of the tearpropagation. In addition, because the origin of the eastwestSandra ridge occurs between 9 and 12Ma (Defant et al., 1992;Lonsdale, 2005), and this structure is collinear with the Caldastear, we propose that the right-lateral lineament defined by theCaldas tear and the Sandra ridge, constitutes a major area oflithospheric weakness along the southern flank of the Panamaarc-indenter. Although there is no evidence for recent activityof the Sandra ridge due to lack of near-bottom instrumentationin the Pacific offshore of Colombia and Panama, recent earth-quakes and the adakite magmatism along the Caldas tear mayindicate that this lineament localizes upper crustal fault con-duits that allowed the upward migration of magmatic fluidsand are associated with elevated geothermal anomalies. Offsetof intermediate- to deep-seismicity that defines the Caldas tearis also coincident with inactive volcanoes of adakite compo-sition and geothermal anomalies (Fig. 2). The adakites of theRuiz volcanic complex with ages ranging from 0.970.05 and1.80.6 Ma (Borrero et al., 2009) and the PaipaIza volcaniccomplex dated 1.92.5 Ma (Pardo et al., 2005) are a likelyconsequence of magmatism related to this progressiveslab tear. Low ratio 87Sr=87Sr (0.705) and the presence ofxenoliths of metamorphic rocks in this last volcanic complex(J. M. Jaramillo, personal comm., 2012) support the proposedbreak-off interpretation south of the Bucaramanga nest andeast of the Caldas tear.

Surficial evidence of this lithospheric tear are restrictedto presence of mineral deposits, hydrocarbon occurrences,and some geomorphological anomalies. High-grade mineraldeposits of platinum, gold, and copper exploited in the min-ing areas called Condoto (Tistl, 1994), Marmato (Ordoez,2001), Quinchia (INGEOMINAS, 1999), La Colosa (Gil-Rodriguez, 2010), and Cerro de Cobre (McLaughlin andArce, 1970) are near, or collinear with, the Caldas tear andexhibit ages ranging between 6 and 20 Ma (see blue hexa-gons on map in Fig. 10a). In addition, significant changes indistribution and trend of oil and gas seeps, as well as thehydrocarbon fields on both sides of the Caldas tear, suggestthat this structure may also affect the geometry of severalsedimentary basins (e.g., Llanos foreland, Eastern Cordillera,Middle Magdalena Valley). But likely the most prominentgeomorphological evidence is coming from hydraulic behav-ior of the main rivers that cross the south-to-north-flowing

Magdalena and Cauca rivers of northwestern South America.After flowing 200 km from their sources, these rivers occupybroad river valleys. Downstream, rivers passing the Caldastear lineament change their morphology from broad valleysto steeper relief gorges near the surface projection of theCaldas tear. The Cauca River near Supia town (Fig. 10a,b)reduces to a narrow channel only 150 m wide whereas theMagdalena River near the town of Honda reduces to a 250 mwide channel (Fig. 10a,c). In contrast to the broader, 42 kmwide valleys observed upstream (e.g., Bolivar and Guamotowns in Fig. 10a,d), in both areas the steeper gradient andmore narrow rivers produce rapids that are an impediment tonavigation. The narrowness of these rivers favored the down-stream economic development of urban settlements such asHonda and Supia from Spanish colonial times. However, thequake that occurred on 16 June 1805 that destroyed Hondaand other nearby towns shows us that the Caldas tear couldform a major source of earthquakes and seismic hazard inthis region.

The Caldas tear may localize large crustal earthquakes,including the recent strong-motion activity associated withthe Tauramena earthquake (19 January 1995, Mw 6.5).Two types of focal mechanisms have been proposed for thisevent, one of which suggests an eastwest-oriented rightlateral movement (Dimate et al., 2003; Fig. 2). Similarly,seismic activity in the area of the Armenia earthquake (25January 1999, Mw 6.2) shows eastwest alignment of after-shocks with the Caldas tear. This long right Caldas tear doesnot explain the Quetame earthquake (24 May 2008, mL 5.7)with left-lateral slip, in which the Caldas tear could haveplayed an important role for controlling the propagationof northsouth-trending faults (such as this event or any re-lated to the Llanos fault system) in a similar way as has beensuggested in the southwest Colombia margin where trans-verse faults reduce coupling between adjacent segments(Collot et al., 2004).

