palinspastic reconstruction of southeastern california and southwestern arizona for the middle miocene a paleogeographic reconstruction of southeastern california and southwestern arizona at 10 ma has been made based on available geologic and geophysical data clockwise rotation of 39 has been reconstructed in the eastern transverse ranges consistent with paleomagnetic data from late miocene volcanic rocks and with slip estimates for left lateral faults within the eastern transverse ranges and nw trending right lateral faults in the mojave desert this domain of rotated rocks is bounded by the pinto mountain fault on the north the model requires that the western part of the late miocene pinto mountain fault was a thrust fault which gained displacement to the west because of the absence of evidence for rotation of the san bernardino mountains or for significant right slip faults within the san bernardino mountains the squaw peak thrust system of meisling and weldon 1989 may be a western continuation of this fault system the sheep hole fault bounds the rotating domain on the east east of this fault an array of nw trending right slip faults and south trending extensional transfer zones has produced a basin and range physiography while accumulating up to 16 km of right slip this maximum is significantly less than the 37 5 km of right slip required in this region by a recent reconstruction of the central mojave desert dokka and travis 1990a geologic relations along the southern boundary of the rotating domain are poorly known but this boundary is interpreted to involve a series of curved strike slip faults and linking normal faults that accommodated noncoaxial extension quaternary movement on the pinto mountain and nearby faults is unrelated to the rotation of the eastern transverse ranges and a hiatus during part of pliocene time followed the deformation which produced the rotation the reconstructed clemens well fault in the orocopia mountains proposed as a major early miocene strand of the san andreas fault projects eastward towards arizona where early miocene rocks and structures are continuous across its trace making large displacements on this structure unlikely the model predicts a 14 clockwise rotation and 32 extension during late miocene and early pliocene time along a nw trending line parallel to the present trace of the san andreas fault palinspastic reconstructions of the san andreas system based on this proposed reconstruction may be significantly modified from current models

early miocene change in direction of least principal stress southwestern united states conflicting inferences from dikes and metamorphic core detachment fault terranes previous investigations have suggested that the direction of least principal stress in the western united states has shifted from northerly to easterly between 50 to 30 ma an effort is made to more tightly constrain the time of this shift using igneous dike host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 ma in such systems the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock these systems indicate that a northerly least principal stress orientation persisted more recently than previously believed until probably after 30 ma possibly about 26 ma and conceivably as recently as 18 ma in some areas the data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift however the change could be diachronous earlier in the southeast and later in the northwest major easterly trending magmatic and tectonic elements of tertiary age in the great basin of utah and nevada are compatible with the dike data however numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes this inconsistency is not clearly resolvable but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by mesozoic compression

formation evolution and inversion of the middle tertiary diligencia basin orocopia mountains southern california the diligencia basin located in the eastern orocopia mountains of southern california contains 1500 2000 m of oligocene miocene continental siliciclastic sedimentary rocks and subordinate limestone and evaporite deposits intercalated with basaltic lavas these rocks are locally intensely folded and faulted defining in present day coordinates an elongate east west trending basin and are unconformably overlain by flat lying late pliocene and pleistocene alluvial deposits the sedimentological stratigraphic and structural history of the basin is compatible with late oligocene early miocene formation as a half graben basin produced by orthogonal basin and range type extension and latest miocene to pliocene basin inversion in either a localized transpressional transverse range regime or a more diffuse compressional regime associated with north south shortening of the entire mojave block facies associations in the lower part of the diligencia basin display a distinctly asymmetric distribution across the basin indicating deposition in a half graben with a steep possibly fault controlled south facing northern escarpment and a more gentle north facing southern slope paleocurrent data particularly from high energy deposits on the northern basin margin indicate stream flow toward the southeast to south southeast oblique to the deformed basin margins and suggest an approximate northeast to east northeast strike for the basin before deformation previously published paleomagnetic data however indicate that the elongate currently east west trending fault block containing the diligencia basin has rotated clockwise by as much as 110 about a vertical axis angle depends on data and model used since deposition ceased if correct this would suggest that the basin may have originally opened to between the northeast and east northeast subparallel to well documented extension directions in miocene age basins in the mojave desert to the east clockwise rotation on east west fault blocks exposed in this region has previously been bracketed as between ca 10 and 4 5 ma structural and paleomagnetic data indicate that inversion of the diligencia basin occurred after block rotation implying a latest miocene to pliocene age for inversion we speculate that basin inversion within a north south and still active compressive stress field resulted from the locking and subsequent internal deformation of this previously passively rotating elongate crustal block

middle cenozoic depositional tectonic and sea level history of southern san joaquin basin california the eocene to lower miocene fill of the southern san joaquin basin contains three complete depositional sequences the tejon san emigdio and pleito each of which corresponds to a formal formation the tejon sequence lower to middle eocene is marine and incorporates nearshore shelf slope and basinal deposits the san emigdio upper eocene to lower oligocene and pleito upper oligocene sequences intertongue eastward with alluvial fan deposits of the tecuya formation the lower part of the san emigdio sequence was deposited by a westward prograding gilbert type delta the upper part of the san emigdio sequence and lower part of the pleito sequence were deposited by a system of shelf fan deltas that prograded at least 10 km to the west the middle and upper parts of the pleito sequence were deposited by a slope fan delta in relatively deep hundreds of meters water regional transgression during the early eocene initiated deposition in the southern san joaquin basin the lower san emigdio gilbert type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 ma relative highstand deposits in the san emigdio and pleito formations consist of widespread progradational shallow marine and nonmarine facies the eocene to early miocene tectonic history of the southern san joaquin basin included three distinct periods of increasingly intense activity rapid early to middle eocene subsidence of the basin was associated with the emplacement of the salinian block late eocene to early oligocene uplift and subsidence occurred in two discrete pulses and a seismically generated submarine slumping episode triggered the change from shelf fan deltas to slope fan deltas during the late oligocene to early miocene the southern san joaquin basin was disrupted by major uplift proximal volcanism and syndepositional faulting as the mendocino triple junction migrated past the study area and san andreas fault related tectonism was initiated in general local tectonic events rather than eustatic ea level events seem to have exerted the predominant control on middle cenozoic sedimentation in the southern san joaquin basin

en echelon miocene rifting in the southwestern united states and model for vertical axis rotation in continental extension two areas of intense early miocene crustal extension in the southwestern united states the colorado river trough and the central mojave desert are separated by a weakly deformed area in the eastern mojave desert we propose that these areas form a left stepping en echelon rift system linked by a ductile detachment at depth the en echelon geometry explains the southward loss of displacement in the central mojave desert and northward loss of coeval displacement in the colorado river trough and it incorporates seismic reflection evidence that mid crustal tertiary extensional mylonites continue beneath the weakly deformed area this geometry also explains clockwise paleomagnetic declination anomalies from lower miocene rocks as recording thin skinned detached rotations large scale block rotations are not required obliquity of the northeast trending crustal extension vector to the east west trending early miocene synextensional volcanic belt may have caused the en echelon pattern to develop