We removed the crustal earthquakes from the databaseof events located by the CNSN between 1993 and 2012 inorder to illustrate the upper surface of the subducted slabsbeneath Colombia (Fig. 11a). The 3D model of this surfaceimages the Caldas tear and flat-slab subduction geometrythat we relate to the presence of the Panama arc-indenter (Ra-mos and Folguera, 2009). The northern border of the indenterbecomes diffuse and does not appear to form a distinctivetear as seen south of the indenter (see dashed line in Fig. 11a).The Caribbean plate changes from flat to steep subductiontoward the northeast. South of the Caldas tear (2:5 N),there is a shift to a new pattern of intermediate and deepseismicity associated with flat subduction along with thedevelopment of a broad area of active volcanoes in southernColombia and northern Ecuador. Figure 11b presents a con-ceptual model that explains the kinematic role of thePanama arc-indenter whose southern boundary is definedby the Sandra ridge and the Caldas tear. In this model thecoupling of the Panama arc with the Caribbean plate couldgenerate a change in buoyancy of the lithospheric system and


consequently the northern region of indentation has condi-tions that favor flat subduction. In the east, the Caribbeanplate suddenly changes its subduction angle and producesa break off of the slab around the location of the Bucara-manga nest. South of the Caldas tear, the Nazca plate is sub-ducting beneath the South American plate with a steeperangle and a faster rate. The Cauca nest is a combined productof eastward decoupling of plates along the Caldas tear andflexure during the subduction process.

A corollary of our model for the Panama indenter andthe formation of the Caldas tear is the eastward indentation ofgeologic features. Inspection of theMap of Quaternary faultsand folds of Colombia (Paris et al., 2000) suggests that some

branches of the Romeral fault zone south of the Caldas tearshow right-lateral offset from parallel faults including thePalestina, Cimitarra, MulatoGetudo, Honda, or Bituimafaults to the north. Other faults with southwestnortheasttrend, such as the Ibague and Garrapatas, could be part ofa transverse strike-slip fault at the surface level overlyingthe Caldas tear (Fig. 1a). These results raise new questionsabout the regional evolution in northwestern South America.For example, the Great Arc of the Caribbean has been de-fined along the south Caribbean region but disappears onceit enters the Guajira basin and the Santa Marta Massif northof Colombia (Ostios et al., 2005). As a consequence of thePanama-arc collision, it is possible that this regional feature

Figure 10. Surficial evidences of the Caldas tear related to mineral deposits, hydrocarbon occurrences, and geomorphological anomalies.(a) Blue hexagons, map of distribution of high-grade mineral deposits of platinum, gold, and copper: (1) Condoto, (2) Marmato, (3) Quinchia,(4) La Colosa, and (5) Cerro de Cobre. Black dots, oil and gas seepages. Purple circles are giant hydrocarbon fields. Blue stars are other oiland gas fields. These hydrocarbon occurrences suggest that the Caldas tear also is affecting the geometrical configuration of several sedi-mentary basins. White stars, hydraulic anomalies of the Cauca and Magdalena rivers on the Caldas tear (rapids on the Supia and the Honda).White circle and square are places upstream the rivers where there are broad valleys (Bolivar and Guamo). (b) Rapids of the Cauca River nearSupia town that overlies the Caldas tear. (c) Rapids of the Magdalena River near Honda town that overlies the Caldas tear. (d) Broad valleyobserved upstream of the Cauca River near Bolivar town. Similar landscape is observed in Guamo town where the valley width of theMagdalena River reaches >40 km wide.


has been offset and displaced eastward by the Panamaindenter.

Conclusions

The eastward-directed collision of the buoyant Panamaarc-indenter with northwestern South America producesa distinctive V-shaped pattern of crustal deformation and

widens the northern Andes in Colombia and Venezuela(Fig. 1b). The Panama collision initiated 10 Ma and con-tinues to be active as shown by GPS data.