regional correlation of extension directions in cordilleran metamorphic core complexes cordilleran metamorphic core complexes exhibit tertiary extensional deformation expressed either as detachment faults or detachment faults plus ductile mylonitic shear zones extension directions as indicated by stretching lineations within mylonite and striations along detachment faults fall into regional groups in which the directions are similar in trend throughout each group asymmetric fabrics on both small and large scale give a sense of shear and indicate that tectonic vergence within groups is directed outward from a central axis the regional consistency of extension directions implies a regional control of extension in metamorphic core complexes

role of subsurface loads and regional compensation in the isostatic balance of the transverse ranges california evidence for intracontinental subduction bouguer gravity anomalies in the transverse ranges of southern california show a pronounced asymmetry in north south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike while individual gravity topography profile pairs could be consistent with a number of isostatic compensation mechanisms the unusual relationship between gravity and topography in the san bernardino mountains severely limits the class of acceptable models if the bouguer gravity field is caused by flexure of the crust mantle boundary the simplest solution consistent with the data in the western transverse ranges the san gabriel mountains and the san bernardino mountains models the great valley and mojave provinces to the north as a 50 km thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south the best fit to the gravity data in the san bernardino mountains requires a terminal bending moment on the southern plate beneath the mountains the magnitude of the terminal bending moment decreases to the west in the san gabriel and western transverse ranges we equate the load responsible for this terminal moment with the high velocity slablike feature mapped in the upper mantle beneath the transverse ranges using seismic tomography the coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small density contrast rather than a small scale convective feature

new estimates of displacement along the san andreas fault in central california based on paleobathymetry and paleogeography studies of depth related benthic foraminiferal biofacies permit the construction of paleobathymetric maps of the la honda and san joaquin basins of central california these maps support the hypothesis that the la honda and san joaquin basins were contiguous during the late oligocene and early miocene and subsequently were separated by about 320 330 km of right lateral displacement on the san andreas fault furthermore these estimates of displacement support the notion that right lateral slip occurred along the san andreas fault during the early miocene

structural and metamorphic evolution of the orocopia schist and related rocks southern california evidence for late movement on the orocopia fault the pelona orocopia and rand schists por schists of southern california and southwesternmost arizona are late mesozoic or early tertiary subduction complexes that underlie precambrian to mesozoic continental basement along the low angle vincent chocolate mountains vcm fault system the vcm faults are often considered to be remnants of the original subduction zone but recent work indicates that many have undergone substantial postsubduction reactivation in the orocopia mountains for example the orocopia schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the orocopia fault structures in the schist include isoclinal folds with axial planar schistosity open to tight folds that fold schistosity penetrative stretching lineations and crenulation lineations all of which show a nearly 360 range in trend folds and lineations that trend approximately ne sw occur throughout the schist and are thought to be part of an early phase of deformation related to subduction folds of this orientation show no consistent vergence folds and lineations that trend approximately nw se are concentrated near the orocopia fault and are interpreted to have formed during exhumation of the schist the nw se trending folds and shear indicators in late stage mylonite at the top of the schist consistently verge ne the exhumation event culminated in emplacement of the schist against brittlely deformed upper plate exhumation of the orocopia schist was accompanied by retrograde replacement of garnet biotite epidote and calcic amphibole by chlorite calcite and sericite matrix amphibole has a lower na al ratio than amphibole inclusions in albite consistent with a late stage decrease in pressure as ne vergence in the orocopia mountains is associated with exhumation of the schist the ne movement along other segments of the vcm fault may also be late and therefore have no bearing on the facing direction of the vcm subduction zone contrary to past interpretations

oligocene and miocene paleogeography of central california and displacement along the san andreas fault recently completed sedimentologic and petrologic studies of oligocene and miocene strata in the temblor range san joaquin basin and santa cruz mountains la honda basin permit detailed reconstructions of paleogeography as well as new estimates of displacement along the san andreas fault during the oligocene and miocene the san joaquin and la honda basins were contiguous the southwestern margin of the san joaquin la honda basin was tectonically unstable and paleogeographically complex with several small deep sea fans fed by sediment derived from nearby subaerial uplifts of franciscan and granitic basement one of these deep sea fans represented by the temblor formation of the southern temblor range and the vaqueros sandstone of the central santa cruz mountains apparently has been displaced about 315 to 320 km by post early miocene post zemorrian about 23 ma right lateral slip along the san andreas fault this new estimate of displacement places an additional constraint on interpretations of san andreas slip history but ambiguities remain the time of initiation of san andreas displacement is poorly known and available data do not permit discrimination between a history of relatively continuous slip versus a history of episodic slip

nature of the southwestern boundary of the central mojave tertiary province rodman mountains california tertiary volcanic and sedimentary rocks in the central mojave desert are largely confined to the barstow bristol trough an elongate tectonic depression filled with lower miocene strata the southwestern boundary of the trough is sharply defined and separates thick 2 km tertiary sequences from an area to the southwest that is virtually devoid of tertiary rocks segments of this boundary have been interpreted as a buttress unconformity as a right slip sidewall fault that formed the lateral boundary of an early miocene extensional terrane and as a dip slip fault caused by flexure of the north american plate above the subducted mendocino fracture zone the silver bell fault in the rodman mountains central mojave desert is the best exposed segment of the boundary and relations exposed along it can be used to test these hypotheses the tertiary sequence north of the silver bell fault is at least 2 3 km thick and comprises at least 1 9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed tertiary section tuff layers intercalated in the lower part of the clastic deposits are 23 24 m y old indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central mojave metamorphic core complex structural and stratigraphic data indicate that the silver bell fault is primarily a dip slip fault the tertiary section is folded into a west trending upright to overturned syncline located near the southern edge of exposed tertiary rocks the syncline records late cenozoic north south shortening along the northwest striking calico fault which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip southwestward dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension however bedding dip does not decrease upward in the tertiary section paleocurrent directions trend obliquely updip and northeast dipping normal faults are rare stratal tilting postdated the accumulation of the exposed section which therefore was not deposited during rotational normal faulting the new data contradict interpretation of the silver bell fault as a major right slip fault bounding the central mojave extensional terrane although they permit small magnitude extension across the fault the amount of early miocene crustal extension in this area therefore is apparently small in spite of stratal tilting and the lateral limit of large magnitude crustal extension in the central mojave block is farther northwest the data are consistent with the fault having formed by failure during plate flexure above the subducted mendocino fracture zone

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

late cretaceous protolith age and provenance of the pelona and orocopia schists southern california implications for evolution of the cordilleran margin the pelona and orocopia schists are southern members of a eugeoclinal terrane that structurally underlies a large part of southwestern north america ion microprobe u pb ages of 100 detrital zircons from three widely spaced samples of these two units indicate that deposition occurred after 70 80 ma moreover the distribution of zircon ages including a significant peak centered ca 1 7 ga implies a major contribution f detritus from the mojave desert and transverse ranges of southern california recrystallization of the schists at depths of 20 35 km occurred within 10 15 m y of deposition which requires underthrusting at minimum rates of approximately 4 12 mm yr for reasonable thrust dips considering that similar processes formed the more northern rand schist at somewhat earlier times results indicate a southward progression in timing of deformation for the schists protoliths this result is at odds with northward migration of deformation implied by the baja british columbia hypothesis the age and provenance of the schist s protoliths are consistent with models that derive the schists from either the franciscan complex or great valley group

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

late mesozoic and cenozoic tectonic history of south central california the late mesozoic cenozoic history of south central california is punctuated by at least five major tectonic events the oldest of these involves late cretaceous drag folding and thrusting of epizonal shelf sediments westward over and to the north along the rising eastern margin of the peninsular ranges batholith these overthrusts with associated drag folds and nappes form imbricate stacks of metasediments slices of plutons and mylonites which overlie a regional core of cretaceous granitic plutons of the peninsular ranges batholith a second distinct kind of epizonal low temperature crustal displacement includes an episode of mid tertiary detachment faulting and folding this represents the westerly extension of the detachment terrane of eastern california and western arizona and utah superimposed upon the cretaceous structural features the mid tertiary motions involve major e w extension and n s compressive strain with formation of east trending antiformal core complexes remarkably parallel to a pervasive set of fold axes and b lineations formed in the cretaceous thrusting and folding all of these features are repeatedly segmented and displaced by step faulting as sial in the salton trough is fragmented and depressed 5 to 10 km below sea level at intervals throughout the cenozoic the irregular step faulted interface of the peninsular ranges batholith and salton trough is accentuated by a series of ne to se striking left lateral and dipslip faults with displacements of several to 10 kilometers these appear to both predate and occur during movements along the san andreas system pliocene pleistocene right lateral faulting a younger extension of the san andreas displacements north of the transverse ranges has broken and displaced all older structural features with total net slip in south central california of perhaps 300 km total right lateral slip along the several strands of the san andreas system are measurable via reconstruction of the mosaic of numerous older predominantly e w trending linear elements mid tertiary antiforms and cretaceous and older rocks right lateral strain on the san andreas appears to either alternate with or to be a byproduct of n s compressional strain from the pleistocene to the present a resultant effect is a large family of e w trending folds and fault scarps in plio pleistocene sediments

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

paleomagnetic evidence for neogene clockwise tectonic rotations in the central mojave desert california paleomagnetic directions from early miocene volcanic flows indicate that a large part of the central mojave desert has been tectonically rotated stratigraphic relations and isotopic ages suggest that two separate clockwise rotations have occurred an early miocene rotation of 50 15 6 is coincident in time and space with opening of the daggett extensional terrane a post early miocene rotation of 30 to 50 may have affected the alvord mountains and the immediate vicinity

neogene tectonic rotation of the san gabriel region california suggested by paleomagnetic vectors previous paleomagnetic work in the western transverse ranges and the islands offshore of california has suggested that these regions have experienced large clockwise rotations since middle miocene time this study is aimed at testing how far east these rotations extend into the continent neogene volcanic rocks from the region bounded by the san gabriel and san andreas faults were selected for paleomagnetic study the sampled units are lavas from the late oligocene to early miocene continental vasquez formation and water laid tuff beds from the middle to late miocene mint canyon formation these rocks possess a stable remanent magnetization which was acquired soon after deposition and is believed to be primary seventy four lava flows from eleven locations in the vasquez formation were studied the average direction for seven locations near the center of the study area suggests that this area has rotated a net clockwise amount of 37 1 12 2 since the early miocene there is no suggestion of north or south translation a location west of the central region is rotated almost 90 clockwise although this is believed to be a local effect detailed magnetic stratigraphy at the western and central location can be correlated over 6 km locations in the east near the san andreas fault yielded highly dispersed directions which cannot now be interpreted in terms of rotation the mean paleomagnetic direction from six sites four different beds in the mint canyon formation west of the vasquez study area is deflected in a counterclockwise sense by 16 30 these data are consistent with a proposed model in which the san gabriel region first was rotated clockwise 53 37 16 in the early miocene and later rotated 16 counterclockwise as it slid northward into the leftward bend region of the san andreas fault the clockwise rotation is believed to be due to dextral shear within the pacific north american plate shear zone counterclockwise rotation in the mojave region is believed to be responsible for the bend in the san andreas fault

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

extensional reactivation of the chocolate mountains subduction thrust in the gavilan hills of southeastern california the ne vergent chocolate mountains fault of south eastern california has been interpreted as either a subduction thrust responsible for burial and prograde metamorphism of the ensimatic orocopia schist or as a normal fault involved in the exhumation of the schist our detailed structural analysis in the gavilan hills area provides new evidence to confirm the latter view a zone of deformation is present at the top of the orocopia schist in which lineations are parallel to those in the upper plate of the chocolate mountains fault but oblique to ones at relatively deep levels in the schist both the orocopia schist and upper plate contain several generations of shear zones that show a transition from crystalloblastic through mylonitic to cataclastic textures these structures formed during retrograde metamorphism and are considered to record the exhumation of the orocopia schist during early tertiary time as a result of subduction return flow the gatuna fault which places low grade supracrustal metasediments of the winterhaven formation above the gneisses of the upper plate also seems to have been active at this time final unroofing of the orocopia schist occurred during early to middle miocene regional extension and may have involved a second phase of movement on the gatuna fault formation of the chocolate mountains fault during exhumation indicates that its top to the ne sense of movement provides no constraint on the polarity of the orocopia schist subduction zone this weakens the case for a previous model involving sw dipping subduction while providing support for the view that the orocopia schist is a correlative of the franciscan complex