The southern edge of the Panama indenter is associatedwith the proposed Caldas slab tear at latitude 5:6 N. Thistear extends for 240 km in an eastwest direction andis collinear with the 912 Ma, now extinct, eastwest-oriented Sandra oceanic spreading ridge on the unsubducted

Figure 11. (a) Seismic surface estimated by interpolation and filtering of 68;000 local earthquakes (h 10:0 km). Blue lines, shore lineof northwest South America. Bold black lines, limits of the convergent margins. Bold red lines, the southern border of the Panama indenterthat includes the Sandra ridge and the Caldas tear. Bucaramanga nest is related to a break-off process that is propagating toward the south-west. Triangles, red (active) and green (inactive) volcanoes. Orange dashed lines, wireframe model suggested for indicating the subductiongeometry of the Caribbean plate. (b) Schematic 3D model suggesting flat subduction on the northern side of the weakness zone formed by theSandra ridge and the Caldas tear. Caribbean plate suddenly changes its subduction angle and promotes a break off of the slab around thelocation of the Bucaramanga nest. South of the weakness zone, the Nazca plate subducts beneath the South American plate with a steeperangle and faster displacement. Probably the Cauca nest is the combined product of eastward decoupling of plates along the Caldas tear as wellas flexion and discrete movements of the plate during subduction. The Murind earthquake nest located in proximity to the Panamanian andColombian border, may be response to convergent accommodation between the Panama arc-indenter and the Caribbean plate.


oceanic Nazca plate to the west (Fig. 1b). We postulate thatthe Caldas tear may have formed as a zone of lithosphericweakness along the now subducted part of the inactiveSandra spreading ridge.

The 240 km long Caldas tear is a narrow, eastwest-trending boundary between two subducted slabs of differ-ent dip. The northern zone is the down-dip extension of thePanama arc, has a shallower dip, and is not associated withactive arc volcanism. The southern zone has a steeper dipand is associated with an active volcanic front (Fig. 11a,b).

The Caldas tear also localizes angular difference of thesubduction geometry in both geophysical sections pre-sented in this work (Figs. 5 and 6). The lineament definedby the 240 km long offset of the deep seismicity along5:6 N; the eruption of the northsouth belt of activevolcanism and presence of extinct magmatic bodies withadakite composition along the lineament; the occurrenceof high-grade mineral deposits and geothermal gradientanomalies; the different patterns associated with oil andgas manifestations; the distribution of major oil and gasdeposits north and south of the Caldas tear as well as theGPS measurements and strong-motion events with right-lateral movements, support the existence of the Caldas tear.

Data and Resources

Waveforms and preliminary hypocentral solutions ofthe Colombian territory were supplied by the Geological Sur-vey of Colombia (INGEOMINAS). Bouguer and magneticdata provided for the National Hydrocarbons Agency ofColombia (http://www.anh.gov.co/es/index.php?id=82, lastaccessed November 2012) were used for modeling geologicsections using the GM-SYS profile module of the OasisMontaj software (Geosoft, 2010). This software calculatesthe gravity and magnetic model response based on the meth-ods of Talwani et al. (1959), and Talwani and Heirtzler(1964). GM-SYS uses a 2D, flat-earth model for the gravityand magnetic calculations. Each structural unit or blockextends to plus and minus infinity in the direction perpen-dicular to the profile. The earth is assumed to have topogra-phy but no curvature. The model also extends plus and minus30,000 km along the profile to eliminate edge effects. The90 m elevation topographic information used for the gravitymodeling is available from the CGIAR-CSI SRTM 90 mdatabase (http://srtm.csi.cgiar.org, last accessed November2012). Focal mechanisms reported by NEIC were used inthis work (http://earthquake.usgs.gov/earthquakes/eqarchives/sopar/, last accessed November 2012).

Acknowledgments

This work was partially funded by the industry sponsors of the CBTHproject of the University of Houston and by fellowship support from theUniversity of Texas at Austin. Earthquake, gravity, magnetic, seismic,and geothermal data were kindly provided by Agencia Nacional de Hidrocar-buros, Universidad Nacional de Colombia, INGEOMINAS, and the followingresearch projects: 1233-333-18664, Contract 201-2006 (COLCIENCIAS);

1233-487-25728, Contract 589-2009 (COLCIENCIAS); CGL2005-04541-C03-02 and CGL2008-00869/BTE (UPC, MICCIN, FEDER). We also thankthe Associate Editor Heather DeShon, and two anonymous reviewers for theirhelpful reviews of this paper.

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Department of GeosciencesUniversidad Nacional de ColombiaSede BogotCarrera 45 No 26-85Edificio Manuel AncizarBogot D.C.Colombia, [email protected]

(C.A.V.)

Department of Earth and Atmospheric SciencesUniversity of Houston4800 Calhoun BoulevardHouston, Texas [email protected]

(P.M.)

Manuscript received 3 November 2012


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Vargas and Mann 2013 Caldas Tear BullSeismSocAmer