middle tertiary extension recorded by lacustrine fan delta deposits plush ranch basin western transverse ranges california abstract the plush ranch formation upper oligocene and lower miocene consists of more than 1800 m of nonmarine sedimentary and volcanic rocks that record the history of an extensional basin referred to here as the plush ranch basin distinctive depositional facies provenance and sediment transport directions along each basin margin suggest an asymmetric basin shape that is consistent with a half graben origin the northern basin margin consists of sandstone dominated alluvial plain deposits 0 1 1 5 m thick normally graded lenticular sandstone beds small deltaic sequences 1 2 m thick were formed where these alluvial systems flowed southward into a lake lenses of massive boulder rich granitic breccia that represent rockslide deposits derived from a nearby northern granitic provenance nterfinger with the alluvial plain facies in contrast to the northern margin the southern basin margin is represented by coarse grained fan delta deposits matrix and clast supported lenticular conglomerate beds 0 2 5 m thick with interbedded trough cross bedded pebbly sandstone represent braided stream and flood flow and or noncohesive debris flow deposits of alluvial fans that drained a highland area to the south the alluvial fan deposits interfinger to the north with several types of subaqueous sediment gravity flow facies including turbidite sandstone beds and matrix supported debris flow conglomerate each of the basin margin depositional systems grades basinward and to the east into lacustrine deposits that include organic rich dark shale evaporite and limestone the lacustrine deposits represent the central and eastern parts of the plush ranch basin which received little coarse siliciclastic sediment basalt deposits that are at least 50 m thick in the west and thicken eastward are interbedded mainly with the lacustrine facies the southern margin of the plush ranch basin formed along a north dipping normal slip fault along which dip separation increased toward the southwest the northern margin developed on the tilted hanging wall block of this fault this fault was later reactivated in post middle miocene time as the present left lateral strike slip big pine fault the plush ranch is one of several extensional and transtensional basins that formed in southern california and western arizona about 25 20 ma as a response to the change from a convergent to a strikeslip tectonic regime along western north america

sandy high energy flood sedimentation some criteria for recognition with an example from the devonian of s w england abstract many modern sand deposits resulting from episodic floods in semi arid zones are dominated by parallel laminations but parallel laminated sands are not a major feature in perennial streams single flood events may deposit over 1 5 m of parallel laminated sand hence ancient parallel laminated sand dominated alluvial deposits may be the product of ephemeral flows the trentishoe formation hangman sandstone group middle devonian of north devon consists in part of laterally extensive beds of parallel laminated fine grained red sandstones that build multistorey sand bodies individual beds are little more than 1 m thick but may extend laterally for hundreds of metres a vertical sequence of erosion surface sparse silt clasts parallel laminations silt drape is often found suggesting high flow stage deposition of plane bedded sand followed by a very rapid waning of flow with silt fallout in the final stages of the flood this sequence bears marked similarity to certain modern ephemeral flow sands

the yaqui ridge antiform and detachment fault mid cenozoic extensional terrane west of the san andreas fault the narrow zone of mid cenozoic detachment terrane in the southwestern united states can be widened and traced across the southern strands of the san andreas fault zone into the eastern margin of the peninsular ranges batholith the yaqui ridge core complex and detachment fault in southern borrego valley california exemplifies the nature of detached terranes in southcentral california the detachment fault dips 10 40 to the northeast and separates a lower core of gneissic late cretaceous granodiorite from an unconsolidated unmetamorphosed megabreccia of eocene to miocene age foliations in the lower plate generally conform to the strike and dip of the overlying detachment fault and become less distinct away from the fault the megabreccia which forms the upper plate is composed of unsorted fairly well rounded clasts characteristic of batholithic and metasedimentary rocks of the region plio pleistocene lacustrine sediments unconformably overlie the megabreccia in some areas the detachment fault and upper plate are expressed as a series of klippen which parallel yaqui ridge along the northeast and southeast flanks the fault is marked by a typically narrow 6 10 cm band of intensely sheared cataclasite a chlorite breccia zone occurs below the cataclasite and gradually grades into the less deformed regionally foliated gneissic granodiorite on the southeast flank what appears to be an extension of the cataclasite and detached upper plate is exposed only locally near the southeast nose of the antiform here the upper plate consists of blocks of pale gray to tan aphanitic fault gouge superimposed upon and cutting through the detachment related features is a broad zone of left oblique slip faulting termed the yaqui ridge shear zone this fault zone trends wnw ese parallel to yaqui ridge and separates the detachment fault from the ridge merging with the detachment fault near the eastern margin further east the shear zone widens into a massive zone of gouge 400 m wide and disappears under the sediments of borrego valley and the salton trough to the west the shear zone slices into the eastern margin of the peninsular ranges batholith and extends the marginal plutons in an e w direction four dominant tectonic episodes can be recognized from exposures at and adjacent to yaqui ridge 1 a late cretaceous synkinematic cataclasis and metamorphism accompanying and following emplacement of the yaqui ridge and related plutons and forming a pervasive regional nw trending foliation and ne trending lineation 2 a subsequent mid cenozoic shallow low temperature event of detachment faulting and folding which produces localized brittle cataclasis which overprints the earlier nw trending fabrics 3 the late miocene early pliocene development of yaqui ridge shear zone along the northeast margin of yaqui ridge shearing rotating and overprinting the fabrics formed during the late cretaceous plutonism and regional deformation 4 a pleistocene episode of high angle faulting and folding which produce a series of e w trending anticlines and synclines in upper pliocene to pleistocene sediments linear elements which have formed throughout the several late cretaceous to cenezoic deformations are concordant

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

microstructural evidence for northeastward movement on the chocolate mountains fault zone southeastern california microstructural analysis of rocks from the chocolate mountains fault zone gavilan hills area southeastern california shows unequivocal evidence for northeast directed transport of the upper plate gneisses over lower plate orocopia schists samples were taken from transects through the fault zone prefaulting fabrics in upper plate gneisses show a strong component of northeast directed rotational deformation under lower amphibolite facies conditions in contrast prefaulting lower plate orocopia schists show strongly coaxial fabrics minimum stretch value of 2 2 formed at greenschist grade mylonitic fabrics associated with the chocolate mountains fault are predominantly northeast directed shear bands that are unidirectional northeastward in the gneisses but bi directional in the schists suggesting a significant component of nonrotational deformation occurred in the orocopia schists during and after emplacement of the upper plate the kinematic findings are in agreement with dillon et al 1989 who found that the vergence of asymmetrical folds within the fault zone indicates overthrusting to the northeast toward the craton in this region the available evidence favors a single protracted northeastward movement on the chocolate mountains fault zone with temperatures waning as deformation proceeded

introduction to special section on the california arizona crustal transect cactis part 3 the cactis california arizona crustal transect interim synthesis workshop in may 1988 brought researchers together in flagstaff arizona to discuss the geologic evolution and crustal structure of the southern cordillera between the san andreas fault in southeastern california and the colorado plateau in arizona sass et al 1988 the first set of papers resulting from the workshop appeared in the journal of geophysical research jgr as the special cactis 1 section in january 1990 howard et al 1990 and a second as cactis 2 in november 1990 haxel et al 1990 twelve papers in this issue form part 3 of the cactis series a final collection of papers will appear in the future as a joint special section with the california consortium for crustal studies calcrust

geologic setting and tertiary structural evolution of southwestern arizona and southeastern california volcanic and sedimentary rocks and structures record the tertiary structural evolution of the lower colorado river region in southwestern arizona and southeastern california a late eocene or early oligocene prior to 33 ma episode of faulting is indicated by medium to coarse grained arkosic rocks in the chocolate and southern trigo mountains much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from 33 to 22 ma but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks a major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 ma and affected the entire area from the palo verde mountains on the west to the kofa mountains on the east extension related faulting quickly waned and had largely ceased by about 20 ma in the kofa mountains elsewhere the timing is poorly constrained by 13 ma thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area volcanism in the lower colorado river region may have been coincident with a broad structural depression now oriented east west the northern limit of the volcanic terrane defines a tilt domain boundary the northern boundary reaching from the new water mountains in arizona to the little chuckwalla mountains in california ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south the southern margin is less well defined but is subparallel to the northern boundary and to the chocolate mountains anticlinorium

evolution of the southern san joaquin basin and mid tertiary transitional tectonics central california a cenozoic tectonic and sedimentary history is proposed for the southern san joaquin basin ssjb and tehachapi mountains that evolved adjacent to the plate margin off central california seismic reflection borehole field biostratigraphic and paleomagnetic data are integrated into geologic and fault structure maps cross sections and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence timing and distribution of complex tectonically linked events the largely buried structures and strata in the ssjb preserve an unusually complete record of the mid tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the pliocene significant structural relief existing across both extensional and contractile features is preserved in the subsurface and an active fold thrust belt propagates basinward along the margin of the u shaped tejon embayment the cenozoic evolution of the ssjb reflects the regional deformation of central california as different tectonic events followed each other along the adjacent north american plate margin five oligocene miocene basin phases are identified in the ssjb 1 late oligocene early miocene extensional subsidence with high and low angle normal faulting accompanied by volcanism and deposition of coarse syntectonic conglomerates 2 middle miocene uplift 3 later mid miocene transtensional subsidence to lower bathyal depths 4 alternating subsidence and uplift until the late miocene and 5 flexural subsidence due to pliocene to recent contraction reconstructions of mid tertiary california place the southern san joaquin tehachapi extensional terrane as a paleotectonic block located between the western mojave terrane then to the east and the western california terrane then to the south and west regional extension occurred during a long transition period between convergent and transform boundaries along the north american plate margin significant slip along the san andreas transform postdates this extensional event its origin apparently coincided with a regional middle miocene uplift event indicating that the san andreas fault is younger than previously supposed the tertiary ssjb has subsided due to extension transtension crustal tilting and thrust related loading the maricopa subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the sierra nevada and locally has subsided beyond 12 km in contrast the relatively stable tejon embayment underlain by sierran crystalline crust achieved its maximum subsidence in the miocene since pliocene time the ssjb has filled and continued to deepen after the basin was tectonically shut off from the pacific ocean

pre transform early miocene extension in western california earliest miocene 20 24 ma faulting and volcanism were widespread in western california including the san joaquin la honda basin coast ranges and transverse ranges evidence for strike slip on faults of this age is lacking their arrangement on a pre san andreas restoration suggests continuity with early miocene extended terranes in eastern california and southwestern arizona deformation was probably not an effect of transform or triple junction migration tectonics because it was synchronous over 300 km of latitude and there is no direct evidence for a transform boundary until late in early miocene time magnetic anomalies in the eastern pacific permit an interpretation that a spreading ridge supplied a still active subduction zone with young oceanic crust until sometime after 20 ma if so extension evidently took place in an arc related setting a transform regime probably formed about 18 ma when most basins underwent episodes of deepening or uplift strike slip faults became active and rotation of the western transverse ranges block began

a model for neogene crustal rotations transtension and transpression in southern california two episodes of crustal rotation have occurred in the regions of southern california and the mojave desert since the beginning of miocene time an early miocene episode of clockwise crustal rotation is associated with widespread north south to north northeast south southwest crustal extension throughout southeast california and western arizona this episode is apparently related to changing subduction parameters of the farallon plate a middle miocene and later episode of clockwise rotation in the transverse ranges and bordering areas is related to shear between the pacific and north american plates in both episodes clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend the effects of extension are most pronounced in the earlier episode but clear in the later one also the pacific north american deforming zone began contracting in width during pliocene time but clockwise rotations continued and are probably in progress today

early history and reactivation of the rand thrust southern california abstract the rand thrust of the rand mountains in the northwestern mojave desert separates an upper plate of quartz monzonite and quartzofeldspathic to amphibolitic gneiss from a lower plate of metagraywacke and mafic schist rand schist the rand thrust is considered part of the regionally extensive vincent chocolate mountain thrust system which is commonly believed to represent a late cretaceous subduction zone the initial direction of dip and sense of movement along the vincent chocolate mountain thrust are controversial microfabrics of mylonites and quartzites from the rand mountains were analyzed in an attempt to determine transport direction for this region but the results are ambiguous in addition the southwestern portion of the rand thrust was found to have been reactivated as a low angle normal fault after subduction reactivation might have occurred shortly after subduction in which case it could account for the preservation of high pressure mineral assemblages in the rand schist or it could be related to mid tertiary extension in the western united states in either event the reactivation might be responsible for the complicated nature of the microfabrics the rand schist exhibits an inverted metamorphic zonation isograds in the schist are not significantly truncated by the reactivated segment of the rand thrust this indicates that other segments of the vincent chocolate mountain thrust should be re evaluated for the possibility of late movement even if they show an apparently undisturbed inverted metamorphic zonation

paleomagnetism and tectonic rotation of the lower miocene peach springs tuff colorado plateau arizona to barstow california we have determined remanent magnetization directions of the lower miocene peach springs tuff at 41 localities in western arizona and southeastern california an unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the colorado plateau to barstow california a distance of 350 km the peach springs tuff was apparently emplaced as a single cooling unit about 18 or 19 ma and is now exposed in 4 tectonic provinces west of the plateau including the transition zone basin and range colorado river extensional corridor and central mojave desert strike slip zone as such the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes from 4 sites on the stable colorado plateau we have determined a reference direction of remanent magnetization i 36 4 d 33 0 95 3 4 that we interpret as a representation of the ambient magnetic field at the time of eruption a steeper direction of magnetization i 54 8 d 22 5 95 2 3 was observed at kingman where the tuff is more than 100 m thick and similar directions were determined at 7 other thick exposures of the peach springs tuff the steeper component is presumably a later stage magnetization acquired after prolonged cooling of the ignimbrite when compared to the plateau reference direction tilt corrected directions from 3 of 6 sites in the central mojave strike slip zone show localized rotations up to 13 in the vicinity of strike slip faults the other three sites show no significant rotations with respect to the colorado plateau both clockwise and counterclockwise rotations were measured and no systematic regional pattern is evident our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post middle miocene strike slip faults in the central mojave desert most of our sites in the transition zone and basin and range province have had no significant rotation although small counterclockwise rotation in the mccullough and new york mountains may be related to sinistral shear along en echelon faults southwest of the lake mead shear zone the larger rotations occur in the colorado river extensional corridor where 8 of 14 sites show rotations ranging from 37 clockwise to 51 counterclockwise these rotations occur in allochthonous tilt blocks which have been transported northeastward above the chemehuevi whipple mountains detachment fault upper plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation from this relation we infer that rotations are accommodated along numerous low angle faults at higher structural levels above the detachment surface

the diagenetic destruction of porosity in plutoniclastic petrofacies the miocene diligencia and eocene maniobra formations orocopia mountains southern california usa

neogene clockwise tectonic rotation of the eastern transverse ranges california suggested by paleomagnetic vectors results from a paleomagnetic study of neogene volcanic rocks situated within and around the pinto basin region of the eastern transverse ranges suggest that the ranges have undergone a clockwise tectonic rotation of 41 4 7 7 since late miocene time both normal and reversed polarities are observed and inclination data indicate no significant flattening the rotation was accompanied by progressive sinistral slip on the several east west trending faults which bound and transect the eastern transverse ranges a computation using the observed amount of cumulative left slip on the east west trending faults of the eastern transverse ranges 50 km and the spacing between the pinto mountain and salton creek faults which bound the region 70 km indicates that 35 of clockwise rotation is required which roughly agrees with the paleomagnetic data removal of the eastern transverse ranges rotation in a reconstruction shows that the distributed dextral shear across the mojave desert region to the north must have been on the order of 100 km since late miocene time from the results of this study we infer that crustal rotation of the eastern transverse ranges began later and is of less magnitude than that of the western transverse ranges

north south contraction of the mojave block and strike slip tectonics in southern california the mojave block of southern california has undergone significant late cenozoic north south contraction this previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate tectonic estimates of pacific north american plate motion and for part of the big bend in the san andreas fault in the eastern mojave block contraction is superimposed on early miocene crustal extension in the western mojave block contractional folds and reverse faults have been mistaken for extensional structures the three dimensional complexity of the contractional structures may mean that rigid block tectonic models of the region based primarily on paleomagnetic data are unreliable

coarse grained deltaic sedimentation in the miocene cuyama strike slip basin california coast ranges abstract the cuyama basin located in the southern coast ranges of california southwest of the san andreas fault developed early in the history of the san andreas transform system the miocene marine basin formed in a transtensional setting along a dextral strike slip fault of the transform system following oligocene non marine basin formation in an extensional setting the lower and middle miocene vaqueros formation in the northwestern part of the basin which represents the first of two transgressive regressive cycles is described here in terms of nine facies in two broad facies groups the 400 m thick soda lake shale member of the vaqueros comprises deep basin and starved basin facies a thin transgressive facies occurs locally at the base of the formation the overlying painted rock sandstone member of the vaqueros which is more than 2200 m thick and consists mostly of coarse grained sandstone and pebbly sandstone constitutes a delta complex of prodelta slope channel delta front tide influenced distributary channel interdistributary bay and fluvial channel facies the basinal depositional system consisted of turbidite mud and sand and hemipelagic and pelagic sediments of the basinal facies deposited in a rapidly subsiding basin the delta depositional system consisted of the delta complex facies that prograded into the deep basin and had a steep prodelta slope that extended to bathyal depths the delta is inferred to be a mixed fluvial wave dominated fan delta analogous in its delta front morphology and processes to a fjord delta in which coarse sediment delivered to the delta front by braided streams was transported down the prodelta slope into deep water by sediment gravity flows transgression and rapid deepening of the basin in the early miocene coincided with rapid tectonic subsidence deepening culminated with deposition of a starved basin facies or condensed section at the time of maximum transgression which was followed by the beginning of a regression and basin shallowing the overall basin history and geometry of the northwestern cuyama basin are typical of strike slip basins the initial rapid subsidence to bathyal depths at rates of more than 500 m m y in the early miocene is interpreted to be a result of extension at the releasing bend of a dextral strike slip fault

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

rare earth and trace elements in igneous and high temperature metamorphic minerals of oceanic gabbros mark area mid atlantic ridge sims analyses have been carried out on clinopyroxenes plagioclases and amphiboles of six gabbroic samples from holes 921 924 of the ocean drilling program leg 153 sited in the mark area of the mid atlantic ridge at the ridge transform intersection to investigate the rare earth trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions the metagabbros underwent granulite to subgreenschist facies conditions through three main tectono metamorphic phases 1 ductile regime 750 t 1000 c and p 0 3 gpa 2 transitional regime 600 t 700 c and p 0 2 gpa 3 brittle regime 350 t 600 c and p 0 2 gpa igneous clinopyroxenes show c1 chondrite normalized patterns depleted in lree and nearly flat for hree the rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as ree reservoirs and comprise the main contribution to the overall rock content the abundances in igneous minerals reflect the degree of fractionation of the parent liquids in metamorphic clinopyroxenes recrystallized in anhydrous assemblages the ree and trace elements patterns mimic those of the primary ones conversely clinopyroxene re equilibrated in amphibole bearing assemblages shows a significant increase in ree ti zr y and v a negative eu anomaly and slight decreases in sr and ba an overall increase of ree and some trace elements is evident in hydrous assemblages with preferential partitioning in the amphibole it shows high ti 18196 22844 ppm lree depleted patterns and lan smn 0 10 0 33 lan ybn 0 10 0 30 amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for tin and negative anomalies for srn and zrn volatiles in amphibole are low with cl f 1 h2o is significantly lower than the stoichiometric ratio 1 33 1 53 the composition of the clinopyroxene and amphibole recrystallized in low strain domains records evidence of incomplete re equilibration and element diffusion and partitioning is in part controlled by the textural site the possible origins of the fluids involved in the metamorphic recrystallization are discussed 1 remobilization from igneous amphibole 2 exsolution from evolved melts 3 introduction of seawater derived fluids modified in rock dominated systems 4 injection of highly evolved hydrous melts during the metamorphic process

timing and extent of neogene tectonic rotation in the western transverse ranges california paleomagnetic data collected from lower miocene rocks throughout the western transverse ranges province have consistently clockwise deflected magnetic declinations which suggests that the entire province has experienced clockwise tectonic rotation during neogene time paleomagnetic declinations in lower miocene rocks increase westward from 33 5 c 11 in the san gabriel block to 91 7 7 in the western santa ynez range suggesting a westward increase in the net amount of clockwise tectonic rotation paleomagnetic data that constrain the tuning of rotation suggest that the entire western transverse ranges rotated uniformly 50 60 clockwise during the middle miocene this implies a minimum of 200 km of distributed dextral shear along the california coast and offshore during middle miocene time little tectonic rotation appears to have occurred in the western transverse ranges during late miocene time since miocene time continued clockwise rotation in the western santa ynez range 31 3 combined with counterclockwise rotation in the san gabriel block 16 has oroclinally bent the transverse ranges to the west of the san andreas fault the pliocene pleistocene clockwise rotation implies an additional 30 60 km of distributed dextral shear across the southern california coast ranges during post miocene time

late oligocene early miocene volcanism faulting and sedimentation in west central california abstract abundant evidence from the rock record indicates that volcanism active faulting and sedimentary basin development were widespread and nearly synchronous in west central california during the late oligocene and early miocene volcanic activity occurred about 24 22 ma at more than 20 eruptive centers in a northwest trending belt extending from point arena to the orocopia mountains volcanism was accompanied by marine transgression rapid bathymetric deepening and accumulation of thick piles of predominantly marine clastic sediment in the point arena la honda san joaquin pescadero ano nuevo salinas husana san luis obispo and cuyama basins as well as in several other basins presently situated offshore concurrent movement along basin margin normal faults resulted in localized deposition of wedges of very coarse nonmarine and deltaic sediment in the la honda cuyama lockwood soledad and diligencia basins also at about the same time right lateral strike slip faulting apparently caused en enchelon folding in both the cuyama and san joaquin basins and formation of a pull apart feature in the la honda basin late oligocene early miocene volcanism faulting and basin development in west central california were nearly synchronous over the entire area rather than sequential from south to north and therefore cannot be explained simply as the effects of the northwestward migrating mendocino triplemore junction an alternative explanation is that volcanism faulting and basin development were caused by an episode of regional extension or transtension strike slip with a component of extension related to a change in plate motions in the circum pacific area less

basin evolution during change from convergent to transform continental margin in central california miocene nonmarine and shallow marine strata exposed east of san francisco bay record a change from convergent margin tectonics to transform margin tectonics during the middle miocene the east bay area occupied the oceanward side of a shelved forearc basin that was progressively incorporated in the evolving san andreas strike slip orogene patterns of deposition in the broad forearc basin were relatively simple andesitic arc derived detritus was transported the full width of the forearc basin from the sierras to the east bay area in contrast the wrench tectonic regime produced complex patterns of sedimentation displaying greater local variability on the basis of stratigraphic data we infer that the west facing slope of the forearc basin in the east bay area was reve sed about 13 ma with uplift of the area between the eventual traces of the san andreas and hayward faults on the site of the present bay a fluvial clastic wedge was shed eastward into the east bay area from this uplifted terrane of mesozoic subduction complex and forearc basin rocks initial rupturing along the hayward fault trend followed the uplift at about 10 ma loci of basaltic volcanism 10 7 ma along these fractures interfinger with the clastic wedge a similar pattern of uplift and drainage reversal apparently presaged the onset of wrenching along the nearby calaveras trend from 8 6 ma expansion of the strike slip orogene segmented the outer forearc basin into local basins some characterized by episodic lacustrine deposition and probable internal drainage by the end of the m ocene sierran arc volcanism waned at the latitude of san francisco bay and arc derived volcaniclastics were fully supplanted by recycled coast range derived detritus in the east bay area certain of these coast range sediment sources particularly blueschist bearing franciscan terranes permit an estimate of 7 27 km 4 17 mi of total right slip on the hayward fault

detailed surface co seismic displacement of the 1999 chi chi earthquake in western taiwan and implication of fault geometry in the shallow subsurface the northern segment of the chelungpu fault shows an unusually large co seismic displacement from the event of the mw 7 6 chi chi earthquake in western taiwan part of the northern segment near the fengyuan city provides an excellent opportunity for characterizing active thrust related structures due to a dense geodetic benchmark network we reproduced co seismic deformation patterns of a small segment of this chelungpu fault using 924 geodetic benchmarks according to the estimated displacement vectors we identified secondary deformations such as local rigid block rotation and significant shortening within the hanging wall the data set also allows us to determine accurately a 3d model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip and the horizontal and vertical displacements at the surface the predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area the successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations

vertical axis rotations in the mojave evidence from the independence dike swarm the 500 km long late jurassic independence dike swarm in california extends from the central sierra nevada through the mojave desert to the eastern transverse range the dikes are mostly vertical and their azimuth in the sierra nevada is 315 however in the mojave region their azimuth varies significantly 300 320 in the argus range the spangler hills and the stoddard well and soda mountain areas and 340 350 in the granite mountain alvord mountain and eagle mountain areas apparently the dikes in the granite alvord and eagle mountains areas have rotated 50 clockwise about vertical axis relative to the sierra nevada and the stoddard well area these rotations are remarkably consistent with known post early miocene paleomagnetically derived rotations the clockwise rotations are associated with the east west trending left lateral strike slip faults in the northeastern mojave and the eastern transverse range regions in contrast the data show that little or no rotation occurred in the central mojave domain which is characterized by northwest trending faults

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

reconciling disparate estimates of total offset on the southern san andreas fault late cenozoic tectonic reconstructions of southern california united states have been hindered for decades by disagreement over the total displacement on the southern san andreas fault disparate estimates of dextral offset range from 160 km to 240 km prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers we present a new reconstruction that integrates published lithologic structural and paleocurrent data with the effects of clockwise rotation in the eastern transverse ranges etr this model yields 200 14 km of cumulative offset on the southern san andreas fault system in the salton trough the disparity between our estimate and a lower estimate of 160 km for the mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the etr transfer of strain to the eastern california shear zone and diffuse crustal shortening in the big bend region this reconstruction reconciles previously incompatible and long debated models and places an important new constraint on the tectonic evolution of the pacific north america plate boundary in southern california

tectonics and alluvial sedimentation of the upper oligocene lower miocene vasquez formation soledad basin southern california the upper oligocene lower miocene vasquez formation marks the earliest sedimentation in the soledad basin central transverse ranges southern california the vasquez consists primarily of alluvial sediments and presently appears as outcrops in three geographically restricted fault bounded sub basins the two southern sub basins vasquez rocks and texas canyon shared similar tectonic and depositional histories the northernmost sub basin charlie canyon appears to have had a distinct history the soledad basin originated as a predominantly orthogonal rift during the latest oligocene incipient subsidence was concentrated in the southeastern region of the basin as debris flow deposits accumulated as small thick alluvial fans draining local source areas adjacent to vasquez rocks sub basin intense rifting volcanism and rapid subsidence produced the half graben geometry of this sub basin as a source area rose to the south southeast across the active soledad fault coeval displacements along the vasquez canyon and pelona faults led to the asymmetric graben geometry of texas canyon sub basin as abundant detritus derived from an eastern southeastern source interfingered with small debris flow dominated fans along the pelona fault margin periodic faulting and source area uplift followed by tectonic quiescence and erosion produced thick upward fining alluvial megacycles in both sub basins major tectonic uplift in the ancestral san gabriel area led to drainage system enlargement increased water discharge into the depositional systems and deposition by hyperconcentrated flood and braided fluvial processes in both sub basins erosional dissection of mint canyon ridge allowed physical interconnection of the two depocenters at this time charlie canyon sub basin the northernmost vasquez depocenter displays no clast suites or alluvial megacyclicity to suggest close affinities with the other two depocenters a sequence of easterly derived braid plain sediments low in the section is overlain by a 1 600 m upward coarsening alluvial sequence this sequence reflects probable inception of the san francisquito fault uplift of a marine sedimentary quartz monzonite quartz diorite source terrane and northward progradation of an alluvial fan system post oligocene clockwise rotation of the soledad basin by as much as 40 degrees is indicated by paleomagnetic data of other workers restoration of the soledad basin to its oligocene orientation indicates that southeast northwest extension caused rifting and heralded vasquez deposition in small rapidly subsiding basins neither compressional deformation nor strike slip deformation seems to have been a significant factor during vasquez sedimentation the sedimentary and tectonic history of the soledad basin is consistent with a plate tectonic model involving extension in the north american plate north of the unstable mendocino triple junction

subduction and exhumation of the pelona orocopia rand schists southern california the pelona orocopia and rand schists of southern california and southwestern arizona are thought to have formed in either the same east dipping subduction zone as the franciscan complex or in a southwest dipping subduction zone related to collision of an outboard continental fragment with north america the principal justification for the collision model has been the observation that continental rocks overlying the schists locally show transport to the northeast field and petrographic studies now confirm that the northeast movement occurred during exhumation of the schist not during subduction combined with regional geologic relations this evidence makes the collision model unlikely

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

contemporary kinematics of the western united states determined from earthquake moment tensors very long baseline interferometry and gps observations using moment tensors from earthquakes between 1850 and 1995 we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western u s plate boundary zone the velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5 of the nuvel ia pacific north american plate motion direction the magnitude of the earthquake related velocity is 62 of the nuvel 1a total pacific north american plate motion if earthquake occurrence is a random poisson like process then the large formal errors associated with the model velocity estimates are an indication that the 144 year interval is too short to define the long term seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long term moment release rate associated with both the seismic and aseismic deformation to spatially resolve the deficit of the earthquake moment release in the last 144 years we have also estimated the total long term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities both with and without the nuvel 1a pacific north american rigid plate motion constraint we find a systematic difference between the southern california earthquake center scec geodetic velocity map and nuvel 1a north american reference frames with a pole at 38 8 n 124 2 w 0 33 m y 1 this difference is significant at the 99 confidence level and indicates that 4 7 mm yr of ne sw convergence is absorbed in offshore areas of southern california if both the scec velocity map and nuvel 1a correctly describe the relative motion between the pacific and north america plates deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long term model strain rate tensors are largest in areas east of san francisco bay in the transverse ranges and along the southern san andreas fault and san jacinto fault the deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an mw 7 earthquake elsewhere the accumulated strain in the last 144 years is small except in northern mojave desert along the garlock fault and in the region southwest of the southern san andreas san jacinto fault zone within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an mw 6 5 7 0 earthquake

collapse of southwestern north america and the evolution of early miocene detachment faults metamorphic core complexes the sierra nevada orocline and the san andreas fault system plate reconstructions indicate that early miocene pacific north american relative motion was not parallel to the transform but was instead oblique and transtensional we propose that the western edge of the north american plate east of the transform gravitationally collapsed to the southwest in response to this divergence the northern and eastern edges of the collapsed region underwent extension between 24 and 20 ma whereas extension in southeastern california and southern arizona during this interval was coaxial with the overall s50 60 w collapse direction the extension direction along the northern oundary mojave extensional belt was north south and the result of the combined effects of the southwestward collapse and the independent northwestward translation of the region to the north collapse continued between 20 and 16 ma and extension persisted in southeastern california and southern arizona extension along the northern boundary waned owing to the apparent cessation of motion of the region to the north thus the kinematics of the northern boundary changed to dextral shear and extension along what is here named the trans mojave sierran shear zone this east west zone of mainly right shear oroclinally folded the southern tail of the sierra nevada and the central mojave desert rocks within this 83 110 km wide zone were rotated approx 40 60 clockwise about vertical axes finally as collapse moved the north american plate progressively west across the locus of transform shear its western edge became increasingly subject to the motion of the pacific plate over time the edge of north america was truncated by faults of the san andreas system and transferred to the pacific plate

eruption of alkali basalts during crustal shortening in southern california the common occurrence of alkali basalts in continental rifts has led to the widespread assumption that late cenozoic alkali basalts of the western united states indicate an extensional tectonic regime however field relations of late cenozoic alkali basalt flows and cinder cones in the mojave desert area including flows that bear mantle xenoliths indicate that many were erupted in contractional or transpressional tectonic settings these observations call into question models of extensional magmatism in which lithospheric extension facilitates transport of magmas to the surface and suggest that an association of alkali basalts and coarse clastic sedimentary rocks need not indicate rifting

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

simple shear of southern california during neogene time suggested by paleomagnetic declinations we have studied the paleomagnetism of various neogene age rocks in southern california as a means of determining the amounts of oligocene and younger tectonic rotation and translation which has occurred in this region our results suggest that fully 25 of this area in particular the transverse ranges has undergone extreme clockwise rotation work in southeastern california implies that 40 of clockwise rotation has occurred here although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200 the crustal block bounded by the san gabriel and san andreas fault has undergone a net clockwise rotation of 35 although the data here can be interpreted to show that an original early miocene rotation of about 50 was followed by a late miocene or pliocene counterclockwise rotation of 15 paleomagnetic results from the offshore islands suggests that san clemente santa barbara and san nicolas islands have not rotated but santa catalina has undergone about 90 of clockwise rotation all of the northern channel islands including anacapa santa cruz santa rosa and san miguel are implied to have rotated 70 or 80 clockwise this result is also found for the santa monica mountains east of anacapa island in the santa ynez range north of the channel islands paleomagnetic study of the monterey formation also indicates large clockwise rotations of near 90 these data also suggest that the santa maria basin is not rotated and that the north boundary of the rotated region is the santa ynez river fault stratigraphic control on the paleomagnetic data from the monterey formation implies that the rotation began about 16 m y ago and may be continuing today in the western region paleomagnetic inclination data from our study show that the northern channel islands in particular may have translated 15 northward since middle miocene time however equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections non dipole magnetic field behavior and right offset on the san andreas fault system palinspastic reconstruction of southern california regions for the early miocene implies that parts of the transverse ranges structures were once aligned with north trending extensional structures in the southwestern united states we propose that pacific american plate interactions both rifted the continental crust to create this pattern and rotated the western most structures within a dextral simple shear zone which had a half width of about 400 km

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region